diff --git "a/txt/美国学生科学读本(英汉双语版)(套装上下册) (西方原版教材之文史经典) - 威廉·H·斯奈德.txt" "b/txt/美国学生科学读本(英汉双语版)(套装上下册) (西方原版教材之文史经典) - 威廉·H·斯奈德.txt"
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+西方原版教材与经典读物•科学系列
+
+
+
+
+
+Authored by William H. Snyder
+
+Principal of the HollywoodHigh School
+
+Los Angeles
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+
+
+
+
+图书在版编目（CIP）数据
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+
+
+
+
+美国学生科学读本/（美）斯奈德著；贺巍译．—天津：天津人民出版社，2013.4
+
+ISBN 978-7-201-08072-7
+
+
+
+
+
+Ⅰ．①美…　Ⅱ．①斯…　②贺…　Ⅲ．①科学知识–中小学–课外读物　Ⅳ．①G634.73
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+
+
+
+
+中国版本图书馆CIP数据核字（2013）第040085号
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+
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+
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+天津出版传媒集团
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+
+
+* * *
+
+
+
+天津人民出版社出版、发行
+
+出版人：黄沛
+
+（天津市西康路35号　邮政编码：300051）
+
+网址：http://www.tjrmcbs.com.cn
+
+电子邮箱：tjrmcbs@126.com
+
+北京建泰印刷有限公司
+
+
+
+
+
+2013年4月第1版　2013年4月第1次印刷
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+710×1000毫米　16开本　35.5印张　字数：500千字　插图：445幅
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+
+
+
+
+定　价：79.80元（上下册）
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+
+
+
+
+目　录
+
+
+FOREWORD
+
+译者序
+
+
+
+
+
+CHAPTER 1
+
+THE EARTH AND ITS NEIGHBORS
+
+地球和它的邻居们
+
+
+
+
+
+CHAPTER 2
+
+THE PLANET EARTH
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+行星地球
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+
+
+
+
+CHAPTER 3
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+THE GIFTS OF THE SUN TO THE EARTH
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+太阳给地球的礼物
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+
+
+
+
+CHAPTER 4
+
+THE EARTH'S CRUST
+
+地球的外衣
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+
+
+
+
+CHAPTER 5
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+THE ATMOSPHERE OF THE EARTH
+
+地球的大气层
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+
+
+
+
+CHAPTER 6
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+THE LIVE PART OF THE EARTH
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+地球上的生命
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+
+
+
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+CHAPTER 7
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+LIFE OF THE EARTH AS RELATED TO PHYSICAL CONDITIONS
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+环境对地球生命的影响
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+
+
+
+
+CHAPTER 8
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+THE SEA
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+海 洋
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+
+
+
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+CHAPTER 9
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+COAST LINES
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+海岸线
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+
+
+
+
+CHAPTER 10
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+WATER SCULPTURE
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+水之妙手
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+
+
+
+
+CHAPTER 11
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+ICE AND WIND SCULPTURES
+
+冰心风吟
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+
+
+
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+CHAPTER 12
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+LOW AREAS OF THE EARTH
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+地球上的低地
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+
+
+
+
+CHAPTER 13
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+THE HIGH AREAS OF THE EARTH
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+地球上的高地
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+
+
+
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+CHAPTER 14
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+VOLCANOES
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+火 山
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+
+
+
+
+APPENDIX
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+附 录
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+
+
+
+
+FOREWORD
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+译者序
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+
+小时候，我们每个人似乎都曾有过一个湛蓝色的科学梦。在那个以天空、大海为背景的梦境中，我们不断追问着一个又一个“什么”和“为什么”：是什么构成了我们的宇宙？为什么世间万物的运动变化这么有规律？地球为什么是圆的？地球的核心里面到底是什么，有神仙吗？为什么大自然有如此绚丽多姿的色彩？为什么生命世界有如此温婉动人的故事？为什么会有我们人类？
+
+今天，科学的发展前沿，已经到了宇宙的边缘和物质最基本的粒子层面，直逼宇宙万物最为核心的部分。我们曾经的这些“梦之问”大多已经有了答案，但是还有不少依然悬而未决。一代又一代的科学家和教育家们，不断地在追寻答案的路上给我们带来好消息，进而又在书本上和教室里娓娓道来，认真而深情地讲给孩子们听。你眼前的这本书，就是这样诞生的。
+
+此书是美国洛杉矶一位名叫威廉·Ｈ·斯奈德（William H. Snyder）的中学校长，和他的同事一起，为中学生编著的一本自然科学入门教材。这位校长虽名不见经传，但他广博而细密的自然科学知识却不得不令人钦佩，几乎将当时所有科学门类的基础知识都融进了本书。从日常物体的运动到太阳系的组成；从江河湖海的欢歌笑语到大陆高山的沉吟咏叹；从微小细菌的自生自灭到动植物与人类的生命活动；从风霜雨雪的翻姿飞舞到春夏秋冬的律吕变换……书中应有尽有，每一章都是孩子们感兴趣的一个领域。其中的每一小节，又都是紧扣着主题的阐发与讲述，语调平实生动，情感真挚感人，宛如一位慈祥的老人注视着孩子们充满稚气的眼睛，讲述着一个又一个动人的故事……
+
+翻开此书，我们便似乎回到了美国洛杉矶的中学课堂上，教堂的钟声还在远处回荡，翻飞的黄叶在微风中飘落到窗台上，这位温厚的老校长正和颜悦色地给我们讲述着地球与太阳、昆虫与花朵、高山与大地、天风与海浪。
+
+这里我还想指出作者在书中有意无意地表露出来的两个重要的“微言大义”。
+
+第一，自然条件对人类文明进程的影响。作者在每一章的末尾，都会拿一小节专门讲述该要素对人类的影响，这是一个重要的史观。历史的演进往往并非完全决定于人为，而是跟自然地理条件大有关系。这样的事例古今中外皆不少见，历史学家黄仁宇先生在谈及著名的淝水之战时，就曾将这一影响后来数百年中国政治格局的战役归因为地理原因，他说：“北人所擅长的骑兵战术，至此地已无法做到有效发挥。南人所长的水军，不仅兵力以舟楫输送，能够争取战场主动，而且将士无行军之劳，粮糈有速达之效。只是这种长处，也不能向北延伸使用。淝水之战时，双方受��形限制的情形，已现其端倪”。本书中这样有趣的例子还有很多，当然也都非常有启发意义，这算是我们在收获科学知识之余，又能体会到的一个史学视角。
+
+第二，对客观世界如何导致生命产生的追问。书中在讲述每一个大自然的环境要素时，作者都会附带提到它对生命诞生的重要性或者促进性。比如大陆与海洋的面积比率、地球与太阳和月亮的距离、大气层的构成、水的特性、土壤的运化、生物体的自身构造等等。虽然作者没有给出这一切为何如此精准巧合的答案，甚至连这个问题也没有正式提出来，但是我们作为后来的读者，不妨对此问题稍稍留意。其实这个问题已经被理论物理学家、宇宙学家、哲学家和神学家们和反复思考和辩论过，并形成了今天的“人择原理”，更进一步产生了强弱不同的数个版本，科学家们至今还在为此争论不已。虽然这个问题到现在依然没有答案，甚至会不会有答案也不知道，但此书从不同侧面给这个问题拉开了序幕，给我们的思考空间也留了很大的余地。
+
+最后要说一说此书的语言。整本书的英语原文平实而舒缓，虽然不乏专业术语，但主体依然醇厚耐读，对于正在学习英语的中学生朋友来说，也是一本上佳的课外阅读范本。
+
+科学梦关乎人类最本真的心灵，对未知世界的好奇与探求更是人类文明永不歇绝的动力。科学的进步也需要一代又一代人前仆后继的努力，这就是科学的托命。正如1988年诺贝尔物理学奖得主莱德曼所坚信的:
+
+
+
+
+
+在全世界60亿人口中，一定有一颗年轻的、与爱因斯坦同样智慧的心，在等待着被发掘。
+
+
+
+
+
+这颗心在哪里呢？会是你吗？
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+
+
+
+
+贺巍（新浪微博@南山薰风）
+
+2012年12月于成都　翠屏湾
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+
+
+
+
+CHAPTER 1
+
+THE EARTH AND ITS NEIGHBORS
+
+地球和它的邻居们
+
+
+1. The Evening Sky. —As the light of the sun fades in the evening, we see the stars coming out one by one until at last the sky is studded with them. We notice, too, that the brighter the star is, the sooner it appears. In the morning, just the reverse of this takes place, the stars begin gradually to fade, and the brightest stars are the last to disappear.
+
+
+
+PART OF THE MILKY WAY.
+
+The plate for this photograph was exposed ten hours and a quarter.
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+
+
+We know how brilliant the light of a match or candle appears in a dark room, and how a light of this kind seems to fade out when it is brought into the presence of a strong electric light. It would seem quite probable that the vast light of the sun might have the same effect upon the light of the stars. This supposition is also supported by the fact that when the sun is covered in an eclipse the stars begin to appear as in the evening. Astronomers are all agreed that if it were not for the greater brilliancy of the sun we should see the heavens full of stars all the time.
+
+
+
+DOME OF THE 60-INCH REFLECTING TELESCOPE AT MT. WILSON SOLAR OBSERVATORY.
+
+Pictures of the heavens are taken through a telescope.
+
+
+
+In the northern hemisphere the stars, except those at the north, which seem to go around in a circle, appear to rise in the east and to set in the west, just as the sun does. If we observe the stars which rise to the east, southeast, and northeast of us, we shall find that these are above the horizon for different lengths of time.
+
+The ancients noticed these facts, and explained them by saying that the earth was at the center of a hollow sphere, upon the inner surface of which were the stars, and that this sphere was continually revolving about the earth and also slightly changing its position in respect to the earth. We of the present day know that it is the earth that is turning around on an imaginary axis, and also gradually changing its position in relation to the stars. We also know that this axis, if extended far enough, would almost strike a star in the center of the northern heavens, which we call the North Star. The points on the surface of the earth through which the axis passes are called the poles.
+
+2. The Earth as one of the Planets. —If we carefully observe the bright points which appear in the sky at night, we shall see that almost all of them shine with a twinkling light. There are, however, three of the brightest which give a steady light like that of the moon. When the positions of these three bodies are carefully observed for some time, it will be seen that they are continually changing their places among the stars, whereas the positions of the stars do not appear to change in relation to each other.
+
+One of these three brightest points has a reddish brown color and has been named Mars, from the Roman god of war. The other two bear the names Venus and Jupiter, one named from the goddess of beauty and the other from the king of the Roman gods. Astronomers call the earth and these three bodies, together with four others, planets, and tell us that they revolve around the sun as a center. They have no light of their own as do the true stars, but the light which comes to us from them is a reflection of the light of the sun.
+
+
+
+MARS.
+
+Most like the earth of all the planets. It is supposed to have a polar ice cap.
+
+
+
+The unaided eye is able at some times to see five of these planets. Astronomers tell us that their change of place in relation to the stars is due to their motion about the sun. If we could stand upon one of these visible planets, our earth would appear to us like one of them. But the surface of some of these planets, like Jupiter or Saturn, is not solid like that of the earth. Our sun, if seen from the distance of one of the stars, would appear like a star.
+
+
+
+THREE VIEWS OF SATURN.
+
+The planet with the beautiful rings.
+
+
+
+The list of the planets in the order of distance from the sun is: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The sun, with the bodies revolving about it, is called the solar system. There is reason to believe that ours is only one of many similar solar systems that exist throughout space.
+
+The planets are by far the nearest of all the starlike bodies, although the distance from the sun to the farthest of the planets is some 2700 million miles greater than the distance from the earth to the sun. The distance of the nearest of the stars however, is probably about 25,000,000,000,000 miles. This distance is so great that it takes light, which travels at the inconceivable rate of 186,000 miles in a second of time, over four and a half years to come to us from this star. From Arcturus, another of the stars, it takes light about 180 years to reach us, and from others very much longer. Sometimes from this outer space comets visit our solar system. Thus we see that our little earth is only a speck in the universe.
+
+
+
+HALLEY'S COMET.
+
+One of the most famous visitors from outer space. The small white dots are stars seen through the tail.
+
+
+
+In the space between the planets Mars and Jupiter, there has been found a group of small bodies which are called planetoids or asteroids. The brighest of these is Vesta, not more than 250 miles in diameter.
+
+A famous theory, called the Nebular Hypothesis, was suggested many years ago to account for the formation of our solar system. This theory supposes that the materials of which the members of the solar system are composed once formed a cloud or nebula of finely divided matter filling an enormous space, and that this matter, by reason of the mutual attraction of the particles, gathered together into what is now our sun with its planets and their satellites. Man is unable to comprehend how matter originated or how matter can either be created or destroyed. But we do know many of the properties of matter.
+
+
+
+
+
+DIAGRAM OF THE SOLAR SYSTEM.
+
+Showing roughly the positions of the various planets and their moons.
+
+
+
+3. Properties of the Matter Composing the Universe.
+
+
+
+
+
+Experiment 1. —Pull out the handle of a compression air-pump or bicycle pump. Close the exit valve or stop up the end of the bicycle pump. Now try to push in the handle. What keeps it from moving easily? Try to shove an inverted drinking glass into a pail of water. Why does not the water fill the glass?
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+
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+
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+All matter as we know it occupies room or space. In other words, it has extension. When we pump up a bicycle tire we find that even the air demands room for itself. In the experiment with the air compressor we found that the space occupied by the air could be reduced only to a limited extent. However great the pressure might have been the air would still have occupied a certain amount of space.
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+
+
+
+
+Experiment 2. —Place a coin on a card extending slightly beyond the edge of a table. Suddenly snap the card horizontally. Does the coin move?
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+
+
+
+
+Fig. 1.
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+
+
+Another of our common observations is that a body does not begin to move unless some force acts upon it, nor when moving does it stop unless some force stops it. When the card was snapped from under the coin, the coin did not appear to move because the friction of the paper was not sufficient to transfer any appreciable motion to it. If the coin had been glued to the card, both coin and card would have moved.
+
+
+
+
+
+Experiment 3. —Revolve around the hand a small weight attached to a strong rubber band. Suddenly let go the band. Does the weight keep on moving in the circular path in which it was revolving?
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+
+
+
+
+Fig. 2.
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+
+
+When a car is moving along a level track we do not expect it to stop until the friction of the track or some other force stops it. When we revolved the weight attached to the rubber band and let go the band the weight started off in a straight line. It did not continue in this straight line because a force, gravity, pulled it down toward the earth. This property which bodies have of remaining at rest unless acted upon by some force, and when in motion of continuing to move in a straight line with the same speed unless acted upon by an outside force, is called inertia. Sir Isaac Newton first stated this fact, and so it is sometimes called Newton's First Law. It is due to inertia that people are thrown out of an automobile if it is suddenly stopped.
+
+
+
+
+
+Experiment 4. —Suspend a heavy ball by a string not much too strong to hold it. (Place a pad beneath it to catch it if it drops.) Attach a similar string to the bottom of the ball. Attempt to lift it suddenly by the upper string. What happens? Suspend it again and pull down gradually on the lower string. What happens? Suspend it again and pull down suddenly on the lower string. What happens?
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+
+
+
+
+Fig. 3.
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+
+
+When we tried suddenly to lift the suspended ball the force of inertia was so great that it broke the string. When the string was attached to the bottom of the ball and the pull gradually exerted, the upper string broke, since it had both the weight of the ball and the pull of the string to withstand; but when the pull was suddenly exerted, the inertia of the ball was sufficient to withstand the pull, and the lower string broke.
+
+It is the inertia of the water which enables the small, rapidly revolving propeller to move the big ship. The same is true of both the propelling and supporting of flying machines. The resistance which the particles of air offer to being suddenly thrown into motion, their inertia, enables the propeller to push the aeroplane along and keeps it from falling to the ground as long as it is moving rapidly. It is inertia which keeps the heavenly bodies moving in space. Once in motion they must keep on forever unless some force stops them.
+
+
+
+A BIPLANE.
+
+The blur shows how swiftly the propeller is revolving.
+
+
+
+Experiment 5. —Place a glass globe partly filled with water on a rotating apparatus. Rotate the globe rapidly. What does the water tend to do?
+
+
+
+
+
+Fig. 4.
+
+
+
+Inertia also manifests itself in the tendency of revolving bodies to move away from the center around which they are revolving. Inertia thus manifesting itself is called centrifugal force. An example of this was seen in Experiments 3 and 5.
+
+Newton many years ago discovered that all bodies of matter have an attraction for each other and that this force of attraction varies as the masses of the bodies, that is, the more matter two bodies contain the more they attract each other. But this attraction becomes less as the distance between the bodies increases. This lessening of the force of attraction on account of the increase of distance is proportional not to the distance, but to the square of the distance. This means that the attraction between the same bodies when twice as far apart is only one fourth as great; when three times as far apart, one ninth as great, and so on. What causes this attraction no one knows, but the name given to this force of attraction is Gravitation. Gravitation is always acting upon all bodies, and their conduct is constantly affected by it. It keeps the heavenly bodies from wandering away from each other just as the rubber band kept the weight from flying away from the hand.
+
+When this attraction is considered in relation to the earth and bodies near its surface the term gravity is used. We are constantly measuring the pull of gravity and calling it weight. This is the cause of bodies falling to the earth. It is the force which causes us to lie down when we wish to sleep comfortably, and frequently makes men fall who try to fly.
+
+If two forces act upon a free body, each will influence the direction of its motion and it will go in the direction of neither force, but in a direction between the two. If there are more than two forces, the path will be the result of the action of all the forces. In the case of the weight and the rubber band we found that the moving weight when not held by the force of the band flew away from the hand. The rubber band continually pulled it toward the hand. The result of these two forces, the "centrifugal force" and the pull of the band, was to make the path of the weight lie between the two. In this case the forces at every instant almost balanced each other, and the path was nearly a circle. It is the force of gravitation acting against the inertia, or the centrifugal force, of the heavenly bodies which holds them in their orbits.
+
+
+
+THREE FORCES IN PLAY.
+
+
+
+
+
+Fig. 5.
+
+
+
+4. Relation of the Earth to Sun and Moon. —Not only do all the planets revolve around the sun, but certain of these themselves have other smaller bodies revolving around them. We call such small bodies satellites or moons. The earth has one of these satellites and Saturn has the greatest number of all, ten, one having been discovered as late as 1905. Our own moon has a diameter of about 2000 miles and a weight of about 1/80 that of the earth. Its average distance from the earth is about 240,000 miles. Compared with the distance of the other heavenly bodies it is indeed very near.
+
+The sun, although a near neighbor as compared with the rest of the stellar community, is at an average distance of about 93,000,000 miles. It is so big that if it were hollow and the earth were placed at its center with the moon as far away as it now is, there would be almost as great a distance between the moon and the sun's surface as there is between the moon and the earth. A good way to get an idea of the relative size of these bodies is to let a pencil dot represent the moon, a circle an eighth of an inch in diameter the earth, and a circle with a diameter of a little more than thirteen and one half inches the sun.
+
+
+
+SURFACE OF THE MOON.
+
+Showing the great crater-like depressions.
+
+
+
+Both the sun and the moon are of the greatest interest to us, as they have much to do with our existence. If it were not for the sun, we should have almost no heat or light on the earth, and life could not exist. If the sun were much nearer, it would be too hot for life as we know it, and if much farther away, too cold. If it were not for the moon, the beauty and variety of our nights would be largely lacking, and we should have no tides strong enough to sweep clean our bays, removing the sewage, and to help vessels over the bars into some of our harbors. If the distance of the moon were changed, the height of the tides would be changed, and this would greatly affect our coast towns.
+
+
+
+HIGH TIDE IN Nova SCOTIA.
+
+
+
+Although we see the moon as a very bright object at night for a part of every month, yet it has no light of itself, and all the light it gives us is reflected from the sun. It has a rough, barren, rocky surface, full of great crater-like depressions. As far as known, it has no air or water upon it. As the earth goes around the sun, and the moon around the earth, the position of these three in relation to each other is constantly changing, and it is these changes which give us the varying heights of the tides and the different phases of the moon. It is profitable to try to picture to oneself the changing phases of the moon. A good way to do this is to carry a ball around a bright light and observe what part of the surface is illuminated in the different positions.
+
+
+
+LOW TIDE AT THE SAME PLACE.
+
+Showing the clean swept sea floor.
+
+
+
+Summary. —We have seen that the earth is one of eight planets which revolve about the sun. The sun and the bodies revolving around it comprise the solar system. The fixed stars, which we see only at night because of the great brilliancy of the sun in the daytime, are suns and may—like our sun—be the centers of separate solar systems.
+
+
+
+THE PHASES OF THE MOON.
+
+Showing roughly the positions of the sun, moon, and earth.
+
+
+
+Everything in the universe is composed of matter, which has certain definite properties, like extension, inertia, gravitation, and so on. The action of some of these properties maintains the relation of the different heavenly bodies—keeps the earth revolving around the sun, and the moon around the earth.
+
+The sun and the moon have more influence upon the earth than do any other bodies. The sun gives us energy in the form of light and heat and so maintains all the life upon the earth. The moon, though shining only by light reflected from the sun, exerts a great attraction upon the earth, causing the tides, which help to keep the waters of our harbors clean.
+
+
+
+
+
+QUESTIONS
+
+
+Why do we see no stars in the daytime?
+
+How is the appearance of the stars explained?
+
+What is the difference between stars and planets?
+
+What starlike bodies make up the solar system?
+
+Name and illustrate three universal properties of matter.
+
+What daily experiences of yours are explained by these properties?
+
+What forces keep the moon moving around the earth in its orbit?
+
+Draw circles illustrative of the size of the earth, moon, and sun.
+
+Why are the sun and moon particularly interesting to us?
+
+How long would it take an express train running thirty miles an hour and stopping neither day nor night to go over the distance from the earth to the moon? From the earth to the sun?
+
+
+
+
+
+【中文阅读】
+
+
+1．傍晚的天空——到了傍晚，夜幕开始降临，太阳的光芒便在大地上渐渐消褪，我们看见星星一颗一颗地眨着眼睛出现了，最后整个天空布满了繁星。这时我们发现越亮的星星出现得越早，而到了早上，整个过程则正好相反。星星开始按部就班地逐渐隐没于黎明的天空，最亮的星星也在最后消失于天际。
+
+我们都有这样的经验，一根火柴或蜡烛的光亮在黑暗的房间里会显得很亮，而在电灯泡面前，它们就黯然无光了。看起来，太阳的万丈光芒对星星的光亮很可能有同样的效应，这个看法也可以从日食的时候星星也像傍晚那样出现得到印证。天文学家们也一定会同意一个结论：要是没有太阳耀眼的光芒，我们便会随时看见满天的星星。
+
+在我们地球的北半球，除了在北极我们看见星星在天空似乎以划圈的方式运动以外，其他任何地方我们都看见星星像太阳一样，东升西落，永不停息。如果注意观察这些从我们的东方、东南���、东北方升起的满天星斗，我们还会发现它们是在不同的时间在地平线上升起的。
+
+我们的先民们也注意到了这一点，他们富有创意地解释说，地球处于一个空心天球的中心，星星就布满在这个天球的内表面上，这个天球还围绕着地球不停地旋转，并轻微改变着它与地球的相对位置。而今天我们已经知道，地球是在沿着一条假想的轴线自转，并不断地改变着与星星的相对位置。而且如果这根自转轴延伸得足够长，它便正好可以穿过在北边天空中心的一颗星星，这就是北极星。地球表面上与这跟轴线的交点就是我们熟悉的南北极。
+
+2．作为行星之一的地球——如果我们仔细观察夜空中的满天星光，就会发现它们几乎都在不停地闪烁，但有三颗最明亮的亮点像月亮一样发着稳定的光芒。如果我们再对这三个亮点的位置做长时间的仔细观察，便可以发现它们在星空中不断地变换着位置，而其他星星相互之间的相对位置却从不会发生变化。
+
+这三个亮点的其中一个有着暗红褐的颜色，它就是火星，它的英语名字Mars来源于罗马神话中战神的名字。另外两个，即金星和木星，在英语中它们分别被叫做Venus和Jupiter，前者是罗马神话中闻名遐迩的美丽女神，后者就是大名鼎鼎的众神之王，即宙斯。天文学家们把它们三个和地球，以及其他四颗类似的星都叫做行星，并且告诉我们，它们也以太阳为中心转动。其实它们并不能像其他星星一样自己发光，我们看见的光亮不过是太阳光通过它们的反射罢了。
+
+有时候肉眼可以看见它们中的五个。天文学家告诉我们，它们相对其他星星的位置改变其实是因为它们围绕太阳的运动造成的。如果站在这些可见行星的其中一个上面，我们的地球看起来也就好像它们中的一个那样。而我们的太阳，如果从其他星星的距离上来看，也像是一颗普通的星星。
+
+我们周围离太阳的距离从近到远的行星依次是：水星、金星、地球、火星、木星、土星、天王星、海王星[1]。太阳连同围绕它转动的所有星球与物体就被叫做太阳系。且有理由相信，我们的太阳系可能在遍布宇宙的所有类似的星系中独一无二。
+
+即使这些行星中最近的一颗，与太阳的距离也比地球到太阳的距离远了27亿英里[2]，虽然这听来很远，但和天上其他星星离太阳的距离相比起来，已经非常近了。最近的星星离太阳的距离大致是25000000000000英里。这个距离非常之远，以至于以光线的不可思议的每秒186000英里的速度从那颗星发出来，到我们这里也要经过4年半的时间。从另一颗叫做大角星发出的光，需要180年才能到达我们这里。而从其他星星发出的光则需要比这长得多的时间，有时彗星也会从外太空造访我们的太阳系。这样看起来，我们渺小的地球在浩瀚的宇宙中真只不过是一粒埃尘而已。
+
+人们发现在火星和木星之间存在许多小的天体，叫做小行星。其中最亮的一颗叫做灶神星，直径还不到250英里。
+
+有一个著名的理论叫做星云假说，它在多年前被科学家们提出来解释太阳系是如何形成的。根据这个理论的假设，最早组成我们太阳系的物质是一些宇宙星云，这些星云在巨大空间中逐渐细微分化，最后在某些相互作用下慢慢形成了今天的太阳，以及行星和它们的卫星。人们还不知道物质到底是如何形成的，或者说还不知道物质是如何被制造又如何被毁灭的，但是对物质的许多特性我们倒是已经知道得很清楚了。
+
+3．组成宇宙的物质的特性
+
+
+
+
+
+实验1：向外拉一个充气泵或者打气筒的活塞。然后堵住出气口，开始推压活塞，是什么让推压非常费劲？将一个喝水用的玻璃杯倒着向装满水的水桶中按压，为什么水不能进到玻璃杯中将它填满？
+
+
+
+
+
+我们知道所有的物体都会占用一定的空间，换句话说，它们都有一定的范围。我们在给车胎打气的时候会发现，即使空气也会占据一定的空间。在这个空气活塞的实验中，我们发现空气占用的空间只能有很小限度的压缩，而且即使你再怎么使劲压缩空气，它始终还是会占据一定的空间。
+
+
+
+
+
+实验2：放一个硬币在一张纸卡片上，让卡片的一端稍微伸出桌子的边缘，现在突然水平地抽出卡片，硬币会移动吗？
+
+
+
+
+
+另一个我们通常所见的现象是，一个物体除非有外力强加给它，否则它不会突然开始移动；同样的，运动的物体除非有外力阻碍它，它也不会停下来。当卡片从硬币下面被抽出时，硬币看上去并没有动，因为它和纸片之间的摩擦力还不足以促使像硬币这样足够大的物体运��，而如果硬币是用胶水粘在卡片上，则就会和卡片一起移动了。
+
+
+
+
+
+实验3：用手甩动一根很结实的另一端系有小重物的橡胶细绳，让其做圆周运动，现在突然松手，小重物还会沿着之前的轨迹做圆周运动吗？
+
+
+
+
+
+当一辆小汽车在水平的路面上行驶的时候，我们不能指望它会无缘无故地停下来，除非是路面的摩擦力或者其他外力导致。当我们甩动这根系有小重物的绳子并突然松手的时候，小重物会沿一条直线的方向被甩出。当然它也不会始终沿着这条直线运动，因为有一种力，重力，会把它拉向地面。一个物体除非有外力施加于它，否则它只会保持静止或做匀速直线运动的特性就叫做惯性。这个事实最早被牛顿勋爵道出，所以有时它又被叫做牛顿第一定律。也正是由于惯性，如果疾速行驶的汽车突然停下来，人可能会被甩出车外。
+
+
+
+
+
+实验4：用一根不是特别结实的细绳悬挂起一个比较重的小球（在球的下方放一个垫子以便它坠落时可以接住它），在球的底部也系一根类似的细绳。现在突然提升上面的绳子，会发生什么？重新悬挂起来，慢慢地向下拉下面的绳子，会发生什么？再重新悬挂起来，突然向下拉下面的绳子，会发生什么？
+
+
+
+
+
+当我们试着突然提升悬挂着的小球的时候，它的惯性力量很大以至于扯断了绳子。当系在小球底部的细绳被慢慢用力向下拉的时候，上面的细绳会断掉，因为它既承受了小球的重量又承受了此时向下拉的拉力；但当向下的拉力是突然作用的时候，小球的惯性便足以抗拒向下的拉力，于是下面细绳会断掉。
+
+也正是水的惯性确保了快速转动的小小螺旋桨可以支撑大船在水上的运动，飞行器的推动力与支撑力也是基于同样的道理。空气微粒被突然卷入运动所形成的抵抗力量，也即它们的惯性，让螺旋桨可以推动飞行器向前运动并且保持飞行器不会向地面坠落，当然必须让螺旋桨始终不停地高速运转才行。也正是惯性让天体在宇宙空间运行。它们一旦运动就会一直保持下去，直到有外力来阻止它们。
+
+
+
+
+
+实验5：将一个球形玻璃容器注入一部分水，放在一个旋转装置上面，现在通过装置快速旋转此玻璃容器，里面的水会出现什么情况？
+
+
+
+
+
+惯性还可以在做圆周运动的物体上显现出来，因为它们总是有离开圆心向外运动的趋势，它显现出来的这种趋势特性就叫做离心力[3]，在实验3和实验5中都可以看到它们的例子。
+
+牛顿在很多年前就已发现物体之间会相互吸引，并且吸引力的大小正比于物体所拥有的质量，也就是说，两个物体的质量越大，相互间的吸引力也就越大，但这个吸引力会随着物体间的距离增加而变小。不过减小的程度不是正比于增加的距离，而是和增加的距离的平方成比例。这就意味着如果同样两个物体间的距离增加一倍，则相互间的吸引力就变成之前的四分之一；要是距离变成之前的三倍，则吸引力就变成了之前的九分之一，以此类推。到底是什么机制产生了这种吸引力还没有人知道，但是我们已经给这种吸引力取了个名字，叫万有引力。万有引力作用于所有物体，物体的一切运动都不断地受其影响。正是它保证了宇宙天体不会按自身运动惯性各自离散，就像实验3的橡胶细绳拉住了小重物不让其飞出去一样。
+
+当我们把引力放入地球与其表面物体的关系中来考虑的时候，重力的概念就派上了用场。我们其实时时刻刻都在感受物体重力的拉拽，并把它叫做重量，这也是物体会落向地面的原因。也正是这个力量使我们在想舒舒服服地睡一觉的时候会躺下来，以及让试着去飞的人屡屡坠落。
+
+当两个力同时作用于一个自由物体上，每一个力都会影响物体的运动方向，而物体最终的运动方向不会是任何一个力的方向，而是在两个力之间的一个方向上。如果作用在物体上的力不止两个，那么物体的运动方向就是所有力相互合成的结果。在上面小重物和橡胶绳的例子中我们发现要是不握住绳子的另一端，小重物会从我们手上飞出去，而橡胶绳则始终把小重物拉向我们手中。这样一来，两个力——“离心力”和绳子拉力共同决定了小重物处在它们两个力向之间的运动轨迹，并且两个力时时刻刻不断地平衡彼此，导致了小重物的运动路径成了一个圆圈。同样的，正是万有引力克服了天体自身的运动惯性，或者说离心力，让它们在各自的轨道上运动。
+
+4．地球与太阳、月亮的关系——在太阳系内，不仅所有的行星都围绕太阳运转，一些行星自己也有其他���天体围绕它们运转，我们把这类小天体叫做卫星。这样的卫星，地球有一个，而土星的最多，有十个[4]，最迟的一个发现于1905年。我们地球的卫星月亮，直径为2000英里，质量只有地球的1/80，到地球的平均距离为240000英里，相比于其他天体之间的距离，这确实已经非常近了。
+
+而太阳呢，尽管和恒星家族的其他成员相比它已经算是我们的近邻了，但它还是距离我们93000000英里。太阳非常之大，如果它是中空的，把地球放到它的中心，再把月亮也放到实际距离地球的位置上，这时月亮离太阳表面的距离还跟它与地球的距离差不多。有个好办法可以让我们直观地感受它们三者之间的大小比例，用铅笔芯在纸上点一下代表月亮，然后画一个直径0.8英寸[5]的圆就可以代表地球，再画一个直径略大于13.5英寸的圆就可以代表太阳。
+
+对太阳和月亮，我们始终抱有极大的兴趣，因为它直接关系到我们的存在。如果太阳离我们再近一些，对生命而言那就太热了，再远一点呢，又太冷了。要是没有月亮，夜晚那令人迷醉的美与浪漫将大打折扣，也将不会再有泛着白沫的潮水去眷恋宁静的海滩、去涤荡浊流、让远离海岸的扁舟回归到温馨的港湾。如果月亮离地球的距离改变，海浪的浪高也将随之变化，而这，将对沿海的城市产生非常深远的影响。
+
+虽然每个月的一部分时间月亮在我们眼里是一个明亮的发光体，但实际上它并不能发光，只是反射了太阳的光而已。月球有一个粗糙、荒芜的岩石表面，布满了火山口一样的凹坑，而且上面没有空气和水。由于地球绕着太阳运动，月亮又绕着地球运动，它们三者之间的位置始终在不断变换，于是我们才有了高低起伏的海浪和阴晴圆缺的月相。单独试着描绘一下月相的改变非常有意义，一个好办法是让一个皮球绕着一个灯泡运转，观察一下在不同的位置，球的哪部分表面会被照亮。
+
+总结——地球是围绕太阳运转的八大行星之一，太阳和围绕它运转的物体一起组成了太阳系。那些在白天被太阳的耀眼光芒所遮蔽，晚上又静静出现在天宇的星星也是极遥远的“太阳”——甚至很可能会很类似于我们的太阳——也处在它们自己的“太阳系”中心。
+
+宇宙的一切都由物质构成，而物质都有确定的特性，比如范围、惯性、引力等等。它们的共同作用决定了天体之间错综复杂的运动关系——让地球绕着太阳运转，让月球绕着地球运转。
+
+太阳和月亮对地球的影响非常显著，远远大于其他任何物体。太阳以光和热的形式给我们传递能量，维持着地球上的生命存活。月亮虽然只能靠反射太阳光来照亮夜空，但它对地球产生了巨大的引力，形成了潮汐，进而才能让我们的海港浪漫而洁净。
+
+
+
+
+
+思考题
+
+
+为什么我们白天看不见星星？
+
+星星是由于什么原因才出现在天空？
+
+星星和行星的区别是什么？
+
+是什么天体组成了太阳系？
+
+说一下物质都有哪些特性，并解释一下他们的含义。
+
+你有哪些日常生活经验可以用这些物质的特性来解释？
+
+是什么力让月亮在围绕地球的轨道上运动？
+
+画一些圆圈来类比说明太阳、月亮、地球的大小。
+
+为什么我们对太阳和月亮始终抱有浓厚的兴趣？
+
+一辆时速为每小时30英里的火车日夜不停地跑完地球到月亮的距离要多长时间？跑完地球到太阳的距离呢？
+
+
+
+
+
+译注
+
+
+[1]冥王星是1930年2月18日才被发现，而本书成书早于这一时间，故此处未提及。但在2006年8月24日，第26届国际天文联合会又将冥王星从太阳系“九大行星”系统中剔除了出来。
+
+[2]1英里=1.6093公里。
+
+[3]其实严格地讲，离心力并不是真正意义上的一种“力”，而是物体自身运动惯性的表现。在这个例子中，小重物只受到两个力：自身重力和绳子拉力，并没有再受到一个“离心力”。作者在这里只是为了说明方便，把离心力类比为了一种施加在小重物上的力作用。希望中学生读者朋友在学习高中物理受力分析的时候，不要受此影响，进而习惯性地给做圆周运动的物体加上一个“离心力”。
+
+[4]此为作者写作当年的数据。到今天为止，已经发现的土星的卫星已多达62个，而且这也不是“最多”的，木星已发现的卫星已达到66个。
+
+[5]1英寸=2.54厘米。
+
+
+
+
+
+CHAPTER 2
+
+THE PLANET EARTH
+
+行星地球
+
+
+5. The Shape of the Earth. —Men who have in different ways made careful measurements of the shape of the earth tell us that it is an oblate spheroid (Fig. 6), that is, a sphere which is somewhat flattened at two opposite points. An ordinary orange has this shape. The earth has been so little flattened, however, that its shape is very much nearer that of a perfect sphere than is that of an orange. Its polar diameter is only 27 miles shorter than its equatorial diameter, so when we consider that each of its diameters is nearly 8000 miles, a shortening of only 27 miles in one of these would not change its shape from that of a sphere enough to be noticed except by the most careful measurements.
+
+
+
+
+
+Fig. 6.
+
+
+
+Experiment 6. —Attach a centrifugal hoop to a rotator apparatus and revolve. The hoop bulges at the center or point of greatest motion and flattens at the top and bottom or points of least motion. The earth revolves in a way similar to the hoop and is very slightly flattened at the poles.
+
+
+
+
+
+Although some of the mountains of the earth rise above sea level to a height of over five miles, and there are depths in the sea which are somewhat deeper than this below sea level, yet these distances are so little in comparison to the size of the earth that the surface is comparatively less irregular than that of an orange.
+
+In these days many men have sailed around the earth, but valiant indeed was that little company which in 1522 first proved that it was possible to sail continually in one direction and yet reach the home port, thus demonstrating that the earth was probably round. Long before, wise men had come to believe that the earth was a sphere, for it had been noted as far back as the time of Aristotle, the famous Greek philosopher, that when the shadow of the earth fell upon the moon, causing an eclipse of the moon, the boundaries of the shadow were curved lines. It was also later noticed that when ships are seen approaching at sea the masts appear first and then gradually the lower parts of the ship. The reverse was seen to be true when ships sailed away.
+
+
+
+
+
+MOUNT EVEREST.
+
+As it would appear if placed in the deepest part of the sea.
+
+
+
+Experiment 7. —Add alcohol to water until a solution is obtained in which common lubricating oil will float at any depth. Insert with a glass tube a large drop of oil below the surface of the solution. The oil will float in the solution in the shape of a sphere. This illustrates the fact that if a liquid is relieved from the action of outside forces, it will take the form of a perfect sphere.
+
+
+
+
+
+A spherical surface is the smallest surface by which a solid can be bounded, so the maximum distance which can separate places located on a given solid will be least when its surface is spherical. Thus the inhabitants of the earth, considering the surface over which they may scatter themselves, are brought into the closest possible relation to one another. One of the most noteworthy consequences of the earth's shape is the ease with which knowledge, news, and the products of both agriculture and manufacture are carried between its most distant parts.
+
+6. The Size of the Earth. —It is easy to say that the polar diameter of the earth is 7900 miles, its equatorial diameter 7927 miles, and its equatorial circumference 24,902 miles, but a true conception of these distances is not so easy. There are, however, distances on the surface of the earth over which we have passed and about which we have real knowledge; and if we can translate other distances into terms of these, then the unfamiliar distances will become appreciable.
+
+One of the best ways to do this is to draw a line which shall represent our known distance and then with this as a measure draw other lines which shall represent the distances of which we wish to get an appreciation. Using as our standard any distance with which we are really acquainted, we shall find that the lines representing the different dimensions of the earth are very long. How vastly greater, then, must be the distances which were mentioned when treating of the stars.
+
+
+
+Fig. 7.
+
+
+
+7. Effect on Life of the Irregularities of the Earth's Exterior. —Although the irregularities of the surface of the earth, when considered in relation to its size, are insignificant, yet in relation to the size of the men and animals that dwell upon the earth they are very great. Some of the mountains rise to heights that are inaccessible, and the oceans in some places sink to depths which until recently were immeasurable. The lofty mountains and broad oceans are barriers which plants and animals have tried in vain to cross and which man himself has had difficulty in surmounting.
+
+
+
+THE HIMALAYA MOUNTAINS.
+
+An insurmountable barrier in central Asia.
+
+
+
+These regions have furnished protected spaces to different species of plants, groups of animals, communities of men, and unmolested in these protected places they have developed their peculiar characteristics. Where man has not succeeded in thoroughly overcoming the barrier, secluded and unprogressive peoples are still to be found. Such are the Highlanders of Scotland and of our own Appalachian plateau. The gentler inequalities of the earth's surface have by their lakes and rivers afforded easy means of transportation from one community to another and have opened paths into hitherto unexplored regions. These waterways have been from the earliest times arteries of commerce and progress, and largely through them have the interior lands been colonized and developed.
+
+
+
+A COTTAGE IN THE SCOTCH HIGHLANDS.
+
+
+
+Where great plains and plateaus stretch their broad and level surfaces over vast areas, plants, animals, and men for ages, unimpeded by natural barriers, have been thrown together and have striven unrelentingly for mastery. They have here developed into great aggregations, not into small distinct communities. Here there has been no shelter for the weaker group except as its identity was lost in merging with the mass.
+
+8. The Interior of the Earth. —Whenever borings have been made into the interior of the earth it has been found, after a depth had been reached where there was no effect from the heat of the sun, that the temperature rose as the boring increased. From this gradual increase of temperature, it must be that far down within the earth the temperature is very high. The pressure within the earth is so great, however, that there are probably no liquid rocks at great depths. If the earth had a liquid interior the attraction of the bodies about it in space would cause changes in its shape, but it is as rigid as steel.
+
+The outside cold part of the earth is called its crust. How thick this is no one knows. This is the part that is of particular interest to us, for it is the only part which we are able to observe and study.
+
+
+
+
+
+9. The Cause of Day and Night.
+
+
+
+
+
+Experiment 8. —(a) In a darkened room place a globe a short distance from a small but strong light. Rotate the globe with its axis at right angles to the line which joins the centers of the globe and light. How much of the globe is illuminated by the light? Is the same part of the globe illuminated all the time? Does any place on the illuminated part receive light for a longer time during a rotation than any other place? Remove the globe to the opposite side of the light without changing the direction of its axis. When rotated, is there any change in the globe's illumination? If so, what?
+
+
+
+
+
+(b) Now make the axis on which the globe rotates parallel to the line joining the centers of the globe and light. Rotate the globe. How much of the globe is illuminated by the light? Is the same part illuminated all the time? Does any place on the illuminated part receive light for a longer time during a rotation than any other place? Remove the globe to the opposite side of the light without changing the direction of its axis. When rotated is there any change in the globe's illumination? If so, what?
+
+
+
+
+
+(c) Place the axis of the globe so that it is inclined to the line joining the centers of the globe and light. Rotate the globe. How much of the globe is illuminated? Is the same part of the globe illuminated all the time? Do any places in the illuminated part receive light for a longer time during a rotation than other places? Remove the globe to the opposite side of the light without changing the direction of its axis. When the globe is rotated is there any change in the length of time of illumination of the places before noted? If so, what?
+
+
+
+MEDIEVAL IDEA OF THE UNIVERSE.
+
+From a fourteenth-century manuscript. Above the earth are the clouds and the moon; then the rays of the sun; next various planets; above these the stars; and finally the signs of the zodiac.
+
+
+
+As has already been stated, the ancients considered the earth as the center of the universe and thought that the sun and stars revolved around it. We of the present day, however, know that it is the rotation of the earth from west to east that causes the appearance of the rising and setting sun and thus makes day and night.
+
+Of course it makes no difference about a person beginning to see a light whether the light is brought toward him or whether he goes toward the light. We are turned into and out of the sunlight by the rotation of the earth. We speak of the sun as rising high in the sky, but what really happens is that we are turned so that the center of the earth, our heads and the sun come nearer and nearer toward a straight line.
+
+When we say down we mean toward the center of the earth, and when we say up we mean in the opposite direction. These are the only two directions that we could be easily sure of, if it were not for the rotation of the earth. This gives the direction of the rising sun, which we call east, and of the setting, which we call west. A line which runs at right angles to the one joining east and west, i.e. one running parallel to the axis of the earth, is said to run north and south. Thus the points of the compass, as well as day and night, are determined for us by the earth's rotation. The north star, which is so important to the sailor in determining his direction, is simply a star which is almost in line with the axis of the earth. It is the rotation of the earth which gives us also our means of measuring time.
+
+As was seen in the previous experiment, the direction of the axis of a rotating globe has much to do with the light which different parts of it will receive from a luminous object. The hemisphere which is inclined toward the luminous object will have a larger part of its surface illuminated and therefore each place on it will be longer in the light during a rotation than when the hemisphere is inclined in the opposite direction.
+
+As the axis of the earth is inclined to a line drawn from the earth to the sun, the light the earth receives is similar to that received by the globe in the last part of the experiment. Thus the days and nights vary in length during the year, because in summer the northern hemisphere is inclined toward the sun and in winter away from it.
+
+10. The Movement of the Earth around the Sun. —The earth not only turns on its axis every day, but it travels around the sun, continually changing its position in relation to the stars. It moves with the tremendous average velocity of about 19 miles a second. It is this revolution around the sun which gives us our measure of time which we call a year. It takes 365 days and a fraction to complete this revolution, so we consider 365 days to be a year, and add a day practically every fourth year to make up the fraction.
+
+In the journey around the sun the earth does not move in a circle but in an ellipse, which is a figure something like a circle but having one of its diameters longer than the other. This figure can be drawn by sticking two pins into a piece of paper, a little distance apart, and tying to each pin one end of a string, the length of which is several times the distance between the pins. Then put a pencil into the loop of the string and draw the curve which will be formed on either side of the pins by the pencil being moved over the paper in the extended loop.
+
+
+
+Fig. 8.
+
+
+
+The points where the pins pierce the paper are called the foci, and, as will be seen, each of these points is nearer one half of the curve than the other. If a body were placed at one of these points and another body moved around it in the line of the curve, the two bodies would be nearest each other when passing one point in the line extending through the foci, and farthest apart when passing the opposite point in the same line.
+
+Now the sun is at one of the foci of the ellipse in which the earth moves, so the distance between the sun and the earth varies during the year. This variation is about three millions of miles, the average distance of the earth from the sun being about 93,000,000 miles. Strange as it may seem, we are nearest the sun in January and farthest away in July (Fig. 9). We are not warmest in the northern hemisphere in January because our hemisphere is then pointed away from the sun and therefore there are fewer heat rays falling upon a given area in this hemisphere than there are when we are farther away in July.
+
+
+
+Fig. 9.
+
+
+
+
+
+Fig. 10.
+
+
+
+11. Latitude Zones. —As the axis of the earth is inclined 23.5° from the perpendicular to the plane of its orbit, the rays of the sun will fail vertically at some times during a year upon all points within 23.5° of the equator both north and south. To this region we have given the name of the torrid zone. There will be a day during the year when no direct sunlight will fall upon points within 23.5° of the poles. The areas inclosed by lines drawn around the earth, 23.5° from the poles, are called frigid zones. The areas between the frigid zones and the torrid zones are called the temperate zones. We live in the north temperate zone.
+
+
+
+ALAPLANDER's HUT.
+
+Made of thick sod to keep out the cold of the frigid zone.
+
+
+
+Although as far as the direct influence of the sun is concerned, these zones are easily separated and bounded by parallels of latitude, yet, on account of other influences, the temperatures of the zones thus bounded are very irregular. For instance, at some places like Hammerfest within the frigid zone the average temperature is much higher than at places like Labrador within the temperate zone, so that as regards temperature, the parallels of latitude are uncertain boundaries. This will be more fully discussed later.
+
+12. The Cause of the Seasons. —Since the earth moves around the sun with its axis inclined to the plane of its orbit, the hemispheres will at different times be inclined toward and away from the sun. When the northern hemisphere is inclined toward the sun, the rays of the sun cover the north pole continuously for six months, so that at this point there is no night for all that time. The days are longer and the nights shorter throughout all the northern hemisphere, and more than this, the rays then fall upon this hemisphere more nearly vertically than during the rest of the year.
+
+
+
+THE PATH OF THE EARTH AROUND THE SUN.
+
+Showing roughly the four positions mentioned in the text.
+
+
+
+The nearer vertical the rays, the greater the number that fall upon a given area and the greater the amount of heat received by that area. More heat is received in the northern hemisphere not only because the rays fall more nearly vertically but also because the length of the day is increased. The amount of heat received from the sun continues to increase as long as the sun appears to move north, or until its rays fall vertically upon the tropic of Cancer. This occurs on the 21st of June, which is called the summer solstice. At this time our days are the longest and our nights the shortest. But the days are not the hottest, as the heat gradually accumulates for some time, more being received each day than is given off.
+
+As the earth proceeds in its orbit from this point, the inclination of the north pole toward the sun becomes less and less, until on the 23d of September the sun is directly over the equator. The north pole now begins to point away from the sun. On December 21 the direct rays of the sun fall upon the tropic of Capricorn, and the sun has reached its farthest point south, our days being then the shortest and the days in the southern hemisphere the longest. From this point until March 21, when the sun is again vertical over the equator, the inclination of the north pole away from the sun decreases. The days when the sun is over the equator are called the autumnal (Sept. 23) and vernal (March 21) equinoxes, since the days and nights are then of equal length all over the earth.
+
+
+
+HUT IN THE TROPICS.
+
+Made of thin walls but a heavy thatched roof to keep out the rain.
+
+
+
+The greater heating of the hemisphere at one part of the year than at another gives us the changes which we call the seasons. Since the change in the length of the day and in the direction of the sun's rays is very small within the tropics, the change in the amount of heat received is very slight, so that in this region there is almost no change of seasons. But at the poles, where for six months there is continuous night and for six months continuous day, the change of seasons is exceedingly great. At middle latitudes the changes, though marked, are not excessive.
+
+There are then two causes which combine to give us our change of seasons: the revolution of the earth around the sun, and the inclination of the earth's axis to the plane of its orbit.
+
+
+
+
+
+13. The Measurement of Time.
+
+
+
+
+
+Experiment 9. —On a fair day place a sundial in an exposed position, and after carefully adjusting it, compare its readings with those of an accurate watch. Probably your watch is set to railroad time and the readings therefore are not alike, unless you are on the time meridian.
+
+
+
+
+
+Although the exact determination of time is a difficult task and requires great skill and very accurate instruments, yet it is not very hard to determine quite satisfactorily the length of a solar day. Before there were any clocks, people told the time of day by sundial (Fig. 11), which consisted of a vertical rod, the shadow of which fell upon a horizontal plane. From local noon, or the time the sun cast the shortest shadow on a certain day, until it cast the shortest shadow the next day, was considered a day's time, or a solar day, and was divided into twenty-four equal parts called hours.
+
+
+
+Fig. 11.
+
+
+
+The direction of the shortest shadow is a north and south line, since the sun must then be halfway between the eastern and western horizon. As the lengths of these solar days vary slightly, for reasons which cannot be explained here, we now divide the mean length of the solar days for the year into 24 parts to get the hours. The civil or conventional day begins at midnight, not noon. The determination of the exact time is very important; for the United States it is done at the Naval Observatories at Washington and at Mare's Island, San Francisco, and telegraphed each day to different parts of the country.
+
+A day may be measured by the interval between the successive passages of a star across the zenith. This would be called a sidereal day, from the Latin word for star. It might also be measured by successive passages of the moon across the zenith. This would be called a lunar day, from the Latin word for moon.
+
+If a person should start at noon and travel around the earth from east to west as fast as the sun does, the sun would be overhead all the time and no solar day would have passed for the traveler, even though 24 hours would be required for the trip. But when he reached home he would find that it was the next day. Thus any one traveling around the earth must drop a day if going toward the west and add a day if going toward the east. The conventional place where this is done is at the International Date Line, a line extending through the Pacific Ocean and in general corresponding with the 180th meridian.
+
+
+
+MAP SHOWING INTERNATIONAL DATE LINE (Dotted Line).
+
+
+
+14. Standard Time. —When railways extending east and west became numerous in the United States and there were many through trains and numerous passengers, it became very inconvenient to use local time, since no two places had the same time. Each railway therefore adopted a time of its own, and when several railways entered the same city these different times became very confusing. Therefore in 1883 the American Railway Association persuaded the Government to adopt Standard Time.
+
+A certain meridian was adopted as the time meridian for a definite belt of country. The meridians adopted were 75° for Eastern, 90° for Central, 105° for Mountain, 120° for Pacific Time. These meridians run through the centers of the time belts and for 7½° on either side the time used is the local time of the central meridian. When a person crosses from one belt to another he finds that the time makes an abrupt change of an hour. This system has been extended to all the United States possessions, and is coming into general use over a large part of the world. In actual practice the changes of time are not made where the boundaries of the time belts are crossed, but at important places near these.
+
+
+
+
+
+MAP SHOWING STANDARD TIME BELTS.
+
+
+
+Experiment 10. —On a day when there appear to be indications of settled fair weather place a table covered with blank paper in an open space where the sun can shine upon it. Make the top of the table level and fix it firmly so that it cannot be moved. Fix vertically upon the table a knitting needle or a slender stick. Mark the line of the sun's shadow and note accurately the time the shadow fell on this line. On the next day note the time the shadow falls upon the same line. If your watch is right, the difference in time it shows between the falling of the shadows the first and the second days is the difference between this particular solar day and the mean solar day. This may be nearly a minute. The shortest shadow of the day marks noon. It extends north and south. (Your watch keeps mean solar time. But twelve o'clock by your watch will probably not be midday or high noon, as your watch is set to Standard Time.)
+
+
+
+
+
+15. Meridians and Parallels of Latitude. —For purposes of measurement, circles of any size are divided into 360 equal parts called degrees. Thus the equatorial circle of the earth is divided into 360 parts. Through each of these divisions there is a semicircle drawn from pole to pole. These semicircles are called meridians. Each meridian is divided into 180 parts called degrees of latitude, and through these points of division are passed circles parallel to the equator. These circles gradually decrease in size as they approach the poles. They are called parallels of latitude and are numbered from o at the equator to 90 at the poles.
+
+
+
+Fig. 12.
+
+
+
+A certain one of the meridians, usually the one passing through Greenwich, England, is called the prime meridian and numbered 0. East and west of this the meridians are numbered from 1 to 180. The degrees thus numbered are called degrees of longitude. Thus we have a skeleton outline by means of which we are easily able to locate the position of any place upon the earth. To secure greater accuracy than could be obtained by giving merely the degrees of latitude and longitude, each of these degrees is divided into 60 equal parts called minutes, and each minute can be divided into 60 parts called seconds.
+
+It will be seen at once that the lengths of the degrees of longitude decrease as the pole is approached, since all the meridians pass through the poles and the distance between the meridians, which is considerable at the equator, becomes nothing at the poles. Of course these lines are simply imaginary lines and do not really exist, but in making a map or a globe we draw them as if they existed. The length of a degree of latitude at the equator is 68.7 miles, at the poles, 69.4. The difference is due to the flattening of the earth near the poles. The length of a degree of longitude at the equator is 69.65 miles, at the poles, 0.
+
+16. Determination of Longitude. —Since the earth turns on its axis once in 24 hours, the interval between the passage of successive meridians under the sun will be four minutes (24 × 60 ÷ 360 = 4). At a point one degree east of us the noon by local time is four minutes earlier than it is with us, and at one degree west it is four minutes later. If an accurate clock were set to twelve o'clock when the sun was nearest vertical at a certain place, and were then carried to a place 15° west, it would indicate one o'clock at the more western locality when it was high noon at this place. Or, changing the statement, if the clock indicated one o'clock when the sun reached the highest point, the place must be 15° west of the place from which the clock started.
+
+Thus we see that by means of an accurate timekeeper we can tell difference of longitude between different places. Every sea captain is provided with one or more accurate clocks called chronometers, which are usually set to the time at Greenwich. Thus all he needs to do to get the difference in degrees of longitude between his position and Greenwich is to determine when the sun has reached its highest point and to multiply by fifteen the difference in hours between the time as shown by the chronometer and twelve o'clock. If the chronometer is too slow he is east, and if too fast, west, of Greenwich.
+
+The determination of latitude is more difficult, but can be easily done by one knowing how and having the proper instrument. The manner of its determination is described in the appendix.
+
+17. Magnetism of the Earth. —There is a peculiar property of the earth which has been of the greatest assistance to geographical explorers and without which it would be very difficult to find a way over the sea. This property is called terrestrial magnetism. In very ancient times pieces of iron ore were found which had the property of attracting iron. Such pieces of ore are called loadstones. Artificial loadstones are called magnets. If a bar of loadstone or a magnetic needle is floated in a basin of water, or if freely suspended, it will invariably assume a definite position.
+
+This discovery was made in the far east at a very early date, but it was put to no particular use in the sailing of ships until about the middle of the thirteenth century. Since then it has enabled sailors to go far out from the sight of land and yet always to know the direction in which they are going. It was supposed even up to the time of the first voyage of Columbus that the magnetic needle always pointed toward the north star or perhaps at some places a little to the east of it, and the sailors of Columbus were greatly alarmed when they found as they sailed west that the needle swung off to the west of the true north.
+
+This difference in the direction of the needle from a true north and south line is called the declination. The westward declination was one of the great discoveries of Columbus. We know now that the reason for the declination of the needle is that the north end of it does not point toward the north geographical pole as was at first supposed but toward a point in the southwestern part of Boothia Felix which is called the north magnetic pole. The south magnetic pole as recently determined is a little to the east of Victoria Land.
+
+
+
+LINES OF EQUAL MAGNETIC DECLINATION IN THE UNITED STATES.
+
+
+
+These magnetic poles do not remain in the same place all of the time but swing slowly back and forth, so that the declination changes for the same place. On account of this it is necessary for surveyors, who use the compass, to find out the declination each year. The annual change in the United States varies from 0 to 5 seconds. In 1910 the declination at Eastport, Maine, was 19.4° west and for Seattle, Washington, 23.5°east. For intervening places there were intervening values. Maps are now made with lines upon them connecting places of equal declination. These lines are called isogonic lines.
+
+
+
+
+
+18. Magnetism.
+
+
+
+
+
+Experiment 11. —Having pushed a long cambric needle through a small disk of cork so that it will float horizontally, carefully place the disk and needle upon the quiet surface of a large dish of water. Does the needle assume any definite direction? Taking the needle from the water stroke one end of the needle from the cork out with the north end of a magnet and the opposite end with the south end of a magnet. When the needle is again floated on the water is it indifferent about the direction in which it points? What has caused the change, if there is any?
+
+
+
+REGION AROUND THE MAGNETIC POLE.
+
+
+
+Experiment 12. —Suspend by a string a short bar magnet in a sling made from a bent piece of wire. Turn it around in several different directions. After each change allow it to come to rest in whatever position it will. Does it prefer any one position to all others?
+
+
+
+
+
+Experiment 13. —Suspend a bar magnet horizontally in a sling and bring one of the ends of another bar magnet toward it. What is the effect? Reverse the ends of the magnet; is there any change in the position of the suspended magnet? Bring a large soft iron nail toward either end of the suspended magnet. What is the effect? Reverse the ends of the nail. (Be careful that the nail has not become permanently affected by the magnet.) Is the effect the same as when the ends of the magnet were reversed?
+
+
+
+Fig. 13.
+
+
+
+Bring pieces of copper, zinc, and other substances toward the magnet. Do these affect it? Notice that the ends of the bar magnet are marked. What can you state about the attraction or repulsion of similar ends of magnets? Of opposite ends? Does it make any difference in its effect on the suspended magnet toward which end the nail is brought? What substances do you find attracted by the magnet?
+
+To the end of a small nail hanging by attraction to a magnet bring another nail. How does the first nail act in respect to the second?
+
+
+
+
+
+So much were some of the ancients impressed with the property of loadstones for attracting iron that one of them suggested building a great arch of this material in a temple so that the iron statue of the goddess would remain suspended in the air without resting upon any support. There is an old legend that the iron coffin of Mohamet rose and remained near the ceiling of the mosque in which it was buried.
+
+It was early discovered that when pieces of steel were rubbed on a loadstone they took on the properties of the loadstone and became magnets. In the experiments with magnets, it was found that like poles repelled and unlike poles attracted, and that iron or steel in contact with a magnet becomes magnetized. Iron and steel are practically the only substances attracted by a magnet, although nickel and cobalt and a few other substances have a little attraction. Thus steel and iron are always used for magnets.
+
+
+
+
+
+Experiment 14. —Wind twenty feet of No. 20 insulated copper wire around the nail used in Experiment 13 as you would wind thread on a spool. Attach one end of this wire to each pole of a dry cell. Bring the nail thus arranged toward a suspended magnet. Reverse the ends of the nail. Does the nail act as it did before it was placed within the coil of wire connected to the battery? Bring another nail in contact with its ends. What happens? What has the nail as arranged become? Disconnect one of the wires from the battery and try the test again. Does the nail act as it did when the battery was connected?
+
+
+
+Fig. 14.
+
+
+
+We found that if a nail is placed in a coil of wire connected with an electric battery it becomes magnetic, but only as long as the connection is maintained. Magnets of this kind are called electromagnets. If the nail had been hard steel and the battery exceedingly strong, the steel would have remained a magnet after being taken out of the coil.
+
+
+
+MAGNETIC CRANE.
+
+The electromagnet is lifting tons of scrap iron.
+
+
+
+Magnets are at the present time ordinarily made by electrical action and not by rubbing on other magnets. Magnetized steel bars are called permanent magnets. Electromagnets have become of almost inestimable use in modern life. The telegraph, telephone, magnetic crane, electric motor, and almost innumerable other mechanical devices are dependent largely upon the principle of electromagnetism for their usefulness.
+
+
+
+
+
+19. The Magnetic Field of Force.
+
+
+
+
+
+Experiment 15. —Place a plate of window glass about the size of a sheet of writing paper above a bar magnet and carefully sprinkle iron filings over it. Describe the behavior of the filings. Sketch on a piece of paper their arrangement. Move a small compass about above the glass plate and note the directions the needle assumes. How do the actions of the needle and of the filings compare? If feasible make blue print.
+
+Holding the small compass two or three inches above the magnet move it parallel with the magnet from end to end. Gently tap the compass occasionally so that the needle will move freely. How does the needle act when it is over the ends of the magnet? How does the direction of the compass needle compare with the direction of the bar magnet?
+
+
+
+
+
+In the above experiment we found that when iron filings were sprinkled above the magnet they arranged themselves in definite lines. The small compass needle also arranged itself along these lines when brought under the influence of the magnet. There is, then, around a magnet a magnetic field of force which affects magnets and magnetic substances brought within it. It is found that magnetic intensity, like the intensity of sound and light, varies inversely as the square of the distance.
+
+When the compass was placed above the ends of the bar magnet one of the ends of the needle was pulled down toward the magnet, or it might be said to dip toward the magnet. When moved near the middle of the magnet it assumed a horizontal position, and when it approached the opposite end of the magnet the opposite end of the needle dipped. This same action is found when a magnetic needle is carried from north to south upon the earth. If a needle is carefully balanced and then magnetized, it will be found no longer to assume a horizontal position.
+
+In the northern hemisphere the north end will dip and in the southern hemisphere the south end. In the northern hemisphere it is customary to make the south end of the needle a little heavier so that it will stay in a horizontal position. At the magnetic pole the needle would stand vertical. If a needle is accurately balanced on a horizontal axis and then magnetized, it will show the angle of dip in any locality. Such a needle is called a dipping needle (Fig. 15).
+
+
+
+Fig. 15.
+
+
+
+20. The Mariner's Compass. —In the ordinary mariner's compass a magnetic needle is arranged so that it will swing freely in a horizontal plane. A circular card is divided into four equal parts the dividing lines of which are marked with the cardinal points of the compass, the intervening spaces being divided into eight equal divisions. The card is attached to the needle and inclosed in a box called the binnacle. This box is arranged so that it will always remain horizontal. A fixed line on the binnacle shows the direction of the keel of the ship. The card being attached to the needle always has its "north" pointing toward the north. To determine the direction of the ship it is only necessary to notice on the card in what direction the keel line is pointing. The mariner of course must know the declination at the place where he is and make the proper corrections. The different governments furnish tables and charts showing these corrections.
+
+
+
+
+
+Fig. 16.
+
+
+
+21. Theory of Magnetism.
+
+
+
+
+
+Experiment 16. —Heat a No. 20 knitting needle red hot and plunge it quickly into cold water. This tempers the needle so that it will break readily. Magnetize the needle as was done in Experiment 11. When it has become well magnetized, break it in the middle. Test each half with a suspended magnet, as was done in Experiment 13. Is each half a full magnet or only half a magnet? Break these halves again and test. What effect does breaking a magnet have upon the magnet?
+
+
+
+
+
+In Experiment 16 it was found that if a magnet is broken in two, each half is a perfect magnet. If these halves are broken, each piece is a perfect magnet, and so on as long as the division is kept up. It is also found that if a magnet is heated or suddenly jarred or pounded it loses its magnetism. If a magnet is filed into filings and these filings are put into a glass tube the tube will have no magnetic properties but will act to a magnet like an ordinary iron bar.
+
+If now the tube is held vertically and tapped several times on a strong magnet, the tube will be found to have acquired the properties of a magnet. The tapping joggled the particles so that they could arrange themselves under the influence of the magnetic pole and when they became so arranged a magnet was the result. If the filings are now poured out of the tube and then put back again, there will be no magnetization.
+
+It was the arrangement of the tiny magnetized particles which must have caused the contents of the tube to become magnetic. It would therefore seem probable that magnetism must be a property of the exceedingly small particles or molecules of which the iron or steel as well as all other substances are supposed to be composed.
+
+It is supposed that when a bar of steel becomes magnetized the molecules arrange themselves in definite directions, as do the filings in the tube. The molecules of magnetic substances are supposed to be separate little magnets. In the unmagnetized bar (Fig. 17) their poles point in all directions dependent upon their mutual attraction, and thus they neutralize each other. When the bar becomes magnetized the molecules tend to arrange themselves so that like poles lie in the same direction (Fig. 18). When the magnet is heated or jarred the molecules are moved out of this alignment and the magnetism is weakened.
+
+
+
+Fig. 17.
+
+
+
+
+
+Fig. 18.
+
+
+
+Summary. —The shape of the earth is spherical with very slight flattening at the poles. Its diameter is almost 8000 miles, more than four times the distance from New York to Denver; and its circumference is nearly 25,000 miles. Though the irregularities of the earth's surface are exceedingly small in comparison with its total area, they are great enough to have a vast effect upon the life of animals and plants.
+
+The revolution of the earth upon its axis causes day and night, and gives us our measurement of time and our points of the compass. The movement of the earth around the sun, combined with the inclination of the earth's axis, gives us the seasons. The inclination of the earth's axis, combined with its revolution, and the movement just mentioned, gives us our latitude zones and causes the variation in the length of our days and nights.
+
+Distance east and west on the earth is measured by meridians of longitude; distance north and south by parallels of latitude. The parallels are about seventy miles apart, and this is also about the distance between the meridians at the equator. Two parallels at the poles are about the same distance apart as two at the equator. But as all meridians pass through the poles, there is no distance at all between them there, while at the equator they are about seventy miles apart.
+
+To find the direction a ship sails we use a compass, a magnetized needle which points toward the north magnetic pole, located northeast of Alaska. Thus, the magnetism of the earth is of infinite value to ocean commerce.
+
+
+
+
+
+QUESTIONS
+
+
+What simple reasons are there for believing that the earth is round?
+
+What effects have the irregularities of the earth's surface had on life?
+
+Draw diagrams illustrating what was discovered in Exp. 8.
+
+Why do we have winter when the earth is nearest the sun?
+
+If a man left Cairo, Egypt, on June 21 and traveled slowly to Cape Town, reaching there on Dec. 21, what changes of seasons would he experience?
+
+How is the length of a day determined? If it were noon Thursday, Sept. 30, with you, what would be the day and date at Yokohama?
+
+Why is Standard Time particularly advantageous in the United States? How much is the difference between local and standard time at your locality?
+
+Suppose that a man at the north pole traveled a degree south and then a degree east. How far would he travel? Suppose he were a degree north of the equator and traveled a degree south and then a degree east. How far would he travel? In both cases he traveled a degree of latitude and a degree of longitude. Do the distances differ? If so, why?
+
+If it is 12 o'clock local time at your home, what time is it at Paris? At Honolulu?
+
+What practical advantages and applications of magnetism do you know?
+
+Why is it necessary for a mariner to know the declination?
+
+
+
+
+
+【中文阅读】
+
+
+5.地球的形状——用各种方法精确测量了地球形状的人告诉我们，地球的形状像一个扁球，好比一个皮球在两端有点泄气凹瘪的样子。一个普通的橘子类似于这个形状，但是地球两极的凹瘪程度非常小，所以总体看来地球更像一个圆球而不像一个橘子，它的两极直径只比其赤道直径短了27英里，因此除非是在极其精确测量的场合，我们都认为地球总体直径就是8000英里，个别地方短出来的那27英里可以忽略不计。
+
+
+
+
+
+实验6：将一个软皮球装在旋转装置上让其高速旋转，可以发现运动幅度最大的球的中心段鼓了起来，而运动幅度最小的转轴两端却瘪了下去，地球的自转与此非常相似，并且在两极处有轻微凹瘪。
+
+
+
+
+
+即使地球上有超过海拔5英里的高山，海底还有比这还深的海沟，但这些参差的长度和地球的尺寸比起来太微不足道了，所以总体来看，地球的表面比橘子表面还要规则平整一些。
+
+现如今很多人已经做过环球航行了，但要说真正坚毅勇敢的，还是要算在1522年首次证明沿着一个方向航行也可以回到起点的那一船勇者，这是第一次证明了地球可能是个圆球。在很久以前，一些智者已经开始相信地球是个球体，这至少可以追溯到亚里士多德时代，这位古希腊著名的哲学家就这么认为，因为他注意到发生月食的时候，月亮的阴影边界总是圆弧形的。后来还有人注意到，从茫茫大海上驶来的帆船，总是先出现桅杆然后才渐渐依次往下出现整个船身；在帆船驶离我们视线的时候，整个过程正好相反。
+
+
+
+
+
+实验7：将适量酒精兑入水中，得到一个让普通油液可以在其中悬浮的溶液。把一根长玻璃管插入溶液中，滴入一大滴油液，让油滴在溶液表面下悬浮成一个小圆球。这个现象说明液体在减轻外部作用力之后，整体会呈现为标准的球形。
+
+
+
+
+
+包裹同样体积的固体，球面拥有最小的表面积。在球面上，不同区域之间的最大距离在各类封闭表面上也是最小的。所以地球上的居民们，表面上感觉他们在地球上天各一方，而实际上因为地球球面的性质，他们相互间的距离其实是最短的[1]。这样一来，地球形状的一个显著结果就是，分散在全球各处的人们互相之间传递知识、信息以及工业、农业产品就变得非常容易了。
+
+6.地球的大小——说出关于地球大小的一串数据其实很容易：它的两极直径是7900英里，赤道直径是7927英里，赤道周长是24902英里，但是要对这些数据有真实的概念可就没那么简单了。还好我们在生活经验中切身感受过地球表面的某些距离，所以如果将其他一些距离拿到经验中来做个比较，也许理解起来会更加容易。
+
+有个好办法就是画一条直线，用它的长度代表我们日常感知的距离，然后按比例画出其他直线代表我们希望感受一下的较长距离。无论用什么样的我们日常接触的距离来比较，地球的尺寸都太大了，据此我们也可以想象，比地球尺寸大得更多的星星之间的距离将是多么的渺远无尽。
+
+7.地球不规则外形对生命的影响——尽管就地球本身的尺度而言，其表面的不规则程度并不显著，但若和生存在上面的人与动物比起来，这影响可就不小了。地球上一些山脉高不可攀，而海底许多地方直到今天依然深不可测。耸立的高山和宽阔的海洋成了动植物无法跨越的障碍，也让人类感觉到要征服它们真的非常困难。
+
+这些被高山大海分割开来的各个地域，无形中为不同的植物种类、动物种群乃至不同人种提供了繁衍生息的保护地带，让它们可以不受干扰地彰显各自不同的特征。在人类还未征服的这些地方，依然有少数与世隔绝的人群在这里过着与世无争的日子，比如苏格兰高地以及美国阿巴拉契亚山区的当地居民们。地球表面温婉多姿的不均衡也导致了湖泊与河流的形成，这也为不同地域之间互相交通打开了方便之门，也为我们进入一些人迹罕至的处女地提供了通道。这些水路作为交通要道，在很早的时候就已促进了商业贸易的进步，慢慢到后来，在很大程度上也为内陆的殖民拓展创造了条件。
+
+在广袤无垠的平原和高原地带，高山大海成了大自然设立的天然屏障，动植物与人类得以在漫长的岁月中共处一隅，并且为了生存各尽其能，努力成为区域的统领。而最终，它们却成为了一个相互依存的整体，并没有发展成为完全独立的生物种群。这里的天地不会为弱者提供避难所，因为除了让自己的特性适应外部世界以外，它们别无选择。
+
+8.地球的内部——人们向地球内部钻探的时候，当钻探达到一定深度，太阳的热能已经无法深入那里，可是人们发现钻探得越深，温度反而越高。从这个温度逐渐升高的趋势来看，地球内部深处的温度一定非常之高，压力也会非常之大，不过，那里应该可能没有液态的岩层[2]。因为如果有的话，地球外部空间物体的引力将会改变液态岩层的形状，但是钻探的结果告诉我们它们却像钢铁一般坚硬。
+
+靠近地球表面的低温部分就是地壳，它到底有多厚还没人知道[3]。地壳是我们最感兴趣的部分，因为我们只能对这部分加以观察与研究。
+
+9.形成白天黑夜的原因
+
+
+
+
+
+实验8：（1）在一间黑暗的屋子里拿一个小球，在球的不远处放置一颗光线强烈的小灯泡。现在转动小球，让转动轴垂直于球与灯的连线。小球有多少部分被照亮了？被照亮的都是相同的部分吗？被照亮的部分中，是否有一些区域的照亮时间总是多于其他一些区域？现在保持转动轴方向不变，把球转到灯泡的对面，转动过程中，小球被照亮的区域是否发生了变化？如果是的话，是什么变化呢？
+
+（2）现在将转动轴平行于球与灯的连线，再转动小球，这时小球有多少部分被照亮了？被照亮的都是相同的部分吗？被照亮的部分中，是否有一些区域的照亮时间总是多于其他一些区域？现在保持转动轴方向不变，把球转到灯泡的对面，转动过程中，小球被照亮的区域是否发生了变化？如果是的话，是什么变化呢？
+
+（3）现在再让转动轴跟球与灯的连线成一定夹角，再转动小球，这时小球有多少部分被照亮了？被照亮的都是相同的部分吗？被照亮的部分中，是否有一些区域的照亮时间总是多于其他一些区域？现在保持转动轴方向不变，把球转到灯泡的对面，转动过程中，小球被照亮的区域是否发生了变化？如果是的话，是什么变化呢？
+
+
+
+
+
+我们之前已经说过，先民们认为地球是宇宙的中心，日月星辰都围绕着地球转动。今天我们已经知道，其实是地球自西向东的自转造成了太阳在我们眼里的东升西落，以及形成了白天与黑夜。
+
+另外，一个人向光源运动和光源向着这个人运动，在这个人自己眼里看来当然是没什么区别的。随着地球的自转，我们也被带着不时面对和背向太阳。我们说太阳高高地升在太空上，其实是因为地球的转动，让地球、太阳和我们的身体慢慢靠近，最后成了直线。
+
+在我们日常的方位观念中，当我们说下的时候，我们指的是朝向地球中心的方向，说上，则指的是相反的方向，这是两个即使地球没有自转，我们也能轻易判别的方向。这样一来，我们判别其他方向也就有了依据，我们把太阳升起的方向叫东，把太阳落下的方向叫西，垂直于东西向，或者换句话说平行于地球自转轴的方向我们叫做南北。于是，指南针罗盘上的方位，以及白天与黑夜，就这样因为地球的自转而可以被我们清晰判别。还有北极星，它对出海航行的船员们非常重要，因为他们就是靠它在茫茫大海上辨别方向，这颗星就正好处在地球自转轴的延长线上。地球的自转同时还为我们判定时间提供了途径。
+
+正如我们在上面的实验中看到的那样，转动轴的方向与球面上不同区域接受光照的多少关系极大。向光源倾斜的半球会有更大的受光面，因此在转动过程中，它上面每一部分接受光照的时间都比背向于光源的半球上的同区域要长。
+
+由于地球的自转轴跟地球与太阳的连线存在一定夹角，所以地球受太阳照射的情形很类似于上面小实验中的最后一种情况。也因为这个原因，一年之中白天与夜晚的时长也会呈现变化，比如夏天是北半球向太阳倾斜，而在冬天则朝向反方向。
+
+10.地球围绕太阳的运动——地球每天不仅自转，还绕着太阳公转，不断变换着与天上其他星星的相对位置。这个公转的速度非常大，达到平均每秒钟19英里。地球这样围绕太阳运转让我们可以用来测定时间，完成一次公转，我们就叫做一年，一共需要365天多一点的时间，因此我们把一年设定为365天，然后每隔四年增加一天来弥补多出来的误差。
+
+地球围绕太阳的运转轨迹不是一个标准的圆，而是一个椭圆，它类似于一个圆但是却有一条直径长于其他所有直径。它可以用下面的办法画出来：在一张纸上用大头针固定两��点，之间隔开一定距离，然后在两点之间系上一根绳子，绳子的长度数倍于两点间的距离，再用一支铅笔抵在绳子的最大弯曲处，沿着绳子划弧完成一个整圈。
+
+大头针穿过的这两点叫做椭圆的焦点，很容易发现，这两点的每一点都比另一点更靠近各自的半边弧线。如果把一个物体放到其中一个焦点上，另一个物体沿着弧线运动，当它经过两焦点连线的延长线的一端时，这两个物体会离得最近，经过另一端时，它们会离得最远。
+
+太阳就位于地球椭圆轨道的一个焦点上，所以地球与太阳之间的距离在一年之中总是在不断变化，其最大差值达到300万英里，而地球到太阳的平均距离是9300万英里。我们可能感到奇怪的是，我们在每年一月离太阳最近，而在七月离太阳最远，但我们并不是在一月最热，这是因为我们北半球在一月份是偏向太阳外侧的，导致我们所接收的光热比在七月偏向太阳时少很多，尽管这个时候我们离太阳最远。
+
+11.纬度区间——由于地球的自转轴与轨道平面成23.5°夹角，所以太阳光线在一年之中总会有一段时间，垂直照射在赤道南北两边23.5°范围内的区域，我们把这个区域叫做热带。一年之中还会有一天，完全没有阳光照射到偏离南北极23.5°的范围之内，这个区域被一条偏离极点23.5°的纬度环线包围，我们把这个区域叫做寒带。把热带与寒带之间的区域叫做温带，我们就生活在北温带里面。
+
+即使由于太阳的直接影响，形成了很容易区分的平行纬度区域，但也因为一些其他因素的影响，不同纬度区域的温度却并不是很有规律。例如，像位于寒带的哈默弗斯特等一些地方，其平均温度远远高于一些位于温带的地方，比如拉布拉多。所以就温度而论，平行的纬度并不是明显分界线，这个我们以后还会详细讨论。
+
+12.四季形成的原因——由于地球自转轴与其轨道平面存在一定的倾斜度，上下半球便会在不同时间侧向或者背向太阳。在北半球侧向太阳的半年中，太阳光将会持续不断地照射北极，所以那里就会一直没有夜晚。这在期间，整个北半球的白天也都会比夜晚更长，阳光也会比一年之中其他时间更加垂直地照射到北半球上。
+
+阳光越是垂直照射，它所传递给照射区的光与热就越多。而北半球能够在夏天接受更多的热量还不仅仅是因为阳光的垂直照射，更由于白天的时长增加了。太阳的光热在其向北半球偏移的过程中持续增加，直到最后太阳完全垂直照射到北回归线上，这个情形会发生在每年的6月21日，也就是我们所称的“夏至”这一天。这一天我们的白天最长，夜晚最短，但还并不是最热的时候，因为热量还会逐渐累积一段时间。在这期间，该区域接受的热量比散发的热量要多出许多。
+
+从这一天之后，北极侧向太阳的倾斜度开始慢慢变小，直到9月23日这一天，太阳完全垂直照射到赤道上。北极也从这一天开始背离太阳，直到12月21日[4]，太阳垂直照射南回归线，也就是它所能到达的最南端，这一天我们北半球白天最短，夜晚最长。然后等到次年3月21日，太阳又垂直照射到赤道上，北极偏离太阳的倾斜度已大为减少。太阳经过赤道的这两天，我们分别叫做“秋分”（9月23日）和“春分”（3月21日），在这两天，地球上白天和夜晚都一样长。
+
+一年之中，一段时间内我们感受到的热量与另一些时间大不一样，这样的时间性的差别我们就叫做季节。在南北回归线之间的地带，一年之中这里白天时长和太阳照射的方向的改变非常小，接收的热量改变得也很轻微，因此这里几乎没有季节的变化。但是在南北极，由于这里有6个月的极昼然后6个月的极夜，因此季节的差别极其显著。中纬度地区，季节变化虽也明显，但并不像极地那么极端。
+
+13.时间的测量
+
+
+
+
+
+实验9：在天气晴朗的时候，在户外放置一个日晷，让其暴露在太阳下，并且精准地校准它，然后将它的读数和一只精确手表的读数进行比较。大部分结果是，手表上跟列车时刻表上的时间一致，而日晷的读数会有些差别[5]，除非你正好处在子午线上。
+
+
+
+
+
+准确测定时间的确是一件非常困难的事情，它需要高超的技巧和非常精密的仪器设备，但若只是判定一个太阳日的长度，也不算太难。以前还没有钟表的时候，每天人们用日晷来判定时间，它是一个垂直标杆立在一个水平刻度盘上，用标杆的影子来读取刻度，进而显示时间。从一天中午，或者标杆阴影最短的时候开始，直到第二天标杆阴影在同样最短位置的时候，中间经过的时间��是一个太阳日，再把它分成24等份，每一等份我们就叫做一个小时。
+
+标杆最短阴影的方向沿着一条南北走向的直线，因为这时太阳正好处在东西地平线的中间位置。由于每个太阳日之间的时间差别很微小，且这里我们暂时不解释其成因，于是我们把一年之中平均的太阳日时长分成24等份，这样就得到了小时的时长。无论民间还是官方，都把一天的开始约定在每天的子夜凌晨，而不是中午。判定精确的时间非常重要，显而易见，美国各地比如华盛顿、马雷岛、旧金山的海军气象观测台必须保持高度精确的统一时间，全国各地每天精确收发的电报也体现了这一点。
+
+一天的时间也可以通过一颗星星两次经过天顶来判定，这叫做恒星日，其英语单词Sidereal就是拉丁语“星星”的意思。一天的时间还可以通过月亮两次经过天顶来判定，这叫做太阴日，其英语单词Lunar也是拉丁语“月亮”的意思。
+
+如果一个人在中午开始自东向西地环球旅行，且航行速度与太阳保持一致，这样他会发现太阳始终在他头顶，即使旅程过去24小时的话，他也没有经过一天。但是当他回到家的时候，他会发现已经是第二天了。所以如果向西做环球旅行，我们必须把时间回拨一天，反之向东的话，则必须往前拨一天。全世界公认的需要调整日期增减的地点就是国际日期变更线，它大致沿着180°经线在太平洋上延伸。
+
+14.标准时间——美国的铁路在全国发展得很快，货车与乘客的数量都越来越多，鉴于各地的时间不同，采用当地时间将会给列车交通带来极大的不方便。比如每趟列车都使用当地时间的话，当不同列车进入同一个城市的时候，时间将会变得极其混乱。因此，1883年，美国铁路总会说服了政府采用标准时间。
+
+我们采用固定的经线作为时区的界定线，比如拿美国来说，75°为东部时间界定线，90°为中央时间，105°为山地时间，120°为太平洋时间。这些经线都穿过各自时区的正中心，两边7.5°的范围内就采用该经线界定的时间。如果一个人从一个时区进入另一个时区，他会发现时间无缘无故变了一小时，这样的分区计时的方式通用于全美国，并已逐渐被全世界大部分国家或地区采用。当然，在实际中并不是只要跨过时区分界线就一定要改时间，但在一些重要的地方多半会如此。
+
+
+
+
+
+实验10：在天气晴朗的时候，在户外放置一张桌子，铺上一张纸，让其暴露在太阳下，同时让桌面保持水平，将桌子固定以免其移动。在桌面的纸上垂直地放置一根指针或者一根细棍，这时给它在太阳下的影子做个记号，并准确记录下当时的时间。第二天当影子到达同一位置的时候，再记录下当时的时间。如果你的表是准确的，两天所记录的时间的差异，就是太阳日和平太阳日之间的时差，差不多有一分钟。当影子最短的时候，其标定的时间就是正午，这时影子正好朝向南北方。（你的手表显示的就是平太阳日的时间，但是手表上显示中午12点的时候，可能并不是刚才所标定的正午，因为手表上显示的是基于平太阳日的标准时间）
+
+
+
+
+
+15.经线与平行的纬线——为了测量方便，我们把任意的圆圈都分成360等份，也就是我们所说的度数。因此地球赤道也被分成了360等份，经过每一等份可以画一条连接南北两极的半圆弧，这就是经线。每一天经线又可以同样地被分成180等份，这就是我们所说的纬度，经过每一个同纬度点的是一环一环的圆圈，它们都与赤道保持平行。这些圆圈从赤道到南北极逐渐变小，它们叫做平行纬线，度数从赤道的0°到南北极点的90°不等。
+
+在经线当中，我们把穿过英格兰格林尼治地区的一条叫做本初子午线，它的标定刻度为0，而它东西两侧的经线则被从1°标定到180°，这些数字就是我们所称的经度。这样我们就有了一个大致的定位系统，可以轻易地给地球上任何地方标定位置。为了在仅有的经纬度的基础上进一步确保定位的准确性，我们还可以把每一度分成60等份，叫做分；再把每一分也分成60等份，叫做秒。
+
+还有一个现象，就是越靠近南北极，经度之间的跨度会越来越短，这是因为所有的经线都要经过南北极点，经线间的距离在赤道最大，到了极点就完全没有了。当然所有的经线纬线都只是假想而并不是真实存在的，但在绘制地图和地球仪的时候，我们会当它是真实存在一样地画上去。在赤道上，一个纬度的跨度是68.7英里，在南北极为69.4英里，这是因为地球在南北两极相对比较扁平；而在赤道附近一个经度的跨度达69.65英里��在南北极为0，因为经线交于一点了。
+
+16.经度的测定——由于地球自转一圈需要24小时，因此以太阳为参照的话，地球转过一个经度需要4分钟（24×60/360=4）。这样在我们东边间隔一个经度的经线上，正午的出现时间就会比我们早4分钟，反之在我们西边的话，就会晚4分钟。如果我们拿一只非常精确的钟，在太阳刚好垂直照射的时候调到正午12点，然后再把它放到西边15°的经度上，当那里正午出现的时候，钟的显示就是1点钟了。或者换句话说，如果这只钟在正午的时候显示的却是下午1点，那它一定是处在间隔校准地西边15°的经线上。
+
+这样一来，我们就可以通过精确的钟表所显示的时间，来确定不同地点的经度差异。每一位随船出海的船长，都会带上一个甚至好几个经纬定时仪，它一般都设定为格林尼治时间。当他想判断其所在地与格林尼治之间的经度差异时，他只需等太阳到达最高点也即正午时，读取经纬定时仪所显示时间，看离正午12点偏差多少时间，然后按照一个小时对应15经度的比例来确定经度。如果定时仪上的时间慢了，则意味着他处于格林尼治东边，快了则处于西边。
+
+17.地球的磁性——地球还有一个十分特殊的特性，探险者们对它抱有极大兴趣，因为如果没有它的话，便很难在茫茫大海上找到航路，这个特性就是地磁。在远古的时候，人们发现一些铁矿石片可以吸引铁器，于是人们把这类铁矿石片叫做吸铁石，书面名称叫做磁铁。如果让一根磁铁条或者磁针漂浮在一盆水中，或者让其自由下沉，它便会始终朝着一个固定的方向。
+
+这个发现发生在早期的东方国家，但直到13世纪中叶才专门应用到航海上，从那之后，它便让船员们能够离开大陆，在浩渺无际的大海上航行而不会迷失方向。据说在哥伦布的第一次航程中，由于磁针总是指向北极星或者偶尔在某些地方指向北极星偏东一点，当他们发现他们在向西航行，而磁针完全偏到了本来应该指向实际北方的西方时，全船船员都大为惊恐。
+
+磁针出现的偏离实际南北向的现象，叫做磁偏角，西向的磁偏角也是哥伦布的重大发现之一。现在我们已经知道了磁偏角的原因，那是因为指针针尖指向的并不是我们之前以为的地理上的北极，而是指向布西亚半岛的西南部，这里就是地球的磁北极。磁南极[6]是直到最近才被发现于位于南极洲的维多利亚地偏东一点的地方。
+
+地球磁极不是始终不变地保持在一个地方，而是会慢慢地前后移动，导致磁偏角也随之发生变化。因此对于常年使用罗盘的检测人员来说，每年确定一次磁偏角是非常有必要的。在美国，磁偏角每年大致变化0到5度。1910年，磁偏角在缅因州的伊斯特波特地区是偏西19.4°，在华盛顿州的西雅图是偏东23.5°，在介于它们中间的地区，磁偏角也在这两个数值构成的区间内相应取值。现在地图上都会将具有同样磁偏角的地方用线连起来，这些线叫做等磁偏线。
+
+18.磁性
+
+
+
+
+
+实验11：将一根缝制麻布的长针塞进一个软木塞材质的小圆片中，让其可以在水面水平漂浮，然后将塞入针的圆木片小心地放在盛有水的小盘子中。这时观察，针会指向特定的方向吗？再将圆木片从水里取出，拔出针，让其针的一端与一块磁铁的北极摩擦，再将另一端与磁铁南极摩擦，然后再重复前面的实验，将针塞入木片漂浮于水盘中，现在针是否有了确定的指向了？如果有，是什么导致了这个变化？
+
+实验12：将一根条形磁铁放置在一个金属丝做成的小挂钩上，再用细绳将其悬挂起来。旋转细绳，让磁铁分别朝向几个不同的方向，每次改变朝向后，都不施加任何其他外力，确保磁铁可以自由旋转到它自身倾向停留的任何方位上。在这个过程中，磁铁会有比较明显的偏向性朝向吗？
+
+实验13：将一根条形磁铁水平地悬挂起来，用另一根条形磁铁的一端靠近悬挂磁铁的这一端，会产生什么现象？换另一端再来靠近刚才悬挂磁铁那一端，悬挂的磁铁的位置会变动吗？拿一根大铁钉的一端分别靠近悬挂磁铁的两端，会产生什么现象？再用大铁钉的另一端分别靠近悬挂磁铁的两端（注意铁钉必须不是长时间被磁铁吸附），结果会跟刚才用磁铁靠近它时所产生的现象一样吗？
+
+再用一些铜片、锌片以及其他物体靠近磁铁，会对它产生作用吗？我们给条形磁铁的两端分别作上不同的记号，你将如何说明条形磁铁之间发生在相同两端的吸引和排斥现象？不同的两端之间呢？用钉子的两端分别靠近悬挂磁铁，磁铁的反应有什么不同吗？你发现什么物体会被悬挂的磁铁吸引？
+
+
+
+
+
+先民们早就对吸铁石能够吸引铁器的特性有了很深刻的印象，于是就有人建议用吸铁石作为材料，在教堂里建造一个大拱门，这样靠它强大的吸力，用铁制成的女神雕像就可以悬置于空中而不需要实物来支撑了。还有一个古老的传说，伊斯兰教先知穆罕默德在下葬的时候，他的铁质灵柩居然升于空中，停留在了靠近清真寺天花板的位置。
+
+人们很早也发现，钢铁片与吸铁石摩擦之后，也会具有与之相同的特性而变成磁铁。在上面关于磁铁的实验中，我们发现不同磁铁条之间，同端相斥而异端相吸，而且钢铁在与磁铁接触之后会被磁化。实际上，虽然还有镍、钴等极少数金属物质也具有轻微的被磁铁吸附的性质，但只有钢铁才真正能被磁铁吸引，所以钢铁也常被用作制造磁铁的材料。
+
+
+
+
+
+实验14：将一段有绝缘层包裹的20号铜线在实验13中使用过的铁钉上沿次缠绕20圈，再将铜线的两端接在干电池的两极上，现在把它靠近一根悬挂起来的条形磁铁，然后再调换一下铁钉的方向，这时发生的现象跟先前铁钉没有被放进线圈中的时候一样吗？再拿一根铁钉靠近它的两端，会发生什么现象？有线圈的铁钉会变成跟什么类似？断开线圈与干电池的连接，再重复上述实验过程，会跟线圈与电池连接时有什么不一样？
+
+
+
+
+
+我们发现，铁钉被放进与电池连接的线圈之后具有了磁性，当然必须保证电池能持续供电。这类磁体我们就叫做电磁铁。如果这里的铁钉是高强度的硬钢，并且给它提供电流的电池组的电力足够强大，那即使将它从线圈中取出，它依然会保持磁性。
+
+现在我们制造磁铁，通常都是运用基于电磁原理的途径，而早已不是通过与吸铁石摩擦来制造了。被磁化之后的钢条我们叫做永久磁铁。电磁铁在现代生活中，已经有了极其广泛的运用，电报、电话、磁力起重机、电动机等几乎无数的机械设备，都是依赖于电磁学原理而发挥作用的。
+
+19．磁场力
+
+
+
+
+
+实验15：将一张书写纸大小的普通玻璃板平放在一根条形磁铁的近上方，然后在上面小心地撒一些细铁屑。请你描述一下铁屑落在玻璃板上的形态，并在纸上画一下它们的分布排列形状。拿一个指南针罗盘陆续放到玻璃板近上方的各个位置，注意观察指针所指的方向的变化，将它和铁屑散落后的排列形状比较，有什么结论？如果可行的话，试着详细说明一下。
+
+将这个指南针罗盘放在玻璃板上方两三英寸远的位置，让它与磁铁保持平行，并从磁铁一端到另一端移动，间或轻轻拍动罗盘，以确保指针可以自由转动，在磁铁端头，指针会有什么反应？罗盘指针的方向与磁铁磁极的方向有什么关联？
+
+
+
+
+
+在上面的实验中，我们发现铁屑撒向玻璃板之后，它们会在玻璃板上按固定的曲线排列；小罗盘被置于磁铁影响范围内后，指针也会按照铁屑的排列曲线变换方向。这是因为在磁铁周围，存在着一个磁性的力场，它会对进入其范围内的磁铁和所有具有磁性的物体产生磁性作用，且磁场中磁力的大小，跟声音与光线的强度一样，也与距离的平方成反比。
+
+当指南针被放置在条形磁铁一端上方的时候，指针的一端被磁力拉向了磁铁，换言之，指针的一端会向磁铁下沉倾斜。当它移动到磁铁中间部分的时候，指针便处于水平状态，而当它移动到磁铁另一端上方时，指针的另一端便也下沉倾斜了。当罗盘的磁针在地球上从北向南移动时，也会出现同样的效应。如果一根针被磁化，并被小心地放置在平衡态，一般情况下它便不会再保持水平。
+
+在北半球，罗盘指针的北端会下沉，在南半球，当然就是南端了。在北半球通常会让指针的南端更重一些，以便让它可以保持水平。而在地球的磁极，指针则会趋于垂直。如果一根被磁化的指针被精确地平衡放置在一根水平轴上，它便会在不同的地方，显示出对应的倾斜角度，这样的指针装置，我们就叫做磁倾针。
+
+20．船员的罗盘——在普通的航海罗盘中，有一根设计精巧的磁针，它可以始终保持在水平面上自由地转动。背后的圆形卡盘的盘面被分成各90度的四等份，分界线被标记上红点以示醒目，介于其间的部分又被分成八等份。卡盘与指针相连，并被封闭在一个叫做罗盘箱的盒体中。这个盒体被精心设置，以便让它可以始终保持水平。在罗盘箱上有一根固定的线，它显示着船的龙骨方向。与指针相连的卡盘的“北向”始终朝着实际的北方，因此判别船的航向，只需观察卡盘上龙骨线所指示的方向。水手们当然必须知道他所在地的磁倾角大小，进而对方向做出适当修正。政府已经绘制了专门表格和航海图，对应显示如何调整偏差。
+
+21．磁学原理
+
+
+
+
+
+实验16：将一根绣花针放在火上加热至红透，然后迅速投入冷水中，这样的淬火过程会让绣花针被轻易折断。将这只绣花针像实验11那样进行磁化，完全磁化之后，用上述方法将其从中间折断，再像实验13那样，用悬挂的磁铁测试每半截针的磁性，这时每一半截是一根完整的磁铁还是只像半截磁铁呢？再把半截针再对半折断，并进行同样的测试，从结论中可以发现将磁铁折断对磁铁本身有什么效应？
+
+
+
+
+
+在实验16中我们发现，如果一根磁铁被分成两半，每一半自身都是一根完整的磁铁，如果把它们继续折断，得到的每一部分依然是完整的磁铁，只是大小变化了而已。如此继续分下去，效果是一样的。而且还发现，如果一根磁铁被加热或者突然猛烈撞击之后，它将失去之前的磁性。如果把磁铁打磨成细屑，再把细屑放进一根玻璃管中，这只装有磁铁粉末的玻璃管也将不再具有磁性，只不过它会像一根铁条一样，依然会被磁铁吸引。
+
+现在如果把这只装有磁铁粉末的玻璃管垂直握住，然后用一个磁性非常强的磁铁去敲击它几次，结果发现，这只玻璃管又具有了像之前的完整磁铁一样的磁性。原因是敲击的过程使得磁铁粉末的每一个颗粒都在磁铁南北极的作用下各自重新调整位置，所以从整体上它们在玻璃管中又变成了一根完整的磁铁。如果这时把它们从玻璃管中倒出来，打散后又填装回去，磁性又会无影无踪了。
+
+正是这些微小的细屑颗粒的组合方式导致了玻璃管内物质序列的变化，从而让整体又变得具有磁性。由此看来，磁性本身很可能是由组成钢铁乃至所有物质的微小颗粒，或者说分子本身的特性而总体构成的。
+
+我们可以认为，一根铁棒被磁化，乃是由于它的组成分子[7]都重新排列了位置，就像刚才实验中玻璃管里面的铁屑一样。磁性物质的分子可以被设想为是独立微小的磁铁，在没有磁性的铁棒中，它们乱七八糟地排列着，凭着相互吸引，磁极胡乱朝着所有不同方向，因此便抵消了各自的磁性。当铁棒变得具有磁性的时候，所有的分子则有序排列，磁极都朝着同一个方向。而当它被加热或者被猛烈撞击之后，分子又从之前的有序排列中移位了，磁性自然也就大大减弱了。
+
+总结——地球的形状大致是在两极略微扁平的椭圆球体，直径大约8000英里，是纽约到丹佛距离的4倍多，其周长大约为25000英里。尽管地球表面的不规则程度相较于它的总体面积极其微小，但依然给动植物的生存带来了极其巨大的影响。
+
+地球绕着自转轴的转动导致了白天与黑夜的形成，也给我们带来了测量时间的办法以及罗盘的确定指向。地球围绕太阳的转动，结合其自转轴的倾角，给我们带来了四季的变换。自转轴的倾斜以及地球的公转与自转，一起让我们有了不同的纬度区，也形成了白天与夜晚千差万别的长度变化。
+
+在地球上，东西向的距离可用经度来衡量，南北向的距离可用平行的纬度来衡量。每条平行纬线间的距离大约为70英里，这也正好是赤道上每条经线间的距离。两条平行纬线的间距在南北极和它们在赤道附近是一样的，但由于所有的经线都要经过极点，所以即使每一度的经线在赤道上均间隔70英里之远，但在南北极，它们就交于一点而不存在距离了。
+
+我们可以用罗盘来为航行的船只寻找航向，其原理是里面有一根磁针始终指向地球的磁北极，它位于阿拉斯加州的最北端。因此，地球的磁性也为海洋贸易打开了方便之门。
+
+
+
+
+
+思考题
+
+
+有哪些简单理由可以让我们相信地球是圆形的？
+
+地球不规则的表面给生命带来了哪些影响？
+
+画一个图对实验8的结果做一下图解说明。
+
+为什么地球距离太阳最近的时候我们却是冬天？
+
+如果一个人在7月21日离开埃及的开罗，慢慢向开普敦航行，到达那里的时间是当年12月21日，他会经历怎样的季节变化？
+
+一天的长度是怎样确定的？如果你的当地时间为9月30日中午12点，那日本横滨市会处在什么日期和时间？
+
+为什么在美国采用标准时间计时会非常有好处？你的当地时间与标准时间之间有多大的差异？
+
+设想一个人从北极点出发，向南行进了1纬度，然后再向东行进了1经度，则他一共行进了多远的距离？再设想如果他是位于赤道偏北1纬度的地方，他向南行进1纬度，然后再向东行进1经度，则他又一共行进了多远的距离？在这两种情况下，他都是行进了1个维度然后又行进了1个经度，他两次行进的距离会不同吗？如果会，为什么呢？
+
+如果你家里的时间为中午12点，这时巴黎的时间是几点？在檀香山又是几点呢？
+
+你知道的磁性对我们的有利之处和应用之途有哪些？
+
+为什么一个航海船员有必要知道磁倾角？
+
+
+
+
+
+译注
+
+
+[1]这段话可能不太容易理解。我们不妨设想如果地球是个正方体，在正方体任何一个面上的人们之间的表面距离是就是直线距离，很短；但是不同平面上的人之间的表面距离就远了。作者在此处强调的是整个地球表面上所有人之间的可能的相互距离最短，很显然，只有球面才能做到。
+
+[2]目前的地质学理论对地球的地核部分存在一个液态外核早已达成了共识，但是这里的“液态”和我们平时说的液体一样的液态有区别，根据地质学家的说法，它虽然是一种可流动的、具有柔韧性物态，但却十分坚硬。
+
+[3]目前已测定地壳平均厚度约为17千米，其中大陆地壳厚度较大，平均约为35千米。高山、高原地区地壳更厚，最高可达70千米；平原、盆地地壳相对较薄。大洋地壳则远比大陆地壳薄，厚度只有几千米。
+
+[4]这一天即为“冬至”，本节中列举的节气日期是就通常而言的，在实际中偶尔也会有1天的误差。
+
+[5]手表上的时间是各个时区的固定时间，是大家共同约定进而共同遵守的；而日规上的时间是当地的太阳时，与时区时间会有一定的出入。
+
+[6]1909年1月16日，英国著名探险家欧内斯特•沙克尔顿（Ernest Shackleton）带领的探险队发现了磁南极。
+
+[7]准确地说，钢铁对应的基本物质构造不是分子而是原子，这里作者是为了避免表达混淆而没有引入更加确切的原子概念。
+
+
+
+
+
+CHAPTER 3
+
+THE GIFTS OF THE SUN TO THE EARTH
+
+太阳给地球的礼物
+
+
+22. Energy. —The capacity for doing work, for overcoming resistance, for causing change, is called energy. If the position of a body or its composition is such that it can exert force or overcome resistance, its energy is called potential. If the body is actually moving, it is said to have kinetic energy.
+
+When a pendulum bob is pulled aside and held higher than the lowest point of its arc, it has potential energy. If it is allowed to swing, it will change this potential energy into kinetic. The potential energy it had when at its highest point is changed into the same amount of kinetic energy when it reaches the lowest point in its swing.
+
+
+
+Fig. 19.
+
+
+
+When a gun is loaded the powder has potential energy due to its composition, but when it explodes this is changed into kinetic energy which is imparted to the bullet. The smallest possible piece of nitroglycerine has potential energy on account of the arrangement of its molecules, and this is capable of being readily changed into kinetic energy.
+
+The sun throughout its existence has been sending vast quantities of energy to the earth. This energy has been mostly in the form of heat and light. The ability of the earth to support plant or animal life or to furnish man the power necessary to carry on his industries is due to the energy furnished by the sun. Plants cannot grow without the energy furnished by the sunlight, and animals could not live were it not for the energy furnished them by the plants.
+
+For untold ages plants utilized the sun's energy and stored it up. It was preserved in the remains of plants in the form of coal. This coal is now being burned to furnish power to carry on man's industries. The water which the sun has evaporated and carried by cloud and shower to the mountain lake is stored there and has potential energy. It is ready to run down the valleys changing its potential energy into kinetic and doing work. We often think that there are many different sources of energy such as wood, coal, oil, waterpower and others, but when these are traced back, their energy is found to have come from one source, the sun.
+
+Energy may readily be changed into different forms, as when the steam engine transforms the energy in coal into mechanical energy, or when this mechanical energy is changed by the dynamo into electrical energy. The most careful investigations have shown, however, that although its form may change, energy can never be destroyed. Mechanical energy frequently changes into heat, as when two surfaces are rubbed together, but when these two kinds of energy are carefully measured there is found to be no loss. This great truth has been determined by a vast amount of most careful investigation and is called the law of conservation of energy.
+
+
+
+TRANSFORMATION OF ENERGY.
+
+
+
+23. Heat and Light. —Every one realizes the importance of the heat and light given to the earth by the sun. If plants or animals are where light is entirely excluded, they begin to sicken and die. If they are placed where it is very cold, they freeze and die. Although the sun gives both heat and light, yet these two are not inseparable. We feel the heat given out by boiling water but there is no light, and we see the light of the moon but there is no heat. We usually say that we feel heat but cannot see it and see light but cannot feel it.
+
+
+
+
+
+24. Heat.
+
+
+
+
+
+Experiment 17. —Fit a glass flask with a one-hole rubber stopper through which passes a glass tube about 20 cm. long. Place this on a ringstand so that the end of the tube extends down into a bottle nearly filled with water. Gently heat the flask. The air expands and bubbles rise in the water. When the flask cools, the air contracts and water rises in the tube.
+
+
+
+Fig. 20.
+
+
+
+Experiment 18. —Fill the flask used in the last experiment with colored water. See that the end of the glass tube passing through the rubber stopper is just even with the bottom of the stopper. Smear the lower part of the stopper with vaseline and insert it in the flask, being careful that the flask and a few centimeters of the tube are filled with colored water and that there are no air bubbles in the flask. Mark, by slipping over a rubber band, the end of the water column in the tube. Heat the flask. The water expands. Why do water heaters always have a pipe at the top leading to a tank?
+
+
+
+Fig. 21.
+
+
+
+Experiment 19. —Pass the ball of a ball-and-ring apparatus through the ring. Notice how closely it fits. Heat the ball in a Bunsen flame for several minutes. See if the ball will now go through the ring. Explain why it does not.
+
+
+
+Fig. 22.
+
+
+
+Experiment 20. —Heat a metal compound-bar. It bends over on one side. The more the bar is heated the more it bends. The two metals do not expand at the same rate. Why are the ends of steam pipes allowed to be free and not attached firmly? Why are the ends of the spans of long iron bridges placed on rollers? When iron tires are fitted to wheels they are heated and then placed on the wheels and allowed to cool. Why? Platinum is the only substance that can be used to pass through the glass in an incandescent lamp. Other metals do not expand the same as glass and when they are fused with it and allowed to cool they break it.
+
+
+
+Fig. 23.
+
+
+
+When heat was first studied it was thought to be an invisible fluid without weight which worked itself into bodies and caused them to expand in the same way that water affects a sponge or a piece of wood. This fluid was supposed to be driven out by pounding or rubbing. Even the primitive savages knew that fire could be obtained by rubbing two dry sticks together.
+
+About the close of the eighteenth century an American, Count Rumford, who was boring some cannon for the Bavarian government, showed that the amount of heat developed seemed to be entirely dependent upon the amount of grinding or mechanical energy expended. The old theory of a fluid prevailed however until about the middle of the nineteenth century, when a great English experimenter by the name of Joule showed conclusively that the amount of heat developed was due entirely to the amount of mechanical energy which apparently disappeared into the heated body.
+
+Every kind of matter is now believed to consist of little particles, or molecules, which are constantly moving about hitting and bumping against each other in the spaces which exist between them. The fact that minute invisible particles may be given off by a substance is readily shown by opening a bottle of ammonia or exposing a piece of musk in a room. Soon in every part of the room the presence of these substances can be recognized by the odor. Yet nothing can in any possible way be seen to have been added to the air.
+
+The molecules are too small to be seen by the most powerful microscope. There are millions of them in a particle of matter as big as the head of a pin. When a substance is heated the molecules move more rapidly and strike each other harder. This causes the substance to expand. Heat is a form of energy due to the motion of these molecules. If a condition could be reached where there was no molecular motion, there would be no heat. The effect of heat in causing expansion of gases, liquids and solids has been shown in the preceding experiments.
+
+25. Measurement of Temperature. —From the experiments it has been seen that gases, liquids and solids expand when heated and contract when cooled. It has been found that most substances expand uniformly through ordinary ranges of temperature, so that if this expansion or contraction is measured, we are able to determine the change of temperature.
+
+
+
+
+
+Experiment 21. —Slightly warm the bulb of an air thermometer tube and place the open end in a beaker half filled with inky water. Allow the bulb to cool. The tube will become partly filled with the water. When the bulb has become entirely cooled mark the end of the water column with a rubber band. Grasp the bulb with the hand, thus warming the air in it. The water column will run partially out of the tube back into the beaker. Cool the bulb with a piece of ice or a damp cloth. The water will come farther up in the tube than it did when simply exposed to the air. We have here an apparatus for telling the relative temperatures of bodies.
+
+
+
+Fig. 24.
+
+
+
+Instruments arranged to show the amount of the expansion or contraction of certain materials due to changes in their temperature are called thermometers. These may be gas, liquid or metal thermometers. There must be some uniform temperatures between which the expansion shall be measured if we are to have a basis of comparison. These definite points have been taken as the freezing and boiling points of water at sea level.
+
+
+
+
+
+Experiment 22. —Fill a four-inch ignition tube with mercury and insert a one-hole rubber stopper having a straight glass tube extending through it and about 20 cm. above it. It may be necessary to cover the stopper with vaseline to keep out air bubbles. When the stopper was inserted the mercury should have risen a few centimeters in the tube. Mark with a rubber band the end of the mercury column. Gently warm the ignition tube. The mercury column rises. Cool the tube and the column falls. We have here a crude thermometer.
+
+
+
+Fig. 25.
+
+
+
+The substance whose expansion is most commonly used to measure the degree of temperature is mercury. This expands noticeably for an increase in temperature and the amount of its expansion can be very readily determined. The ordinary thermometer consists of a glass tube of uniform bore which has a bulb at one end. The bulb and part of the tube are filled with mercury. The remaining part of the tube is empty, so that the mercury can freely rise or fall. When the temperature rises, the mercury expands and rises, when the temperature falls, the mercury contracts and sinks.
+
+There are two kinds of thermometer scales commonly used. In one, the point to which the mercury column sinks when submerged in melting ice is marked 32°, and the point to which it rises at sea level when immersed in unconfined steam, the boiling point, is 212°. The distance between the boiling and freezing points is divided into 180 equal parts. Each one of these parts measures a Fahrenheit degree of temperature. This is the common household thermometer of this country and England.
+
+
+
+Fig. 26.
+
+
+
+Another kind of thermometer scale, which is used almost exclusively in scientific work and in those countries where the metric system of weights and measures has been adopted, is called the Centigrade. In this scale, the point at which ice and snow melt is marked o and the point at which water boils, 100. A degree Centigrade then is 1/100 the distance the column expands when heated from freezing to boiling, instead of 1/180 of this distance as in the Fahrenheit scale. There are a number of different designs of thermometers. Some are for measuring very high, others for measuring very low, temperatures. Thermometers are also constructed so as to be self-recording.
+
+
+
+THERMOGRAPH.
+
+This device makes a continuous record of the temperature for a week at a time.
+
+
+
+26. The Three States of Matter. —There are three states or conditions in which substances exist: solid, liquid and gas. Examples of these are: the solid metal ball, the liquid water, the liquid metal mercury, and the gaseous air. These have already been dealt with experimentally. Almost every one knows that water is a liquid, or a solid, ice, or a gas, steam, depending only on the temperature to which it is subjected. It is not so generally known that the state of all other substances depends also upon their temperature.
+
+Many substances are capable of existing in all three states. Iron, for instance, may be solid as we ordinarily see it, or liquid as it comes from a blast furnace, or a gas, as it exists in the tremendously hot atmosphere of the sun. Substances usually expand in volume as they change from the solid to the liquid state and they always do as they change from the liquid to the gaseous. Ice is a notable exception to the general rule, since when water freezes its volume increases. If it were not for this, ice would not float. Metals that are suitable for casting must have the property of expanding when cooling or at least of shrinking but a trifling amount. This is a most valuable property of type metal and cast iron.
+
+
+
+
+
+27. The Transference of Heat.
+
+
+
+
+
+Experiment 23. —Cut off 15 cm. of No. 10 copper and No. 10 iron wire and the same length of glass rod of about the same diameter. Holding each of these by one end place the opposite end in the flame of a Bunsen burner. Which of the three conducts the heat to the hand first?
+
+
+
+
+
+Experiment 24. —Fill a test tube about 3/4 full of cold water. Holding the tube by the bottom carefully heat the top part of the water until it boils. Be sure that the flame does not strike the tube above the water, else the tube will break. A little piece of ice in the bottom of the test tube makes the action more apparent. A bit of wire gauze or a wire stuffed into the test tube will prevent the ice from coming to the surface. Water conducts heat poorly. The hot water does not sink. It must be lighter than the colder water.
+
+
+
+Fig. 27.
+
+
+
+Without the heat of the sun there would be no life upon the earth, no flowing streams, no changing winds, none of the restless energy which makes the world as we know it. It is therefore essential to understand how heat is transferred from one place to another.
+
+Through solid substances, such as metals, heat travels quite readily, through others such as glass, less rapidly. In Experiment 23，we found that heat traveled along some rods faster than it did along others. In no case, however, was there any indication that there was a transference of the particles composing the rods. In the boiling of the water at the top of the test tube, there was no indication that the water particles moved to the bottom of the tube. In these cases, the heat is simply transferred from molecule to molecule.
+
+This kind of heat transference is called conduction. Conductors may be good or bad, as was shown by the different materials used in the experiments. We use iron for our radiators so that the heat of the steam may readily be given out to the room, and we cover our steam pipes with asbestos when we wish to retain the heat, because asbestos is a poor conductor and will keep the heat in the pipes.
+
+
+
+
+
+Experiment 25. —Hold a piece of burning paper under a bell jar held mouth downward. Notice the air currents as indicated by the smoke. Paper soaked in a moderately strong solution of saltpeter and dried, burns with a very smoky flame.
+
+
+
+Fig. 28.
+
+
+
+Experiment 26. —Fill a 500 cc. round-bottomed flask half full of water and place on a ring stand above a Bunsen burner. Stir in a little sawdust. Some of it should fall to the bottom of the flask. Gently heat the bottom of the flask. Notice the currents.
+
+
+
+Fig. 29.
+
+
+
+When the water was heated at the bottom of the flask and when the burning paper was held under the bell glass, currents were seen to be developed. The heated and expanded water and air rose. Here the heat was transferred by the upward movement of the heated water and air. This method of heat transference is known as convection. The efficiency of the hot water and hot air furnaces which heat our houses is due to the convectional transference of heat. We shall find later that if it were not for convection there would be no winds or ocean currents.
+
+If an incandescent electric lamp is turned on and the hand held immediately below the lamp, it will be warmed, although the glass bulb itself, a poor conductor of heat, remains cool. The white-hot filament is surrounded by an almost perfect vacuum. It can set up no convection currents, neither does the cool glass. The sensation of heat cannot be due to conduction because the air which surrounds the bulb is not in contact with the filament. It is also a poorer conductor than glass and the glass itself does not become hot for some little time.
+
+There must therefore be another mode of transferring heat beside conduction and convection. It also appears that in this method of transferring, no material substance is necessary, as the hot filament is surrounded by an almost perfect vacuum. Now astronomers tell us that there is no material medium between our atmosphere and the sun and that the heat of the sun travels to us with the tremendous speed of light, 186,000 miles per second, and does not warm the intervening space. The convection and conduction processes are, when compared to this, very slow. Radiation is the name given to this method of heat transference. If heat did not travel in this way the earth would be uninhabitable.
+
+
+
+HOT WATER FURNACE.
+
+The hot water rises from the top, passes through the radiator and returns as colder water to the bottom.
+
+
+
+If a body, heated to ordinary temperatures, is surrounded by substances which do not readily permit of conductional or convectional heat transference, the heat is retained within the body. Application of this is made in the fireless cooker and the thermos bottle (Fig. 30). In one, the hot substance is surrounded by felt, wood fiber, asbestos or similar nonconducting substances, and in the other by glass and a space from which the air has been nearly exhausted.
+
+
+
+Fig. 30.
+
+
+
+Both of these arrangements prevent the transference of heat from the hot body. The cooking therefore continues in the fireless cooker and the liquid in the thermos bottle remains warm for a long time or, if cold when put into the bottle, it remains cold, as the heat from the outside cannot reach it. Clothing is placed upon the body in order to prevent the body heat from being conducted to the surrounding air.
+
+
+
+
+
+28. The Measurement of Heat.
+
+
+
+
+
+Experiment 27. —In each of two beakers or tin cups weigh out 100 g. of water. Carefully heat one of the beakers until the water when thoroughly stirred shows a temperature of 90°C. Cool the other beaker till the temperature of the water is 10°C. Pour the water from one beaker into the other, and after thoroughly stirring note the resulting temperature. Use a chemical thermometer to determine the temperatures.
+
+Weigh out 100 g. of fine No. 10 shot in a tin cup and 100 g. of water in another. Place the cup containing the shot in boiling water and allow it to remain, stirring the shot occasionally, until its temperature is 90°C. Cool the water in the other beaker until its temperature is 10°C. Determine the temperatures exactly and then pour the shot into the water. After thoroughly stirring determine the temperature of the mixture. Which has the highest temperature, the mixture of water and water or the mixture of shot and water?
+
+
+
+
+
+Since heat plays such an important part in the activities of the earth we need to know how to measure it. There is a great difference between temperature and the amount of heat. The amount of heat in a spoonful of water at 100° would be very much less than in a pailful of water at 10°. It would require more heat to raise a pond of water a small part of a degree than to raise a kettleful many degrees. That is why large bodies of water, although their temperatures never greatly change, are able to absorb and to give out great amounts of heat.
+
+Not only does the amount of heat necessary to raise the temperature of different quantities of the same substance vary, but the amount of heat necessary to raise the temperature of equal quantities of different substances also varies. If a pound of water and a pound of olive oil were placed side by side in similar dishes on a stove, it would be found that the olive oil increases in temperature about twice as fast as the water, i.e. it takes about twice as much heat to raise water as it does to raise the same weight of olive oil one degree. In fact, it takes more heat to raise a given weight of water one degree than it does to raise the same weight of almost any other known substance.
+
+In Experiment 27, the resulting temperature from the water mixture was much higher than from the shot mixture. The shot has much less capacity for heat. The quantity of heat required to raise the temperature of a certain mass of a substance one degree compared to the quantity of heat required to raise the same mass of water one degree is called the specifie heat of that substance. The specific heat of olive oil is .47, of shot .03. That is it takes .47 as much heat to raise a given mass of olive oil and .03 as much heat to raise a given mass of shot one degree as it does to raise corresponding masses of water one degree. In order to compare different quantities of heat, physicists have taken as the unit of measure the quantity of heat required to raise the temperature of one gram of water through one degree C. This unit is called a calorie.
+
+29. Light. —The sun is not only the source of almost all the heat of the earth but also of its light. We have developed artificial self-luminous bodies such as candles, lamps, electric lights, but none of these compares with the light given by the sun. The stars also furnish a little light. Most of the bodies that we know are dark and non-luminous. Sometimes some of these which have polished surfaces reflect the light from a luminous body and thus appear themselves to be furnishing light.
+
+
+
+Fig. 31.
+
+
+
+An example of this is often seen about sundown when the sunlight is reflected from the windows of a house, making them look as if there were a source of light behind them. Any dark body whose surface reflects light appears itself to be luminous as long as the source of light remains, but grows dark again when the source is removed. This is the case of the moon. At new moon, the moon is so situated with respect to the sun, that light is not reflected to the earth and we cannot see it. At full moon, half of the moon's entire surface reflects the sunlight, and it appears very bright.
+
+
+
+
+
+30. Direction of Light Movement.
+
+
+
+
+
+Experiment 28. —Point the pinhole end of a camera obscura or pinhole camera (this consists of two telescoping boxes, the larger having a pinhole at the end and the smaller a ground glass plate) at some object and move the ground glass plate back and forth until a sharp image of the object is formed. Sketch on a piece of paper the object and the image, showing the direction in which you think the rays of light must have traveled through the pinhole to form the image.
+
+
+
+Fig. 32.
+
+
+
+A photographic camera is constructed in the same way as this little camera, only a lens is placed behind the pinhole to intensify the image, and it is possible to exchange the ground glass plate for a photographic plate.
+
+
+
+
+
+There are certain properties of light which seem readily apparent from our daily experiences. We cannot see objects in the dark, but if a light is brought into the room so that it can shine upon them, they become visible. We see them because the light is reflected to us from them. All objects except self-luminous bodies are seen by reflected light.
+
+If a candle is held in front of a mirror and we look into the mirror, we see the candle behind it. We know that the candle is not there but that its light is reflected by the mirror in such a way as to make it appear to come from behind the mirror. We see the candle by the light the mirror reflects.
+
+
+
+A LAKE MIRROR.
+
+
+
+If we wish to see whether the edge of a board is straight, we sight along it. If we wish to hit an object with a bullet, we bring the rifle barrel into our line of sight. We therefore feel confident that if light is traveling through a uniform medium, such as air usually is, it goes in a straight line.
+
+
+
+
+
+Experiment 29. —Place a penny in the center of a five-pint tin pan resting on a table. Stand just far enough away so that the outer edge of the penny can be seen over the edge of the pan. Have some one slowly fill the pan with water. How is the visibility of the penny affected?
+
+
+
+
+
+Experiment 30. —Fill a battery jar about two thirds full of water. Place a glass rod or stick in the jar. Does the rod appear straight? Pour two or three inches of kerosene on the top of the water. What effect does this have on the appearance of the rod?
+
+
+
+FIG. 33.
+
+
+
+Experiment 31. —Hold an ordinary spectacle lens such as is used by an elderly person, or any convex lens, between the sun and a piece of paper. Vary the distances of the lens from the paper. The heat and light rays from the sun are bent so that they converge to a point. Try the same experiment with a lens used by a short-sighed person, or a concave lens. This lens does not have the same effect as the convex lens. The rays are made to diverge. Why cannot long-sighted and short-sighted persons use the same glasses?
+
+
+
+
+
+In the experiment of the penny in the dish, the water in some way bent the ray of light and made the penny come into the line of sight when it could not be seen before the water was there. This experiment shows that when light is passing from one medium to another it does not always travel in the same straight line. Certain media offer more resistance to the passage of light than others and are called denser media. It is this resistance which causes the bending of the ray.
+
+Suppose that a column of soldiers marching in company front are passing through a corn field and come obliquely upon a smooth open field. The men as they come on to the open field are unincumbered by the cornstalks and will move faster, and thus the line of march will swing in toward the edge of the corn field. It can easily be seen that the bending of the line would be in the opposite direction if the soldiers were marching from the smooth field into the corn field. If the company front was parallel to the edge of the corn field, then the men would reach the open field at the same time and there would be no swinging of the line.
+
+
+
+Fig. 34.
+
+
+
+The above illustration roughly explains what happens when light passes from one medium to another. Refraction is the name given to this bending of light in passing through different media or through a medium of changing density. Twilight, mirage, the flattening of the sun's disk at the horizon and other appearances we shall find later are due to this property of light.
+
+
+
+
+
+31. The Intensity of Light.
+
+
+
+
+
+Experiment 32. —Take two square pieces of paraffin about an inch thick, or better two squares of parawax, and place back to back with a piece of cardboard or tinfoil between them. When a light is placed on either side of this apparatus the wax toward the light will be illuminated, but not that on the other side of the cardboard. If lights are placed on each side, it is easy to see when both pieces of wax are equally illuminated, or receive the same amount of light. In this way the strengths of lights can be compared.
+
+
+
+Fig. 35.
+
+
+
+Place a candle about 25 cm. in front of one side of this apparatus, and 4 candles, placed close together on a piece of cardboard so that they can be readily moved, about 90 cm. away on the other side. Move these candles back and forth till a position is found where both pieces of wax are illuminated alike. Measure the distance of the four candles from the wax. How many times as far away are they than the one candle?
+
+
+
+
+
+The brightness of the sun's light is so great that even an arc light placed in direct sunlight appears like a dark spot. So great, however, is the sun's distance that the earth receives only a minute portion, less than one two-billionth of the light and heat it gives out. It is impossible to express the greatness of this light in ordinary terms. The standard measure for intensities of light is the candle power. This is the light given out by a standard candle, which is practically our ordinary No. 12 paraffin candle. The ordinary incandescent electric light is sixteen candle power.
+
+No comprehensible figures will express the intensity of the sun, using the candle power as a measure. The intensity of light, like that of heat and electricity, and all forms of energy which spread out uniformly from their point of origin, varies inversely as the square of the distance from the source. This rapid decrease in the brightness of light as the distance increases is the reason why so small a change in the distance of a lamp makes so great a difference in the ease with which we can read a book. If we make the distance to the lamp half as great, we increase the amount of light on the book four times (Fig. 36).
+
+
+
+Fig. 36.
+
+
+
+32. Reflection of Heat and Light.
+
+
+
+
+
+Experiment 33. —In a darkened room reflect by means of a mirror, a ray of light from a small hole in the curtain, or from some artificial source of light, on to a plane mirror lying flat upon a table. If there is not sufficient dust in the air to make the paths of the rays apparent, strike two blackboard erasers together near the mirror. Hold a pencil vertical to the mirror at the point where the rays strike it. Compare with each other the angle formed by each ray with the pencil. Raise the edge of the mirror, and notice the effect on the reflected ray. Place the pencil at right angles to this new position of the mirror, and compare the angles in each case. How do the sizes of the angles on either side of the pencil compare?
+
+
+
+
+
+It has already been stated that the moon shines by reflected light. It is a matter of common observation that objects on the earth reflect both heat and light. In the summer, the walls of the houses and the pavements of the streets sometimes reflect the heat to such an extent that it becomes almost unbearable. In countries where the sun shines brightly nearly all of the time, as in the Desert of Sahara, reflectors have been so arranged as to reflect the heat of the sun on to boilers and to run steam engines.
+
+
+
+A REFLECTION ENGINE.
+
+This engine used the rays of the sun instead of coal to heat its boiler.
+
+
+
+The smooth surfaces of houses often reflect so much of the light falling upon them that the glare is thrown into the windows of surrounding houses into which the sun itself cannot shine. If one stands in the right position, the reflection of trees and other objects can be seen in a smooth lake. But the reflection cannot be seen if the position of the spectator is much changed. The reflected ray must therefore maintain a certain relation to the ray that strikes the surface from the object.
+
+In Experiment 33, when the pencil was held perpendicular to the mirror at the point where the rays touched the mirror, it was seen that both the ray from the window and the reflected ray made about the same angle with it. These two angles are respectively called the angle of incidence and the angle of reflection. By most careful experimentation it has been found that the angles between each of these two rays, and the line drawn perpendicularly to the reflecting surface are always equal, or in other words the angle of reflection is always equal to the angle of incidence. This explains why, if you are standing in a room beyond one side of a mirror, you can see in the mirror only the opposite side of the room.
+
+
+
+Fig. 37.
+
+
+
+33. The Speed of Light. —In the latter part of the seventeenth century a Danish astronomer by the name of Roemer, after carefully watching the brightest of Jupiter's satellites or moons as it revolved around the planet, noticed that the time of occurrence of its eclipses or passages behind the planet showed a peculiar variation. He accurately determined the interval between two eclipses or the time it took for a complete revolution of the satellite around the planet.
+
+
+
+Fig. 38.
+
+
+
+Using this interval he computed the time at which other eclipses should take place and found that as the earth in its revolution around the sun moved away from Jupiter the eclipses appeared to take place more and more behind time. Determining the exact time at which an eclipse took place when the earth was nearest to Jupiter, and computing the time an eclipse should take place six months later when the earth was farthest from Jupiter, he found that the actual time of the eclipse was 22 minutes behind the computed time. This slowness he said must be due to the time required by the light in crossing the earth's orbit.
+
+Many determinations of this kind have been made since those of Roemer, and it has been found that he was somewhat in error, as the time required by light in traveling across the earth's orbit is about 16 minutes and 40 seconds, or 1000 seconds. Since the diameter of the earth's orbit is about 186,000,000 miles the speed of light must be about 186,000 miles per second. Determinations of the speed of light have been made in several other ways with almost like results.
+
+34. Theories Concerning Light. —Although it is very easy to perceive light and to examine many of its properties, yet to determine just what it is that produces the light sensation has been found vastly difficult. Sir Isaac Newton thought that light consisted of streams of very minute particles, or corpuscles, thrown off by the luminous body. Since about 1800, it has been considered a form of wave motion which is transmitted through the ether which fills all space.
+
+
+
+
+
+35. Sound.
+
+
+
+
+
+Experiment 34. — Arrange a large widemouthed bottle with a small bell suspended in it from the stopper and a delivery tube extending through the stopper. Attach the delivery tube by a thick-walled rubber tube to an air pump and exhaust the air from the bottle. Shake the bottle so that the bell can be seen to ring but does not strike the sides of the bottle. Can the sound be heard distinctly?
+
+
+
+
+
+Fig. 39.
+
+
+
+Although sound is not related to the sun's energy it seems best for certain reasons to consider it briefly in this place. In Experiment 34, it was found that if the air was exhausted and the bell did not touch the sides of the bottle, almost no sound was heard when the dapper of the bell showed that the bell was ringing. This shows that the sounds we usually hear are transmitted in some way by the aid of the air.
+
+Sound is found to be a vibratory wave motion in a material medium. If a scratch is made on the end of a long log, it can be heard if the ear is placed at the other end of the log, when it cannot be heard if the ear is away from the log. In this case the medium is the wood.
+
+
+
+Fig. 40.
+
+
+
+If a stone is dropped into a quiet pond, the rippling waves developed will extend often to the farthest shore of the pond, but a chip floating near where the stone fell will not be moved from its position except up and down. Thus the waves traveled outward from the point of origin, but there was no outward movement of the water. If a long rope, attached at one end and held in a horizontal position, is suddenly struck with a stick, a wave motion will travel along the rope from end to end, but the particles of the rope will simply move up and down. It is in a similar way to this that the sound waves travel, but the particles which transmit the sound only move back and forth through small distances.
+
+
+
+
+
+Summary. —All energy upon the earth is due to the sun. There are two kinds of energy, kinetic and potential. Energy may be changed in countless ways but it cannot be destroyed.
+
+Heat is a form of molecular energy. Heat is shown in changes in temperature and these are measured by thermometers, of which the Fahrenheit and the Centigrade are the commonest. Heat affects the state of matter: the same substance may be solid, liquid or gaseous, depending on the amount of heat to which it has been subjected. This is shown in ice, water and steam. Heat is transferred by conduction, convection and radiation. The unit for measuring the amount of heat is called the calorie.
+
+Light moves at the almost unbelievable rate of nearly two hundred thousand miles a second. It goes straight except when passing at an angle through media of different densities. It is then refracted. Its intensity varies inversely as the square of the distance from the source. When light is reflected from any smooth surface like a mirror, the angle of reflection is equal to the angle of incidence.
+
+Sound, like light, is a wave motion. But sound waves can travel through substances that shut out all light, and on the other hand light waves can travel through a vacuum that shuts out all sound. So the intensity of light or sound depends largely upon the medium through which it travels.
+
+
+
+
+
+QUESTIONS
+
+
+Why would it be true to say that all artificial light is the sun's light?
+
+Germany uses the Centigrade thermometer scale. If the temperature of Berlin is reported as 20°C. what would the corresponding temperature be in the thermometer scale generally used in the United States?
+
+Why are iron and type metal better suited for casting than copper and zinc?
+
+In what three ways is heat transferred?
+
+Describe how you could prepare from the ordinary materials you have at hand a crude, inexpensive fireless cooker.
+
+Ponds near the Great Lakes freeze entirely over. Why do not the Great Lakes freeze?
+
+What experiences have you ever had illustrating refraction?
+
+If a boy is reading two feet from a light and moves to a distance of eight feet, how much ought the light to be increased to enable him to read with the same ease?
+
+When the sun is shining brightly, why is it hotter standing on a smooth pavement than on the grass?
+
+How long does it take light to come from the sun to the earth?
+
+
+
+
+
+【中文阅读】
+
+
+22．能量——我们把所有能完成工作、克服阻力、导致改变的能力叫做能量。如果是由物体的所在位置或者自身结构带来的可以发挥效力或克服阻力的能力，我们叫做势能。如果一个物体在实际运动，我们则可以说它具有动能。
+
+当一个摆锤被拉向一边，高于它摆弧的最低点时，它便具有了势能。如果让它就此开始摆动，它便会将自身势能变成动能。它在摆弧最高点拥有的势能，会在经过摆弧最低点的时候，完全变成等量的动能。
+
+一支枪由于自身构造，在被施加负载之后会具有势能，但当火药爆炸后，它便会变成动能并传递给子弹。在这个过程中，极其少量的硝酸甘油炸药由于自身分子结构而具有了潜在能量，可以在一定条件下转变为动能。
+
+太阳通过其自身的存在，给地球输送了��大能量，其能量形式，主要是光和热。地球之所以能够让动植物生存，让人类有能力拓展工业文明，都是缘于太阳所提供的能量。没有阳光带来的能量，植物便不能生长；而没有植物所提供的能量，动物也不能存活。
+
+经过远古漫长的岁月，植物利用了太阳的能量并将其储存起来，这样的能量保存在植物的残骸中，形成了煤矿。在今天，煤通过燃烧，给我们人类的工业发展提供了能量。水则被太阳蒸发，变成云，进而云腾致雨洒向大地，部分水资源则被储存在山地湖中，具有了势能，它可以穿越峡谷倾泻而下，将自身的势能转化为动能而做功。通常认为，能量有很多不同的来源，比如木材、煤、石油、水力等等，但若追溯回去，所有的能量均来自于太阳。
+
+能量可以被转换成不同的形式，比如蒸汽机可以将煤的化学势能转变为机械能，而这机械能又可以被发电机转换成电能。经过极其严密的研究发现，尽管能量的形式可以改变转化，但能量本身却永远不会消灭。机械能常常会转变为热能，比如两个物体表面相互摩擦即可，但当这两种能量被精确测算之后，发现能量的总和一点也没有减少。这个具有重要意义的事实已经被大量极其精确严密的研究所证实，并被叫做能量守恒定律。
+
+23．光与热——我们每个人都知道太阳无私奉献给地球的光与热有多么重要。要是把动植物置于完全没有阳光的地方，它们便会病变死亡；而要是将它们置于非常寒冷的地方，它们也会被冻死。虽然我们能够同时感受到太阳的光与热，但它们二者并不是不可分割。我们可以感受到沸水的热量，但是它不会发光；我们也可以看见月亮的光，却感受不到它的热量。因此也常说，我们能感到热而看不见它，能看见光却感觉不到它。
+
+24．热量
+
+
+
+
+
+实验17:将一个玻璃烧瓶安上一个单孔橡胶塞，再将一根20cm长的玻璃细管通过塞孔插入瓶中，然后将其放在一个环架上，让玻璃管向下伸入一个烧杯的水中。现在给烧瓶慢慢加热，可以看见空气向烧杯中的水里面膨胀并形成许多气泡。然后等烧瓶冷却下来，里面的空气收缩导致杯中的水会上升到玻璃管中。
+
+实验18:将上面实验里面的玻璃烧瓶装进有颜色的水，记住伸进瓶中的玻璃管刚好要穿进橡胶塞的位置，然后给玻璃管伸进瓶中的这一部分涂上凡士林，再插进烧瓶中，注意这时烧瓶和几公分玻璃管里面已经被有颜色的水填充，且烧瓶中没有一个气泡。用一根橡皮筋给玻璃管中的水柱顶端位置做上记号，加热烧瓶，可以看见液体开始膨胀。现在想想为什么烧水的用具的结构总是在一个水槽的顶部连接一根管子？
+
+实验19:将一对球环装置的球穿过环，注意观察二者严丝合缝的契合形态。然后在本生煤气灯上将球加热几分钟，看看加热后的球还能否穿过之前的环。解释一下为什么不能？
+
+实验20:将一根合金金属条加热，它会变弯，加热越久，弯曲程度越大。原因是不同的金属受热膨胀的比率不一样。现在想想为什么蒸汽机的蒸汽管端头要自由松开而不固定死？为什么钢结构大桥的长长桥架要安置在轮毂上？为什么安装轮胎的时候，先给轮胎加热才安装到钢轮毂上，然后又故意让它冷却？铂丝是唯一的一种可以穿过炽热玻璃灯管的金属物质，其他金属由于和玻璃的热胀冷缩程度不一样，当它们熔化冷却后，会让玻璃管炸裂。
+
+
+
+
+
+在最初研究热的时候，它被认为是一种没有重量的无形流体，可以进入物体内部，让物体像海绵和木头被水泡胀那样受热膨胀。人们认为这种流体可以通过物体间的撞击和摩擦而被带出来，因为即使原始人也知道用两根干木棍就可钻木取火。
+
+18世纪晚期，一个叫康德•拉姆福德的美国人，在为巴伐利亚政府钻磨加农炮时发现，热量的增加似乎完全根源于机器的刺耳声和机械能的消耗。之前流行的旧有流体理论到了19世纪中期，终于要寿终正寝了，一位名叫焦耳的伟大英国实验物理学家给出了定论：物体热量的增加完全是由于机械能的转化。
+
+现在我们已经相信所有物质都是有各自的微粒或者说分子组成的，分子会在物体的空间区域内，相互之间无序地运动碰撞。对于物体的无形微粒，可以通过一个实例来感知，比如在一个房间里打开一个氨气瓶，或者放一块麝香，过不了一会儿，整间屋子每个地方都能闻到氨气或者麝香的气味，而我们并没有给房间的空气中加入其他什么物质。
+
+由于分子极其微小，即使用最强大的显微镜我们也看不见它们[1]。针尖大小的物质里面就能包含数百万个分子。当物体被加热的时候，分子会加速运动，相互之间的碰撞会更加激烈，这就让物体开始膨胀。热之所以也是能量的一种形式，就是缘于分子的运动。如果物体分子完全没有运动的状态可以达到的话，那物体将不会有任何热量[2]。热量能使气态、液态和固态的物质膨胀的现象，一如上面的实验所示。
+
+25．温度的测量——从上面的实验中，我们已经知道所有的气态、液态和固态物质都存在热胀冷缩的现象，而且还发现，绝大多数物质都会在普通的温度范围内有着一致比率的膨胀幅度。因此我们可以通过测量物质的膨胀或者收缩程度来确定温度的改变。
+
+
+
+
+
+实验21:轻微地给一个测温玻璃管的球端加热，并让玻璃管端头伸进装有半杯墨水的烧杯中。让玻璃球端冷却，管中便会有一部分墨水上升进来。当完全冷却后，用橡皮筋给管中的墨水顶端做个记号。现在再用手握住球端，给里面的空气加热，玻璃管中的墨水又会被压回一部分到烧杯中，再用冰块或湿布给球端降温，墨水又会在管中上升，且比第一次更高。这样，我们就算有了一个测定物体温度的简易装置了。
+
+
+
+
+
+测定物体由于温度改变而发生的热胀冷缩程度的仪器叫做温度计。温度计有很多种类型，有气体、液体还有金属温度计。如果我们想对不同温度下物体膨胀程度有一个基本的比较标准的话，那就必有一个统一的温度范围。因此，人们选定了在0海拔地区，水结冰和沸腾的两个温度作为统一参照。
+
+
+
+
+
+实验22:给一个试管装满水银，塞上一个单孔橡胶塞，通过孔插进一根玻璃管，留出大概20cm在外面。为了隔绝气泡，有必要给胶塞抹上足够的凡士林。当胶塞被塞进试管的时候，水银会在玻璃管中上升几厘米，用橡皮筋给玻璃管中水银柱顶端做上记号。现在若轻轻给试管加热，可以看到水银柱会上升；若让试管冷却，水银柱则会下降。这样，我们就有了一个简单的温度计了。
+
+
+
+
+
+有一种物质，我们通常通过它的热胀冷缩来测定物体的温度，它就是水银。它能很明显地在温度的增加下随之膨胀，进而我们可以轻易地通过测定它膨胀程度来测定温度。普通的温度计都是有一根均匀中空的玻璃管加上一个底部的球端构成。球端和部分管体里面填塞着水银，剩下的管体部分是空的，水银可以在里面自由升降。当温度升高时，水银便膨胀上升；反之温度下降时，水银便收缩回落。
+
+我们通常使用的温度计标度有两种。一种标度是，在0海拔地区，温度计涉入冰水混合物时水银柱会收缩下降至32°的刻度上，而在蒸汽中，即水的沸点温度，会显示在212°，二者之间被分成180等份，每一等份叫1华氏度。这种标度被广泛应用在英国、美国的家用温度计上。
+
+还有另外一种标度被广泛用在科研用途以及实行公制测量单位的国家和地区，叫做摄氏度。在这种标度中，冰水混合物的温度被标定为0，而水的沸点温度则被标定为100。每1度摄氏温度便是将冰水加热至沸腾的温度变化过程的1/100，而每1华氏温度则是这个过程的1/180。温度计的设计构造也很多，有专门测量超高温的温度计，也有专门测量超低温的温度计，还有可以对温度变化进行自动记录的温度计。
+
+26．物质的三态——物质有三种物态：固态、液态和气态。各自对应的例子有：固态的金属球，液态的水和水银，气态的空气。科学家们早已通过实验对物态进行了研究，我们都知道水是一种液体，也可以是固体，比如冰块，还可以是气体，比如水蒸气，这完全依赖于它所处的温度。但是所有的物质在不同温度下都会呈现这三种物态，就不被大多数人知晓了。
+
+许多物质都可以在这三种物态下存在，比如铁，我们平常看见它是固态，而一旦进了高温熔炉，它便会成为液态；如果在太阳表面的超高温环境中，它也会成为气态。物质在从固态变为液态的过程中，体积会膨胀，在液态变气态的过程中也是如此。然而水结冰的过程却是例外，水结成冰体积反而膨胀，但如果不是这样，冰也就不能漂浮在水面上了。适合铸造的金属必须遵守这个降温时体积收缩的规律，这也是我们在各种类金属铸造过程中，所利用的一个十分有价值的特性。
+
+27．热量的传递
+
+
+
+
+
+实验23:分别切一段15cm长的10号铜丝和10号铁丝，再找一根同样长度和粗细的玻璃棍。现在依次拿着它们的一端，将另一端放在本生煤气灯上加热，哪一根会最先将热传递到手上？
+
+实验24:取一支试管，装进其容积四分之三的冷水，握住试管底部小心地将水的顶部放在火焰上加热，直到其附近部分的水开始沸腾。注意确保火焰不要烧烤到水面以上的试管，以免其受热炸裂。要是在试管底部加少许小冰块，整个实验的效果会更加明显，当然可以将一小块铁丝网或者少许金属丝塞进试管底部，阻止小冰块浮到水面附近。从实验结果可以看出来，水的传热能力是很差的，上半部分热水不会下沉，也说明它们一定比下半部分的冷水更轻。
+
+
+
+
+
+要是没有太阳给我们的热量，地球上将不会有生命，也不会有潺潺流动的溪流，不会有纵横驰骋的风，以及流变无穷的能量所造就的这个异彩纷呈的大千世界。因此我们很有必要了解热量是怎样从一个地方传输到另一个地方的。
+
+热量可以通过固态物质传输，比如金属，热量在其中会传得很快，在其他物质中比如玻璃，就会传得慢一些。在实验23中我们发现，热量会在其中一根小棍上传输得比在其他两根上快一些。同时也没有任何迹象显示，组成小棍的微粒在这个过程中进行了传输移动。在试管加热的试验中，也没有任何征兆显示，存在水的粒子从沸水部分传递到了试管底部的冷水部分。在这些例子中，热量都只是简单地从一个分子传递给另一个分子。
+
+这样的热量传递过程叫做热传导。物体的热传导能力有好有坏，我们在刚才使用不同材料的试验中也已经感受到了。我们用钢铁来做散热器就是为了让热量能更快更容易地被传到屋外，而用石棉来包裹暖气管以保存热量，是因为石棉是热量的不良导体，可以防止暖气管中热量的散发。
+
+
+
+
+
+实验25:将一张燃烧的纸放在一个口朝下的钟罩中，注意通过烟雾来观察空气的流动。纸张在硝酸钾的强溶液中适度浸泡再拿出来晾干，点燃之后将会有极其浓烈的烟雾。
+
+实验26:给一个500毫升的烧杯装半杯水，然后放到环架上用本生煤气灯加热，再不断给杯中撒入少量锯末，适当搅拌，让一些木屑沉入杯底。继续给烧杯缓缓加热，注意观测杯中水的流动。
+
+
+
+
+
+当烧杯中的水被加热以及纸张在钟罩下燃烧的时候，水和空气加剧流动，加热之后便开始翻腾上升。这时，热量便被加热后的空气和水在翻腾的运动中进行了传递。这样传输方式也是我们之前提到的热传导。热水与暖气炉给我们的房间加热的效率关联于热量的对流程度，我们之后还会发现，要是没有对流，地球上便不会有风，也没有海上的洋流。
+
+打开一盏高温炽热的电灯，然后把手立即置于其下，即使玻璃灯管由于是热量的不良导体而依然保持冰冷，我们也能立刻感受到热量带来的温暖。白热化的灯丝外面包裹着一层完全的真空层，它可以排除一切对流，像冷玻璃管一样，可以阻止热量的快速传递。因此，感觉上似乎觉得，热量并不是由于接触而进行传导的，因为灯管周围的空气并没有接触到灯丝，何况空气是比玻璃更迟钝的不良热导体。还有玻璃管，也只是会在刚开始的极小段时间内没有变热。
+
+看来除了传导和对流之外，一定还存在着别的热量传递方式。而且似乎在这样的热传递方式中，中介物并不是必要的，因为灯丝周围全是真空，没有任何物质。如今天文学家已经告诉我们，在地球大气层与太阳之间没有任何中介物质，但太阳的热量依然以光的极大速度，每秒186000英里传输到我们地球，而且还并没有让中间的巨大空间变热。热的对流、传导和这样的方式比起来就显得太慢太慢了，这样的热传递方式就叫做辐射。如果热量不能以这样的方式传递，地球也将会变得不适合人类居住。
+
+如果一个物体被加热到一定温度，而包围它的物体既不能进行热传导，也不能进行热对流，那该物体的热量就会保存在自身之中。这个原理的应用例子有无焰灶和保温瓶，前者是将热源物体用毛毡、木质纤维、石棉等类似的不传热物质包裹起来，后者是用玻璃以及一层被抽尽空气的隔热层包裹起来，两种办法都是为了防止热量从热源物体传递出来。只有这样我们才能在无焰灶上面做饭，保温瓶中的液体才能长时间保温，当然如果将冰冷液体放进去，它也会保持冰冷，因为外部的热量完全不能触碰到它。这种情况下人们往往还会给保温瓶一类物体搭上厚重衣物，以防止其热量传递到周围的空气中去。
+
+28．热量的测量
+
+
+
+
+
+实验27:在两只空烧杯中各装入100克水，给其中一只烧杯缓缓加热并小心搅动杯中的水，让其均匀加热至90°C，将另一只烧杯降温至10°C。然后将一只烧杯中的水倒入另一只烧杯中，并将水均匀搅动，��后记录下混合水的温度。最好用化学温度计来进行测温。
+
+再取两只空烧杯，一只装入100克低度白酒，一只还是装入100克水。将装有白酒的烧杯放入沸水中加热，保持其稳定并不断搅动白酒直至温度达到90°C,装水的烧杯还是降温至10°C。确定各自温度后，将白酒倒入另一只装水的烧杯中，均匀搅动，最后记录下混合液的温度。前后两次混合物哪个温度更高？是水与水的混合物温度高，还是水与白酒的混合物温度高？
+
+
+
+
+
+既然热量在地球的各类活动中扮演了如此重要的角色，那我们就有必要知道如何来测量它。温度与热量之间存在着巨大差别，一汤匙100℃的水的热量远远少于一桶10℃的水的热量。让一池水升高1度所需要的热量，也会远远多于让一壶水升高许多度的热量。这就是为什么巨量的水即使温度改变很小，也能吸收、消耗超大量的热量。
+
+不仅不同数量的相同物质温度升高所需要的热量不一样，相同数量的不同物质温度升高所需要的热量也大不相同。如果将同样体积的水与橄榄油各自放在同样的盘子中一起在烤炉上加热，会发现橄榄油温度上升的速度几乎是水的两倍。换句话说，温度同样增加1度的话，水需要的热量是橄榄油需要的热量的两倍。实际上，让一定量的水温度增加1度所需要的热量，比相同重量任何其他物质温度增加1度所需要的热量都多。
+
+从实验27的结果来看，水与水的混合物的温度远远高于水与白酒的混合物的温度，白酒对热量的容积要小得多。将一定量物质温度升高1度所需的热量，相较于将等量的水的温度升高1度所需的热量，我们叫做该物质的比热[3]。橄榄油的比热是0.47，白酒的比热是0.03，意思就是，将等质量的三者温度升高1度，橄榄油需要的热量是水的47%，白酒则只需水的3%。为了比较热量的大小，物理学家引入了一个单位来表示将1克水升高1°C所需要的热量，这个单位叫做卡路里。
+
+29．光——太阳不仅是地球几乎所有热量的来源，更是光的渊薮。我们已经有了人造的发光物体，比如蜡烛、油灯、电灯，但它们的光线相较于太阳发出的光，完全不可同日而语。星星也能发出一点微弱的光线，但大多数我们所知的物体都是暗淡无光的。有时候，某些物体由于自身的光滑表面可以反射发光体发出的光线，因此看起来好像它们也能发光似的。
+
+这样的例子很多，比如黄昏日落之时，阳光反射在房间窗户的玻璃上，让我们觉得似乎有个光源在窗户玻璃窗后面。任何因光滑表面而反射光线的物体，看起来像自己能发光一样，这主要是缘于存在真实光源，如果这个真实光源移走了，它们便黯然失色了。月亮就是如此，农历月末月初的时候，月亮太偏离于太阳以至于不能反射太阳的光，我们也就看不见它；而当月中满月时，半个月亮表面都在反射太阳的光，因此这时看起来十分明亮。
+
+30．光的运动方向
+
+
+
+
+
+实验28:将一个针孔照相机暗室（它由两个大小不同的镜筒盒组成，大的尾部有一个针孔，小的有一块精致玻璃平板）的成像孔对准一个物体，并且前后移动其中的玻璃板直至上面出现清晰的物像。在一张纸上绘出该物体以及所成的像，标示出你认为光线为了通过小孔成像而必须经过的路径方向。
+
+
+
+
+
+照相机就是用与上面成像设备相同的原理设计制造的，只是在小孔前增加了一个镜头来增强成像效果，并且可以用照相底片来切换这里的精致玻璃板。
+
+光的许多固有特性已经在日常生活中被我们熟知。我们在黑暗的房间中看不见任何东西，但若光线照进来，所有的物体便立即呈现。我们能看见它们，是因为光线被它们反射进到我们的眼睛。除了自己发光的物体以外，所有物体都是因为反射光线才能被我们所见。
+
+若将一支蜡烛置于镜子前面，我们看看镜子便能发现有一支蜡烛在它里面。我们当然知道蜡烛其实并不在那里，只是它的光线被镜子反射成那样一条光路，让它看起来就像是从镜子后面发出来一样。我们正是通过镜子反射的光才看见了里面的蜡烛。
+
+如果我们想看一块木板的边缘是否笔直，我们只需用眼睛沿着边缘瞄上一眼；如果我们想用子弹射击一个物体，我们会把枪管抬起，用视线瞄准。因为我们很确定地认为，在像空气一样的均匀介质中，光是沿着直线运动的。
+
+
+
+
+
+实验29:将一枚硬币放置在一个5品脱[4]的平底锅锅底的正中心，再将锅静置于桌上。然后你站在顺着锅沿刚好可以看见硬币边缘的地方。让一个人慢慢往锅里倒水，这时硬币好像有什么变化？
+
+实验30:给一个空电池槽装入三分之二满的水，斜插进一根玻璃棒或者木棍，棍子看起来还是直的吗？在给水面上倒入两三英寸厚的煤油，棍子看起来又会有什么不一样？
+
+实验31:取一片普通的远视眼镜片比如老年人的老花镜片，或者凸透镜片，放在一张纸上面，置于太阳下。变换镜片与纸张的距离，可以看见太阳的光热被透镜弯曲而集中到一点上。现在用近视镜片，或者凹透镜片做同样的实验，可以发现光线被发散了。为什么近视眼和远视眼不能用相同的镜片呢？
+
+
+
+
+
+在上面锅底硬币的实验中，水在一定程度上让光线发生了弯曲，进而让整个硬币都进入了我们的视线，而没有水的时候我们是看不见硬币全貌的。这个实验说明，光从一种介质进入到另一种介质的时候，它不会沿着同一条直线传播。我们把那些对光线阻力比较大的物质叫做密集介质，光线在其中经过时所遭受的阻力比在其他物质中要大得多。正是这样的阻力变化导致了光线的弯曲。
+
+设想一队士兵以纵队形式穿出一片玉米地，然后又斜着行进到旷野上。当士兵们在旷野行进时候，由于没有玉米秆的阻碍他们会行进得快一些，所以在靠近玉米地边缘的地方，行进队伍会显得更加歪斜零散一些。而如果行进路线是反方向的，即从旷野走进玉米地，则我们也想象得出来，队形会和刚才相反。若队伍的排列平行于玉米地边缘，那每一个士兵都会同时到达旷野边缘，队形也就不会有摇摆变形了。
+
+上面的类比粗略地解释了光从一种介质进入到另一种介质时所发生情形。我们把光从一种介质进入另一种介质，或者进入同一种介质但密度发生改变时所出现的弯曲现象，叫做折射。日出日落的美景，海市蜃楼的虚幻，太阳在地平线上扁平的脸以及不久我们将会看到的其他现象都是由光的这个特性所致。
+
+31．光的强度
+
+
+
+
+
+实验32:取两块厚约1英寸的方石蜡片，背靠背地贴在一起，中间加一张硬纸片或者锡箔纸。当在其一边放一个光源时，向着光源那边的石蜡会被通体照亮，另一边隔着纸片的石蜡则不会。如果两边都放有光源，则会看见两边的石蜡都被差不多地照亮了，或者说都接受了等量的光照。这样我们就可以轻松比较两边的光照强度。
+
+
+
+
+
+在石蜡一边25cm远的地方放1支蜡烛，在另一边90cm远的地方放4支蜡烛，都一起放在一张纸板上以便整体移动。然后通过下的纸板前后移动这4支蜡烛，到两片石蜡的亮度都差不多为止。测量一下这时4支蜡烛到石蜡的距离，它是另一边那支蜡烛到石蜡距离几倍？
+
+太阳光的强度极其巨大，我们哪怕拿个弧光灯放在太阳下，它也显得像个芝麻点儿。不过由于地球到太阳的距离太远，以至于地球所接受的太阳光热只是它的总体光热的一丁点，不到十亿分之二，我们已经不能用普通的言语来描述太阳光热超乎想象的强大程度。我们对光线强度的标准测量可以用蜡烛的光亮强度来做单位，指定为一支12号的普通石蜡蜡烛所发出光为一个单位标准。一只普通的白炽灯的亮度大约是16个蜡烛单位。
+
+如果用蜡烛单位来衡量太阳光强度的话，那这个数字将远远无法估量。光的强度跟热与电的强度一样，都是决定于从发射源这一点源源不断地发射出来的能量强度，且大小的变化都与距离的平方成反比。反映在现实中，与光源的距离增加，则光的亮度会迅速减小，这就是当稍微把台灯移远一点，我们看书的视觉舒适度就大打折扣的原因。如果这时我们把台灯的距离再拉近一倍，则书本所接受的光照就成了原来的四倍。
+
+32．热与光的反射现象
+
+
+
+
+
+实验33:在一个黑暗房间的桌上放置一面镜子，再从一个暗幕的小孔中，或者从一个人造小光源，投射进一缕光线到镜子上让其发生反射。如果房间中没有足够的灰尘让你看见光线传播路径的话，可以在镜子上方将两个黑板刷拍几下。这时拿一支铅笔竖直地放在镜子与光线的接触点上，比较一下铅笔两边与光线的夹角大小。然后抬起镜子的一边，注意观察光线反射路径的变化，再把铅笔垂直于镜面放在反射点上，同样比较一下两边的夹角，大小一样吗？
+
+
+
+
+
+之前我们已经说过，月亮发光是因为反射了太阳的光线。日常的基本观察也告诉我们，地球上的物体也都可以反射光与热。在炎热的夏天，房子的外墙和街道的路面所反射的热量让人有时觉得难以忍受。在一些日照时间非常长的国家和地区，比如撒哈拉沙漠，人们便安装了反射镜专门反射太阳光热来给物体加热，甚至用来运转���汽机。
+
+房屋的细致表面可以极大程度地反射太阳光，将太阳本身照不到的周围其他房屋的窗户照亮。一个人站在湖边合适的位置，便可以看见树木和湖光山色的倒影。但若他大范围地改变一下所站的地方，刚才的一切也许就看不见了。这说明反射的光线一定与射向反射物表面的入射光线有密切的关系。
+
+在实验33中，当铅笔垂直放置于光线与镜面的接触点上时，可以明显发现，入射的光线跟反射的光线与铅笔的夹角是完全相等的。这两个角分别叫做入射角与反射角。经过极为精确的实验研究已得出结论，这两条光线与反射面垂线的夹角总是相等的，换句话说，反射角等于入射角。这就是为什么你站在房间里镜子的一边只能看见房间另一边的原因。
+
+33．光的速度——在17世纪晚期，一位名叫罗默的丹麦天文学家在仔细观测木星最亮的一颗卫星围绕木星的运动时，发现接连两次木卫食，或者说卫星经过木星背影之间的时间间隔有奇怪的变化。他于是精确测定了两次木卫食[5]之间的时间间隔，即这颗卫星围绕木星完成一次公转的时间。
+
+利用这个间隔，他计算出了其他木卫食将要发生的时间，并且发现，由于地球自身绕着太阳公转，离木星越来越远，而看见木卫食发生的时间似乎也越来越滞后。接下来他又精确地测算了地球最靠近木星时木卫食发生的时间，又推算出6个月之后，地球离木星最远时木卫食将会发生的时间，最后他发现这次木卫食实际发生的时间比推算的时间滞后了22分钟。他说这个现象一定是因为光需要一段时间才能横穿地球轨道而导致的。
+
+从罗默的这些计算开始，人们又做了很多这类的研究与测算，人们发现他的计算还是有一些误差，光线横穿地球轨道的时间其实只要16分40秒，或者说1000秒。由于地球轨道直径为186000000英里，因此光的速度也一定是每秒18600英里。通过其他办法对光速的测定也给出了大致相同的数据结果。
+
+34．关于光的理论——尽管对光的感知以及检测光的许多特性都很容易，但是说清楚光的本质到底是什么却极其困难。艾萨克•牛顿勋爵认为光是由极微小的粒子流组成，它们被发光体释放出来。但自从1800年，人们又认为光是一种波动现象，它在充斥于整个空间的一种名叫以太[6]的介质中传播。
+
+35．声音
+
+
+
+
+
+实验34:取一个广口瓶，在里面悬挂一个小铃铛，系在瓶塞上。再通过塞子插进一根厚壁橡皮管，将橡皮管另一端接在一个空气泵上，抽完瓶子中的空气。现在摇动瓶子，让铃铛看起来会发出响声，注意不要让铃铛碰到瓶子。你能清晰地听到铃响吗？
+
+
+
+
+
+尽管声音与太阳的能量没有什么关系，但也似乎有必要在这里简单地谈一谈它。在实验34中我们发现，当瓶中空气被抽出，铃铛被摇动但没有碰到瓶子，即使看起来铃铛好像在响一样，我们却几乎听不见任何声音。这说明我们平常所听见的声音，是依赖于空气而传播的。
+
+研究发现，声音其实是一种在物质介质中传播的振动波。在一根长圆木的一端刮擦一下，如果这时把耳朵贴在圆木另一端，我们便能听见刮擦的声音，若耳朵离开圆木便听不见。在这个例子中，介质就是木头。
+
+如果把一块石头投入平静的池塘水面，可以看见层层涟漪荡漾开来，直达最远的池塘边缘，但浮在水面上靠近投掷点的地方的一片小树叶，除了上下跌宕以外并没有移动到别的地方。这是因为虽然水波是从中心向外传播，但是水本身并没有向外运动。如果我们拿住一根长绳子，另一端固定在一个地方然后让绳子整体保持水平，这时我们用木棒突然敲击一下绳子，便可以看见一个绳波会沿着绳子从一端传到另一端，而绳子的每一小部分都是上下移动了一下而已。这就很类似于声音的传播方式，只不过声音传播介质的组成微粒，是在小范围内前后移动而不是上下运动。
+
+
+
+
+
+总结——地球上的所有能量都来自于太阳。能量有两大类，动能与势能。能量可以转化为无数的形式，但不会消灭。
+
+热是分子能的一种形式。热可以在温度的变化中显现出来，温度计可以测定温度的变化，华氏度和摄氏度是比较常见的两种温度单位。热可以改变物质的物态：同一种物质可以呈现固态、液态以及气态，取决于自身蕴涵的热量，冰、水、水蒸气就是对应的例子。热量可以通过传导、对流以及辐射三种方式进行传递，热量的测量单位叫做卡路里。
+
+光以难以置信的将近每秒200000英里的速度运动。除了经过不同密度的介质时光会发生一定角度的弯折��外，光总是沿着直线运动。光经过不同密度介质时发生的弯折叫做折射。光线强度与到光源的距离的平方成反比。当光线在光滑表面，比如镜面上发生反射现象的时候，反射角一定等于入射角。
+
+声音像光一样，也是一种波动。声音可以在能遮住光的物质中传播，而光也可以在阻止声音的真空中传播。光和声音的强度，在很大程度上决定于它们穿行而过的传播介质。
+
+
+
+
+
+思考题
+
+
+为什么我们可以说“一切人造的光都是太阳光”这句话是正确的？
+
+德国用的是摄氏温度，如果柏林的预报温度是20°C,它在美国常用的温度计上的相对应的温度是多少？
+
+为什么钢铁与铅合金比铜与锌更适合锻造？
+
+热有哪三种传递方式？
+
+如果你手里的材料只有一块粗麻布、一个便宜的无焰灶，请描述一下你如何利用他们来做饭？
+
+当五大湖区附近的池塘都已结冰封冻时，为什么五大湖却没有千里冰封？
+
+你有过哪些经历可以用来阐释光的折射现象？
+
+一个男孩正在离灯光2尺远的地方读一本书，然后他走到了离灯光8尺远的地方继续读，若要让他保持之前的光照舒适度，他接收的光线的强度应该增加多少？
+
+在烈日当空的时候，为什么站在大马路上比站在草地上更热？
+
+光从太阳到地球要花多少时间？
+
+
+
+
+
+译　注
+
+
+[1]现在早已不是如此，我们已经可以通过电子显微镜窥见比分子更小的原子的完全面貌。
+
+[2]根据热力学定律和量子力学原理，物体分子没有运动这一状态是永远也不可能达到的。
+
+[3]如今物理学对“比热”的定义已有了变化，它是指单位质量的某种物质升高单位温度所需的热量。其国际单位制中的单位是“焦耳每千克开尔文”，即令1千克的物质的温度上升1摄氏度所需的能量。
+
+[4]美制单位的品脱有两种，湿量品脱与干量品脱，各不相同。1湿量品脱=473.176473毫升，1干量品脱=550.61047毫升。大致而言，一品脱差不多就是半升的容积。
+
+[5]也即木星上的“月食”，这颗卫星相当于木星的月亮。
+
+[6]1881〜1884年间，著名的“迈克尔逊-莫雷实验”已经否定了以太的存在。进入20世纪以后，物理学家们对光的本质进行了大量研究，并成为整个百年物理学理论演进的主战场之一，波动说和粒子说都各领风骚彼此消长。到今天为止，我们对光的认知依然还停留在“波粒二象性”，简言之，它既是波又是粒子，其实在结果取决于我们的观测方式。
+
+
+
+
+
+CHAPTER 4
+
+THE EARTH'S CRUST
+
+地球的外衣
+
+
+36. Land and Water Areas. —The surface of the earth has an area of about 197,000,000 square miles, about 28 percent of which is land. Such areas are too vast for us to conceive, but it may help us toward a conception to know that the area of the United States, exclusive of Alaska and islands belonging to it, is about 1/18 of the land area of the earth.
+
+
+
+LAND AND WATEE HEMISPERES.
+
+Notice that London is about at the center of the land hemisphere.
+
+
+
+It is possible to divide the surface of the globe into two hemispheres, one of which contains the larger part of its land and the other the larger part of its water surface. This bunching of the land has brought the people of the earth near together and has greatly facilitated their intercourse, especially since land transportation has become so easy. Under the climatic conditions which exist, it is very advantageous also for the inhabitants of the earth that the greater part of the polar lands are around the north pole instead of the south. The fact that the land masses have irregular outlines and are separated by water areas instead of being in one continuous extent is also, as we shall see, of benefit to the earth's inhabitants.
+
+37. Interchange of Land and Water Areas. —It has been found from numerous observations that the land and sea do not always maintain the same relation to each other. Areas which at one time were land have since become sea and those which were once sea are now land. Sea shells are found imbedded in the rocks far from the sea and old river valleys are found by soundings under the sea at considerable distances from the present mouths of the rivers. What were once sea beaches are now found hundreds of feet above the sea.
+
+From some such marks on the coast of northern Sweden it appears that the coast has risen about seven feet during the last 150 years. The Netherlands are sinking. Observations along the coast of Massachusetts give reason to believe that it is sinking very slowly. Indications of the movement of the land in respect to the sea are found in all parts of the world.
+
+
+
+OLD SEA BEACHES, SAN PEDRO.
+
+Three old beaches can be distinctly seen on the promontory.
+
+
+
+Old sea beaches are found rising one above the other along the entire slope of a high hill at San Pedro, near the port of Los Angeles, California. Suess, the great Austrian geologist, thinks that the great changes of level between the sea and the land are due to a rising and falling of the sea and not a rising and sinking of the land. However this may be, there have been marked changes of level between the two and the boundary between sea and land has been a varying line. Sea and land areas have frequently interchanged, although deep sea bottoms were probably never dry land.
+
+38. Characteristics of Land Surfaces. —The surface of the land differs from that of the sea in being at least comparatively immovable. It is rough and irregular, and is composed of many different kinds of rocks and soils. For the larger part of its area it rises above the level of the sea, but in a few places it sinks below, as in the Salton Sea, a part of Imperial Valley, California, and near the Dead Sea. Its surface is eroded by wind and water and is thus constantly but slowly changing its features. Travel upon the land, over most of the earth's surface, is difficult because of the irregularities.
+
+
+
+OLD ROOK BEACH, IMPERIAL VALLEY, CALIFORNIA.
+
+Formerly part of the coast line of the Gulf of California.
+
+
+
+
+
+SALT WORKS ON THE SHORE OF THE SALTON SEA.
+
+In 1905 the Colorado River broke through into this depression, which is below sea level, and completely covered the salt works seen in the picture.
+
+
+
+Surface conditions also vary greatly over small areas. Great temperature changes occur on the land between day and night and between summer and winter. Land animals must exert considerable muscular force to move about, yet they must all move to get their food. They must therefore be highly organized to maintain themselves upon the land. Water animals are not subjected to the same difficult conditions. In fact, the conditions of life on sea and land surfaces are entirely different.
+
+
+
+
+
+39. Characteristics of Water.
+
+
+
+
+
+Experiment 35. —Place in a dish of fresh water a density hydrometer, or stick loaded with lead at one end, so that it will float upright. Mark with a rubber band the point to which the hydrometer sinks in the water. In a dish sufficiently deep for the hydrometer to float dissolve a considerable quantity of salt in water. After the salt has become thoroughly dissolved taste the water at the top and then after pouring off the larger part of the water taste that at the bottom of the dish. The salt is present in all the water but the appearance of the water has not changed.
+
+
+
+Fig. 41.
+
+
+
+Now place the hydrometer in the water containing the dissolved salt. It does not sink to the same depth that it did in the fresh water. What can be said about the buoyancy of water which contains substances in solution? Does a fish need to exert muscular force to float in water? Since water contains many substances in solution, it is possible for a water animal that does not move to be continually supplied with food.
+
+
+
+
+
+Experiment 36. —(Teacher's experiment). Place a small handful of zinc scraps in a strong wide-mouthed bottle. Fit the bottle with a two-hole rubber stopper having a thistle tube extending through one hole and a bent delivery tube through the other. The thistle tube should reach nearly to the bottom of the bottle. Connect the delivery tube with the shelf of a pneumatic trough by a rubber tube. Have several inverted 8 oz. wide-mouthed bottles filled with water on the shelf of the trough. Pour enough water through the thistle tube to partly cover the zinc and then pour on commercial hydrochloric acid or sulphuric acid diluted 1 to 10.
+
+
+
+Fig. 42.
+
+
+
+Chemical action will take place between the zinc and the acid and hydrogen will be freed. Allow the gas to escape for several minutes, as this is largely the air which was in the bottle. Collect several bottles full of the hydrogen. Keep the bottles inverted. Examine the hydrogen in one of the bottles. Has it color or odor? Holding the mouth downward thrust a lighted splinter into another bottle. The splinter does not continue to burn in this gas but the gas itself burns. Place another bottle mouth up on the table and allow it to stand for several minutes. Insert a lighted splinter. Why is not the hydrogen still present?
+
+Draw out a glass tube so that the bore will be about as large as the point of a pencil and insert it in the rubber delivery tube. Pour more acid into the bottle and after this has been working for several minutes touch a lighted match to the glass tip of the rubber delivery tube. A jet of burning hydrogen will be formed. Hold a cold dry beaker over this burning jet. Water drops will collect in the beaker. The hydrogen is combining with the oxygen of the air and water is being formed.
+
+
+
+
+
+Pure water is a chemical compound of two gases, hydrogen and oxygen. The oxygen we have always been familiar with, as it makes up about one fifth of the air by which we are surrounded. The hydrogen was prepared in the previous experiment. It is a colorless transparent gas, the lightest of all substances, and must be handled carefully. If it is mixed with oxygen or air and the mixture ignited, it explodes with much violence, forming water.
+
+
+
+
+
+Experiment 37. —Fill a small beaker with fresh water. Heat it slowly. Bubbles collect on the bottom and sides. When the water becomes cold these bubbles do not disappear. If they were steam, they would change back to water. What are they? Where did they come from? Does water contain dissolved air? How can water animals that do not come to the surface obtain the air they need?
+
+
+
+
+
+Experiment 38. —Put a piece of ice in water. What part of its volume sinks below the surface of the water? Is it heavier or lighter than water? From Experiment 24 do you conclude that cold water is heavier or lighter than warm water?
+
+
+
+
+
+The water that we usually see has air and other substances dissolved in it, for water is the greatest solvent known. Another property of water which is very important is its practical incompressibility. No matter how much pressure may be put upon water its volume is little decreased and its density little changed. So it happens that substances which readily sink in the upper part of the sea sink to the bottom no matter how deep the water may be, as the bottom is so little denser than the top.
+
+The substances that are dissolved in water mix thoroughly together. In isolated bodies of water there are often great differences in the amount and kinds of dissolved materials, but over the whole ocean from top to bottom the composition of the water is practically uniform.
+
+From previous experiments we have learned some of the chief physical properties of water, so perhaps we can understand the different effects that water and land have had upon the development and activities of living things upon the globe. Some water animals move about easily to get their food, but others have it brought to them in solution and so obtain it without muscular effort. The air that they breathe is in solution and they cannot as easily obtain a large quantity of it as can the land animals. Since the energy of all animals depends upon the amount of oxygen they use in their bodies, the water animals are generally less energetic than the land animals. Since they also have such an easy time in moving or floating about to get the things they need they have not developed as high organisms as the land animals.
+
+
+
+CORALS.
+
+These are fixed animals whose food is brought to them in solution by the ocean currents.
+
+
+
+Water is readily moved by the winds and becomes a means of cutting down the land and carrying away its material. When heated by the sun or any other source of heat it evaporates and, rising into the air, floats away to be condensed and to fall as rain or snow. It takes a great deal of heat to evaporate water and all this heat is given off when it condenses. Water seeks the lowest place it can find, giving out energy as it flows. In fact, the earth has been likened by some writers to a water engine, since water has played such an important part in its history.
+
+Another property of water which is of great importance is its power to take up heat. This was shown in Experiment 27. When it cools, it gives out the heat it took up when its temperature was raised. It is for this reason that hot water bags are used to keep people warm, and that farmers sometimes in winter, when they fear that their cellars will freeze, carry down tubs of water to keep their cellars above the freezing point. This is why orange groves are often irrigated just before there is danger of a heavy frost.
+
+A pound of water in cooling one degree gives out about as much heat as a pound of iron in cooling 9 degrees. This capacity for holding heat makes bodies of water warm up slowly in the summer and cool off slowly as winter approaches. As they cool they give back to the air the heat they have taken up. During the early part of the summer the air above them is kept cool and in the fall it is warmed. This property of water will be found later to be of great importance.
+
+
+
+
+
+40. Materials Composing the Land.
+
+
+
+
+
+Experiment 39. —Obtain specimens of the igneous rocks, lava, obsidian, basalt, granite; of the sedimentary rocks, sandstone, fossiliferous limestone, conglomerate, peat; of the metamorphic rocks, shale, schist, marble, anthracite coal. Examine these carefully with the eye and with the lens, noting whether they have a uniform composition or are made up of different particles. Are the particles composing the rocks crystalline? Are they scattered irregularly or arranged in layers? Test with a file or knife-blade the hardness of the rock as a whole and of its different constituents. Try a drop of hydrochloric acid on the different rocks to see whether they are affected by it. Describe in a general way the characteristics of each specimen.
+
+
+
+
+
+The composition of different land areas varies greatly. Many different kinds of rocks are often found crowded together, or it may happen that the same kind of rock covers a large area. There is no uniformity. The soil on top of the rock is also variable. In some places it contains the minerals which are in the rock below and in other places its composition is not at all dependent upon the bed rock.
+
+
+
+GRANITE.
+
+Igneous rock formed deep below the surface of the earth.
+
+
+
+The great variety of rocks of which the crust of the earth is composed has been divided into three great groups in accordance with the manner in which they were formed. These groups are igneous, sedimentary and metamorphic.
+
+
+
+FOSSIL-BEARING LIMESTONE.
+
+A sedimentary rock formed from sea shells.
+
+
+
+The igneous rocks are those which have solidified from a melted condition. They may have solidified deep down within the crust, or on the surface, or somewhere between the depths and the surface. If these rocks cooled slowly, they will have a crystalline structure, as in granite, and if very rapidly, a glassy structure, as in obsidian. Their structure can vary anywhere between these two extremes. A common dark colored variety of this kind of rock is called basalt. There are many varieties of igneous rocks, but they need not be considered here.
+
+The sedimentary rocks are those that are made by deposition in water. When rocks are worn away into fragments and these fragments are deposited in water they will, under certain conditions, harden into rocks. The shells and remains of sea animals also accumulate, and after a time consolidate into rock. The remains of plants may accumulate under such conditions that they will not rot but will solidify into rock which we call bituminous or soft coal. About four fifths of the land surface of the earth is composed of sedimentary rocks. They vary greatly in color, durability and usefulness to men.
+
+
+
+CONGLOMERATE.
+
+A sedimentary rock formed from old gravel beds.
+
+
+
+The sandstones, which are composed of little grains of sand cemented together, are used for buildings and for many other purposes. The limestones, which are mostly made from the remains of sea animals, are the source of our lime and are also used for building and for other purposes. The shales are finely stratified mud deposits often having many layers in an inch of thickness. Bituminous coal, which is formed from plants of former ages, is the most useful and valuable of all mineral products. None of these rocks is crystalline. They are composed of fragments of other rocks or remains of plants or animals and usually occur in layers or strata.
+
+
+
+OIL WELLS.
+
+Tapping the rock layers containing petroleum.
+
+
+
+Petroleum is probably a result of the accumulation in the sea of layers of animal and plant remains. These were covered by other layers and, during the ages since their formation, they have decomposed and changed into oil and gas.
+
+The metamorphic rocks have a crystalline structure, often contain well-formed crystals imbedded in them and often bands of crystalline substances extending through them. These rocks are not in the condition in which they were originally laid down, but are modified forms of either the igneous or sedimentary rocks. The rocks originally laid down have been subjected to changes which have rearranged their mineral constituents and changed the structure.
+
+
+
+GNEISS.
+
+Probably metamorphosed granite.
+
+
+
+These changes are generally due to heat and pressure. Marble is a crystallized limestone and gneiss generally a metamorphosed granite. Slate and mica-schist are greatly changed clay rocks and anthracite coal is a metamorphosed form of bituminous coal. The rocks of this group are often hard to distinguish from igneous rocks.
+
+
+
+
+
+41. Structure of the Land Areas.
+
+
+
+
+
+Experiment 40. —Take a copper ball having a ring just large enough to encircle it, the same apparatus as used in Experiment 19. (Fig. 22.) Place the ball within the ring and heat them both to a high temperature. Remove the ball from the ring and plunge it into a dish of water. Place the cooled ball again within the ring. The ring will be found too large to fit snugly upon it.
+
+If the ring had been a cold hollow sphere fitting tightly to the surface of the hot ball and the ball had then been cooled until its temperature approached the temperature of the cold surrounding spherical surface, it would have shrunk away from this spherical surface. This would leave an unfilled space between the two into which the spherical shell must have shrunk if not strong enough to support itself. This shrinking would cause wrinkling in some parts of its surface.
+
+
+
+
+
+Experiment 41. —When at home measure the greatest and least circumference of a large smooth apple by winding a string around it and then unwinding and measuring the length of the string. Bake the apple. Measure its circumferences again. Are they greater or less than before? Is the skin of the apple as smooth as it was before?
+
+
+
+
+
+Not only do the land areas differ greatly in the kind of rocks of which they are composed, but also in the way in which these rocks are placed. Some of the rocks lie nearly in the condition in which they were originally formed while others have been folded and warped and twisted. Vast layers of rocks have been worn away by the forces which are continually wearing away and removing the rocks at the surface of the earth, and thus rocks which were once at great depths below the surface have been exposed. Even granite rocks which were originally formed at a depth of thousands of feet below the surface now appear at the surface and are being quarried in many places.
+
+
+
+STRATIFIED ROCK.
+
+These layers have remained horizontal as originally formed.
+
+
+
+The folding and warping of the rock layers has brought some of the stratified beds which were originally horizontal into an almost vertical position so that we now find at the surface the worn-off edges of these beds. The different kinds of rocks and the different positions in which the rock layers are presented to the forces which are active in wearing them away cause great variety in the forms of the surface features.
+
+
+
+FOLDED ROCKS.
+
+Stratified rocks which have been folded since they were formed.
+
+
+
+It is not necessary to consider all the causes which may have disturbed the position of the rock layers, but the most important of them deserves attention. It has already been found that, although the exterior of the earth is cool, the interior is hot. Now it is known that almost all substances contract when cooled. If the interior of the earth is cooling, and there is every reason to believe that it is, then it must be contracting. As the crust is already cool it has ceased to contract and thus the interior shrinks away from it and it must fold up in order still to rest upon the shrinking interior. The cooling of the earth is so slow that the folding under ordinary conditions disturbs the surface but little.
+
+
+
+
+
+42. Rock Weathering.
+
+
+
+
+
+Experiment 42. —Weigh carefully a piece of dry coarse sandstone or coquina. Allow this to remain in water for several days. Wipe dry and weigh again. Why has there been a change in weight?
+
+Experiment 43. —Fill a test tube or small glass dish about half full of limewater, made by putting about 2 ounces of quicklime into a pint of water. Blow from the mouth through a glass tube into the limewater. There is formed in the limewater a white substance which chemists tell us is of the same composition as limestone.
+
+
+
+ROCKS WEATHERING AND FORMING DEEP SLOPES.
+
+
+
+Experiment 44. —Continue to blow from the mouth for a considerable time through a tube into a dish of limewater. The white substance disappears. A gas in our breath called carbon dioxide dissolves in the water, forming a weak acid and causes the change. Now if we heat the water, thus decomposing the acid and driving out the gas, the white substance again appears. This gas is found everywhere in the air and is given out in the decay and burning of substances.
+
+
+
+
+
+CLEOPATRA'S NEEDLE, CENTRAL PARK, NEW YORK.
+
+
+
+Rocks which are exposed to the atmosphere, especially in moist climates, undergo decomposition. If the climate is warm and dry, rocks may stand for hundreds of years without apparent change, whereas the same rock in another locality, where the weather conditions are different, will crumble rapidly. A striking example of this is found in the great stone obelisk, called Cleopatra's Needle, which was brought from Egypt to Central Park, New York, some time ago. Although it had stood for 3000 years in Egypt without losing the distinctness of the carving upon it, yet in the moist and changeable climate of New York it was found necessary within a year to cover its surface with a preservative substance.
+
+Not only do different climates affect differently the wearing away of rocks, but different kinds of rocks themselves vary much in the rate at which they crumble. It has been found that while marble inscriptions, in a large town where there is much coal smoke and considerable rain, will become illegible in fifty years, that after a hundred years inscriptions cut in slate are sharp and distinct.
+
+
+
+
+
+Experiment 45. —Allow a test tube filled with water and tightly corked to freeze. What happens? If the temperature of the air is not cold enough, place the test tube in a mixture of chopped ice and salt, or better, chopped ice and ammonium chloride (sal ammoniac), and allow it to remain for some time.
+
+
+
+
+
+ROCKS SPLIT BY ROOTS OF A TREE.
+
+
+
+Water getting into the cracks of rocks and expanding when it freezes splits them apart and aids much in their destruction. Plant roots penetrate into the crevices of rocks and by their growth split off pieces of the rock. Water, especially when it has passed through decaying vegetable matter, has the power of dissolving some rock minerals. Certain minerals of which rocks are composed change when exposed to the air somewhat as iron does when it rusts.
+
+
+
+WIND-CUT ROCKS.
+
+These rocks have been fantastically cut by wind-blown sand.
+
+
+
+Where the temperature varies greatly during the day the expansion and contraction due to the heating and cooling sometimes cause a chipping off of the rock surfaces. In some localities, the winds, by blowing sand particles against the rocks, cut them away quite rapidly. All these agencies and others tend to break up and decompose the rocks, thus forming soil. The actions of some of these agencies were seen in the previous experiments.
+
+
+
+
+
+43. Soil.
+
+
+
+
+
+Experiment 46. —Into a 16 oz. bottle nearly full of water put a small handful of sand, and into another bottle about the same amount of pulverized clay. Shake each bottle thoroughly and allow the water to settle. Which settles the more rapidly? Which would settle first if washed by a stream whose current was gradually checked?
+
+
+
+
+
+Wherever the inclination is not too steep, we find the surface of the bed rocks covered for varying depths with a loose material which we call soil. It is upon this that plants grow and in it lies the wealth of our agricultural communities. On examining this soil, it will be found that in some places it grows coarser and coarser the farther down we dig. The coarser the pieces become, the more they resemble the bed rock, until finally they pass by imperceptible stages into it. This kind of soil is called local or sedentary soil.
+
+
+
+LOCAL SOIL.
+
+This soil is being weathered from the underlying rock.
+
+
+
+In other localities the coarseness of the soil does not materially change as we dig into it, but suddenly we come upon the surface of the bed rock, which may contain few if any of the constituents which were in the soil. This soil, which in no way resembles the underlying rock, is called transported soil. We shall find out later how most of it reached its present position.
+
+
+
+DIGGING PEAT IN IRELAND.
+
+Peat is cut in small brick-like squares and dried, before being used as fuel.
+
+
+
+The first kind of soil has evidently been made in some way from the rock below, since it gradually shades into this rock. This kind of soil changes with the change of the bed rock. A striking illustration occurs in Kentucky, where the rich and fertile "Blue Grass" region is bounded by the poor and sandy "Barrens." The one is underlaid by limestone and the other by sandstone.
+
+The soil at the surface is usually finer than the soil a foot or so below the surface, and sometimes it has a great deal of decayed vegetable matter mixed with the decomposed rock, and to this its fertility is often largely due. Some soils are made up almost entirely of decayed vegetable matter, peat and muck. The underlying coarser and lighter colored soil, which contains little if any vegetable matter, is usually called the subsoil.
+
+
+
+
+
+Experiment 47. —Examine under a strong magnifying glass samples of sand, loam, clay, peat and other kinds of soil. Notice the different kinds of particles composing the different soils and the shapes of these particles.
+
+
+
+
+
+Experiment 48. —Put a handful of ordinary loamy soil into a fruit jar nearly full of water and allow it to stand for a day or two, shaking occasionally. At the end of this time shake very thoroughly and after allowing it to settle for a minute, pour off the muddy water into another jar. Allow this to stand for about an hour and then pour off the roily water and evaporate it slowly, being careful not to burn the material left. Examine with the eye, by rubbing between the thumb and fingers, and with a magnifying glass, the three substances thus separated. These three separates will be composed largely of sand, silt and clay.
+
+
+
+Fig. 43.
+
+
+
+If a compound microscope is available, mix a bit of the silt and of the clay in a drop of water and put these drops on glass slides. Examine the drops under the low power of the microscope. Notice the little black particles of decayed vegetable matter, also the little bunches of particles that may still cling together. Why was it necessary to soak the soil so long? Draw the shapes of a few of the particles. Describe the composition of the soil you have examined.
+
+
+
+
+
+If we examine most soils with a microscope, we shall find that they are composed, as was seen in Experiment 48, of many different kinds of material. Some of these materials dissolve slowly in water and thus furnish food for plants; others are insoluble.
+
+In different soils the particles vary greatly in size as well as in composition. In gravel the particles are large and in a gram's weight there would be but few; in sands there are many more, dependent upon the fineness; and in a gram of clay there are several billion particles. Agricultural soils, intermediate between sand and clay, are usually called loams. There are sandy loams and clayey loams, with many intermediate varieties. As the mineral part of the soil is derived entirely from the rocks, only those minerals which were present in the underlying rock can be present in sedentary soils, whereas in transported soils the underlying rock has had no influence upon the soil.
+
+The minerals composing the soil must furnish certain substances if the soils are to support plants. The substances needed in most abundance are nitrogen, phosphoric acid, potash and lime. Practically all soils except the quartz sands contain more or less of these substances.
+
+The chemical make-up of the soil is, however, only one of the qualities necessary for it to support plant life. It must contain water. Plants require a very great deal of water. Yet few plants absorb the proper amount of water if they are submerged in it, or even if their roots are submerged. They must have the soil only partly saturated with water.
+
+
+
+
+
+Experiment 49. —Take about a quart of soil from a few inches below the surface of the ground and after sifting out the large chunks, put it in a sheet iron pan and carefully weigh it to the fraction of a centigram. Place the pan containing the soil in a drying oven or ordinary oven, the temperature of which is but little above 100° C. The soil should be spread out as thin as possible. Allow it to remain in the oven for some time, until it is perfectly dry throughout. Weigh again. The loss of weight will be the weight of water contained in the soil. As there was no free water in the soil how was this water held? Dip your hand into water and notice how the water clings to it after it is withdrawn. Examine with the eye and the lens several particles of the original soil as taken from the ground and see if there is a water film on each of these as there was on the wet hand.
+
+
+
+
+
+Experiment 50. —Take the soil after it was dried and weighed in the previous experiment and heat it throughout to a red heat over a Bunsen burner or in a very hot oven. Weigh again. If there is still a loss of weight this must be due to the burning of the organic matter, rotten twigs, roots, leaves, etc., which was in the soil. Soils differ greatly in the amount of water they contain and in the amount of organic substance present.
+
+
+
+
+
+We have seen from Experiment 49 how the soil takes up water, and how each little particle has a film of water around it. Little hairs on the plant roots are prepared to take up these little films of water which surround the soil particles. These water films have probably dissolved a minute amount of material from the soil particles, and this material enters into the plant and can be used for food.
+
+
+
+
+
+Experiment 51. —Fill an 8 oz. bottle with soil taken from a few inches below the surface. Fit the bottle with a two-hole rubber stopper having the long neck of a three or four inch funnel pushed as far as possible through one hole and a bent delivery tube just passing through the other hole. See that there is no air space between the soil and the stopper. The soil in the bottle should be as hard packed as it was originally in the ground. If necessary, push a wire down through the neck of the funnel so as to free all hard-packed particles of soil in it.
+
+
+
+Fig. 44.
+
+
+
+Connect the delivery tube with a bottle full of water standing inverted on the shelf of a pneumatic trough. Pour water into the funnel until it is full, and keep it full during the rest of the experiment. Allow the apparatus thus arranged to stand for some hours. Air will collect in the bottle over the pneumatic trough. Where did it come from? When the soil in the bottle has become entirely saturated with water, roughly compare the amount of air collected with the volume of the bottle containing the soil. What part of the soil's volume is the air?
+
+
+
+
+
+The smaller the soil particles are, the more surface they present to water, the more they are dissolved, the more food the plant hairs can reach, and the more fertile is the soil, other things being equal. We have also seen by experiment that soil contains air as well as water. Air is needed if plants are to flourish, and it is necessary that it be changed frequently, just as it is necessary to change the air in a room if people are to flourish. The soil must be ventilated. Plant roots must have air to breathe.
+
+44. Fertile Soils. —Rock disintegration does not furnish all the complex materials needed for the growth of agricultural plants. Only the lower orders of plants, such as lichens, can grow on soil as at first formed. A fertile soil is the product of ages of plant and animal life, labor and decay.
+
+
+
+MOLEHILLS.
+
+Showing how animals dig up the soil and make it porous.
+
+
+
+Plants send their filaments and roots among the rock particles, prying open their crevices and pushing the pieces apart so that the agents of disintegration can more readily attack them. By their decay plants provide the humus so necessary for making soil fertility.
+
+
+
+ANTHILL.
+
+This soil has been brought from below and piled up by the ants.
+
+
+
+Animals like moles and gophers plow their holes through the soil, mixing up the particles and making the soil porous, so that the water can readily get in to aid in breaking up and decomposing the soil particles. These holes also provide openings through which plant roots and soil organisms can obtain the oxygen and dissolved food they need. Ants each year move vast quantities of fine material to the surface, and in some places change the surface soil in a few years.
+
+Angleworms, the most important animal soil builders, channel the soil with their burrows, thus providing readymade openings for the growing roots, and by increasing the porosity of the soil aid in its ventilation and drainage. They swallow the soil as they make their burrows, in order to get the decaying vegetable matter for food, and they grind it fine as it passes through their bodies. Every year they bring to the surface great quantities of this finely ground soil mixed with the undigested vegetable matter. Darwin estimated that the angleworms in English soil deposited one fifth of an inch of these castings each year over some parts of the surface. This is the finest kind of fertilizer. It is a common saying that the more angleworms the better the soil.
+
+Besides harboring these visible plants and animals the soil teems with germ life. Some of these germs increase the fertility of the soil and some decrease it. It has been estimated that there are 50,000 germs of various kinds in a gram of fertile soil. It is these that cause the decay of the vegetable and animal matter in the soil. In the course of this decay various acids and gases are formed which help to decompose the rock particles and other compounds which are needed for the food of the plants.
+
+
+
+MUD CRACKS.
+
+Showing the way clay cracks when it dries.
+
+
+
+The most important of these germs to agriculture are the nitrogenfixing bacteria. Plants must have nitrogen if they are to grow, but they are unable to take it from the air where it exists in greatest abundance. For the use of the plants it must be chemically combined with other substances and these compounds must be soluble in water. Saltpeter is a compound of this kind and is often used to fertilize plants. But soluble compounds of nitrogen are not abundant, and these would be soon removed from many soils unless in some way replaced.
+
+
+
+LUMPY SOIL.
+
+The result of cultivating at the wrong time.
+
+
+
+This is most often done by adding manures to the soil. In these there are nitrogen compounds which the bacteria of the soil work over and get into shape so that the plants can use them. Some bacteria are even able to take the nitrogen from the soil-air and combine it so that it can be used by the plants. If this varied and teeming life of the soil is to thrive, certain conditions must be maintained, and it is the skill of the agriculturist in maintaining and increasing these favorable conditions which determines his success or failure.
+
+45. Agricultural Soils. —As has already been shown, soils differ greatly in fineness, mineral composition and water-holding capacity. They also differ greatly in the amount of decayed vegetable material or humus in them. The humus is a most important soil ingredient. It helps in holding water, it furnishes plant food and it keeps the soil from getting too compact.
+
+
+
+ADOBE SOIL.
+
+A heavy clay soil, very fertile, but hard to cultivate.
+
+
+
+In sandy soils there is usually little humus, the water soon drains out of them and plants become parched. Such soils warm up quickly in the spring and dry out rapidly after long wet spells. When humus and plant food in the form of manure are added they are especially adapted for growing early crops and crops that do not require a great deal of moisture, such as grapes. The "Fresno Sand" of California and the sandy coast plains of the eastern United States are soils of this kind.
+
+In clay soil the particles are extremely small, as are also the spaces between the particles. Water is therefore taken up very slowly. It is, however, held tenaciously. When clays become wet, they are very sticky and cannot be worked. When they dry, they become very hard and crack. If cultivated at the wrong time they break into hard lumps and render further cultivation difficult. The adobe soil of the west is of this character. If the soil is nearly pure clay, it is useless for farming. If sufficient sand or humus can be added, it becomes valuable, since clays usually contain the elements needed by plants.
+
+
+
+THE CRACKING OF ADOBE SOIL WHEN DRY.
+
+
+
+
+
+PRAIRIE SCE NE.
+
+Showing modern methods of harvesting the crops from the fertile silt soil.
+
+
+
+A soil having grains about midway in size between sand and clay is called a silt. This is usually a most fertile soil. It is the soil of the western prairies and the great grain-producing states of our country. It holds water well, contains an abundance of plant food, and is easily cultivated. Between these three types—sand, silt and clay—there are all grades of soils presenting problems of various degrees. The problem of the farmer, however, is to maintain a soil which holds water but is well-drained, which contains the elements plants need, and which is mellow enough to be well aired and to let the plant roots grow.
+
+46. Soil Water. —Although many soils contain every thing needful for the production of agricultural plants, yet the rainfall is insufficient or so unevenly distributed that these plants are unable to grow. This is true over a large area of the United States, and the same conditions often prevail over the usually well-watered part of the country in times of drought. The question of increasing the water-holding capacity and of preventing the loss of water by evaporation, or in other ways is a very important one.
+
+
+
+
+
+Experiment 52. —Weigh out equal amounts (about 100 g. each) of dried gravel, coarse sand and very fine sand. Put each of these into a four-inch funnel which has been fitted with a filter paper. Pour water upon each until all that can be absorbed has been absorbed. Allow each to stand until water ceases to drop from the funnel. Weigh again, balancing the weight of the wet filter paper retainer by a similar wet filter paper placed on the weight side of the scales. Which of these substances is capable of holding the most water? Since water does not penetrate into the grains composing these different substances, the difference in water-holding capacity must be due to the different sizes of the grains.
+
+
+
+
+
+ALFALEA ROOT.
+
+Along root which has gone deep to seck water.
+
+
+
+If we dig deep enough into almost any soil we shall find water. Wells show this. Certain trees and plants have such long roots that they can reach the underlying water and flourish where other plants will die. When wetlands are so drained by tilling that the plants can send their long roots down to this constant water supply or water table, as it is called, they stand a drought much better than plants grown on undrained land where the water table has not so uniform a depth.
+
+
+
+
+
+Experiment 53. —Place small glass tubes of several different bores in a dish of colored water. In which is the surface of the water higher, in the tubes or in the dish? In which tubes is it the higher, those of large or small bore?
+
+
+
+
+
+Experiment 54. —Place two wide-mouth 4 oz. bottles side by side and fill one partly full of water. Put a coarse piece of cloth, or better, a lamp wick, into the water bottle and allow the other end to hang over into the empty bottle. Allow the bottles to stand thus for an hour. What happens? The force that causes the rising of water up small tubes, wicks and crevices is called capillarity.
+
+
+
+Fig. 45.
+
+
+
+Experiment 55. —Tie pieces of cloth over the ends of four lamp chimneys. Fill one of the chimneys with coarse sand, another with fine sand, another with clay, and the fourth with a deep black loam. Stand each chimney in a shallow pan of water. Allow them to remain for a week, keeping water in the pan all the time. Note how high the water has risen in the different chimneys at the end of an hour; two days; a week.
+
+
+
+
+
+Fig. 46.
+
+
+
+It was found in Experiment 49 that each little particle of soil was surrounded by a film of water, even though there was apparently no water in the soil. This film will be replaced if removed just as the water in the top of the wick (Experiment 54) was replaced by water flowing up the wick. Roots get a large part of their water by absorbing the water films of the soil particles.
+
+If a region is well supplied with forests so that the rain as it falls is held by the moss, leaves and roots and protected from evaporation by the foliage, soil water will continue to be supplied to the surrounding open land long after it would have become dry had the forests been removed. Mountain soils have been found which hold back five times their own weight of water.
+
+
+
+A NATURAL SPRING.
+
+Coming to the surface between rock layers.
+
+
+
+Gravity is continually pulling the soil water deeper and deeper into the ground. This deep soil water is frequently diverted to lower ground by impervious layers of soil or rock and comes to the surface as springs, or it may come gradually to the surface over a broad area a long distance away from where it fell and make a region, otherwise barren, fertile by subirrigating it.
+
+It is often very essential to stop as far as possible this downward passage of water, or seepage, as it is called. The water in seeping through the soil dissolves plant food and if allowed to drain off would decrease the fertility of the soil. Whatever decreases the porosity of the soil will decrease the seepage and thus help to retain the plant food. This may be done by adding humus, and sometimes where the soil is very porous by rolling. At the time rain is likely to fall, however, the soil must be kept loose and mellow so that the water can sink into it.
+
+
+
+AN ARTESIAN SPRING.
+
+A deep water-layer has been pierced and the water diverted to the surface.
+
+
+
+Evaporation is, however, the cause of soil's losing the greatest amount of water. Soil water is constantly moving toward the surface on account of capillary action, and is being evaporated. This loss by evaporation must be counteracted, if in arid countries or during dry spells agricultural plants are to be provided with sufficient moisture.
+
+
+
+
+
+Experiment 56. —Fill full of soil four tin cans having small holes punched in the sides and bottom. Water each with the same amount of water. Cover the first with about an inch of grass and the second with about an inch of sawdust, and weigh carefully. Weigh the third and fourth. Record the weight of each. Thoroughly stir the surface of the third, as soon as it is dry enough, about an inch deep. Keep this stirred. Let the fourth stand undisturbed. Weigh all four every school day for two weeks. Keep a record of the loss of weight of each. Why have they lost weight? How do the grass, the sawdust, and stirring of the earth affect the loss? Suggest ways to keep soils from losing their moisture.
+
+
+
+
+
+In Experiment 56, it was seen that if a layer of grass or sawdust was put on the top of the soil, the moisture did not evaporate as rapidly as it did when the soil was not covered. The grass could have been replaced by shavings, manure, or any substance which would protect the ground from the sun and wind. Protections of this kind are called mulches. They are most frequently used around trees, vines and shrubs. It is impracticable to use them extensively on growing crops.
+
+
+
+"DRY FARMING" IN EGYPT.
+
+
+
+It was also found that soil water was not readily evaporated where the top of the soil was kept stirred, so that the little capillary tubes by which the soil water reaches the surface were broken and the sunshine and air were kept from the under part of the soil by a layer of finely divided soil mulch. When the surface of the soil is thoroughly stirred or cultivated the particles are separated so far apart that the water cannot pass from one grain to another, and so is retained in the under layer ready for the plant roots. Thorough tillage of agricultural crops is perhaps the best way to assure the plants sufficient moisture in regions subject to droughts.
+
+In some parts of the arid region of the United States dry farming is practiced. The soil is deeply plowed and the plow often followed by a bevel wheel roller called a soil packer, in order to pack the under soil or subsoil so that the air cannot circulate through it and dry out the upper soil. The surface soil is then most thoroughly cultivated so as to make as perfect a soil mulch as possible. Thus, whatever moisture falls is kept from seeping below the reach of the plant roots and from evaporating from the surface. In this kind of farming the aim is to use more than one year's moisture in growing a crop.
+
+
+
+KAFFIR CORN.
+
+A plant suitable for dry farming.
+
+
+
+Crops are usually planted only every other year, two years' moisture being retained for one crop. The soil is, however, kept thoroughly cultivated all the time. Of course plants requiring the least amount of moisture are best adapted to dry farming.
+
+Irrigation is the most efficient means of raising crops in regions of insufficient rainfall or of droughts. Water is brought to the land from distant sources, or from flowing artesian wells, or is pumped from wells which have been sunk to an available water table. In this way water can be supplied to plants whenever needed. Where the ground is quite level it is often flooded, sometimes in larger or smaller squares, with little ridges separating the squares. A great deal of water is lost in this way by evaporation.
+
+
+
+IRRIGATION IN SQUARES.
+
+
+
+Another way is to plow furrows eight to ten inches deep in the direction of the surface slope and run the water into these from the irrigation ditch. In either case the water is allowed to soak in until the soil is thoroughly wet. The surface is then cultivated so as to check surface evaporation. In the last few years the government and many private companies have spent millions of dollars in putting in irrigation plants. By this means thousands of acres of land which would otherwise have been valueless for agriculture has been made exceedingly productive.
+
+
+
+IRRIGATION IN FURROWS.
+
+
+
+47. Alkali Soils. —In dry regions where the rainfall all sinks into the ground and after remaining for a time rises to the surface and is evaporated, large areas are found upon which almost nothing can be made to grow even when sufficient water is provided. Often in the dry season white or brown crusts appear scattered over the surface in large patches. The white crust usually tastes like Epsom salts and the brown like salsoda. The salts forming these patches have been dissolved out of the soil by the soil water and left on the surface when it evaporated.
+
+
+
+ALKALI SOIL.
+
+Few plants can grow here because of the excess of alkaline salts.
+
+
+
+Such substances are not found in wet regions because they are carried away by the water which runs into the streams. About the only way soil of this kind can be treated to make it productive is to irrigate and drain it, thus washing the salts out of the soil. This is just what is done by nature in well-watered regions. Sometimes if there is not much alkali deep plowing or the planting and removal of certain plants such as sugar beets, which are capable of growing in such soils, will sweeten it.
+
+
+
+RECLAIMING ALKALI SOIL IN THE SAHARA.
+
+
+
+
+
+ROMAN PLOWING.
+
+Showing primitive methods.
+
+
+
+48. Soil and Man. —Although nature through countless ages has been preparing the soil, and generation after generation of plants and animals has been contributing to its fertility, yet it will not continue profitably to produce agricultural crops unless carefully handled by man. The materials taken from it must be replaced by manures. It must also be thoroughly tilled in order (1) to keep in the moisture, (2) to prepare a mellow place where the roots of the plants may spread, (3) to provide air and water and humus needed by the germs which build up the soluble nitrogen compounds, and (4) to kill the weeds which would use the space and plant foods needed by the growing crops and would choke them out. Proper tillage probably has more to do with thrifty and productive farming than any other one thing. By careful tillage much expense for fertilizers can be saved and the value of the crop produced greatly increased.
+
+
+
+STEAM PLOW.
+
+Showing modern methods.
+
+
+
+
+
+GOOD SOIL.
+
+A truck farm.
+
+
+
+49. Value of Soils. —Many different factors enter into the determination of the value of a soil. Soils which in one locality would be of great value are almost valueless in other localities. Light sandy soil far from a market, unless transportation facilities are exceptionally good, is almost worthless; the same soil near a city where fertilizers can be easily procured and where early vegetables find a ready market is of great value.
+
+Different soils are adapted to different crops, and where a soil, although not good for many crops, is adapted for raising a crop which in its locality is valuable, the soil is called good. Thus the soil in many parts of Florida, although unsuited for raising most crops, is suited for orange trees and early vegetables, and so is a good soil. The stony soil in certain of the orange regions of California would be an exceedingly poor soil for most crops, but it is good for oranges and therefore it is most valuable.
+
+Summary. —Only a little more than a quarter of the earth's surface is land, and only a little over a twentieth of this land belongs to the United States. Though there are depths in the ocean greater than our highest mountains, most sea animals live near the surface, while land animals are distributed over hills and valleys. This gives greater variety to life on land.
+
+Water is simply a compound of oxygen and hydrogen. It has many valuable and interesting properties: it is practically incompressible; it is the greatest dissolver of substances that there is; it evaporates readily, giving us rain, and, when put under pressure, our steam power; it has great power of taking up heat, thus regulating the climate of the land near which large bodies of it lie.
+
+The land is made up of various animal, mineral and vegetable substances. But four fifths of it consists of sedimentary rocks, that is, those that were deposited by water, as distinguished from igneous or rocks formed from melted materials within the earth, and metamorphic or rocks that have been changed from sedimentary or igneous rocks.
+
+The soil comes chiefly from the decaying and weathering of rocks. It is divided into local or sedentary soil, that which is formed from the rocks directly beneath it, and transported soil, that which is generally brought down and deposited by water, ice and wind. Soils are classed roughly as gravel, sand, clay, and loam.
+
+The fertility of the soil depends largely upon composition, air and water, ventilation and drainage. Fertile soil must contain nitrogen, potash and lime. The roots of plants must have air to breathe, and water must dissolve the nourishing substances in the soil and bring them to the roots to be absorbed. For this reason the soil should not be packed hard like clay, nor should it be loose like coarse gravel, as clay does not let the water soak through readily, while gravel lets it seep down too fast.
+
+To maintain its fertility, the soil must be frequently cultivated to decrease seepage and evaporation. It must be supplied with fresh nourishment by manures or other fertilizers. In some districts where the rain supply is inadequate, irrigation and dry farming are practiced. Different soils are suited to different crops. Without the fertility of the soil, life on the earth would cease.
+
+
+
+
+
+QUESTIONS
+
+
+For what would you look if trying to determine whether a land surface had ever been under the sea?
+
+What are the characteristic differences between land and water surfaces and between the conditions to which the animals and plants of each are subjected?
+
+What reasons can you suggest for likening the earth to a water engine?
+
+To what great classes do the rocks in your neighborhood belong?
+
+What examples of rock weathering have you ever seen? Describe.
+
+Is the soil in your neighborhood local or transported? Does its character vary much in different places? Does the fertility vary?
+
+What would you suggest as the causes of these variations?
+
+What is necessary to produce a fertile soil?
+
+How can the fertility of a soil be maintained?
+
+How can the supply of soil water be maintained in dry regions and at times of drought?
+
+What determines the value of a soil?
+
+
+
+
+
+【中文阅读】
+
+
+36．陆地与海洋——地球的表面积为197000000平方英里，其��28%是陆地。它的广阔无垠与绵延无尽，似乎已超出我们的感知范围，但尽管如此，它还是让我们对美国本土的面积有了一些概念，除去阿拉斯加及其附属岛屿，美国的陆地面积是它的1/18。
+
+我们可以把地球表面划分为两个半球，一个囊括了大部分的陆地，另一个则涵盖了大部分的海洋。大陆板块的这种紧凑分布，让不同地域的人们更加相互靠近，促进了他们的相互往来与沟通交流，因为大陆为他们的交通运输提供了极大便利。在地球的气候条件下，大部分陆地分布在北极附近而不是在南极附近，也对地球上的居民们极为有利。我们之后还将看到，陆地板块的不规则分布状况，以及它们被海洋分割开来而不是连在一起，同样对地球居民很有好处。
+
+37．沧海桑田的变换——人们通过大量的研究考察发现，大陆与海洋的各自分布并不是一成不变的。一个地方曾经是陆地，后来也可以变成海洋；而那些曾经是海洋的地方，现在也可能是陆地了。在远离海洋的内陆，人们发现了嵌在岩石中的海洋贝壳；通过深水探测，一些河流流经的峡谷与河床也被发现曾经处于海底，而这些地方距离它们现在的入海口非常遥远。一些曾经是海滩的地方，现在也被发现已经远远高出海平面数百英尺了。
+
+从一些类似的迹象看，瑞典的北部海岸似乎在过去的150年里升高了大约7英尺，而荷兰的陆地却在逐渐下沉。对马萨诸塞州海岸线的长期观测表明，它似乎也在以很慢的速度下沉。类似的陆地与海洋的相对升降运动的现象，在全世界各个地方都有被发现。
+
+在加利福尼亚的靠近洛杉矶港口的圣佩罗，一些古老的海滩也被发现正在逐渐抬升，依次错层并形成斜坡丘陵。苏斯，这位伟大的奥地利地质学家认为，海洋与大陆之间显著的相对运动，是由于海洋自身的地质升降运动，而不是由大陆的地质升降运动造成的。尽管很可能如此，但是海洋与大陆之间的升降变化也有很多显著的实例，二者之间的海岸线也始终是一条变化中的曲线。
+
+38．陆地表面的特征——相比于海洋表面，陆地表面与之大不相同，因为陆地表面至少是固定的。它凹凸不平、粗糙而不规则，由一些不同种类的岩石与土壤构成。它的大部分表面都高出海平面，但也有一小部分地方下沉到了低于海平面的高度，比如在靠近死海的加利福尼亚帝王谷，有一个叫沙顿海的地方就是如此。陆地表面长期经受风霜雨雪的侵蚀，长此以往便慢慢改变了最初的模样。要想轻松地走遍地球大陆的每一个角落，真的很难，原因便是它千差万别的参差嶙峋。
+
+陆地表面的特征状况即使在很小的区域内也呈现着丰富多样的变化。白天黑夜，春秋冬夏，温度在陆地上变化很大。陆地上的动物必须尽可能地发挥它们的肌肉力量，四处游走，到处觅食。因此它们必须高度有组织地群居，以维持它们在陆地上的生存。水生动物则完全没有这样困难的生存环境，与陆地上的动物相比，二者之间真可谓天壤之别。
+
+39．水的特性
+
+
+
+
+
+实验35：在一个装有新鲜水的大烧杯中放入一支密度计作为浮标，或者用一根细线拴住一段小木棍的一端，将其放入水中也可以，主要是让它可以竖直地漂浮，用橡皮筋给浮标的入水位置做上记号。在另一个水的深度足够深的容器中溶解大量的盐，当盐完全溶解后，尝一下水面部分水的味道，然后再把其中的水倒出去一大部分，再尝尝容器底部的水的味道。现在盐已完全溶解在水的各个部分里面，但外观并无任何改变。
+
+在刚才的盐溶液中再放入密度计浮标，会发现其与之前的下沉深度不一样。你对能支撑物体的溶液浮力有什么看法？水里的鱼需要靠发挥自身肌肉力量才能在浮在水中吗？正因为水溶液中可以包含很多物质，所以水生动物即使不四处移动，也能在其周围获得食物以维持生存。
+
+实验36（老师的教学实验）：在一个结实的广口瓶中放入少量锌屑，用一个双孔橡胶塞塞紧瓶口，从一个孔中插入一只长颈漏斗，让长颈漏斗末端靠近瓶底，从另一个孔中插入一根弯曲的玻璃导管，用一根橡皮管将其与一个集气水槽的隔板孔相连。取几个8盎司广口瓶，装满水后倒置于集气水槽的隔板上。通过长颈漏斗先向装有锌屑的瓶中注入适量的水，让其能淹没一部分锌屑，然后再注入稀释10倍后的工业盐酸或硫酸。
+
+锌遇酸会发生化学反应，并产生氢气。先让瓶中的气体排放几分钟，因为其中大部分都是之前瓶中的空气。然后通过水槽收集数瓶氢气，注意保持瓶子必须倒置。取一瓶进行检测，观察它是否具有颜色和气味？将另一瓶氢气瓶口朝下地握住，插入一片点燃的小纸片，会发现小纸片停止了燃烧，瓶中的氢气却燃烧了起来。再取一瓶氢气，在桌上瓶口朝上放置几分钟，再插入燃烧的小纸片，为什么瓶中的氢气全然不见了？
+
+将一个玻璃管管口加温拉拔到只有铅笔尖大小，形成尖嘴后插入氢气流经的橡胶导管，再给反应瓶中注入更多的酸液并让其持续反应几分钟，然后将一根点燃的火柴靠近氢气流出的尖嘴导管口，会看见氢气形成了喷焰。将一只干燥的常温烧杯罩在氢气的喷焰上，会发现烧杯壁上逐渐累积水珠，这说明氢气与空气中的氧气正在发生化合反应，水便在这个化学反应中形成了。
+
+纯净的水是由氢气和氧气两种气体化合而成。氧气我们已经非常熟悉了，我们周围环境中的空气有1/15就是氧气。氢气就是刚才实验中收集到的气体。它是一种无色的透明气体，也是世界上最轻的物质，因此必须小心处置。如果它与空气或者氧气混合并发生燃烧，将可能形成强烈爆炸，并形成水。
+
+
+
+
+
+实验37：给一只小烧杯装满新鲜水，慢慢在火上加热，一会儿便会看见杯底和杯壁上会累积许多小气泡，这些小气泡在水变凉的时候也不会消失。如果它们是水蒸气气泡的话，在水变冷时它们就应该会变成水，那它们是什么呢？它们从何而来呢？水里面含有溶解于其中的空气吗？水里面的动物是怎样做到即使不浮出水面也能获得它们必需的氧气的？
+
+实验38：将一小块冰放入水中，它哪一部分会淹没在水面以下？它比水轻还是比水重？通过实验24，请你总结一下，冷水是比热水轻还是比热水重？
+
+
+
+
+
+我们日常所见的水，其中溶解了空气以及其他许多物质，因为水是一种我们目前所知最强大的溶剂。水还有另一种极其重要的特性，就是它的不可压缩性。不管多大的压力加之于水，它的体积也极少收缩，密度自然也极少改变。这样一来，物体只要在水面下沉，就会一直沉到水底，因为底部的水的密度和水面的水的密度相比，增加的程度极其微小。
+
+溶解在水中的物质，与水均匀地混合在一起。但在水的分解物中，各类溶解在其中的物质真是千差万别，不过就海洋整体来说，从洋面到海底，水的构成成分都是一致的。
+
+从之前的实验中，我们已经了解了水的一些物理特性，因此便也可能初步理解了陆地与海洋的天壤之别，以及这对生存在其中的生物的进化活动会带来多么大的影响。一些水生动物在水里四处游动以寻找食物，但还有一些水生动物只是靠水里面的溶解物来维持生命，当然，这不需要自身的肌肉力量。它们的呼吸也依赖于溶解在水中的空气，并且它们不能像陆地动物那样，可以很轻松地获得大量空气进行呼吸。所有动物自身的能量都依赖于它们体内新陈代谢所需的氧气量，因此，水生动物普遍地比陆地动物拥有较低的能量。同时由于它们在水中游动沉浮以获取所需相对比较容易，因此它们自身生物机能的发达程度也远远低于陆地动物。
+
+水可以被风吹动，然后变成水蒸气随风而去；它也可以冲蚀陆地，并将陆地的沙石悄悄带走。当它被太阳或者其他热源加热之后，会变成气体升于空中，飘向远方；进而又云腾致雨、露结为霜，变成雨雪降落大地。它在蒸发时会吸收大量的热，而在凝结时又会全部释放出来。水总是寻求最低的地方以自处，并在其流动的过程中释放自身的势能。在很多作家笔下，地球被描述为一个水的发动机，因为在地球的漫长历史中，水真的扮演了一个极其重要的角色。
+
+水的另一个极其重要的特性，就是它对热量的保持能力，这在实验27中我们已经有所了解了。在它降温的时候，它会释放在升温过程中吸收的热量，正是因为这个原因，热水袋才可以让人们保持温暖，农民如果担心地窖可能结冰的话，便会在地窖里放上一大桶水，以让地窖的温度保持在0度以上。霜冻可能来临之前，果农经常会给橘子林进行提前灌溉，也是因为这个原因。
+
+一磅[1]水温度降低1度所释放的热量，可以让一磅铁的温度降低9度。水的这个保持热量的特性自然让它在夏天升温更慢，而在冬天来临时，降温也很慢，因为在其降温的过程中，它会把自身持有的热量完全释放到大气当中。故而在初夏，水面的空气依然凉爽；在深秋，它也依然荡漾着暖意。后面我们还会发现，水的这个特性有着极大的重要性。
+
+40．组成陆地的物质
+
+
+
+
+
+实验39：取几组岩石标本，包括：火成岩、岩���岩、黑曜石、玄武岩、花岗岩、沉积岩；沉积岩、砂岩、石灰岩、砾岩、泥炭；变质岩、页岩、片岩、大理石、无烟煤。用放大镜仔细观察它们的外观，看它们是构成成分相同，还是由不同的颗粒构成。这些组成岩石的颗粒是透明的吗？它们是不规则地分散，还是有规律地排列？用锉刀或者刀背分别擦拭一下整块标本和它一部分的硬度。给每个标本滴上一滴盐酸，看它们各自是否都会发生反应。最后总体描述一下各个标本的特征。
+
+
+
+
+
+陆地的不同区域的构成成分千差万别，许多不同种类的岩石经常被发现聚集于一处，但也有时候发现同一种岩石的分布覆盖了很大的区域，因此谈不上有什么一致的规律。岩石层表面的土壤也极其多变。在有些地方，土壤中含有下面岩石层中的许多矿物质，而在其他一些地方，土壤成分可能跟下面岩石层成分风马牛不相及。
+
+尽管如此，地壳岩石还是存在着三种主要分类，分类依据是它们一致的形成方式。这三种分类是：火成岩、沉积岩与变质岩。
+
+火成岩是熔岩凝固而成的岩石。它们可以在地壳深处固化而成，也可以在地壳表面，或者在二者之间的任何一个深度形成。如果这类岩石在形成过程中冷却很慢，它们便会拥有结晶状的构造，比如花岗岩；如果冷却很快呢，会有类似玻璃状的构造，比如黑曜石。它们的总体构造便可以在这两个极端条件下产生丰富的变化。这类岩石中，有一种最普通的暗色岩石就是玄武岩，当然火成岩还有很多种种类，这里就不一一列举了。
+
+沉积岩是水中的沉积物累积而成的岩石。有些岩石在漫长的岁月中被磨损成碎片，而这些碎片又在水中慢慢沉积，在一定的自然条件下，它们便又固化成了坚硬的岩石。在深深的海底，贝壳和海洋动物的残骸也会逐渐累积，经过足够的时间，也会慢慢形成岩石。而海洋植物的残骸也会在漫长的时间内逐渐累积，但不会腐烂，在一定条件下它们会变成一种沥青煤，或者叫烟煤。地球上大约五分之四的陆地表面都是由沉积岩构成的，它们颜色丰富多样，十分坚固，在人类生活中大有用处。
+
+由微小的沙石颗粒聚合而成的砂岩，被用来作为建筑的主要材料以及其他许多用途。由海洋动物残骸沉积形成的石灰岩，便是石灰的来源，也被经常用于建筑以及其他用途。页岩是由泥沙分层沉积而成，每一沉积层大约有一英寸左右的厚度。沥青煤是由植物残骸在极其漫长的时间中积累而成，是一种极其有用的矿物质。以上所有的沉积岩种类都没有晶体结构，因为它们都是由岩石碎片或动植物残骸沉积而成，一般在地层中以分层分布的形式存在。
+
+石油就可能是海洋中动植物残骸分层累积的结果，它被其他沉积层覆盖，在形成过程中又不断腐化分解，经过漫长的时间便形成了石油与天然气。
+
+变质岩一般拥有透明的晶体结构，并且一般还会有结构精致细腻的结晶物质嵌在它们内部，甚至包含一些延伸距离很长的水晶类的物质。这类岩石不是生来就是如此，而是一般由火成岩或者沉积岩转化而来。这些改变是在最初状态下，岩石自身的矿物成分被打乱，改变了内部结构之后，慢慢形成的。
+
+改变的直接原因，总体而言无非热量与压力，变化的类型也多种多样。大理石是一种结晶的石灰岩，片麻岩总体上是一种变质的花岗岩。板岩和云母片岩变成黏土岩和无烟煤的过程，其实是一种沥青煤的变质形式。这一类岩石有时也很难与火成岩类的岩石严格区分开来。
+
+41．陆地区域的结构
+
+
+
+
+
+实验40：取一个小铜球以及一个刚好可以让其通过的环，就跟我们已经在实验19中使用过的装置一样。把球放在环中一起加热到高温，取出球将其投入一盆水中冷却，然后再放入环中，会发现环比之前大了许多，二者之间的空隙范围很大。
+
+如果这个环有一个常温的凹槽，紧紧箍住高温小球的表面，则小球的表面会被环面降温，直到二者温度逐渐趋近，被环包围的球面也会发生收缩。这便会在二者之间形成空隙，因为球形表面支持力不够的话，便会在降温过程中收缩，并同时在表面形成皱折。
+
+实验41：在家里用一根细绳测量出一个表面光滑的大苹果的最大和最小周长，然后将苹果进行烘焙，之后再测量一次，测量结果比之前更大还是更小？苹果的表面还像之前那样光滑吗？
+
+
+
+
+
+陆地区域不仅在岩石的构成类型上差别很大，其存在形态上也是如此。有些岩石层一直保持着它们形成后的状态，有的则被折叠、弯曲、扭裹得十分严重。数目众多的岩层被巨大力量不断磨损，并被不断地挤压到地球表面，许多本来在地球很深处的岩石也就被推到了浅表地层。即使像花岗岩这类岩石，本来形成于地表数千英尺之下，现在也能在地表看见，许多地方都已经挖到了它。
+
+岩石层的折叠弯曲形成了明显的岩床，这些岩床最早是水平状的，而现在则可能成了垂直状的，我们也因此有机会可以看见岩床的边缘。这些大不相同的岩石种类和它们千差万别的存在形态，也为我们提供了一个窥探大自然鬼斧神工的侧面，正是这样的自然力量，参与形成了陆地表面丰富多彩的特征地貌。
+
+虽然我们没有必要对所有影响岩层形态的因素都加以深究，但其中最重要的原因是值得注意的。我们已经发现，尽管地球表面温度不高，但其内部温度却非常高。我们也知道所有物体在冷却时都会收缩，因此如果地球内部的温度是在逐渐下降的话，当然有理由相信这很可能是真的[2]，那么它也必定一直在慢慢收缩。又由于地壳温度已经相对较冷，因此地壳的收缩已经停止，于是地球内部核心会逐渐向内坍缩进去，地壳则会随之而缓慢坍塌，以同步于坍缩的内核。不过地球的冷却过程极其缓慢，因此它对地壳的影响也虽有如无，我们大可放心。
+
+42．岩石的风化
+
+
+
+
+
+实验42：小心地给一片干燥的粗砂岩或者贝壳灰岩称一下重量，然后让其在水中浸泡几天。取出擦干，在进行称重，为什么重量会改变？
+
+实验43：给一支试管或烧杯中注入一半容积的石灰水，石灰水的浓度差不多按照两盎司石灰兑一品脱水的标准即可。然后用嘴朝石灰水中吹气，会看见石灰水中形成了一些白色物质，化学家们告诉我们，它们和石灰岩的构成成分是一样的。
+
+实验44：继续通过玻璃管向刚才的石灰水中吹气，持续吹一段时间后会发现，之前的白色物质消失了。我们呼出的气体中有一种成分叫做二氧化碳，它溶解在水中，让水变成了弱酸性，进而导致了这一变化。现在我们如果将石灰水继续加热，这样会分解弱酸并排除水中的气体，白色的物质又会重新出现。二氧化碳气体在我们周围的空气中随处可寻，物体在燃烧或腐烂的过程中也会释放这种气体。
+
+
+
+
+
+暴露在大气中尤其是潮湿气候条件下的岩石会逐渐分解变质，而如果在干燥温暖的气候条件下，岩石也可以原封不动伫立数百年。不过即使同样的岩石，如果将它们换到一个气候条件截然不同的地方，它们也照样可能会迅速分解成碎片。一个极为显著的例子就是那个名叫克娄巴特拉方尖碑的著名石碑，它在多年以前被从埃及运至纽约中央公园。即使它在埃及已经模样不改地伫立了3000年，但在纽约潮湿多变的气候环境中，不到一年就发现它周身已经覆盖上了一层变质保护层。
+
+不仅不同的气候条件会对岩石造成不同的磨损侵蚀，不同种类的岩石自身的破坏速度也大不一样。人们发现一张刻有文字的大理石碑，如果被放置于一个多雨的、煤烟很重的城市的话，不到50年，上面的文字就难以辨认了；而在正常条件下，这些铭文即使过上100年，也依然会非常清楚而容易辨认。
+
+
+
+
+
+实验45：将一支试管装满水，用木塞塞紧瓶口，然后将其冷冻至结冰，会发生什么？如果玻璃管的温度不够低，可以将其放在冰屑与盐的混合物中降温一段时间。有条件的话，放在冰屑与氯化铵（卤砂）的混合物中降温效果更好。
+
+
+
+
+
+水浸入到岩石的缝隙中，冷冻结冰时便膨胀外撑，这样无形之中加速了岩石的分崩离析。植物的根茎也会伸入岩石的裂缝，不断向内生长，最终也会将岩石彻底分裂。水在流经一些腐烂的植物残骸时，由于一些化学物质溶入其中，它们便对岩石有了更强的腐蚀能力。岩石所含有的一些矿物，若长时间暴露在空气中也会像铁生锈那样发生变质。
+
+在日夜温差较大地方，岩石的热胀冷缩也会导致其表面的碎裂剥落；而在风沙较大地方，强劲的风卷裹着沙粒吹打在岩石上，对其破坏力尤其显著而迅速。所有这些对岩石的破坏侵蚀作用，最终都让岩石变成了土壤。关于这些侵蚀作用的某些实例，我们已经在之前的实验中做了演示。
+
+43．土壤
+
+
+
+
+
+实验46：取两只容积为16盎司的玻璃瓶，装满水，给一只瓶子中放入少量沙子，给另一只放入等量的泥土粉末，然后都使劲摇匀后等待其慢慢沉淀。哪个沉淀得更快？如果在逐渐变小的水流的冲洗下，又是哪个先沉淀完毕？
+
+
+
+
+
+在陆地上，只要不是特别陡峭嶙峋的地方，我们总会发现岩石层被一层��度不一的松软物质层所覆盖，这就是土壤。植物就植根于土壤，我们的大部分农业活动也跟土壤有密切关系。通过对土壤的观察研究，人们发现在有些地方我们朝下挖得越深，土壤便越来越粗糙，而越是粗糙的土壤便越像岩石的岩床，直到最后完全达到岩石层的内部，这类土壤我们叫做局部土或原地土。
+
+但在其他一些地方，土壤的粗糙程度在我们向下深挖的过程中并不会改变，且若一直挖下去的话，会不知不觉突然达到下面的岩床。而在岩床中，覆盖在上面的土壤的成分含量可能极其微小。这类与下面岩石成分完全不同的土壤，我们叫做运积土。后面我们还会讲到，它们中的大部分是怎样流转到现在的所在之处的。
+
+前面的那种土壤很明显是由下面的岩石成分构成的，因为它们的分布本身就与岩石层在逐渐融合。因此如果下面的岩床发生改变，这类土壤自然也就会跟着发生变化。这个现象有一个很好的例子，与肯塔基州美丽富饶的“蓝草”地区紧挨着的便是一块贫瘠荒芜的不毛之地，原因是它们一个处在石灰岩之上，另一个却在砂岩之上。
+
+地表的土壤一般都比地表一英尺以下的土壤更细腻，且很多时候它们都混杂着腐烂的植物以及分解后的岩石成分，而这些，便是土壤肥沃与否的标志所在。有些土壤几乎完全是由腐烂植物、泥炭以及淤泥构成，而其下相对更粗糙、颜色更浅的土壤，一般就几乎不含植物腐烂物，这类土壤我们叫做下层土。
+
+
+
+
+
+实验47：在一个高倍放大镜下仔细观察沙土、肥土、粘土、泥炭以及其他种类土壤的标本，着重观察它们不同的组成微粒的特征及形状。
+
+实验48：取少量的肥土放入一个装满水的罐头盒中，放置一两天，期间偶尔摇一摇盒子。时间差不多后，使劲将其摇匀，然后让其沉淀一分钟。将浑浊的水再倒入另一个罐头盒子，又让其沉淀大致一个小时，然后再将浑水倒出来，对其加热以让其缓慢蒸发，但注意不要烧焦了析出的物质。现在用放大镜仔细观察剩下的物质，并用手指搓捻，会发现有三种分离出来的物质，可大致分为：沙粒、淤泥以及粘土。
+
+如果条件允许的话，取一台复式显微镜，将一滴混有淤泥和粘土的水滴在载玻片上，在低倍镜下仔细观察混合液滴，注意观察腐败植物的黑色颗粒，以及它们聚集在一起的团块。为什么实验之前要用水长时间浸泡土壤？画出一些显微镜下小颗粒的形状。描述一下你观察到的土壤的构成状况。
+
+
+
+
+
+如果我们用显微镜观察更多的土壤，会发现它们就像我们在实验48中看到的那样，都是由许多种不同的物质构成的。其中有些物质在水中溶解得很慢，这便给植物提供了养分，还有些其他物质则根本无法溶解。
+
+不同的土壤，其构成颗粒的大小与组合方式都截然不同。碎石的颗粒非常大，因此1克碎石里的颗粒数量极少，而1克的沙土中，颗粒就非常多了，而且沙的纯度越高，颗粒越多。颗粒最多的是粘土，1克粘土中的颗粒可以达到数十亿。栽种农作物的耕土的土质，就介于沙土与粘土之间。由于土壤中的矿物都来源于岩石，因此只有存在于岩石下半部分的矿物质才能进入到原地土壤中，但在运积土中，岩石与土壤本身就几乎没什么关系了。
+
+如果土壤上面有植物生长，则土壤中必然会有矿物质给它们提供营养，其中需求量最大的矿物养分有氮、磷酸、碳酸钾以及石灰。实际上，所有类型的土壤中，除了石英砂以外，都或多或少地含有以上几类物质。
+
+在土壤的化学构成物中，只有一种是植物生长发育必不可少的，那就是水。且植物对水的需求量很大，个别种类的植物还可以通过在水中生长或者将根茎伸入水中来吸收水分，故而它们都必须要求部分土壤要含有水分。
+
+
+
+
+
+实验49：在地表数英寸的下方取大致一夸脱[3]的土壤，用筛子筛去较大的土块，然后放进一个平底铁锅中，小心称量其重量，精确到厘克，然后将装有土壤的平底锅放置到干燥炉或普通烤炉上，让烤炉温度略高于100摄氏度，土壤尽量在锅中摊开并碾压平整。在炉上放置一段时间，让土壤彻底干透，然后再进行称量。减少的重量便是土壤中所含水分的重量。土壤中并没看见流动的水，那这些水是怎样浸含在土壤中的呢？将手伸入水中，取出时注意观察手上所附着的水分。然后仔细地用眼睛分辨、再用透镜观察部分刚才从地面下取出的土壤，看看是否也有像从水里取出的手上所附着的水膜那样，也有水膜附着在土壤颗粒上？
+
+实验50：把上一个实验中烤干的土壤在本生灯或者烤炉上继续加热至红透，然后再进行称量，如果依然发现重量减轻了，那一定是因为土壤中的有机物、植物腐烂的根茎叶等在加热过程中燃烧了。不同类型的土壤不仅含持的水分数量差别很大，含有的有机物数量也不尽相同。
+
+
+
+
+
+我们已经在实验49中看到了土壤如何保持水分，以及土壤颗粒附着水膜的情形。植物的根须也是这样，通过周围的土壤颗粒来吸附水膜进而吸收水分，而且这些水膜中很可能溶解了少量土壤中的矿物质，正是它们为植物提供了养分。
+
+
+
+
+
+实验51：将一只8盎司的玻璃瓶装满从地表下几英寸取出的土壤，用双孔胶塞塞紧瓶口，从一个孔中插入一支三四英寸长的漏斗，尽量插深一些，另一个孔中插进一支弯曲的玻璃导管，稍稍插入即可。这时要保证土壤与瓶塞之间没有缝隙，并且尽量将土壤压紧，保持其在地面以下的原初状态。有必要的话，还可以通过漏斗插入一根金属丝，以便可以搅动可能堵塞的土壤颗粒。
+
+将弯曲导管与一个装满水并倒置于集气水槽隔板上的集气瓶相连，然后通过漏斗给装有土壤的瓶子里面注水，将漏斗倒满并且在实验过程中一直保持这样的状态，持续几个小时。然后便会看见集气瓶中充满气体，它们从何而来？当瓶中的土壤被水完全浸泡之后，大致比较一下收集的空气量与瓶中土壤的体积，空气能占到多大比例？
+
+
+
+
+
+土壤的颗粒越小，水能附着的总体面积便越大，溶解物便越多，植物根茎便也能获得更多的养分，土壤自然也就越肥沃，其他可以以此类推。从实验中我们已经得知，土壤中不仅持有水分，还有空气。如果要让植物繁荣生长，水分必不可少，且要保持空气的更换流通，就像我们为了健康要保持房间内空气流通一样。因此土壤也必须保证能足够通风，植物的根才能够呼吸。
+
+44．肥土——岩石分解后的产物并不能提供农作物生长所需的全部化合物养分，只有一些很低级的植物，比如青苔，可以只依赖极少养分而直接在土壤中存活。因此，肥沃的土壤来之不易，它是动植物的生命见证，更是人类劳动与万物运化的结果。
+
+植物将它们的纤维组织与根须沿着岩石的缝隙深入其中，不断撕裂分离岩石，让岩石最终分崩离析。然后植物的残骸于无形中为土壤提供了腐殖土，这便是土壤变得肥沃的必要条件之一。
+
+像鼹鼠和衣囊鼠一类的动物可以在土壤中挖洞，这样也就混合了土壤的成分，并让土壤得以通风，水也能更方便地进入土壤，促进土壤的分解。这些洞穴还为植物根须以及土壤中的有机生物体获得氧气打开了通道，进而让它们能够得到所需的养分与食物。每一年，蚂蚁族群都会搬运大量的细小物质颗粒到地表上来，因此在某些地方，对表层土壤的改变，其影响会持续数年。
+
+蚯蚓，这个最重要的土壤调理师，不断地隧穿土壤来制造洞穴，也为植物的根茎提供了更舒适的生长空间，同时还增加了土质的疏松程度，让土壤更加通风透气，排水性能也更好。它们钻洞的时候会把土壤吞进身体，以获取其中腐烂的植物残骸作为食物，然后经过消化，成为细小颗粒排出体外。每一年它们会为陆地表面制造出极大数量的这类排泄物，并混杂着未经消化的植物残渣。达尔文认为，在英国某些地区，每年蚯蚓所制造的排泄物差不多是五分之一英寸的地表土壤。这是难得的上好肥料，所以才有俗语说：蚯蚓越多，土壤越肥。
+
+土壤除了可以为我们日常所见的动植物提供家园，它同时也是各类细菌微生物的超级大本营。有些微生物会增加土壤的肥沃程度，但有些则相反，会让土壤失去养分。在1克肥沃的土壤中，估计有50000个各种各样的微生物，正是它们导致了动植物残骸在土壤中的腐烂。且在动植物的腐烂过程中，会产生各类酸液与各种气体，它们又正好能帮助岩石分解成更小的颗粒以及其他类化合物，植物正需要这些来提供养分。
+
+对农作物而言，这些微生物中最重要的要数固氮细菌。植物生长，氮元素必不可少，但是它们却不能在含氮量非常丰富的空气中获取它。能被植物所吸收的氮，必须是与其他物质进行化合后的产物，且必须能溶于水。硝酸钾就是这样的一种化合物，因此经常被用于给农作物施肥。不过可溶的含氮化合物并不多，且若非转变为其他类物质，它们在土壤中便会很快流失。
+
+人们经常也给农作物施加粪肥，这种肥料含有土壤细菌可以分解的含氮化合物，细菌可以将它们分解成为合适的成分，以便于植物吸收。个别��类的细菌甚至能将土壤空气中的氮也合成到化合物中，以供植物利用。如果想让土壤生机勃勃，则自然需要保持土壤的生物活性与多样性，这也是农民在土地上耕耘所必需的经验知识，因为这关乎他们的收成。
+
+45．农耕土——在实验中我们已经知道，各类土壤的精细程度、矿物成分以及含水量均各不相同。同时，它们各自所含的腐败植物残骸，以及腐殖土的含量，也大不一样。腐殖土是土壤最重要的组成成分，它们可以帮助土壤吸收水分、给植物提供营养以及让土壤免于结块。
+
+在砂土中，腐殖土含量极少，因此水会很快地渗透而去，植物也会变得焦渴枯萎。而且这类土壤在春季升温很快，在雨季之后也迅速变得干燥。但如果在砂土中施上粪肥，粪肥中的腐殖土成分与植物养分与砂土的特质相结合，特别有利于早春农作物的生长，也极适合对土壤水分需求很大的作物，比如葡萄。加利福尼亚州的弗雷斯诺沙地，以及美国东部的海岸沙地平原便是由这类土壤构成。
+
+粘土的颗粒极其微小，颗粒之间的空间也非常细微，因此对水的吸收非常慢，不过吸收水的持续能力却很好。当粘土湿润的时候，它会有很明显的粘连性，并且基本上不能被利用；干燥的时候，则会变得很坚硬，并极易龟裂破碎。如果这时不合时宜地进行耕种的话，土壤会进一步干燥结块，让以后的耕种变得更加困难。美国西部广袤的干燥粘性土壤区便具有上述特征。如果土壤的成分全是粘土的话，那便几乎不能用于耕作，但若加进足够的沙子和腐殖土的话，那就大有可为了，因为粘土本身就包含有植物所需的诸多微量元素。
+
+其微粒大小介于沙子与粘土之间的土壤叫做淤泥，一般都是十分肥沃的土壤。美国西部大草原和重要农产地便分布着这类土壤。它保持水分的能力很强，并且含有大量植物所需的养分，且极容易耕种作物。砂土、粘土和淤泥这三种土壤，其优势劣势均各有千秋，但毕竟也都不完美。对农民而言，他们希望土壤吸水性好但又渗透快、富含植物所需养分、足够肥沃且通风透气，以便让农作物茁壮成长。这就不是上面任何单独一种土壤能办到的了。
+
+46．土壤中的水分——即使有很多种类的土壤都含有农作物所需的各种养分，但若没有风调雨顺的气候，农作物的生长依然会大受影响。这在美国大部分地区都是一个十分现实的问题，即使在干旱期进行了充分人工灌溉的土地，也依然有水分缺失的问题。因此，如何增加土壤保持水分的能力，以及减少水分的流失，已经成了一个迫在眉睫的问题。
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+
+
+
+
+实验52：称取等量的砂砾、粗沙、细沙各100克，分别放进一个4英寸大小的漏斗，漏斗中垫一层滤纸，然后往漏斗中倒入足量的水，让土壤标本充分吸收，保持漏斗竖直，直到水全部过滤滴出漏斗。这时再次对三种标本进行称量，由于是与滤纸一起称量，所以在天平秤的计量盘中也放上一片类似的湿滤纸以平衡重量。比较称量结果，哪个标本保持水分的能力最强？由于水并不能穿过样本土壤的颗粒，因此保持水分的能力只会与颗粒的大小有关系。
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+
+
+
+
+在任何土壤区域，只要我们挖得足够深，便一定会发现水。井就是这样的例子。有一些树种和特别的植物，可以把根伸向很深的地下，去靠近地下水源，因此在其他植物可能不能存活的地方，它们依然能够枝繁叶茂。当湿润土壤中的水分通过瓦沟渗透完毕之后，植物便能将它们的根须向下伸至这类水源，或者叫地下水。因此它们即使在干旱期也会有很强的生命力，而那些生长在不能渗透水分的土壤层中的植物就逊色多了，因为那里地下水的分布会相对混乱。
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+
+
+
+
+实验53：在一盘有颜色的水中，放入几支孔径大小不一的细小玻璃管，管中和盘中的水面，哪个更高？大孔玻璃管和小孔玻璃管中，哪个水面更高？
+
+实验54：取两个4盎司的广口瓶，并排放在一起，给其中一个装半瓶水。将一根粗布条，或者有条件的话，拿一根灯芯，一端插进瓶子里的水中，另一端悬吊在空瓶中，这样保持一个小时的时间。看着会发生什么现象？这种让水上升到一些细玻璃管、灯芯以及小裂缝中的现象，叫做毛细现象。
+
+实验55：将四支灯管的一端蒙一块布，封紧，然后分别放入足量的粗沙、细沙、粘土以及深褐色亚粘土，然后将它们竖立放置在一个装有水的浅底平板锅中，令其保持一周的时间，但注意须保证平板锅中一直有水。注意不同时间，灯管里面水所上升的高度，可专门记录一下一小时后、两���后、一周后，各管中的水位位置。
+
+
+
+
+
+在实验49中我们已经得知，土壤的每一个颗粒都被一层很薄的水膜包裹着，即使土壤整体看上去并没有水分也依然如此。但这层水膜也可能会像实验54中所显示的那样，通过灯芯的毛细现象而被吸走。岩石就是这样，它就是通过吸收土壤颗粒的水膜而获取大量水分的。一片土地若被森林覆盖，那里的降雨则会被苔藓、树叶以及植物的根饱和地吸住，并且树叶也会呵护水分，防止其过快蒸发。这样一来，土壤中的水分便可以更长时间地滋润周边地区，除非彻底干枯或者森林消失。也正因如此，山地的土壤可以为大地回馈五倍于自身重量的水分。
+
+重力总是不断地将土壤中的水分拉向地下深处，因此深层土壤中的水，总是不断地向下渗透，当遇到不能渗透的土层或岩石层的时候，它们便会以泉水形式又回到地面，或者以润物细无声的方式悄悄地滋润周围的大地，甚至惠及离当初它们向下萃集的区域极其遥远的地方，让干涸的土壤变得肥沃湿润，这便是大地的化育之灵。
+
+但另一方面，经常也有必要阻止大量水的向下渗透，因为在渗透的过程中，水会溶解并带走植物的养分，进而降低土壤的肥沃程度。因此，降低土壤的疏松程度，便能减弱水的渗透效应，进而帮助土壤保留可供植物利用的养分。增加腐殖土就是一个办法，同时对渗水性强的土壤进行夯实滚压也能起到类似效果。但在雨季来临的时候，土壤则必须保持疏松，这样水才能浸入土壤。
+
+蒸发会导致土壤失去大量水分，它们先通过毛细现象被吸附到土地表面，然后被蒸发掉。在干燥的地区以及干旱季节，农作物生长需要足够的水分，因此蒸发导致的水分流失是人们需要阻止的现实问题。
+
+
+
+
+
+实验56：找4个大小相同的锡皮罐，都装满土壤，在罐子周围和底部都戳出一些小孔。给每一罐土壤都浇上等量的水，给第一个罐子上覆盖一英寸青草，第二个罐子上覆盖一英寸锯末，然后小心地称出重量。再对第三罐和第四罐称重，记录下每一罐的重量。将第三个罐子的土壤表面均匀搅动，直到表面变干，搅动深度大致一英寸左右。然后持续不断搅拌一会儿，并让第四罐土壤保持原样。每天放学后将四个罐子都称量一下，持续两周，并做好重量减少的记录。为什么重量会减少呢？青草、锯末以及表面的搅动是怎样导致土壤重量减少的？请你建议一些让土壤保持水分的办法。
+
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+
+
+
+从实验56我们可以看到，在表面覆盖了青草或锯末的土壤，其水分的蒸发要比什么都没覆盖的土壤来得慢。刨花、粪肥以及其他可以避免让土壤日晒风吹的物质都可以代替这里的青草，起到同样的效果，这类保护性物质叫做覆盖物。它们常被用来覆盖树木、藤蔓以及灌木周围的土壤，但若要广泛用于庄稼的耕种土地，目前还不太现实。
+
+我们还发现，要是土壤表层被持续搅拌，土壤中的水分也会蒸发得很慢，这是因为水分上升到表面的途径——毛细管被破坏，且阳光和空气又到不了下层土壤，因为搅拌层将它们隔开了。如果土壤表层被充分搅拌或者被翻动而耕种农作物，那土壤的颗粒便也被分开了，水分便不能从一个个土壤颗粒上进行转移，自然就为植物的根须保证了水分供应。因此，在干旱地区，开发耕地并栽种农作物，恐怕是让土壤保持水分，进而满足植物生长需要的一个最佳途径。
+
+在美国的一些干旱地区，一直推行着一种叫做旱地耕作的农耕方法。它要求耕作时深耕土壤，且耕犁前面有一个切削轮毂，叫做镇压器，主要作用是压紧下层土壤，让空气不能流通到土壤的翻动层，进而带走上层土壤的水分。表层土壤则须被充分耕作，以形成一层完美的土壤覆盖物。这样一来，就能保证雨水既不向农作物根须以下的土壤渗漏，又不从土地表面蒸发。这种耕作方法的目的就在于，可以让土壤的水分能够支持一年生农作物的生长。
+
+在美国大部分地区，庄稼一般两年种一次，也就是说，成熟的庄稼需要两年水分的持续滋养，而土壤则须在这期间不断地被充分耕作。毋庸置疑，旱地耕作法保证了土壤至少能提供植物生长所需的水分的最小值。
+
+在降雨量稀少或者常年干旱的地方，灌溉便成了种植庄稼最有效的方式。撒向大地的水通过远途输送，或者自流井水渠的传送，还有通过打井从地下水获取的水源。这样就保证了对农业生产所需水量的随时供应。在地势非常平坦的地方，一般容易发洪水，有时在被丘陵山脊分隔开的大大小小的平原地区��会发洪水。大量的水就是这样流失掉了。
+
+另一个办法，开挖一条8到10英寸深的犁沟，让水可以从灌溉沟溢出后，通过犁沟流向土壤。这两种办法，目的都是让水能充分滋润土壤，然后表面土壤被充分耕作以控制土壤表层的水分蒸发。近些年，政府和一些私人企业投入了巨资用于农作物的灌溉，通过这样的努力，许多贫瘠的地区数千英亩的土地变成了千里沃野，成为了农业的主产区。
+
+47．碱性土壤——在一些干旱地区，雨水浸入地下一段时间后，又慢慢被向上吸入表层土壤进而被蒸发掉。人们发现这些地方有大片大片的土地，即使提供足够的灌溉，也依然寸草不生。在干燥的季节，这些地区的地面上会大范围零星地出现白色或棕色的霜痂，如果尝一下的话，白色的像泻盐的味道，棕色的像天然苏打粉的味道。正是盐类物质形成了这些霜痂，它们溶解在土壤的水分中，水分蒸发后便留在了土壤的表面上。
+
+在湿润的地区则找不到这类物质，因为它们在水变成水蒸气的过程中被带走了。因此，对付这类土壤的唯一办法，就是对其反复灌溉并充分排水，把土壤中的盐分彻底洗干净。在雨量充沛的地方，这个过程已经被大自然完成了。而在没有太多碱类物质的土地上，对土壤的深度耕作，或者栽种一些在这类土壤上可以存活的植物，比如甜菜，也能慢慢中和土壤的碱性。
+
+48．土壤与人类——虽然大自然经过漫长的岁月生成了土壤，一代又一代的植物和动物也让土壤变得越来越肥沃，但如果人类不以敬畏之心对待我们脚下的土地的话，土地也不会一如既往地化育万物。我们从土壤中获取的物质，必须通过施肥填补回去，且必须对土壤充分耕作来保证土壤的以下特性:（1）保持水分；（2）保持疏松柔软以便让植物根须可以自由伸展；（3）为菌类微生物提供空气、水和腐殖土以让它们能够制造可溶性氮氧化物；（4）消除野草以免占用农作物空间、消耗农作物所需的养分，甚至让农作物窒息死亡。因此，合理的耕作，可能是比其他任何方式都有效的、多快好省的对土壤的利用途径。同时，对土壤的精心调理耕作，可以节约大量施肥的成本，农作物的产量也会大幅提高。
+
+49．土壤的价值——决定土壤的农业价值的因素有很多，因此在一个地方极有价值的土壤，换到另一个地方则可能一钱不值。比如很轻的砂土，基本上无人买卖，除非有极好的交通运输条件，否则没人会对它感兴趣；而在另一些城市，如果那里能轻易获得肥料，且早季蔬菜也十分走俏的话，砂土就变得极有价值了。
+
+不同类型的土壤都有各自适合栽种的农作物，而且即使一种土壤不适合许多农作物生长，但只要它非常适合某一种经济作物，那这样的土壤依然很有价值。比如在佛罗里达州大部分地区，尽管那里的土壤不适宜栽种其他许多农作物，但却十分适合载橘子树或早季蔬菜，故而同样是不错的土壤。加利福尼亚州的橘子产区的石质土壤也是如此，其他作物在这里毫无收成，但这里却非常适合橘子树生长，也因此这里的土壤同样极具价值。
+
+
+
+
+
+总结——地球表面只有四分之一多一点的面积是陆地，而陆地面积中只有二十分之一多一点是美国领土。海岸中一些海沟的深度，甚至超过了陆地上最高山峰的高度，大多数海洋动物都生活在海洋的表层水域，而陆地动物则分布庞杂，从山地丘陵到平原峡谷，都有它们身影。因此，陆地上的生物多样性体现得更加丰富多彩。
+
+水是氢与氧的简单化合物，它有许多十分有价值同时也有趣的特性：它不能被压缩；它是最强大的溶剂；它容易蒸发，然后形成雨水回到我们身边，同时在巨大压力下它能成为蒸汽动力；它有极大的保存热量的能力。因此，水可以通过海洋调节大陆的气候变化。
+
+陆地由种类繁多的动物、无机物、植物构成。其总量的五分之四是沉积岩，它由水中沉积物积累而成。另外还有火成岩，它是由地球内部的熔岩物质构成的；以及变质岩，它是由前两种岩石变化而来的。
+
+土壤主要来自于腐蚀和风化的岩石，总体可分为原地土与运积土两大类。原地土是由下面的岩石直接变化而来，运积土则是由水流、冰川、风沙带来的土壤与岩石物质积累形成的。土壤从形式上也可以粗略地分为砂砾、砂土、粘土以及壤土。
+
+土壤的肥沃程度与土壤的成分直接相关，同时与其中所含的空气和水，以及其自身的通风与排水情况不无关系。肥沃的土壤必须含有氮、碳酸钾以及石灰。植物的根要有空气进行���吸，还要有水来溶解营养物质，以便让根须可以吸收。所以，土壤最好不要像粘土那样结块，也别像砂砾那样彻底松散，因为粘土的吸水性很差，砂砾的渗透性又太强。
+
+为了保持土壤的肥力，必须让土壤保持频繁的耕作翻动，以降低水分的蒸发与渗漏，同时还必须通过施加粪肥以及其他肥料来为土壤提供养料。在一些降雨量不足的地方，人们使用灌溉和旱地耕作法来让土地保持水分。不同类型的土壤都有各自适合栽种的农作物。没有肥沃的土壤，地球上的一切生命都将会消失殆尽。
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+
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+
+
+思考题
+
+
+判断一片陆地是否曾经沧海，你会从哪些地方着眼？
+
+陆地与海洋有哪些不同特征？它们给生活在其中的动植物带来了哪些不同影响？
+
+你觉得有哪些理由说地球是一个水发动机？
+
+你周围的岩石属于岩石的哪个大类？
+
+你见过哪些岩石风化的例子？请描述一下。
+
+你周围的土壤是原地土还是运积土？它们所处的地理位置的差异大吗？肥沃程度差别悬殊吗？
+
+以上的差别你认为是由什么原因形成的？
+
+让土壤变得肥沃有哪些必要性？
+
+怎样保持土壤的肥力？
+
+在干旱的地区以及干旱的季节，怎样保持土壤的水分？
+
+哪些因素决定了土壤的农业价值？
+
+
+
+
+
+译注
+
+
+[1]1磅=0.45359237千克，约等于1斤。
+
+[2]目前还没有任何观测证据支持作者这一观点。
+
+[3]夸脱与品脱一样，有干量与湿量之分，1美制湿量夸脱=946.352946毫升，1美制干量夸脱=1101.220毫升，总体而言，大致与1升相当。
+
+
+
+
+
+CHAPTER 5
+
+THE ATMOSPHERE OF THE EARTH
+
+地球的大气层
+
+
+50. The Origin of the Atmosphere. —As the earth cooled down from the intensely hot condition which it is supposed to have had at first, the substances which had not chemically combined and formed liquids and solids, or which required a low temperature for their con-solidation were left still in the gaseous state around the solid core. This gaseous envelope composed of these substances surrounding the earth we call the atmosphere. Some of these gases are inert and do not readily combine with other substances. Others have formed extensive combinations, but they exist in such large quantities that they were not thereby exhausted.
+
+
+
+
+
+BLUE HILL OBSERVATORY, MILTON, MASS.
+
+One of the first places in America where conditions of the upper atmosphere were studied.
+
+
+
+51. The Composition of the Air.
+
+
+
+
+
+Experiment 57. —(To be performed only by the teacher.) Having rounded out a cavity in a small flat cork, cover the cavity and surface around it with a thin layer of plaster of Paris. After the plaster has set and become thoroughly dry float the cork on a dish of water with the cavity side up. Place a piece of phosphorus as large as a pea in the cavity and carefully light it. (Great care must be taken in handling phosphorus as it ignites at a low temperature and burns with great fierceness. It must always be cut and handled under water.)
+
+
+
+Fig. 47.
+
+
+
+As soon as the phosphorus is lighted, cover it with a wide-mouthed bottle. Be sure that the mouth of the bottle is kept slightly under water. The water will be found to rise in the bottle. The phosphorus soon ceases to burn. White fumes are formed, but these soon clear up. A clear gas is left in the bottle, but this cannot be air, for if it were, the phosphorus would have continued to burn in it, since it burns in air. If it were not for this property of not permitting phosphorus to burn, the gas left in the bottle could not be distinguished by ordinary means from air.
+
+The gas fills more than three fourths of the bottle, so that more than three fourths of the air is composed of a gas which does not support combustion. This gas is called nitrogen. The other constituents of the air must also be transparent colorless gases, since the air is transparent and colorless. The most important of these is called oxygen. The phosphorus united with this and formed the white fumes. These fumes dissolved in the water, leaving the nitrogen.
+
+Be careful to put the cork on which the phosphorus was burned in a place where it cannot cause a fire.
+
+
+
+
+
+Although the air appears like a simple gas and was so considered until the end of the eighteenth century it has been shown to be composed of several different colorless gases. One of these, oxygen, supports combustion; another, nitrogen, neither burns nor supports combustion. Chemists have found that these two gases are mixed in the air in the proportion of about one part of oxygen to four parts of nitrogen.
+
+Another heavy colorless gas called carbon dioxide is found in the air in the proportion of about 3 parts to 10,000. There are in addition very small quantities of several other gases, but these are not of sufficient importance to be studied here. Besides the gases, the air contains other matter, such as water vapor, dust particles and microbes.
+
+
+
+
+
+Experiment 58. —Obtain four bottles of oxygen from the chemical laboratory. If not obtainable, place a piece of sodium peroxide (oxone) about as large as the end of a finger in a side necked test tube provided with a medicine dropper filled with water, as shown in Fig. 48. Put the end of the delivery tube under the mouth of an inverted bottle filled with water arranged on the shelf of a pneumatic trough. Drop water slowly on to the sodium peroxide and collect the gas generated. Fill several bottles. Oxygen can also be prepared by heating a mixture of about one part manganese dioxide and two parts potassium chlorate in a test tube and collecting the gas over water (Fig. 49). Does the appearance of this gas differ in any way from air? Smell of it. Has it any odor? Into one of the bottles of oxygen insert a splinter of wood having a spark at the end. It bursts into flame. Does the same thing take place when the stick with the spark upon it is held in a bottle of air?
+
+
+
+Fig. 48.
+
+
+
+
+
+Fig. 49.
+
+
+
+Hold a lighted match at the mouth of another of the bottles containing oxygen. Does the gas itself burn as illuminating gas does when a match is applied to it? If the oxygen in the air were increased or decreased, it would have a great effect upon combustion. Attach a piece of sulphur to a short piece of picture wire. Ignite it and place the wire in a bottle of oxygen (Fig. 50). Does the sulphur burn strongly? How about the wire? Does it burn too?
+
+
+
+
+
+Fig. 50
+
+
+
+The oxygen is the most important part of the air to animals, for without it they could not live. They breathe in oxygen and breathe out carbon dioxide. All their heat and energy is due to the power they have of combining oxygen with carbon and forming carbon dioxide. Plants also need it.
+
+Plants need carbon dioxide as much as animals need oxygen. By far the greater part of plants is made from the carbon which they get from this gas. The growth of a plant is due to the power it has of tearing apart the carbon dioxide by the help of the sun and building the carbon into its structure. It returns the oxygen to the air to be used again by the animals and plants.
+
+
+
+
+
+Experiment 59. —Get two or three bottles of carbon dioxide from the chemical laboratory, or prepare it by pouring dilute hydrochloric acid upon pieces of limestone in a bottle and collecting the gas over water. Does the appearance of this gas differ in any way from that of air? Smell of one of the bottles that has stood over water for some time. The gas has no odor. Plunge a lighted match into one of the bottles containing the carbon dioxide. What happens? Does the gas burn or support combustion? Slowly overturn a bottle of the gas above a lighted candle. The candle is extinguished. The gas falls out when the bottle is overturned, thus showing that it is heavier than air. If the amount of carbon dioxide in the air were largely increased, what effect would it have upon combustion?
+
+
+
+
+
+Animals smother in carbon dioxide. It is known to coal miners as choke damp, because frequently after they have escaped from an explosion they are smothered by it. It occurs at a few localities, as at the Dog Grotto near Naples and in Death Gulch, Yellowstone National Park, in sufficient quantities to be fatal to animals passing through these places.
+
+As a rule, however, the proportion of oxygen, nitrogen and carbon dioxide is the same for all places on the surface of the earth and it is only where for some peculiar cause carbon dioxide is emitted from the ground in a place sheltered from the wind, that it can accumulate. As animals and men breathe it out, rooms where they stay must have proper ventilation.
+
+The nitrogen is needed to dilute the oxygen. If oxygen were undiluted, animals could not live, and a fire once started would burn up iron as readily as it now does wood. As has already been stated, certain bacteria take nitrogen from the air and prepare it so that plants can use it.
+
+Plants and animals both need water vapor. Were it not for this form of moisture there would be no rain, and without rain life could not exist. Thus the air which contains oxygen and water vapor for both plants and animals, carbon dioxide for the plants, and nitrogen to dilute the oxygen, is one of the greatest factors in the life history of the earth.
+
+
+
+
+
+52. Weight of Air.
+
+
+
+
+
+Experiment 60. —Into a five pint bottle insert a tightly fitting rubber stopper through which a glass tube extends. To the outer end of the glass tube tightly fit a thick-walled rubber tube of sufficient length for the attachment of an air pump. Put a Hoffman's screw upon the rubber tube. See that all connections are air-tight. Weigh carefully the apparatus as thus arranged. Now attach the rubber tube to the air-pump and extract the air from the bottle. When all the air that can be exhausted has been removed, close the rubber tube tightly with the Hoffman's screw and weigh again. Unclamp the Hoffman's screw and allow the air to enter the bottle. The weight should be now the same as at first. Or, instead of weighing a bottle of air, weigh an incandescent light bulb. Make a hole in it with a blowpipe and weigh again. Is the weight now the same as before?
+
+
+
+
+
+Fig. 51.
+
+
+
+We have found by the previous experiment that air has weight. With the apparatus used it was impossible to tell exactly the weight of the air extracted or to determine the weight of a definite volume of the air. If we had been able to do this, we should have found that on an average day, at sea level, the weight of a liter, a little more than a quart, of air, is about 1.2 grams. 12 cu. ft. weigh about 1 lb. The air extends to so great a height that although very light, the weight of so great a mass of it is considerable.
+
+Now that air has been found to have weight, it can be seen why a light body like a balloon will float in it in the same way that a stick will float in water. The weight of the air varies with the pressure and temperature, as we shall find later.
+
+
+
+BALLOON.
+
+The gas in the balloon is lighter than air, so the balloon floats in air as a piece of wood does in water.
+
+
+
+53. Expansion of Air when Heated. —Air expands very much when heated, as was seen in Experiment 17. It is found that if air at freezing is heated to the temperature of boiling water, it will expand about 4/11 of its volume. The force with which air expands is so great that sometimes when buildings are on fire and there is no opening for the confined air to escape, the walls are blown out or the roof blown off by the expansion of the hot air, and great injury done to those fighting the fire. That air expands upon being heated is readily seen when a toy balloon is brought from the cold outer air into a hot room, —the cover ing begins at once to tighten and the balloon to swell.
+
+
+
+
+
+54. Weight of Air as Affected by Heat and Cold.
+
+
+
+
+
+Experiment 61. —Take two open flasks of nearly the same weight and capacity and balance in as nearly a vertical position as possible at the ends of the arms of a beam balance. Bring the flame of a Bunsen burner to the upper side of the bulb of one of the flasks so that the hot air currents that are generated will have no upward push on the flask. Do not allow the hot air to get under the flask. What is the effect?
+
+
+
+
+
+Fig. 52.
+
+
+
+As the previous experiment shows, and as we should expect from the fact that air has been found to expand when heated, it follows that hot air is lighter than cold air. A liter of air at freezing under ordinary pressure weighs about 1.293 grams, but at the temperature of boiling water it weighs only about .946 grams. So a volume of cold air, being heavier, will exert more pressure at the surface of the earth than an equal volume of hot air.
+
+As air is a gas whose particles can move freely among themselves we should expect that a heavier column of cold air would sink down and distribute itself along the surface under surrounding lighter air just as a column of water falls when its supports are withdrawn and forces up the lighter air which surrounds it.
+
+
+
+HOT AIR FURNACE.
+
+The hot air rises through the pipes and registers, and cold air presses in from outside.
+
+
+
+A similar action is seen when water is poured upon oil; the water sinks to the bottom and forces the oil to rise. Thus if air is heated at any place, we should expect that there would be a rising current of hot air and a current of colder air creeping in to take its place. The winds of the earth are due to this property of air. It is this tendency of heated air to rise that makes hot air furnaces useful for heating houses. Valleys are generally colder than the surrounding hillsides, so that delicate crops can be grown successfully on the hillsides although those in the valley are frost bitten.
+
+
+
+
+
+55. Pressure of Air.
+
+
+
+
+
+Experiment 62. —Use a convection apparatus or take a tight chalk box and in two places on the top punch holes in a circle not quite as large as the bottom of a lamp chimney. Place a small lighted candle at the center of one of the circles of holes and a lamp chimney, tightly sealed to the box, about each circle. Hold a smoking piece of paper above the chimney which does not inclose the candle. (If a pane of glass is put into one of the vertical sides of the box, better observations can be made.) What happens? Put out the candle and carefully heat the chimney with a Bunsen burner. Is there the same action as before? Why is it that sparks rise from a fire? What is meant by the draft of a stove? Why in order to ventilate a room is it best to open a window at the top and bottom?
+
+
+
+Fig. 53.
+
+
+
+Experiment 63. —If a tin can with a tightly fitting screw cap can be easily procured, boil a little water in it, having the screw cap open so that the steam can readily escape. While the water is still strongly boiling, quickly remove from the heat and tightly cork. Be sure not to cork before removing entirely from the heat. Set the tin thus corked upon the desk and observe. What happens as the steam condenses? Why?
+
+
+
+
+
+Experiment 64. —By means of an air pump exhaust the air from a pair of Magdeburg hemispheres. Now try to pull the hemispheres apart. It cannot be done as easily as before the air was exhausted. Why?
+
+
+
+Fig. 54.
+
+
+
+Experiment 65. —Fill a glass tumbler even full of water and press upon it a piece of writing paper. Be sure that the paper fits smoothly to the rim of the tumbler. Take the tumbler by its base and carefully invert it over a pan. Does the water fall out? If not, why not? While the tumbler is in the inverted position, insert the point of a pencil between the paper and the rim of the tumbler. What happens?
+
+
+
+Fig. 55.
+
+
+
+Anything that has weight must exert pressure upon the surface upon which it rests. The air has been found to have weight, therefore it must exert pressure at the surface of the earth. Air is a gas and its particles easily move over each other, therefore this pressure is exerted equally in all directions. No one feels the pressure, however, because the air is within us as well as about us. Those that have measured this pressure find that it is about fifteen pounds to the square inch at sea level. If two egg shells from which the insides had been removed, one of them with the holes left in it and the other completely sealed, were sunk to a considerable depth in water, which one would be crushed and which one would not? This illustrates why we are not crushed by the pressure of the air upon us.
+
+
+
+
+
+56. Decrease of Volume due to Pressure.
+
+
+
+
+
+Experiment 66. —In a Mariotte's tube cause about a centimeter of mercury in the short arm to balance the same amount in the long arm. The pressure inside the short tube will then be equal to that outside the long tube and will be that of the air upon the day of the experiment. The short arm will now be sealed with mercury so that no air can get in or out. Pour mercury into the long arm. The air in the short arm will be gradually compressed and will occupy less and less space. If we remember that the pressure upon the air in the short arm is the air pressure of the day plus the height that the mercury column in the long arm exceeds that in the short arm, we can show by careful measurement that the volume of the air decreases just as the pressure increases.
+
+
+
+
+
+Fig. 56.
+
+
+
+As was seen in Experiment 1, the volume of the air can be very much decreased by pressure, but when the pressure is removed, it regains its original volume. It cannot be told from this experiment whether the volume of the gas decreases as the pressure increases or whether it decreases much more rapidly when first pressed upon than afterward. This can be best shown by the use of the Mariotte's tube as in Experiment 66. But if the bicycle pump is a good one, it will answer the question of the rate of decrease quite accurately. It is found that the volume decreases directly as the pressure increases.
+
+57. Barometers. —As the measurement of the atmospheric pressure is of great importance in the study of atmospheric conditions, it is necessary to have an instrument by which these measurements can be readily made. An instrument designed for this purpose is called a barometer. There are two kinds of barometers in common use, called the mercurial and the aneroid.
+
+
+
+
+
+Fig. 57.
+
+
+
+Experiment 67. —(Teacher's Exp.) Take a thick-walled glass tube of about 1/2 cm. bore and about 90 cm. long and slip tightly over the end of it about 10 cm. of a thick-walled flexible rubber tube 30 cm. in length. Firmly secure the rubber tube to the glass tube by winding tightly around them many turns of string, making it impossible for the rubber tube to slip or admit air. Completely close the rubber tube with a Hoffman's screw just beyond the place where it leaves the glass tube. Placing this closed end in a large dish so as not to waste any mercury, fill the glass tube with mercury. Place the thumb over the open end of the tube and invert it in a cup of mercury. If the connections were made tight, the mercury will not fall far below the end of the glass tube. The air pressure keeps the mercury up. This is a simple form of barometer.
+
+While the tube is still standing in the mercury cup take another glass tube similar to the first and attach it to the open end of the rubber tube in the same way as the first was attached. Place the free end of this tube in a dish of colored water and gradually open the Hoffman's screw. The water rises in the tube. Why? What is meant by sucking water up a tube?
+
+
+
+
+
+Fig. 58.
+
+
+
+
+
+Fig. 59.
+
+
+
+Experiment 68. —Fill a bottle with clean water and fit it tightly with a rubber stopper having two holes in it. Plug one of the holes tightly with a glass tube one end of which has been closed by heating in a Bunsen burner. Through the other hole put an open glass tube 10 to 15 cm. long. See that both tubes fit tightly in the stopper and that the stopper fits tightly in the bottle. Now attempt to "suck" the water out of the bottle through the open tube. Does it come out freely? Pull out the glass plug. Does it come out any better? If so, why?
+
+
+
+
+
+The mercurial barometer we have already made in a rough form. The best form of these instruments consists of a glass tube of uniform bore about eighty centimeters long and closed at one end. After being carefully filled with pure mercury, it is inverted in a cistern of mercury. The cistern of mercury has a sliding bottom easily moved up and down by means of a set screw. At the top of the cistern there is a short ivory peg. The lower end of the ivory peg is at an exactly measured distance from the bottom of a scale. The scale is placed beside a slit near the top of a metallic tube which is firmly fastened to the cistern and surrounds and protects the glass tube.
+
+
+
+Fig. 60.
+
+
+
+When it is desired to read the barometer, the sliding bottom of the cistern is raised or lowered until the top of the mercury in the cistern just touches the bottom of the ivory peg. The height of the top of the mercury column is then read from the scale. In order to determine the height with great precision there is generally attached to the metallic tube a sliding vernier which moves in the slit.
+
+The aneroid barometer consists in general of a corrugated metallic box from which the air has been partially exhausted. Within the box is a stiff spring so that the pressure of the air will not cause it to collapse. Attached to the box are levers by which any change in the volume of the box will be multiplied and indicated by a pointer arranged to move over a dial. The dial has a scale upon it and thus the air pressure is registered.
+
+
+
+BAROGRAPH.
+
+This is arranged so as to record the air pressure automatically for a week at a time.
+
+
+
+Instruments called barographs are constructed in which a long lever provided with a pen point is attached to the aneroid and made to record on a cylinder revolved by clockwork. Thus a continual record is made of barometric readings.
+
+
+
+
+
+58. Determination of Height by a Barometer.
+
+
+
+
+
+Experiment 69. —Carry an aneroid barometer from the bottom of a high building to the top. Note the reading of the barometer at the bottom and again at the top. Why is the barometer lower at the top of the building?
+
+
+
+
+
+As the pressure of air at any surface is due to the weight of the air above that surface, it happens that as we go up the pressure decreases, since there is a continually decreasing weight of air above. If the rate of this decrease is determined, then it is possible to determine the elevation by ascertaining the pressure.
+
+Although the height of the barometer is continually varying with the changing air conditions, yet if these conditions remain about the same, it may roughly be estimated that the fall of 1/16 of an inch in the height of the mercury column indicates a rise of about 57 feet, and that the fall of a millimeter indicates a rise of about 11 meters. These values are fairly reliable for elevations less than a thousand feet, under ordinary temperatures and pressures.
+
+At the height of 25 miles the barometric column would probably not be more than 1/25 of an inch high. Several measurements made in different ways indicate that the air is at least 100 miles in depth, probably more. Nearly three fourths of the atmosphere however is below the top of the highest mountain. The highest altitude ever reached by man was about 7 miles.
+
+To study air conditions small balloons to which meteorological instruments are attached have been sent to a height of 21 miles. It is found that the minimum temperatures occur at a height of from 6 to 10 miles. Conditions affecting weather, however, seem to extend to a height of not much over 3 miles.
+
+
+
+
+
+59. Adiabatic Heating and Cooling of Air.
+
+
+
+
+
+Experiment 70. —Have a five-pint glass bottle fitted with a two-hole rubber stopper. Pass through the holes in the stopper a chemical or air thermometer and a short glass tube the lower end of which extends into the bottle not near the bulb of the thermometer, so that when the air is exhausted or allowed to enter the bottle there will be no movement of the air near the bulb of the thermometer. The end of the column of the thermometer must be visible above the stopper.
+
+
+
+Fig. 61.
+
+
+
+Attach the glass tube to an air pump by means of a thick-walled rubber tube. Note the temperature of the thermometer within the bottle and also of the air outside. Quickly exhaust the air from the bottle, carefully noting the action of the thermometer. See that the temperature of the air in the room does not change during the experiment. Allow the air quickly to enter the bottle and note the action of the thermometer. The temperature inside the bottle changes as the air is quickly exhausted, or as it is allowed to enter the bottle again and thus to increase the density of the air in the bottle.
+
+
+
+
+
+It has been found that when air expands its temperature falls and when it is compressed its temperature rises. This heating and cooling of the air without the application of external heat or cold, but simply by a change in the density of the gas itself, is called adiabatic heating or cooling. It is taken advantage of in the manufacture of liquid air and is the same principle which is utilized in cold storage plants. This property of air has much to do in developing our wind circulation and storms.
+
+The heating effect of compressing air can be well seen when a pneumatic tire is filled. No matter how well the piston of the pump may be oiled, as the density of the air in the tire begins to increase, the pump will grow warm rapidly. This rapid heating cannot be due to friction, as the pump is not being worked any more swiftly than at first. It is due to the greater compression of the air. As this compression increases, the heating increases, the effect of friction in a well-oiled pump being of small value.
+
+
+
+
+
+60. Effect of Temperature on the Capacity of the Air to hold Moisture.
+
+
+
+
+
+Experiment 71. —Take a liter flask and put into it just sufficient water to make a thin film on the inside of the flask when shaken around. Now warm the flask gently, never bringing its temperature near to the boiling point, until the water disappears from the inside and the flask appears to be perfectly dry. Having tightly corked the flask, allow it to cool. The flask appeared dry when warm and on account of having been corked tightly no moisture could have entered it. The air in the flask was perfectly transparent both before and after heating. The film of water around the inside of the flask was taken up by the air when it was warmed but the moisture reappeared when the flask was cooled.
+
+
+
+
+
+Experiment 72. —Fill a bright tin dish or glass beaker with ice water and after carefully wiping the outside allow it to stand for some time in a warm room. Can water go through the sides of the dish? Does the outside of the dish remain dry? If water collects upon it, from where does the water come? See if the same results will happen if the water within the dish is as warm as or warmer than the air in the room.
+
+
+
+
+
+Experiment 73. —Partially fill a dish or beaker like that in the previous experiment with water having a temperature a little warmer than that of the room. Gradually add pieces of ice, continually stirring with a chemical thermometer. Note the temperature at which a mist begins to appear upon the outside of the dish. When the mist has appeared, add no more ice but stir until the mist begins to disappear. Note this temperature. Take the average of these two temperatures. This average is probably the temperature at which the mist really began to form. This temperature is called the dew point.
+
+
+
+
+
+When we wish to dry clothes, we place them in a warm room or in the sunshine. Soon we find that the water has left the clothes. It must have gone into the air. It would thus appear that when the temperature of the air is raised, it has the capacity of taking up more moisture than when it is cold. The previous experiment has shown this, and the one in which the dew point was determined showed that when heated air was cooled it deposited moisture.
+
+This property that air has of taking up a large amount of water when heated and giving it out when cooled is the cause of our clouds and rain. If it were not for this there would be no circulation of moisture over the land, no rain, and without rain there could be no vegetation and no animal life. Thus this simple property of the air furnishes the means for the support of practically all the animate life on the earth.
+
+
+
+
+
+61. Moisture in the Atmosphere.
+
+
+
+
+
+Experiment 74. —Carefully weigh a dish of water and place it in a convenient place where there is a free access of air. After some hours weigh it again. What causes the change of weight? Try this experiment with a test tube, watchglass and a wide-mouthed beaker under various conditions and in various places.
+
+
+
+
+
+The atmosphere at all times and under all conditions contains some moisture. When its temperature has been raised, its capacity to hold moisture is increased, but at no place is it so cold that it cannot contain a certain amount of moisture. When water in the solid or liquid condition is exposed to the air, it gradually disappears and is taken up into the air.
+
+If the water surface is large and the temperature high, there is a large amount of evaporation and the water rapidly rises into the air. In the tropics the evaporation from the water surface amounts to perhaps eight feet per year. This means that the energy of the sun lifts about five hundred pounds of water from every square foot of the surface every year. In the polar latitudes the amount of evaporation is perhaps a tenth of that in the tropics.
+
+From every water surface on the globe, however, a large amount of water is evaporated each year. In many places much of the water evaporated falls upon the same surface from which it came, but a considerable part of it is carried by the winds to other places and falls upon the land surface, furnishing the moisture needed for the land life of the world.
+
+62. Humidity. —The condition of the air as regards the moisture it holds is called its humidity. If the air contains all the moisture it can hold, it is said to be saturated or to have reached its dew point. The amount of vapor present in the air is called its absolute humidity. The amount of vapor in the air divided by the amount that it would contain if it were saturated is called the relative humidity. If the air contains much moisture, its humidity is said to be high. When air which has a high humidity is cooled, it can no longer hold all the moisture which it previously held, and some moisture will be deposited.
+
+
+
+CUMULUS CLOUDS.
+
+Typical low level coluds, indicating showers.
+
+
+
+The moisture in the air may form into little droplets high above the earth's surface, making clouds, or these droplets may be near the surface of the earth. In this case we name the moisture fog. If it collects on objects attached to the surface we call it dew.
+
+
+
+FOG.
+
+A low cloud formed near the surface of the earth.
+
+
+
+By determining the dew point as was done in Experiment 73， and comparing this with tables which have been prepared by meteorologists from many observations, the relative humidity can be readily determined. An instrument for determining the humidity of the air is called a hygrometer (Fig. 62).
+
+
+
+
+
+Fig. 62.
+
+
+
+63. Effect of Atmospheric Conditions on Light.
+
+
+
+
+
+Experiment 75. —Allow sunlight to pass through a glass prism and fall upon a white wall or piece of paper. How has the white sunlight been affected? Where did the colors come from? In what order are the colors arranged? This group of colors into which a prism separates white light is called the spectrum.
+
+
+
+
+
+If the light that comes from the sun is passed through a glass prism, as in Experiment 75，it will be seen to be composed of many different colors. In fact it is the absorption of some of these colors and the reflecting of others which make objects appear of different colors.
+
+Light itself is a vibration which has the power of affecting the optic nerve, and the different colors are vibrations of different lengths. Now the sunlight is affected by the air through which it comes. If there is smoke or dust in the air, the sun will appear to be red. When the sun sets at night and the rays come to us through a considerable thickness of air which is near the surface of the earth and contains dust, the light often appears red. On the top of a high mountain or on a clear day or when the sun is high overhead the sky appears blue. Both these colors are due to the effect of the atmosphere on transmitted light.
+
+Sometimes after a shower an arch appears in the heavens composed of beautiful colors; we call this a rainbow. In this case the sunlight is broken into different colors by the drops of water which still fall in the distance, just as it is when passing through a prism.
+
+
+
+LICK OBSERVATORY.
+
+As light is affected by the atmosphere, observatories must be placed where atmospheric conditions are the best. This famous observatory is on a mountain in the clear air of California.
+
+
+
+Sometimes the sun or moon is surrounded by bright rings called—when of small diameter—coronas, and when of great diameter, halos. These rings are due to the effect of water or ice particles on the light coming from the sun or the moon.
+
+Under certain conditions it may happen that light coming from objects at a distance is so refracted and reflected by the layers of air of different density, through which it comes to the eye of the observer, that objects appear to be where they are not, like the image of a person seen in a mirror. This phenomenon is called mirage or looming. It occurs most frequently on deserts and over the sea near the coast.
+
+Sometimes in high latitudes arches and streamers of colored light are seen illuminating the northern sky. The brilliancy and colors of the illumination vary. Sometimes it is bright enough to be seen even in the daytime. This display is called the aurora borealis or "northern lights" and is believed to be an electrical phenomenon in thin air. The heights of the streamers have been calculated to be more than a hundred, perhaps several hundred miles, so that it is probable that air in a rare condition extends to this elevation.
+
+64. The Warming of the Atmosphere. —The sun transmits both light and heat to the surface of the earth through the atmosphere. On the top of a high mountain the temperature is found to be colder than on the lower levels. The amount of sun radiation, technically called insolation, that falls upon a given surface on the mountain is about the same as that which falls upon an equal surface in the valley. If the heating effect is less, it must be due to something besides the number of heat rays intercepted.
+
+In the spring when gardeners wish to hurry the growth of their plants, they cover them with boxes, the tops of which are made of glass (Fig. 63). It is found that the temperature within the boxes is higher than that outside. The heat rays coming from the sun are in some way affected by the absorption of the ground so that they are not able to get out through the glass as readily as they get in.
+
+
+
+Fig. 63.
+
+
+
+Now the atmosphere affects the heat rays radiating from the earth in the same way that the glass does, and keeps them from flying back into space and leaving the surface cold. Where the atmosphere is thin, as on the mountains, this effect is not so great, and therefore the surface is colder and often covered with snow. When there are clouds in the air, they help to hold in the heat. That is why in the fall, when it is getting cold enough for frosts, the farmers say that the frosts are likely to come on clear nights but not on cloudy ones.
+
+For the same reason plants are covered by pieces of paper and smoky fires are built around cranberry bogs to cover them with an artificial cloud of smoke on nights when there is likely to be a frost. Thus the atmosphere acts as a blanket to the earth and keeps in the heat of the sun just as blankets on a bed keep in the heat of the body. If there were no atmosphere on the earth, its surface would become intensely hot during the day, when the sun shines directly upon it, and intensely cold at night, so that it would not be possible for life to exist.
+
+It has been estimated that, if there were no atmosphere, the mean temperature of the earth's surface during the day would be 350° F. and during the night -123° F. Thus the atmosphere is not only needed for the breathing of plants and animals and for carrying moisture, but also for keeping in the heat of the sun. On the moon, where there is no atmosphere, there can be no life as we know it.
+
+
+
+
+
+65. Cause of the Variation in Atmospheric Temperatures.
+
+
+
+
+
+Experiment 76. —Cut a hole 4 in. square in the center of a board 12 in. square. Fit tightly into this hole one end of a wooden tube 4 in. square and 1 ft. long. Paint the inside and outside of the tube a dull black. Hinge the opposite end of this tube 10 in. from the end of a baseboard 2 ft. long and 16 in. wide, having 6 in. of the board on either side of the tube.
+
+
+
+Fig. 64.
+
+
+
+On a clear day place this apparatus out of doors on a table freely exposed to the sun with a piece of paper on the baseboard under the end of the tube. Point the tube directly at the sun in the early morning, in the middle of the forenoon, at noon, in the middle of the afternoon and about sunset. Mark on the paper the amount of surface illuminated by the sunlight passing through the tube at each of these different times. Why are different amounts of surface covered at these different times?
+
+Place a thermometer in the centers of the surfaces covered by the sunlight passing through the tube at these different times. Note the different readings of the thermometer. Can you suggest a reason why they are not alike? The opening exposed to the rays has been the same throughout the experiment. Draw diagrams illustrating the action of the sun's rays in the different positions.
+
+
+
+
+
+The number of rays of the sun which fall upon a given area depends upon the angle at which they strike the surface. Figure 65 shows that the same number of rays fall upon a much smaller surface when the direction of the sun is vertical than when it is nearly horizontal. In the 30-degree arcs there are 2½, 7, and 9½ ray spaces respectively. The sun is here considered to be vertical at the equator, as it is on March 20 and September 23. Thus on these days, other conditions being the same, about one fourth as much heat from the sun falls upon the 30° about the pole as upon the 30° north of the equator.
+
+
+
+Fig. 65.
+
+
+
+The latitude of a place has much to do with the amount of heat that it receives. As the sun becomes vertical to places north of the equator, the length of the day in the northern hemisphere increases and the time that a place is in the sunshine is greater, so that it receives more heat from the sun. On the 21st of June all points within 23.5° of the north pole, as at North Cape, have 24 hours of sunshine, and the amount of heat received at the pole during these 24 hours is greater than that received at the equator where the day is only about half as long.
+
+
+
+A WINTER SCENE IN VENICE.
+
+
+
+Although the latitude of a place has much to do with the amount of heat received, there are also many other things which affect its temperature. This will appear when we consider that Venice, Italy, with its mild and equable climate is in almost the same latitude as Montreal, Canada.
+
+As has been seen, the height above the sea makes a difference with the temperature, since there is less thickness of air above and therefore a thinner blanket to hold the heat. Then, too, the kind of soil affects the temperature. If the soil is sandy and there is little or no vegetation, it becomes rapidly heated in the daytime and radiates back the heat into the air very rapidly, thus making the temperature of the air near the surface very hot during the day; while at night, when the sun is not adding heat, it rapidly loses the heat acquired during the day, and so the temperature of the air becomes low. In the daytime on great sandy deserts the heat is almost unbearable, but at night it is so cold that heavy blankets are needed to keep the traveler warm.
+
+The nearness to the sea and the direction of the wind also greatly affect the temperature of a place. In some parts of the earth these are the principal causes in determining the temperature. Thus the temperature of the atmosphere at any place is not due to a single cause, but is the result of many and complex causes, such as latitude, height, direction of prevailing winds, ocean currents, nearness to the sea, and kind of soil.
+
+
+
+A WINTER SCENE IN MONTREAL.
+
+The famous Ice Palace, built entirely of blocks of ice.
+
+
+
+Maps are sometimes constructed showing heat belts where tropical, temperate and frigid conditions are found. These belts do not correspond very closely to the torrid, temperate and frigid latitude zones.
+
+66. Graphic Method of Showing the Temperature of a Region. —It is often quite essential that the temperature over a considerable region should be known and a record of it made and preserved. This might be done by taking a map and writing their temperatures above the different places marked on the map. This would make a map full of small figures and very difficult to read.
+
+
+
+HEAT BELTS.
+
+Notice how these heat belts vary from the latitude zones shown on Figure 10.
+
+
+
+A much better method has been developed and is now almost universally used. In making this map the temperatures are first written on the map and then lines are drawn through places which have the same temperature. These lines are called isotherms and the map is called an isothermal map. By the use of such a map it is possible at a glance to determine the temperature prevailing at any place and to see the relation which this has to the temperature of other places on the map. As a rule the isotherms are not drawn for each degree, but only for each ten degrees.
+
+When the map has been constructed, copies are made in which the figures are left off and only the isotherms are preserved. In Figure 66 we have a plan before the isotherms are drawn, and in Figure 67 after the isotherms are drawn. Figure 68 is a typical isothermal diagram. If the map itself were sketched, it would be an isothermal map.
+
+
+
+Fig. 66.
+
+
+
+
+
+Fig. 67.
+
+
+
+
+
+Maps recording barometric conditions are made in the same way as the isothermal maps, only their lines pass through places of equal barometric pressure instead of places of equal temperature. These lines are called isobars.
+
+Weather maps are prepared by the United States Weather Bureau every day, on which are both the isotherms and isobars for that day. The data for these maps are telegraphed each morning from stations scattered all over the settled part of North America.
+
+67. Weather Maps. —Expensive weather bureaus are maintained not only by the United States, but by all the other highly civilized countries of the world. Records are kept also by sea captains and by other observers throughout the world, and these are gathered together by scientific men and from them are made charts of the weather conditions over the entire surface of the earth. Every year more and more data are being collected and these charts are becoming more and more reliable.
+
+
+
+Fig. 68.
+
+
+
+These charts are of great value, since they aid in the explanation of climatic conditions in different parts of the world. The results of the data thus gathered together have been of untold service to commerce and each year have saved many lives and a vast amount of wealth. On the previous page are isothermal maps of the world for the months of January and July.
+
+68. Land and Water Temperatures. —As was seen in Experiment 27, water has the power to hold a great amount of heat. During the summer, water is heated less rapidly than the air above it, so it continually extracts heat from the air, making the air cooler than it otherwise would be. In the winter, water loses its heat less rapidly, so, being warmer than the air above it, it constantly gives heat to the air. Consequently the air over large bodies of water changes its temperature less rapidly than does the air over the land.
+
+When air moves in wind from the ocean to the land, it cools the land in summer and warms it in winter. It is found therefore that lands which border on the ocean usually have a smaller range of temperature than those which are far from the sea. On some islands the range of temperature throughout the year is almost imperceptible, whereas in the interiors of continents the average temperature of some of the summer months is more than a hundred degrees higher than that of some of the winter months.
+
+69. Distribution of Air Pressure over the Earth. —An examination of the isobar maps for January and July shows that atmospheric pressure, like temperature, is greatly affected by land masses. In the southern hemisphere, south of 40° latitude where there is little land, the isobars are very regular in their directions and nearly parallel to the parallels of latitude. North of this in the same hemisphere they are somewhat affected by the land, but the sea is still the predominant influence.
+
+In the northern hemisphere, however, the land and water are much more equally divided and here the effect of the land masses is at once apparent. In the winter the high-pressure areas and the low-temperature areas are found over the land, but in the summer, the low-pressureareas areas and the high-temperature areas are over the land. This illustrates what we have already learned; that land heats and cools much more rapidly than water and that hot air is lighter than cold air.
+
+
+
+
+
+In both summer and winter there is an area of comparatively high pressure on either side of the equator, but this area is not fixed; it moves north and south. In summer it is farthest north in the northern hemisphere.
+
+The winds are simply a transfer of air from a place where the pressure is high to a place where it is low, or a transfer of air along what are called barometric gradients from a high barometer to a low barometer. So the above-mentioned changes in the relation between the pressure on the land and on the sea must have an effect upon the directions of the winds. As a rule the wind blows out from the land interiors in the winter and into these interiors in the summer. It is thus seen that isotherms and isobars are closely related to each other, and that the wind is but a result of the atmospheric conditions which they represent.
+
+
+
+
+
+70. Wind.
+
+
+
+
+
+Experiment 77. —On a day when the temperature in the room is considerably higher than that outside, open a window at the top and bottom and hold a strip of tissue paper in front of the opening. Is there an air current, and if so, in what direction does it move at the top and at the bottom of the window? What causes "drafts" in a room?
+
+
+
+
+
+Experiment 78. —Procure two similar dishes about 15 cm. high and 5 or 6 cm. in diameter with short tubes of about 1 cm. in diameter opening out from near the top and bottom. Connect the bottom tubes of the two dishes with a tightly fitting rubber tube. Do the same with the top tubes. Place a Hoffman's screw upon each of the rubber tubes and screw it tight so that no liquid can flow through either tube. Fill one of the dishes with colored water and the other with kerosene or some light oil.
+
+
+
+Fig. 69.
+
+
+
+Although the two liquid columns are similar, yet the pressure at the bottom of the dish of water will be greater than that at the bottom of the dish of oil since the water is heavier than the oil. These are the conditions that exist on the surface of the earth at two places one of which has a high and the other a low barometer. Release the Hoffman's screw upon the top tube and then the one at the bottom. Notice carefully what happens as the lower tube is allowed to open. The dishes are not now filled with oil and water respectively. In the transfer of the liquids, through which tube did each pass? (If part of each rubber tube is replaced by a glass tube, the action in the experiment can be seen to better advantage.)
+
+
+
+
+
+Experiment 79. —Fill a convection apparatus with water, putting in a little sawdust and mixing it well with the water. Heat one side of the tube and observe the convection currents set up.
+
+
+
+
+
+Fig. 70.
+
+
+
+In Experiment 78 the interflow from one dish to the other is due to the fact that the water is heavier than the oil and runs under it and pushes it up so that the oil overflows into the dish that the water has left. The same thing happens in the atmosphere when from any cause the column of air above one place becomes heavier than that above another place. There will be under these conditions a transfer of air, along the surface, from the place where the pressure is greater to that where it is less great, and this movement of the air we call wind.
+
+The wind on the surface of the earth is not usually in the same direction as that high up. The strength of the wind depends upon differences in air pressures. As the air pressure is measured by the barometer, the wind is commonly spoken of as due to a difference in barometric pressure or to the barometric gradient. Winds are named from the direction from which they come. A west wind is a wind that blows from the west.
+
+If there were no other forces that affected the movement of the air, except the high and low pressures, the transfer would be in a straight line from one place to the other, and it could always be told in what direction the high and low pressures were, by direction of the wind. But obstacles like mountains and hills deflect the air currents. There are also other causes which influence the direction of the movement; chief among these is the rotation of the earth on its axis.
+
+71. Velocity and Effect of Wind Action. —The velocity of air movement varies from a gentle breeze which has not force enough to stir the leaves, to the terrific and almost irresistible blast of the tornado, which sometimes attains a velocity of a hundred miles an hour and sweeps everything before it. The velocity of ordinary wind is measured by an instrument called an anemometer, which usually consists of four aluminum cups attached by horizontal arms to a vertical spindle, the number of revolutions of which is recorded on a dial by a train of cogwheels geared to the spindle (Fig. 71).
+
+
+
+Fig. 71.
+
+
+
+When the wind has great velocity, it can be estimated only by the pressure which it exerts. A measure for the velocity of wind which needs no apparatus is given by Professor Hazen and is as follows:
+
+0. Calm.
+
+1. Light; just moving the leaves of trees.
+
+2. Moderate; moving branches.
+
+3. Brisk; swaying branches; blowing up dust.
+
+4. High; blowing up twigs from the ground, swaying whole trees.
+
+5. Gale; breaking small branches, loosening bricks on chimneys.
+
+6. Hurricane, or tornado; destroying everything in its path.
+
+Although the wind in its great paroxysms of rage is sometimes very destructive, it is ordinarily a most beneficent force. It is the circulatory medium for the earth; as the blood is for the animal and the sap for the plant. Without it the activities of the earth would stagnate. It spreads over the land the water evaporated from the sea. It cools the hot regions with the invigorating breath from the mountains and the uniformly tempered sea. It warms the cold places by bearing to them the heat taken from the warm ocean waters and the parched places of the earth. It bears man's commerce across the seas and by the power of the water which it has borne over the land, furnishes him the means for his manufacturing. It scatters the seeds over the fields and sweeps the smoke and foul air away from his cities.
+
+
+
+
+
+A DUTCH WINDMILL.
+
+Windmills are widely used to pump water.
+
+
+
+
+
+EFFECT OF WIND ON THE GROWTH OF TREES.
+
+The trees have grown in the direction in which the prevailing wind blows.
+
+
+
+72. The Effect of the Earth's Rotation on Winds.
+
+
+
+
+
+Experiment 80. —Revolve a globe from left to right and while it is revolving draw a piece of chalk from the pole toward the equator. Does the line as marked on the globe follow a meridian? What is its general direction in lower latitudes?
+
+
+
+
+
+The rotation of the earth affects the direction of movement of all bodies free to move over its surface. Thus if a current of air starts from the north pole to flow south, it will, as it goes along, tend to move toward the right, and so when it reaches middle latitude it is no longer moving south but southwest. Why this is so can be fairly well understood if the conditions of this moving body of air are considered.
+
+
+
+A SAILING VESSEL.
+
+Showing how the wind is used in commerce.
+
+
+
+As the earth is about 25,000 miles in circumference and turns on its axis once in 24 hours, a body situated at the equator is carried from west to east at the rate of about 1000 miles per hour, whereas a body at the poles simply turns around during a revolution. Thus as we go on the surface from the poles toward the equator, each point has an increasing west to east velocity.
+
+A body of air, not being attached to the surface, will have this west to cast velocity imparted to it very slowly by friction. Thus as it goes from higher to lower latitudes, it will lag behind particles on the surface which have this west to east velocity, and so will appear to have an east to west motion, just as a person sitting in a train that is just starting appears to be sitting still and the objects outside seem to move in the opposite direction. The combination of the north to south movement with the apparent east to west movement gives a northeast southwest direction to the air current.
+
+It can be proved mathematically that all freely moving bodies on the earth's surface are deflected toward the right in the northern hemisphere and toward the left in the southern hemisphere. This statement is called Ferrel's law.
+
+73. Planetary Wind Belts. —As the air at the equator receives a large amount of heat, it becomes warm and light, while that near the poles is cold and heavy. The air would thus have a constant tendency to move along the surface of the earth toward the equator and in an upper current from the equator toward the poles, just as in the dishes where water and oil were connected. But this direct movement is affected by the rotation of the earth and by certain atmospheric conditions so that between 25° and 35° both north and south of the equator there is an area of high pressure. These high-pressure areas can be seen on the isobar maps of January and July.
+
+From these areas of high pressure the surface currents move both toward the equator and toward the poles. On account of the earth's rotation the directions of these movements are not north and south but in the northern hemisphere northeast and southwest. Winds of this kind must occur on every revolving planet having an atmosphere; hence these winds are called planetary winds.
+
+As the rotation of the earth and the heating of the air near the equator are conditions that do not change, among the most permanent things about our planet are the belts into which the wind circulation is divided. The change in the position of the heat equator, —the belt of highest temperature, due to the apparent movement of the sun north and south, modifies the conditions in these wind belts during the year. The planetary winds thus modified are sometimes called terrestrial winds.
+
+74. Wind Belts of the Earth. —Near the heat equator where the air is rising there is a belt of calms and light breezes called the doldrums. As the air here is rising and cooling, thus having its capacity to hold moisture decreased, this is a cloudy rainy belt of high temperature in which much of the land is marshy and the vegetation so rank and luxuriant that agriculture is exceedingly difficult.
+
+Extending north and south of the doldrums to about 28° of latitude are belts in which constant winds blow toward the doldrum belt and supply the air for the upward current there. In the northern hemisphere these winds have a northeast to southwest direction and in the southern hemisphere a southeast to northwest direction. They are the most constant winds on the globe in their intensity and direction, and are called trade winds. Since they blow from a cold region to a warmer region, their power to hold moisture is constantly increasing and clouds and rains are not usual. The places where they blow are dry belts and in them are found the great dry deserts of the world.
+
+
+
+WIND BELTS OF THE EARTH.
+
+
+
+On the poleward sides of the trade-wind belts lie the areas of high pressure already referred to. These are called the horse latitudes or belts of tropical calms and are rather ill-defined. The air is here descending and the surface movements are light and irregular. These, like the doldrums, are regions of calms. But unlike the doldrums, they are dry belts, since the temperature of the descending air is increasing, owing to adiabatic heating (section 59), and thus its power to hold moisture is increasing. Therefore the tendency in these belts is to take up moisture rather than to deposit it.
+
+
+
+
+
+In the middle latitudes there is a belt of irregular winds which have a prevailing tendency to move from west to east or northeast. This general eastward drift of the air is constantly being interrupted by great rotary air movements having a diameter of from 500 to 1000 miles. These are called cyclones and anti-cyclones. In this region of the "westerlies," since the air tends to move from lower to higher latitudes, an abundance of moisture is usually supplied.
+
+In the anti-cyclone the air movement is slowly downward and outward from the center and in the cyclone it is inward toward the center, and upward. The center of the anti-cyclone is a place of clear sky and high pressure, while that of the cyclone is a place of cloudy sky and low pressure. The anti-cyclones, or high-pressure areas, have dry, cold, light winds, while those of the cyclones, or lowpressure areas, are usually strong and wet.
+
+75. Land and Water Winds. —As the land is much more rapidly heated by the rays of the sun than is the water, the land during the daytime becomes hotter than the water near it. On this account the cool air over the water flows in over the land and displaces the lighter warm air. Therefore near large bodies of water when the temperature is high there is often in the daytime a wind blowing from the water to the land. At night, as the land loses its heat more rapidly than the water, the wind blows in the opposite direction. These water winds temper the climate of the tropics near the coasts and also render seaside resorts popular in summer.
+
+76. Monsoons. —Over the interior of the great continent of Asia the temperature becomes so high in the summer months that the air above it is greatly expanded and decreases in weight. This causes a strong indraft from the colder ocean. The high temperature also brings the heat equator far north of the earth's equator and causes the southeast trade winds to cross the earth's equator. These swing to the right on the north side of the equator and proceed as southwest winds (Fig. 72), thus greatly strengthening the air movement toward the heated continental interior.
+
+
+
+Fig. 72.
+
+
+
+The winds, being heavily loaded with moisture from their passage over the tropical seas, are forced to rise when they come upon the high lands of India near the coast. There they become cooled and deposit a great amount of rain, making this southern part of Asia the place of greatest rainfall on the earth.
+
+In the winter, when the heat equator has moved south and when the continental interior has become exceedingly cold, there is a strong movement of air out from it toward the warmer ocean (Fig. 73). This strengthens the northeast trade winds over the Indian Ocean. It thus happens that near the southern coast of Asia there are strong seasonal winds that blow toward the northeast in summer and toward the southwest in winter. These winds are called monsoons. In the early sailing voyages to India they were very important, the trip to India being made so as to utilize the summer monsoons and that from India so as to utilize the winter monsoons. On this account these winds had much to do with the conquest of India by the nations of Europe.
+
+
+
+
+
+Fig. 73.
+
+
+
+77. Rainfall and its Measurement.
+
+
+
+
+
+Experiment 81. —Place a dish with vertical sides in a large open space so that the rim is horizontal and at a height of about one foot above the ground. Fasten the dish so that it cannot be overturned by the wind. After a rain, measure the water that has collected in the dish to the smallest fraction of an inch possible. This will be the amount of rainfall for this storm.
+
+
+
+
+
+The amount of rainfall during the year varies greatly in different places. It amounts to nothing or only a few inches over some regions, as in parts of Peru where rain falls only on an average of once in five years. But in the Khasi Hills region of India it has been known to be over 600 inches; and over 40 inches, or about the average yearly rainfall for the eastern United States, has been known to fall in 24 hours. This was in the season of the southwestern monsoons.
+
+The rainfall in different parts of the earth has been carefully measured and maps showing its average amount prepared. As agriculture is largely dependent upon the amount of rain and the season of the year in which it falls, these maps tell much about the relative productivity of different regions of the earth. An annual total of eighteen or more inches is necessary for agriculture; and this must be properly distributed throughout the year.
+
+
+
+RAINFALL MAP OF THE WORLD.
+
+
+
+On examining the map of the mean annual rainfall, we see that there are large areas where it is not sufficient for agriculture without irrigation. Such areas are within the belts of dry winds or in continental interiors far from large bodies of water. The rain-bearing winds coming from the water are forced to rise and cool so that their moisture is deposited before reaching these interior regions.
+
+The rainfall of a place depends largely: (1) upon its elevation, since most of the rain-bearing clouds lie at low altitudes; (2) upon the direction and kind of winds that blow over it; and (3) upon the elevation of the land about it. The sides of mountains toward the direction from which the rain-bearing winds approach will be well watered, while the opposite side may be a dry desert. Explain the cause of the dryness of five of the great dry regions as found on the map above.
+
+A cylindrical vessel having vertical sides, called a rain gauge (Fig. 74), is used to determine the amount of rain. It is placed in an open space away from all trees and buildings and after each rain the amount collected is measured. Snow is melted before it is measured. As a rule eight or ten inches of snow make an inch of rain.
+
+
+
+Fig. 74.
+
+
+
+If the temperature is below the freezing point, 32° F., when condensation takes place, the moisture of the air will form into a wonderful variety of beautiful six-rayed snowflakes. These float downward through the air and often cover the ground with thick layers of snow. Although snow is itself cold, yet it keeps in the heat of the ground which it covers, so that in cold regions soil which is snowcovered does not freeze as deeply as that without snow. Therefore, to keep water pipes from freezing, it is not necessary to bury them as deeply in localities where snow is abundant as in places equally cold where snow seldom falls.
+
+If raindrops become frozen into little balls in their passage through the air, they fall as hail. Hail usually occurs in summer and is probably caused by ascending currents of air carrying the raindrops to such a height that they are frozen and often mixed with snow before they fall. Sometimes hailstones are more than a half inch in diameter. They occasionally do great damage to crops and to the glass in buildings.
+
+Sleet is a mixture of snow and rain.
+
+78. Rainfall of the United States. —An examination of a rainfall map of the United States (following page) will show that the distribution of rainfall can readily be divided into four belts which, although gradually shading the one into the other, are yet quite distinct. These belts may be called the north Pacific slope, the south Pacific slope, the western interior region, and the eastern region.
+
+In the north Pacific coast region the storms of the "westerlies" are common, particularly in winter, when the westerly winds are strong and stormy. The yearly rainfall here amounts to about seventy inches.
+
+
+
+SALMON RIVER DAM, IDAHO.
+
+A typical irrigation dam in the United States.
+
+
+
+
+
+From central California south the rainfall of the Pacific slope decreases until, in southern California, there is almost no rain in summer and the entire rainfall for the year averages about 15 inches. By reference to the isobar map of the world (section 69) it will be seen that the high-pressure area of the dry tropical calm belt moves sufficiently far north in summer to take this region out of the influence of the wet westerlies and into that of the drier belt.
+
+The western interior region, extending from the Cascade and Sierra Nevada mountains to about the 100th meridian, is dry over the larger part of its surface, since the winds have deposited most of their moisture in passing over the mountains to the west. On the mountains and high plateaus, however, there is a considerable fall of rain, as the winds are cooled sufficiently in passing over these to deposit their remaining moisture. In most of this region, as in southern California, irrigation must be resorted to if agriculture is to succeed. The fall of rain on the mountains and high plateaus supplies rivers of sufficient size to furnish water for extensive irrigation, and so a considerable part of the area which is now practically a desert will in the future be reclaimed for the use of man. The government is at present engaged in extensive irrigation work in this territory.
+
+
+
+EAST END OF THE ASSUAN DAM ACROSS THE NILE.
+
+The greatest irrigation dam in the world.
+
+
+
+From about the 100th meridian to the Atlantic Ocean there is a varying rainfall, but it is as a rule sufficient for the needs of agriculture. It gradually increases toward the east, moisture being supplied plentifully from the Gulf of Mexico and the Atlantic Ocean by the southerly and easterly winds. The rainfall is well distributed throughout the year and averages from thirty to sixty inches.
+
+
+
+
+
+79. Electricity.
+
+
+
+
+
+Experiment 82. —Place some small pieces of paper or pith balls on a table and after rubbing a glass rod with silk bring it near the pieces. Do the same with a stick of sealing wax or a hard-rubber rod rubbed with flannel or a cat's skin. Note the action of the pieces.
+
+
+
+
+
+Experiment 83. —Rub a glass rod briskly with silk and place in a wire sling such as was used in Experiment 12. Bring toward one end of the glass rod another glass rod which has been rubbed with silk. Do the rods attract or repel each other? Bring toward the suspended rod a piece of sealing wax or a vulcanite rod which has been rubbed with flannel or a cat's skin. Does this repel or attract the glass rod?
+
+
+
+
+
+Experiment 84. —Suspend a pith ball by a silk thread from the ring of a ring stand. Rub a glass rod with a piece of silk and bring it near the pith ball but do not allow the two to touch. Note the action of the ball. Touch the pith ball with the rod. Does it behave now as it did before? Rub a vulcanite rod with a piece of flannel or cat's skin and bring it near a suspended pith ball. Does the pith ball act as it did with the glass rod? Touch the pith ball with the rod. How does it act? Bring a glass rod rubbed with silk near a pith ball which has been in contact with a vulcanite rod after it was rubbed with flannel or a cat's skin. Does the glass rod repel or attract the ball?
+
+
+
+
+
+Experiment 85. —Suspend a pith ball from the ring of a ring stand by a very fine piece of copper wire no larger than a thread. Wrap the wire around the pith ball in several directions. Bring a rubbed glass rod toward the pith ball. Does it act as it did when suspended by silk? Allow the ball to touch the rod. Does the ball now act as it did when suspended by silk? Try these same experiments, using the vulcanite rod.
+
+
+
+
+
+It was known by the ancient Greeks that when certain substances, one of which was amber, were rubbed, they had the power of attracting light objects. This property was afterward called electricity, from the Greek word for amber. From the previous experiments it has been seen that when glass is rubbed with silk, and vulcanite with flannel or a cat's skin, they seem to have two different kinds of electrical charges. The like kinds repel each other and the opposite kinds attract. These two kinds are called positive and negative respectively.
+
+Whether there are really two kinds of electricity has not yet been fully determined, but electricity acts exactly as it would if there were two kinds, and it has become customary to speak as if there were. In Experiment 84 it was found that pith balls suspended by a silk thread could be charged with electricity if brought in contact with a charged body. Experiment 85 showed that this was not possible when they were suspended by a copper wire. The wire conducted the electricity away. Substances like copper that conduct electricity are called conductors, and those substances like silk which will not conduct it, nonconductors.
+
+
+
+
+
+Experiment 86. —Having started the electrical action in a static electrical machine (Fig. 75), pull the knobs as far apart as the spark will jump and notice the course taken by the spark. Does it travel in a straight line? Hold a piece of cardboard between the knobs so that its edge is just within the line joining them. What effect does the cardboard have upon the direction taken by the spark? Place the cardboard so that it entirely covers one of the knobs. Is the spark able to pass through the card? Attach a wire with a sharp point to each of the knobs and extend it vertically two or three inches above the knob. Start the machine. Do sparks now jump across between the knobs? Why are houses provided with lightning rods?
+
+
+
+
+
+Fig. 75.
+
+
+
+
+
+A FLASH OF LIGHTNING.
+
+Showing it takes different paths of least resistance.
+
+
+
+About the middle of the eighteenth century, Benjamin Franklin proved by his notable kite experiment that lightning was simply an electrical discharge between the clouds and the earth, or between different clouds. This discharge is similar to that which takes place on an electrical machine. The electricity in the clouds attracts as close as possible the opposite kind of electricity on the earth's surface and tends to hold it accumulated on high objects. If the attraction is sufficient, the electricity discharges between the cloud and the object, and we say the object was struck by lightning.
+
+If a sharp point, such as a lightning rod, is present on the object where the electricity tends to accumulate, it allows the electricity to pass off gradually before enough accumulates to cause damage. Lightning rods, however, must be continuous conductors and properly terminated in the ground.
+
+80. Thunder-storms. —Often on a hot, sultry summer afternoon large cumulus clouds are seen to rise and spread out till they cover the sky. The wind soon begins to blow quite strongly toward the cloud-covered area, the clouds moving in a direction opposite to the surface wind. As the storm clouds approach, a violent blast of wind, often called the thunder squall, blows out from the front of the storm. Soon flashes of lightning appear and thunder is heard. As the storm comes nearer, the rain begins to descend and for a short time, usually about half an hour, it rains heavily. Then the clouds roll away and the sky becomes clear with perhaps a rainbow to heighten the beauty of the clearing landscape.
+
+
+
+TREE COMPLETELY SHATTERED BY A STROKE OF LIGHTNING.
+
+
+
+Thunder-storms are caused by hot moist air rising over certain areas and causing an updraft, which is increased by the inflow and upward movement of air from the surrounding regions. The condensation of the moisture in the rising air quickly forms clouds, and these become charged with electricity. As the electrical charge increases, discharges take place which cause lightning flashes. These discharges occur along the lines of least resistance and are often very irregular and forked. As tall objects are likely to offer good paths for the discharge, it is safest to keep away from trees and walls during a thunder-storm.
+
+The air becomes greatly agitated by the lightning discharges and makes us aware of this by the noise of the thunder, just as the agitation of the air caused by the discharge of a gun is made apparent to us by what we call the noise of the report. Since sound travels at about the rate of a mile in five seconds and the lightning discharge is practically instantaneous, the noise from different parts of the discharge will reach us at different times and to this and the echoing from clouds or hills is due the roll of the thunder. The distance of the flash can be told approximately by dividing the number of seconds between seeing the flash and hearing the thunder by five.
+
+
+
+THUNDER-STORM CLOUDS.
+
+
+
+Frequently in the evening flashes called heat lightning are seen near the horizon. These are due to the reflection on clouds of flashes of lightning in a storm which is below the horizon. Thunder-storms occur sometimes in winter. They are very prevalent in the tropics.
+
+
+
+
+
+81. Electrical Communication.
+
+
+
+
+
+Experiment 87. —Attach one end of a wire to a pole of a dry cell and the other end to one of the binding posts of a telegraphic sounder. From the other binding post of the sounder lead a wire to the binding post of a telegraphic key. Connect the free binding post of the key with the free pole of the battery (Fig. 76). When the key is pushed down, the circuit is closed and the sounder clicks. If a relay can be procured, remove the sounder and connect two of the binding posts of the relay in the same way that the sounder was connected.
+
+
+
+Fig. 76.
+
+
+
+Connect one of the free binding posts of the relay with a binding post of the sounder and the other binding post with the pole of a dry cell. Connect the other pole of the dry cell with the free binding post of the sounder. When the key closes the circuit through the relay, the circuit through the sounder and its dry cell is closed by the relay (Fig. 77) and the sounder clicks. This is the usual arrangement in a simple telegraph office. The sounder in the first part of the above experiment can be replaced by an electric bell (Fig. 78) and the key by a push button, thus showing the arrangement of the ordinary doorbell.
+
+
+
+
+
+Fig. 77.
+
+
+
+
+
+Fig. 78.
+
+
+
+Electricity can be developed by chemical action as well as by friction, and many different kinds of electrical cells have been invented. The most simple of these is a sheet of copper and a sheet of zinc placed so that they do not touch and put in a dish containing dilute sulphuric acid (Fig. 79). The current developed by this cell is very weak. At the present time dry cells are used for almost all ordinary purposes in which electric batteries are needed.
+
+
+
+Fig. 79.
+
+
+
+The history of the development of our knowledge of primary cells and current electricity is exceedingly interesting and important, but it cannot be dwelt upon here. In 1832 an American, Samuel F. B. Morse, invented the commercial telegraph. This was the first step in the wonderful progress that has been made during the last century in communicating rapidly between distant points. The necessary instruments used in this form of communication are a sounder (Fig. 80) and a key (Fig. 81). The sounder is simply an electro-magnet such as was made in Experiment 14, arranged to attract a piece of soft iron held at a short distance from it by a spring. When this piece of iron is attracted toward the magnet, it strikes on another piece of iron, making a click, and so remains drawn to the magnet as long as the circuit is kept closed. Thus long and short clicks can be made. Morse arranged a combination of these long and short clicks to represent the alphabet. Thus he was able to send words from one station to another. Experiment 87 illustrates how a simple telegraph can be arranged.
+
+
+
+Fig. 80.
+
+
+
+
+
+Fig. 81.
+
+
+
+Many improvements have been made since Morse first sent a dispatch between Washington and Baltimore, but his dot-and-dash alphabet and the electro-magnet sounder and the key are still in use. Since 1832, the land has been strung with telegraph wires and the ocean girdled with cables, and now an important event occurring in any part of the earth is known almost instantly in all other parts. The telephone, the wireless telegraph and the wireless telephone, all electrical devices, have added to the ease of communication so that the whole earth is brought into such close relation that every part knows what all the other parts are doing. No other form of energy which man has discovered is of such diversified usefulness as electricity.
+
+
+
+WIRELESS TELEGRAPH STATION, LOS ANGELES.
+
+
+
+82. Tornadoes and Waterspouts. —Sometimes causes like those which produce a thunder-storm are so strongly developed that the indraft is exceedingly violent and a furious whirling motion is produced. Such storms are called tornadoes. The warm moist air rises rapidly and spreads out into a funnel-shaped cloud with the vertex hanging toward the earth. In the center of the whirl the air pressure is much diminished and the velocity of the inrushing whirling wind is tremendous, being often sufficient to demolish all obstacles in its path.
+
+
+
+A TORNADO.
+
+Notice the funnel-shaped cloud.
+
+
+
+The length of the path swept over by a tornado is rarely over thirty or forty miles and the width generally less than a quarter of a mile. The rate of progress in the Mississippi valley is from 20 to 50 miles an hour, usually in a northeasterly direction. These storms are often wrongly called cyclones. When storms of this kind occur at sea, a water column is formed in the funnel-shaped part of the storm and they then receive the name of waterspouts.
+
+83. Cyclones. —In the belt of westerly winds are found, as has already been noted, large storm areas called cyclones. As the barometric pressure in the center of these areas is lower than that of the surrounding region, they are marked "Low" on the weather maps. Into these lowpressure areas the air from all directions is moving, but on account of the deflection due to the rotation of the earth, the wind does not blow directly into them, but produces great whirls in which the air moves spirally inward and upward.
+
+The rate at which the wind blows varies in different parts of the whirl, but is never very great. In the northern hemisphere the rotary movement is in the direction opposite to that in which the hands of a watch move, while in the southern hemisphere it is with the hands of a watch. As these are areas of ascending air, they are storm areas. The extent of the precipitation varies in different parts of an area according to the direction from which the ascending air has come. Note the direction of the wind and the rainfall area as shown on the map (section 85).
+
+
+
+THE EFFECTS OF A TORNADO.
+
+
+
+Air which comes from the continental interiors is dry, while that from the oceans contains much moisture, some of which it deposits when made to ascend. These whirls move in a general eastward direction with varying velocities, but averaging about 20 or 30 miles per hour. To these is due the larger part of the rain which falls in middle latitudes. Areas of high pressure in which the air moves spirally downward and outward from the center as has already been stated (section 74) are called anti-cyclones. These are areas of dry, cold weather.
+
+
+
+REMAINS OF FARM BUILDINGS DESTROYED BY A TORNADO.
+
+
+
+84. Paths of Cyclonic Storms across the United States. —The map on secton 85 shows the paths of a large number of cyclonic storms across the United States. It will be seen from this that although these paths vary considerably, yet the general direction is a little north of east. The movement of the cyclone is in the direction of the prevailing winds of the middle latitudes.
+
+In summer time the average rate of motion of the cyclone across the continent is about 500 miles per day, while in winter it is 800. The velocity of the wind in the cyclone is also much less in summer than in winter, as the difference in pressure between the low and high areas is much less. The changes in temperature as the storms pass are greater in winter than in summer since the regions from which the northerly and southerly winds flow in toward the center of low pressure vary more in their temperatures.
+
+
+
+WATERSPOUT SEEN OFF THE COAST OF NEW ENGLAND.
+
+
+
+85. Sudden Weather Changes. —In middle latitudes there often occur, particularly in winter, sudden changes in the temperature of 20° or more in a few hours. In our own country, if the temperature falls 20° or more in 24 hours, reaching a point lower than 32° F. in the north or lower than 40° in the south it is known technically as a cold wave, and there is a special flag (Fig. 82) displayed by the Weather Bureau to indicate the approach of such a change.
+
+
+
+Fig. 82.
+
+
+
+When these waves extend over the southern part of the country, they are very destructive to the orange groves and delicate crops and are known as "freezes." A notable freeze of this kind occurred in 1886 and did tremendous damage to the orange groves of Florida. So great was the effect upon this important industry throughout the orange belt that for years afterward the "freeze" was the date from which events were reckoned.
+
+If the northwesterly wind which brings on the cold wave is accompanied by snow, it is called a blizzard, and on the plains and prairies, where the wind has a clear sweep, it is much dreaded. Cattle and men, when caught in it, frequently perish. In southern Europe the coldest winds are from the Siberian plains and are therefore northeasters. In the United States the cold area is at the southwest and rear of the cyclone, whereas in Europe it is at the north and front.
+
+
+
+CYCLONES AND ANTI-CYCLONES.
+
+
+
+
+
+When, instead of the strong, cold northwest winds which blow into the rear of a cyclonic area and in the colder seasons may produce a cold wave, there is a prolonged movement of highly heated air from the south into the front of the low pressure, as sometimes occurs during the warm months, the "hot spells of summer" are caused. The air is sultry, exceedingly hot and oppressive. Sunstrokes and prostrations from heat are common. The "hot winds" of Texas and Kansas, the Santa Ana of lower California and the siroccos of southern Italy are intensified examples of these winds. All sudden weather changes of this kind are due to atmospheric conditions related to areas of low pressure.
+
+86. Weather Forecasting. —The data necessary for forecasting the weather are telegraphed to the Weather Bureau Stations every day, and a record of them placed on the weather map. The observations recorded on these maps furnish the forecasters with all the information obtainable as to what the weather of the future is to be. It has already been stated that the dominant cause of our weather conditions is the eastward movement of cyclones and anti-cyclones.
+
+If the direction and rate of motion of these can be determined the weather of those places which are likely to come under their influence can be foretold with a good deal of accuracy. If a cyclone were central over the lower Mississippi valley with an anti-cyclone to the west of it, we should expect that the southerly and southeasterly winds and rains to the east and southeast of the Mississippi would gradually change to fair weather and westerly winds with increasing cold, as the cyclonic area was replaced by the anti-cyclonic.
+
+The rate at which the change would take place would depend upon the rapidity of the movements of the two areas of high and low pressure, and the order of change in the direction of the winds would depend, for any place, upon the directions taken by the centers of these areas. The direction of movement and the rapidity of movement of the cyclonic areas are, therefore, two of the chief factors which enter into the prediction of the weather. There is usually an increase in the intensity of the storm as the Atlantic coast is approached.
+
+87. Climate. —The average succession of weather changes throughout the year, considered for a long period of years, constitutes the climate. Thus, if the average temperature of a place throughout the year has for a long period been found to be high, and the rainfall large and uniformly distributed, the place is said to have a hot and humid climate. The climate is a generalized statement of the weather. Two places may have the same average temperature throughout the year without having the same climate, as in one the temperature may be quite uniform and in the other very high at one season and very low at another. Many factors enter into the making up of a comprehensive statement of climate.
+
+88. Effect of Climate upon Animals and Plants. —Plants are greatly affected by climate. The ornamental palm and orange trees, which are sometimes cultivated in the north, have to be protected from the winter cold with great care, whereas in southern climates they grow and flourish as the apples and pears do in the north. Corn and wheat are the staple agricultural products of the northern part of the United States, while cotton, rice and oranges are of the southern part.
+
+
+
+A SOUTHERN COTTON FIELD.
+
+
+
+If plants are to flourish, the heat and cold and amount of moisture must be such that the seeds can ripen and find suitable conditions for preservation and growth in succeeding years. Plants like the cactus and the Yucca Palm which thrive in the dry and desert regions of New Mexico and Arizona would soon die in the moist climate of Louisiana.
+
+As animals live upon plants or upon other animals, the plants must supply the food of the plant-eating animals and through these of the animal eaters. Thus, the distribution of plants has a great effect upon the animal life. Animals that eat grass will not live in a desert, neither will animals that eat nuts live in a prairie, where there are no nut-bearing plants.
+
+
+
+YUCCA PALM.
+
+
+
+Temperature and moisture also affect animals as well as plants, although animals can hide away from the scorching sun and move about for water as plants cannot. An animal like the polar bear, whose coat has become thick to protect him from great cold, would soon pine away and die, if transferred to the jungles of Africa, where his fellow flesheater, the lion, revels in joyful existence. To the camel of the desert the damp, grassy savannas would be, indeed, a dreary waste and verdant cemetery. Thus, when once plants and animals have become adapted to certain climatic conditions, they cannot flourish if placed under very different conditions.
+
+
+
+CAMEL.
+
+
+
+89. Effect of Climate upon Man. —Since man can change his out er covering of clothes whenever he desires and is able to carry with him and store for long periods his necessary food, and by artificial means raise or even lower the temperature of the space in which he lives, he is not nearly so dependent upon climate as are either plants or animals.
+
+The same man can, for a time, live in arctic regions or in the tropics. Men can, moreover, by centuries of effort become accustomed to the climate of almost any part of the globe. The Laplander and the South Sea Islander both flourish in their adopted homes. Neither of these, however, has attained to the highest development of which man is capable. The rigorous severity of the climate saps the energies of one and its uniform geniality lulls the ambition of the other.
+
+
+
+A LAPLANDER.
+
+
+
+In temperate latitudes, where there is need of providing for the winter when plants do not grow and when food is hard to find, where the blood is stirred by the invigorating cold, and where nature in her ever changing mood gives zest to living, is the place where man has attained his highest achievements. Here the fight for existence does not require all man's energy, and the bounty of nature does not free him from strenuous effort. Thus it is seen that even upon man the influence of climate is great. How great, it is impossible fully to realize, so complex are his relations.
+
+
+
+
+
+Summary. —The atmosphere is just as important to life upon the earth as are energy, light, heat, water and land. Air contains oxygen from which we get heat and energy, carbon dioxide, from which plants build up their tissues, and nitrogen which dilutes these two.
+
+The weight of air is not usually realized because it presses uniformly in all directions. Air expands when heated; so a cubic foot of warm air weighs less than a cubic foot of cold air. Warm air will also hold more moisture than cold air.
+
+The pressure of air, due to its weight, may be measured by a barometer. The heights of mountains may also be measured by this instrument, as there is less air above a high mountain than above a low one. The winds are caused by changes in atmospheric pressure; their prevailing direction is affected by the earth's rotation. Certain winds common to all Planets are called planetary winds; when modified by certain peculiarities of the earth they are called terrestrial winds. Because of their constancy and their aid to traffic, some of these winds are called trade winds. South of Asia there are winds called monsoons.
+
+
+
+A SOUTH SEA ISLANDER.
+
+
+
+When very moist air cools, it cannot hold as much moisture as when it is warm, so this falls as rain, hail, sleet or snow. The rainfall varies from nothing at all in some places to over fifty feet a year in others. In the United States the north Pacific slope has a rainfall of about seventy inches a year; the south Pacific slope about fifteen inches; the eastern slope of the Rockies is very dry; and the Mississippi valley and the country to the east of it have a rainfall of from thirty to sixty inches.
+
+Rainstorms when accompanied by thunder and lightning are called thunder-storms. Thunder and lightning are caused by certain clouds having a higher charge of electricity than others. The higher charge bursts across to the lower charge, making a flash of lightning and a roll of thunder.
+
+When the wind blows spirally and with great violence, sweeping everything before it, it is called a tornado. This is popularly known as a cyclone, but, properly speaking, a cyclone is a very large circular storm. Real cyclones usually do no damage.
+
+All these storms have a marked effect upon the weather, the changes of which are forecast by the weather bureau. The general weather conditions of a place determine its climate. The climate of any place has a great effect upon plants, animals and man.
+
+
+
+
+
+QUESTIONS
+
+
+What are the characteristics and principal uses of the three most abundant gases in the atmosphere?
+
+How can it be shown that air has weight and exerts pressure?
+
+What effect has heat upon the weight and volume of air?
+
+What effect has pressure upon the weight and volume of air?
+
+How do the two kinds of barometer ordinarily used differ in construction?
+
+Where have you ever observed the effects of adiabatic heating?
+
+What experiences have you ever had which show that hot air will hold more moisture than cold air?
+
+How are the light and heat rays from the sun affected by the atmosphere?
+
+Name and explain the chief causes that affect the temperature of a place.
+
+What is the cause of wind and how is its velocity measured?
+
+How are the winds influenced by the earth's rotation?
+
+In going from Boston to Cape Horn through what wind belts would a sailing vessel pass and how would her progress be affected by the winds in these belts? What weather conditions would she probably encounter?
+
+At what season of the year would a steam vessel equipped with sails make the best time to India by way of the Suez canal? Why?
+
+Upon what does the rainfall of a place largely depend?
+
+How is the rainfall of the United States distributed?
+
+What is the relation between lightning and electricity? What is thunder?
+
+With what electrical devices are you familiar?
+
+What are the principal differences between a tornado and a cyclone?
+
+What are the chief effects of climate upon plants and animals?
+
+State the climatic conditions which are best for man's development.
+
+
+
+
+
+【中文阅读】
+
+
+50．大气层的起源——据推断，地球最早形成之时处于极端高温的状态，然后慢慢冷却。在其初期，物质还没有完全化合成液态和固态，或者也有可能许多炽热气体需要较低的温度才能化合在一起形成其他物质，因此便在内部的固态核心周围留下了一个气态物质层。这一气态圈层所包含的物质就是我们今天所讲的大气层。这其中有一些气体有明显的化学惰性，并不与其他物质发生化合反应。其他的气体则形成了种类繁多的化合物，但由于它们数量巨大，故也并没有因发生这类化学反应而消耗殆尽。
+
+51．空气的成分
+
+
+
+
+
+实验57：（由老师操作完成）在一个小软木塞上旋出一个小孔洞，将整个软木塞表面用熟石膏敷裹一遍，干定后让它浮在一盆水中，小孔洞朝上。在小孔洞中放置豌豆大小的一小片磷，小心地将它点燃。（操作时必须特别小心，因为磷的燃点很低，但燃烧起来却十分激烈，因此磷块必须切割开，且必须在水中操作）
+
+磷被点燃之后，在软木塞上罩一个广口瓶，让瓶口轻轻保持在水面以下。一会儿会发现水将慢慢升进瓶中，磷也会停止燃烧，瓶中会形成白烟，然后又慢慢消失。最终瓶子里会剩下透明的气体，但这不是空气，因为如果是的话，磷便会继续燃烧。要不是这一特性，瓶中的气体和空气还真不好区分。
+
+瓶中剩下的气体大致占到四分之三多一点的空间，这说明空气中有四分之三多一点的气体成分是不支持物体燃烧的，这种气体叫做氮气。由于空气无色透明，因此除了氮气以外的其他成分的气体也必然是无色透明的。其中最重要的一种便是氧气。实验中的磷片就是与它混合后才产生了白烟，这种白烟可溶解在水中，因此瓶中便只剩下了氮气。
+
+实验做完之后，必须小心处置软木塞上没有燃完的磷，务必将其放在不能燃烧的环境中。
+
+
+
+
+
+空气看起来似乎只是一种非常简单的气体，且人们也就是一直这么认为的。直到18世纪晚期，才发现空气是由几种不同的气体组成的。其中有一种便是氧气，它支持燃烧；还有一种是氮气，它既不能燃烧也不能支持其他物体燃烧。化学家们研究发现，空气中氧气和氮气所占的比例大致为一比四。
+
+另一种较重的无色气体叫做二氧化碳，大约占到空气总量的万分之三。除此之外，空气中还有其他几种含量极其稀少的气体，但其重要性不大，我们也就不在此细说了。除了气体，空气中还含有其他物质，比如水蒸气、粉尘颗粒以及细菌微生物等等。
+
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+
+
+
+实验58：到化学实验室取4瓶氧气。如果没有这个条件的话，可以取指头大小的一块过氧化钠，放入一个集气试管中，瓶塞上插入一只医���滴管，将集气导管与一个倒置于集气水槽隔板上装满水的集气瓶相连，缓慢地向试管中的过氧化钠上面滴水，这样便可收集到数瓶氧气。还可以通过在试管中加热二氧化锰和氯化钾的混合物来收集氧气，二氧化锰与氯化钾的比例为一比二，同样通过集气水槽装置收集。从外观上看，氧气与空气有区别吗？它有气味吗？在一瓶氧气中放入一根末端有火星的小木条，会看见木条进入瓶中后立即燃起了火焰，而如果将它放入一瓶空气中，会发生同样的现象吗？
+
+
+
+
+
+拿一根点燃的火柴放在另一瓶氧气的瓶口，瓶中的气体会自己像发光气体那样燃烧起来吗？如果空气中的氧气含量发生变化，它将对燃烧现象带来极大影响。在一段金属丝末端粘少许硫磺，点燃后插入氧气瓶中，硫磺会很强烈地燃烧吗？金属丝呢，也会燃烧吗？
+
+对动物而言，氧气是空气中最重要的气体，没有氧气，动物便不能生存。它们的呼吸离不了氧气，无时无刻不在吸入氧气，然后呼出二氧化碳。它们所有的热量与能量，都来自于碳氧结合生成二氧化碳的过程中所释放的能量。植物，也是需要氧气的。
+
+植物对二氧化碳的依赖，就像动物对氧气的依赖一样。几乎所有植物都是由碳元素构成，而这些碳元素就是从二氧化碳中得来的。植物的生长发育，就是依赖于分解二氧化碳所获取的能量，在这个过程中，阳光会起到重要作用，分解后的碳元素便也就植入了植物体的自身结构当中。这之后，植物又会把氧气排放到空气中，让动植物再一次利用。
+
+
+
+
+
+实验59：到化学实验室取几瓶二氧化碳，或者也可以通过向装有石灰石的瓶子里注入稀盐酸，然后用集气装置收集。从外观上看，它与空气有区别吗？取一瓶二氧化碳在水中倒置一段时间，然后闻一闻，会发现它没有气味。将一根点燃的火柴投入装有二氧化碳的瓶中，会发生什么现象？瓶中的气体会燃烧吗，或者会支持燃烧吗？在一支燃烧的蜡烛上面慢慢倾倒一瓶二氧化碳气体，蜡烛会慢慢熄灭。当瓶子倾斜的时候，瓶中的二氧化碳气体会倾倒出来，这说明它比空气更重。如果空气中的二氧化碳含量大幅增高，将会对物体的燃烧产生什么影响？
+
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+
+动物在二氧化碳气体中会窒息而死。煤矿工人对窒息性气体都很熟悉，因为他们都有逃离瓦斯爆炸的经历，那些气体是可以导致人窒息的。这在一些很少的特殊地区也有发生，比如那不勒斯附近的犬穴，以及黄石国家公园里的死亡之谷，动物经过此地时，大量的窒息性气体对它们非常致命。
+
+尽管如此，空气中氮气、氧气、二氧化碳三者之间的比例在整个地球表面几乎都是一样的，只在一些极个别的地方，由于特殊原因，排放的二氧化碳由于没有接触流动的空气而逐渐积累，便会导致二氧化碳过量。人和动物的呼吸都会产生二氧化碳气体，因此我们所在的房间应该保持良好的通风。
+
+氮气对氧气有稀释作用，而这，恰恰也是必不可少的。如果氧气没被氮气稀释的话，动物同样不能生存，而且一旦有火焰燃烧，它就差不多会像火焰烧毁木头那样，一发不可收拾地将钢铁都化为灰烬。前面我们也提到过，某些细菌也会利用空气中的氮气，生成植物所需要的氮氧化合物，以作为植物生长发育的养分。
+
+动物和植物也都需要水蒸气。因为要是没有以这种形态而存在的水分，也就不会有雨露霜雪，而如果没有雨水，生命同样也不会存在。这样看来，大自然的空气为动植物准备了氧气和水蒸气，为植物准备了二氧化碳，又让氮气稀释了氧气，正是这一伟大的好生之德，让生命得以孕育。
+
+52．空气的重量
+
+
+
+
+
+实验60：取一个容积为5品脱的瓶子，用橡胶塞塞紧瓶口，然后通过瓶塞插入一根玻璃管，玻璃管的外端紧密连接一根厚壁橡胶管，橡胶管的另一端接一个抽气泵。在橡胶管上安一个螺旋夹，并让整个装置的所有接口都保持密闭不漏气。现在先称量整个装置的重量，然后用抽气泵开始对瓶子抽气，当所有空气抽尽以后，紧紧地关闭螺旋夹，再次对装置进行称量。然后再松开螺旋夹，让空气又回流到瓶中，这时装置的总重量应该与最初的重量是一致的。或者简单一点的话，也可以称量一只白炽灯灯管，然后用一支吹风管在上面刺出一个小孔，让内外空气流通，再次进行称量，结果与刚才一样吗？
+
+
+
+
+
+通过刚才的实验，我们知道空气也有重量。但用这些装置，要想精确测量抽取空气的重量，或者测定一定体积空气的重量，都是不可能的。通过真正精��的称量，我们发现，在正常情况下，零海拔地区一升的空气，重量为1.2克，12立方英尺的空气重量大致就有1磅重。而在很高的高空，空气即使很轻，但由于它巨大的体积，它的重量依然不可小觑。
+
+现在我们已经发现空气有重量，自然也能明白为什么热气球之类的较轻的物体也可以像木棍浮在水上一样，飘浮在空气中。空气的重量与压力和温度有很大关系，后面我们还会细讲。
+
+53．空气的受热膨胀——空气受热后会膨胀得非常显著，我们在实验17中已经见识过了。研究发现，如果将空气从水结冰的温度加热到水的沸点温度，其体积会膨胀4/11。空气膨胀的力量也极其强大，因此在一些房屋建筑物着火时，要是没有空气逃逸的通路，墙面和屋顶就会被受热空气的巨大膨胀力量炸开撕裂，并对救火人员造成严重伤害。空气受热膨胀的现象也可以从玩具气球的一些现象上看出来，当它从寒冷的屋外进入到温暖的房间里面后，气球表面会立刻绷紧，整个气球也立刻膨胀得胀鼓鼓的。
+
+54．冷热变化对空气重量的影响
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+
+
+
+实验61：取两只同样的烧瓶，挂在杠杆秤两端并处于平衡状态，尽量保持两个烧瓶都瓶口朝上垂直悬挂。用一架本生煤气灯对一只烧瓶加热，让受热空气能膨胀上升并慢慢通过瓶口逸出，注意不要让热空气向下逸出。会发生什么现象？
+
+
+
+
+
+我们通过空气受热膨胀这一现象，完全可以推论出热空气比冷空气轻这一结论，刚才的试验结果也完全为我们证实了这一点。在平常气压下，1升空气在0摄氏度的重量为1.293克，但在100摄氏度时，其重量变得只有0.946克。因此一定体积的冷空气相对同样体积的热空气更重，自然也会比热空气施加给地球表面的气压更大。
+
+由于空气的分子可以自由移动，我们因此可以猜想，一股冷空气形成后会逐渐下沉，遇到固体表面后会散布开来，并最终停留在较轻的空气层下面。就像一股水流，在撤掉支撑物后会倾泻而下，并最终将支撑物表面空气排开而占据其位置。
+
+将水倾倒入油液中也会看到类似的现象，水会下沉到油的底部，把原来在底部的油排挤到上面来。类似地，如果空气在一个地方受热，必将产生一股上升的热空气气流，然后一股较冷的空气会随之而来，占据热空气先前的位置。地球上的风，就是由于空气的这个特性而形成的。也正是由于空气受热后会上升这一原理，暖气炉才能发挥作用，让房间可以升温而且保暖。山谷一般都比周围的山坡要寒冷一些，因此精细作物在山坡上都长势良好，而在山谷中则可能被霜冻戕害。
+
+55．气压
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+
+
+
+
+实验62：取一个对流装置，如果办不到，取一只粉笔盒，在顶部钻出两个圆孔，圆孔不要超过普通玻璃灯管横截面大小，放一只点燃的小蜡烛在一个圆孔的中心，然后取两支玻璃灯管，紧紧地卡进这两个圆孔中。现在拿一张冒烟的纸，放在没有蜡烛的灯管上方（如果有一张玻璃片嵌在盒子的一面上的话，观察效果会更好）。会发生什么现象？把蜡烛取出来，用本生灯小心地加热灯管，产生的现象跟刚才一样吗？为什么会有火花从火焰上飘升起来？这个暖炉模型实验说明了什么？为什么让房间通风最好的办法是开天窗和落地窗？
+
+实验63：取一个盖子上有螺丝旋钮的罐子，在里面放少量的水，将其煮沸，保持螺丝旋钮是打开的，让水蒸气可以自由冒出。当水在强烈沸腾的时候，突然将热量排开旋紧螺帽，注意不要在散热之前就锁死了旋钮。将罐子放到桌上仔细观察，水蒸气凝结时会发生什么现象？为什么？
+
+实验64：通过抽气泵将一对马德堡半球中的空气抽空，现在用力向外拉两个半球，会发现远不如刚才里面有空气时容易拉开，为什么会这样？
+
+实验65：给一只平底玻璃杯装满水，然后在上面平放一张信纸，让信纸与杯沿刚好紧密贴合。现在握住杯子的底部，小心地将杯子连同纸片翻转倒置在一个平底锅里面。水会流出来吗？如果不会的话，这是为什么呢？这时再拿一支铅笔，将笔尖伸进杯沿和纸片之间，会发生什么？
+
+
+
+
+
+一切有重量的物体都会对其支撑面施加压力，既然我们已经知道空气也有重量，那它也必将对地球表面施加压力。由于空气是一种气体，其特点是，组成空气的分子可以相互间自由移动，因此它所施加的压力在各个方向上都会显得非常均衡。似乎没有人感觉到空气压力的存在，其实这是因为空气除了在我们周围，同时也在我们身体内部，两者刚好抵消了。据测量，在海平面，气压的大小为15磅/平方英寸。假设有两个鸡蛋壳，里面是中空的，一个在壳上钻一个小孔，一个完全密封，现在让这两个鸡蛋壳都沉入水下，哪个会被压碎哪个不会？这个小实验中体现出的原理，就说明了为什么我们不会被周围空气的气压所压碎。
+
+56．气压下的体积收缩
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+
+
+
+实验66：在马略特气压管中，短臂管里面一厘米的汞柱与长臂中同样高度的汞柱代表着相同的压力，因此短管中的汞柱刻度便可以显示长管中的压力值，进而得出实验当时的大气压。由于短管是密封的，空气不能进出，因此在长臂中倒入水银后，短管中的空气会被逐渐压缩，体积也会越来越小。如果我们还记得短管中空气的压力就等于长管中汞柱的压力再加上当时的大气压力的话，我们便可以精确地通过短管中空气体积的减小来测量大气压力的增加了。
+
+
+
+
+
+我们在实验1中已经看到，在压力下，空气的体积可以急剧收缩，但压力撤消后，它又会恢复原样。但这个实验并不能告诉我们，空气体积的减少是否与压力的增加成比例关系，或者换句话说，空气体积减少的速度会在压力增加的过程中越来越快吗？这个疑问在实验66中得到了清楚的显示。如果自行车的打气筒质量很好的话，它也能精确回答上面关于空气体积减少比率的疑问。研究发现，体积的减少与压力的增加成正比。
+
+57．气压计——对大气压力的测量对研究大气状况具有重要意义，因此很有必要拥有专门的仪器能轻易对其测量，这样的仪器就叫做气压计。常用的气压计一般有两种，一种叫水银气压计，一种叫无液气压计。
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+
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+实验67：（老师的教学实验）取一根孔径为0.5cm、长90cm的厚壁玻璃管，在一端接上一根厚壁橡胶管，让玻璃管紧紧插入橡胶管大约10cm，并用绳子拴牢实，以免橡胶管滑动进而让空气进入。在橡胶管上安装一个螺旋旋钮并彻底旋紧，让这一端保持彻底封闭，最好在封闭端下面放一个大盘子，以防万一水银泄漏造成浪费。然后向玻璃管中注入水银，再将大拇指扣在管口，将玻璃管倒置于一个装有水银的烧杯中。如果各处连接都很结实的话，水银不会在玻璃管中下降太多，空气压力会托住它，这就是一个简单的气压计。
+
+让这支玻璃管继续保持该状态，再取一支同样的玻璃管，将其一端与橡胶管的另一端相连，要求与之前一样，务必紧密结实。然后将玻璃管的另一端放入一杯有颜色的水中，慢慢松开螺旋旋钮，水会上升到玻璃管中，这是为什么呢？这又说明了什么呢？
+
+实验68：取一瓶干净水，用一个双孔胶塞将瓶口塞紧，一个孔中插入一根10到15cm的玻璃管，另一个孔也插入一支玻璃管，但其外端口与本生灯的加热装置相连，保持各处连接紧密无缝，现在通过短玻璃管试着用嘴吸水，能吸出来吗？将另一支玻璃管拔出来，再吸，好些了吗？如果是的话，这是为什么呢？
+
+
+
+
+
+在之前的实验中，我们实际上已经构建了一个粗略的水银气压计。当然了，标准的水银气压计是由一根80厘米长、孔径尺寸统一的玻璃管构成，一端封闭。在小心注入水银之后，开口端插入到一个水银槽中。水银槽有一个可滑动的底座，旁边有螺丝旋钮来控制其上下滑动。水银槽顶部有一个乳白色的小钉桩，其尾端与水银槽底部刻度的距离被精确设置。刻度在一个包围玻璃管的金属外皮的夹缝中显示，这层金属外皮紧紧地包裹并固定着玻璃管，主要起保护作用。
+
+当要从气压计读取数据的时候，便滑动水银槽的底座，让水银柱的上端刚好接触小钉桩的下端，这时水银柱的高度便可以从刻度标记上读出来。为了提高测量的精确度，一般还会在金属外皮的夹缝中安装一个游标，它可以在夹缝中根据需要自由滑动，以便读取更精确的数据。
+
+无液气压计主要由一个外表面起皱的金属箱体构成，通过这个金属箱，空气可以部分地被排压出来。其中有一根硬质弹簧，因此空气的压力不会使其塌缩。箱体中还装有杠杆装置，空气体积的变化可以通过它们进行传动，进而让与之相连的指针在刻度盘上显示读数。这样，气压值就可以被记录下来了。
+
+还有一种叫做自动记录式气压计，它是由一根长杠杆与一个空盒气压表的指针相连接，并通过一根上有发条的圆柱的旋转来提供实时记录的动力。这样，圆柱不停旋转，气压值便得到了连续的记录。
+
+58．用气压计测量高度
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+
+
+
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+实验69：将一只无液气压计从一幢高建筑物的底部拿至顶部，并在两处分别记录其气压值，为什么在顶部的气压值要低一些？
+
+
+
+
+
+任���表面的气压都由这个表面的空气重量所导致，且随着海拔高度的增加，气压会逐渐减小，这是因为空气的重量减轻了。如果高度增加与气压减小这二者之间的比例关系很清楚的话，那就可以通过对气压的测量来判断海拔高度了。
+
+尽管气压计所测定的高度与当时的空气状况有直接关系，但就这种变化的普遍性而言，还是有粗略的规律可循。据测算，水银柱下降1/16英寸，表明高度增加57英尺；差不多水银柱下降1毫米，则高度增加11米。在正常温度和气压下，这个数值在海拔1000英尺范围内基本可靠。
+
+在海拔25英里的高空，气压汞柱还不到1/25英尺。不同的测量结果已经显示，大气的空气圈层有高达100英里的厚度，甚至更高。但差不多四分之三的大气都分布在地球最高山峰的高度以下，我们人类目前所达到的最高高度也不过海拔7英里[1]。
+
+为了研究大气状况，附着了气象仪的小气球已被发送到海拔21英里的高空。人们发现，最低的温度，一般出现在海拔6到10英里之间。同时还发现，虽然大气状况会影响到天气，但这个影响只局限于海拔3英里范围之内的大气层。
+
+59．空气的绝热升温与绝热冷却
+
+
+
+
+
+实验70：取一个5品脱容积的瓶子以及一个双孔胶塞，在瓶塞的一个孔中插入一支化学温度计或者空气温度计，另一个孔中插入一支短玻璃管，让其下端不要靠近温度计的玻璃球，以免空气进出时，玻璃管口与温度计玻璃球之间没有空气运动。另外温度计在瓶外的部分也须足够长，以方便读数。
+
+
+
+
+
+将短玻璃管的外端通过一根厚壁橡胶管与抽气机相连，记录一下此时瓶内与瓶外的温度。然后迅速抽尽瓶中的空气，小心观察温度计上的反应，会发现房间温度几乎无变化。这时再往瓶中充气，依然小心观察温度的变化。会发现往外排气时，瓶中空气温度变化了，往里充气时，温度也变化了，这是空气的密度变化而导致的温度变化。
+
+人们发现，空气膨胀时温度会下降，而受压收缩时温度会上升。这种不是通过改变气体内部的热量，而是通过改变其密度进而让其升温或降温的办法，就叫做绝热升温或绝热冷却。此效应一般被用来制造液态空气，一些冷藏装置一般也会用到这个原理。同时，它对我们开发空气循环应用系统也有极大的提示作用。
+
+压缩空气产生的加热效应，可以在给车胎打气的时候明明白白地显现出来。不管打气筒的活塞再怎么抹润滑油，当车胎里空气密度不断增大时，打气筒也会很快变热。这个热量绝不可能是摩擦所致，因为活塞运动的频率已经越来越慢，不像刚开始打气那么快了。这个热量其实就是由于空气被压缩导致的，压力增加，温度升高。一个被润滑油充分润滑的活塞，其摩擦力对加热的影响是微乎其微的。
+
+60．温度对空气保持水分能力的影响
+
+
+
+
+
+实验71：取一个容积为1升的烧瓶，放入适量的水，让水在烧瓶摇动时可以在其内壁上形成一层水膜。现在缓慢地给烧瓶加热，注意不要加热至沸腾，最后让水完全蒸发消失，烧瓶这时看起来完全干燥。然后用塞子将瓶口塞紧，让烧瓶慢慢降温。这时水分已不能进入瓶中，但与之前一样，瓶中的空气都是透明的。但随着温度冷却，刚才加温时消失的水膜，又会慢慢出现在烧瓶内壁上了。
+
+实验72：在一个玻璃烧杯中装入适量冰水，在这之前必须保证烧杯外壁被彻底擦干，然后让这杯冰水在一间温暖的房间中放置一会儿。水能够穿过杯壁吗？这时杯子的外壁还是干燥的吗？如果上面布满了水珠，这些水是从哪里来的呢？如果让杯中的水的温度等于或高于房间的温度，还会产生同样的现象吗？
+
+实验73：与刚才实验类似地，在一个玻璃烧杯中加入适量的水，让水的温度稍微高于房间温度，然后逐渐往杯中加入适量冰块，并用一支温度计持续搅动。当烧杯外壁开始出现一层薄雾时，记下此时的温度，并停止向杯中加冰块。当薄雾又开始散去时，又记下此时的温度。将前后两次温度取平均值，这个平均温度就可能是薄雾真正开始形成时的温度，我们把它叫做露点。
+
+
+
+
+
+当我们想让衣服晾干时，可将它们放在温暖的房间中或者置于阳光下。不久，我们就会发现衣服里面的水不见了。很自然，它一定是跑到空气中去了。这说明，当空气温度升高时，空气会比它们在低温时容纳保持更多的水分，刚才的实验也显示了这一点。关于露点的实验更说明，当热空气被降温时，会部分地释放出它们所保持的水分。
+
+空气的这个升温时能保持大量水分、降温���又会释放出来的特性，就是云和雨形成的原因。如果没有这个特性，便没有陆地上的水循环，也就没有雨雪风霜，自然也就不会有植物，不会有动物以及一切生命。因此，这个简单的特性，其实为地球上所有生命的诞生与繁衍奠定了基础。
+
+61．大气中的水分
+
+
+
+
+
+实验74：小心地给一盘水称重，然后将其放在一个通风的地方，静止放置数小时后，再次进行称重。是什么让重量变化了？分别用试管、表面皿、广口烧杯，在不同地方进行刚才的实验。
+
+
+
+
+
+大气在任何时候任何条件下，都会保持一定的水分。当其温度升高时，其保持水分的能力也会显著增强，而且不会出现因为太冷而让大气无法保持哪怕一丁点水分的情况。当水处于固态或液态时，它便一直暴露在空气中，并不断地挥发消失于空中。
+
+如果水面的表面积很大，温度也很高，那水的蒸发量也就会非常大，且蒸发的速度也会很快。在热带地区，每年蒸发的水量差不多达到表面积8英尺以下的体积。这意味着，太阳的能量每年在热带地区每平方英尺的水面上，可以消散500磅的水。而在极地区域，水的蒸发量差不多只有热气区域的十分之一。
+
+每一年，地球上的各个水域都会蒸发掉大量的水。在许多地方，这些蒸发掉的水分又会降落到它们原来所在的地方，但依然还有一大部分被风吹送到其他地区，并降落在那里的陆地表面上，滋润着地球上的生命世界。
+
+62．湿度——空气所保持的水分的多少，我们叫做湿度。如果空气已经含有了它能保持的最大水量，则我们就说它已经饱和，或者说它已达到了露点。空气中所含的水分的量叫做绝对湿度，而其所含水分的量与饱和时所含水分的量的比值，叫做相对湿度。如果空气中含有大量的水分，我们就可以说它的湿度很高。当含有大量水分的空气被降温时，它便不再能保持之前那么多水分，其中一部分便会以凝结的方式释放出来。
+
+空气中的水分可以凝结形成很微小的水滴。在高空，便形成了云，但这些小水滴也可以存在于地球表面附近。这种情况下，我们把这样的水分形态就叫做雾，如果它在物体表面凝结，我们便叫它露水。
+
+正如我们在实验73中测定露点一样，气象学者们通过许多观察与实验，也将各种情况下空气的露点数据做成了表格，通过对比这些数据，我们便可以测定空气的相对湿度。用来测定空气湿度的仪器就叫做湿度计。
+
+63．大气状况对光的影响
+
+
+
+
+
+实验75：让太阳光穿过玻璃三棱镜，投射在白色的墙壁上或者一张白纸上，会出现什么现象？这些缤纷的色彩是从何而来的呢？不同的颜色是按怎样的次序排列的？我们把这组由三棱镜折射开来的有色光序列，叫做光谱。
+
+
+
+
+
+如果来自太阳的光通过一个三棱镜折射出来，便会像上面实验中看到的那样，分成不同的颜色。实际上，物体之所来看起来有颜色，就是因为它吸收了那种颜色的光，而反射了其他颜色的光。
+
+光自身就有振动的能量，它能刺激我们的视神经，让我们感觉到它。光的不同颜色，是由其各自不同的振动波长所导致。光从太阳传到地球，自然会被它必须穿过的大气层所影响。如果空气中有大量烟雾或者灰尘，太阳看起来就是红彤彤的。每当太阳下山，阳光从地平线上射向我们的时候，由于它穿过了因靠近地面而含有大量灰尘的空气层，太阳总是呈现着红色的脸庞。如果站立在高高的山峰之上，或者在一个大晴天，阳光普照万里无云的时候，我们仰望天空，便会看见那一望无尽的蔚蓝。这两种不同的颜色，都是由于大气层对穿行于其中的光线施加了影响所致。
+
+有时在阵雨过后，天空会出现一道由不同颜色组成的美丽弯拱，这就是彩虹。这是因为阳光被正在其他较远的地方降落的雨滴折射，分离出了不同颜色的光，就像穿过了一个从旁边经过的三棱镜一样。
+
+有时太阳和月亮会环绕着一圈光晕，其直径较小时，我们这叫做日冕；直径较大时，我们叫做光环。这些光圈都是由于大气中水与冰的小颗粒对其施加影响所致。
+
+在某些特殊情形下，从远方物体过来的光被不同密度的空气层轮番反射折射，观测者看到的影像，似乎存在于真实物体之外的另一个地方，就像照镜子时看到里面有个人一样。这就是海市蜃楼，或者叫幻影。它在沙漠地带或者大洋海岸一带时有发生。
+
+在高纬度地区，有时还会看见拱形或长条形的彩色光带照亮在北方的天空，色彩和亮度千变万化绚丽多姿，有时它们极其明亮，在白天也能看见，��就是极光，或者叫北极光。一般认为，它是带电粒子流穿越稀薄的空气层而形成的。据测算，这些光带的高度可以达到数百甚至数千英里，因此稀薄的空气层也很有可能延伸到这个高度。
+
+64．温暖的大气层——太阳传输给地球表面的光与热都会穿过大气层。不过人们发现，在高山顶部的温度一般都比山下的温度低一些。然而太阳的辐射，即专业术语讲的日照，投向山顶单位表面积上的数量，应该是与投向山谷单位表面积上的数量是一样的。既然热量减少了，那一定是由于除了热辐射可能被遮蔽的另外一些原因。
+
+每到春天，园丁想让植物生长快一点的时候，他们会给植物盖上一个盒盖，其上表面由玻璃制成。盖上之后，盒中的温度会比外面的温度更高一些。这是因为太阳的热辐射会被地面以某种方式吸收一部分，因此它们便不能像穿进玻璃时那样容易地穿出去。
+
+大气层对地表热辐射的影响就类似于这里玻璃的作用，它会阻止热量散播出去，便也让大气外表面依旧保持寒冷。在山顶这样的空气较为稀薄的地方，类似的效应会不那么明显，因此那里一般都相对较冷，并常年覆盖着皑皑白雪。天空中漂浮的朵朵白云，会加强热量在大气中的保存。每当秋风渐起天气转凉霜降已至，农夫会说，霜都是在晴朗的夜晚降下，而不是多云的晚上。其原因就在于此。
+
+也是基于同样的原因，人们经常会给农作物盖上一层纸给他们保暖，还会在种越橘的沼泽地周围架起烟火来制造烟雾，以防止晚上结霜。因此大气层就像一张地球的毛毯，保持了太阳辐射过来的热量，让地球表面得以温暖，跟晚上毛毯保持我们身体的热量让我们温暖是一个道理。如果地球没有大气的保暖作用，白天就会极其炎热，因为太阳光热会完全直接照射在地球表面上；而晚上又会极其寒冷。这样一来，生命同样不会在地球上诞生。
+
+据测算，如果地球没有大气，地球表面的疯狂温度在白天会达到350华氏度，夜晚则为零下123华氏度。因此大气不仅仅对动植物的呼吸以及水分的保持是必需的，同时对太阳热量的保持也是必不可少的。月球就没有大气层，如我们所知，那里一片死寂，毫无生机。
+
+65．大气温度变化的原因
+
+
+
+
+
+实验76：在一张边长12英寸的木板中心挖出一个边长4英寸的正方形孔洞，在孔洞中牢固地安插一根1英尺长的正方形木管，边长也为4英寸，边缘与孔洞正好吻合。将木管的内外表面都刷上黑色油漆，将木管另一端用合叶铰链连接在一张2英尺长、16英寸宽的木板上，让铰链与木板的端头距离10英寸，铰链与木管位于木板中线上，两边各留出6英寸木板。
+
+找一个大晴天，将此装置放在户外的一张桌子上，并在木管口的桌面上放一张白纸，让另一端始终朝着太阳，并分别在早上、中午、晚上找几个不同时间，给白纸上被从木管中照射进来的阳光照亮的区域做上记号。为什么不同时间白纸上被照亮的区域大小不一样呢？
+
+在被管中阳光照亮的区域中心放一支温度计，记录下它在不同时间点的读数，你能说说为什么这些记录的温度都不一样吗？整个实验中，暴露在阳光下的区域都是一致的，故而请你画一个图表，来阐述一下太阳光热在不同状态下的变化情况。
+
+
+
+
+
+太阳光在一定表面上的辐射量，取决于它投射的角度。从插图65中可以看出来，同样的辐射量，当其垂直射向物体表面的时候，远比其水平照射时所照射的面积小。每30度的圆弧上，接受的照射量分别是2.5、7、9.5个单位。当然这里假定太阳正好垂直照射地球赤道，就跟每年的春分秋分的时候一样。在这两天，其他条件都不变的情况下，太阳照射到极地30度范围内的光照数量，只有其照射到赤道偏北30度范围内的四分之一。
+
+纬度与该地区接收的太阳热量有很大关系。当太阳垂直照射赤道北边地区的时候，北半球白天长度就会增加，光照也更强，因此这期间该区域会接收到更多的热量。在每年六月21日，北极点23.5度范围内，也即是北角范围内的区域，会出现24小时的日照，北极点在这24小时中接收的热量，比赤道上某一点在此日白天接受的热量要多出一倍。
+
+尽管纬度与太阳热量的接收关系密切，但还是有很多其他因素决定着不同地方的温度。就拿意大利威尼斯的例子来说吧，那里一年四季不冷不热，气候非常温和。但与它差不多同纬度的加拿大蒙特利尔可就大不一样了，那里的温度与降雨量会随着季节发生显著变化。
+
+我们已经知道，随着海拔高度的变化，温度也会随之��同。这是因为高度越高，那里的空气便越稀薄，好比只有一张薄薄的毛毯在保持热量，于是就“高处不胜寒”了。同样的，不同的土壤类型也会影响当地的气温。如果土壤类型是砂土，且当地也没什么植物的话，那白天便会受热极快，同时也会把热量很快地反射到空气中，自然会让靠近陆地表面的空气在白天温度很高；到了晚上，没有了太阳热量，白天的热量会很快散去，因此空气的温度也会变得很低。在白天，一望无际的沙漠上的高温，让人完全难以忍受；而在晚上，旅行的人们却必须盖上厚毛毯来保暖。
+
+一个地方与海洋的距离，以及当地的风向，也会影响当地的温度。在地球上有些地方，这甚至是影响气温的主要因素。因此，大气在任何地方的温度都不是取决于单一的原因，而是很多因素综合作用的结果，比如纬度、高度、主风风向、洋流、与海洋的距离、土壤类型等等。
+
+地图上常常标明了不同的温度带，热带、温带、寒带，一目了然。但是，这些不同的温度带，却与各自纬度区域并不是完全一致的。
+
+66．显示区域温度的图解法——通常很有必要对一个地区的温度变化情况有所了解，包括对其做记录并且长久保存。有一个办法就是拿一张地图，在每个区域都标注上那里的温度，但这样一来，这张地图便会密密麻麻地布满数字，难以辨认了。
+
+还有一个更好的办法，而且它现在已经被广泛应用。那就是在绘制地图时，先标注一个温度，然后画一条线让它穿过具有同样温度的所有区域，这样的线我们叫做等温线，这种形式的地图便叫做等温线地图。在这样的地图上，一眼便可以看出每个地方大致的温度情况，也能明白不同温度区域之间的关系。有一个规则是，等温线不是每隔一度就画一条，而是每隔十度才画一条。
+
+在绘制这类地图时，还会绘制一些只保留这些等温线而不标注数字的副本。插图66中就是画出等温线之前的样子，只有温度数据，插图67就是画出了等温线的样子了。而插图68，则是典型的只有等温线的图表型副本。如果我们要给这类地图做个速写图的话，那多半应该是只有等温线的略图了。
+
+用同样的方法，还可以绘制出记录各地气压状况的地图，只需将这里的等温线换成一条穿过相同气压地方的线条即可，这些线条就叫做等压线。
+
+美国国家气象局每天都会绘制出当天全国的气象地图，那上面既有等温线，又有等压线。每天早上这些数据都会通过电子传输方式，发送到整个北美地区的各个接收站。
+
+67．气象地图——耗资甚巨的气象局不光美国有，世界上几乎所有发达国家都有。气象数据的记录除了气象局的科研人员以外，远洋航行的船长们以及其他许多气象观测者都会参与进来。他们将许多气象信息收集在一起，科研人员们会将其汇总分析，然后制成覆盖全球的气象图。如此年复一年，气象信息与数据收集得越来越多，因此在它们基础之上绘出的气象图也便越来越可靠了。
+
+这类气象图表有极大的应用价值，因为它们为我们展示了世界每一个地区的气候状况。这些汇集在一起的数据与信息，每年都为人类的商业贸易活动起到了巨大的参考作用，同时还挽救了无数人的生命以及数额巨大的人类财富。上两页的插图便分别是每年一月和七月的全球等温线地图。
+
+68．陆地与海洋温度——我们在实验27中已经知道，水具有很好的保持热量的能力。在炎炎夏日，水升温的速度远远赶不上水面空气的升温速度，因此它会持续地从空气中吸取热量，从而也降低了空气的温度。在寒冷的冬季，水散热的速度也相对较慢，因而也会比空气更温暖一些，自然也会持续地向外散发热量。这样的话，广阔水域上面的空气的温度变化，自然就比陆地上的空气来得缓慢。
+
+当空气随着海风从海洋吹到陆地，便让那里夏天变得凉爽，冬天变得温暖。这样，靠近海洋的地方的气温变化，便与海洋的冬暖夏凉产生了关联。而远离海洋的地方，四季的温差就明显得多了。在海洋中的一些小岛上，一年四季几乎感觉不到气候的变化，而在内陆一些地区，夏天的平均温度可能比冬天的平均温度高出一百多度。
+
+69．大气压在地球表面的分配——从每年一月和七月全球等压线地图可以看出，大气压跟温度类似，被大陆板块的分布深深影响。在南纬40°附近，几乎没有什么陆地，那里的等压线就很有规则，方向平顺且几乎与纬线平行。在北半球对应的地方，等压线就明显被陆地所影响，当然海洋对它们的影响力依然是��定性的。
+
+在北半球，陆地与海洋的分布比较均衡，但是陆地对大气压力的影响也一目了然。冬天，高压区和低温区都出现在陆地上，但是在夏天，却是低压区和高温区出现在陆地上。从这里我们可以得知，陆地的升温与冷却都远远快于海洋，并且热空气比冷空气轻得多。
+
+在冬夏两季，赤道两边各有一个显著的高压区，但它们并不固定，会南北移动。在夏天，北半球的高压区，会到达它可能出现的最北边。
+
+风会很自然地卷裹着空气从高压地区吹向低压地区，或者说它会沿着气压梯度，从高气压向低气压流动。因此，之前提到的陆地和海洋的气压变化，自然就会对风向产生影响。有一个规律就是，冬天风会从内陆吹向海洋，夏天则会从海洋吹向内陆。我们其实还能看出来，等温线和等压线在地图上的分布与走向都很类似，风其实就是大气总体状况的一个综合表征。
+
+70．风
+
+
+
+
+
+实验77：当某一天室内温度明显高于室外温度的时候，打开房间顶部和底部的窗户，然后拿一张小薄纸条靠近窗户。会感觉到气流吗？如果会的话，它在顶部和底部的窗户之间会朝什么方向运动？是什么原因导致了房间内的气流？
+
+实验78：找两个类似的锡罐，15cm高，直径差不多5~6cm，在靠近顶部和底部各有一个直径1cm的开口管，用两根橡胶管把两个罐子的上下两个开口管各自对应紧紧地连接，并在每根橡胶管上安一个螺旋开关夹，先将开关夹紧紧关闭，让液体不能从橡胶管中流向另一个罐子。在一个罐子里加入足量的有颜色的水，另一个则放入足量煤油或者轻质油品。
+
+尽管罐子中两个液柱在形态上很类似，但装水的罐子底部的压力会比油罐底部压力更大，因为水比油更重一些。这就和地球上两个地方一个气压高一个气压低的情形有点类似。这时先后将上下两个开关夹打开，注意观察下面的开关夹被打开之后，发生了什么现象。水和油混合到了一起，液体在传输的过程中，是通过上下哪个管道传输的？（如果橡胶管中能有部分玻璃管，实验效果观察起来会更清楚明白。）
+
+实验79：给一个对流装置装满水，再放进少许木屑，让其与水均匀混合。给管子的一端加热，注意观察管中的对流现象。
+
+
+
+
+
+在实验78中，我们看到了两种液体交互流动的现象，这是由于水比油更重，进而挤压了底部的油向上面溢流，又通过上端的开口管流到了另一个罐子里，并占据着流到油罐里的水的空间。类似的情况也会发生在大气层中，当一个地方上空的空气比另一个地方上空的空气更重的话，空气对流就会发生了。这时便会有一股空气沿着地表，从气压高的地方流向气压低的地方，这样的空气运动就是我们所熟悉的风。
+
+在地球上，靠近地表的风，一般不像它们在高空那样有固定的方向。风力的大小也决定于空气压力的具体状况。气压可以通过气压计进行测量，因此我们也常说风是由于气压计压力差，或者说是由气压梯度引起的。人们以风吹来的方向来给风命名，如西风就是从西边吹来的风。
+
+如果除了气压高低之外，没有其他因素影响空气的流动的话，那风便会从一个地方到另一个地方沿直线运动，并且人们还可以通过高气压区的所在地，知道风的确切方向。但实际并不如此，山脉与丘陵就是干扰风向的重要因素。除此之外，还有很多因素影响风向，其中最主要的，便是地球的自转。
+
+71．风速及其影响——空气运动的速度差别巨大，轻柔拂面的微风连树叶都吹不动，狂暴的龙卷风则所向披靡，可以席卷沿路一切物体，其风速可达到每小时上百英里。平常用来测风速的仪器叫做风速计，其通常结构为四个铝制圆杯，通过四个水平支撑臂连接在一个垂直的转轴上，转轴旋转的圈数通过转轴上一系列齿轮传动，在一个刻度盘上得到记录，进而得出风速。
+
+当风速特别巨大的时候，风速计就不管用了，这时只能通过它向外施加的压力来进行估算。海森教授发明了一种不需要仪器设备就可以给风速定级的办法[2]:
+
+0．无风——风平浪静。
+
+1．轻风——只能吹动树叶。
+
+2．和风——可以吹动树的枝干。
+
+3．凛风——可以让树枝猛烈摇摆，能扬起灰尘。
+
+4．疾风——能从地面吹起树枝，可以撼动整棵大树。
+
+5．狂风——吹断小树枝，让烟囱的砖块出现松动。
+
+6．飓风——摧毁沿途一切。
+
+尽管风有时会突发性地上演几次极具毁灭性的狂暴，但就其平常面目总体而言，它依然不失温柔。它是地球的循环系统，就像血液之于动物，以及汁液���于植物。没有风的话，地球上的许多活动都会陷于停滞。所谓风行天下，它的足迹不仅遍布于陆地的每一个角落，还经行过海洋的每一处洋面，并将水从江河湖海蒸发到空中。它夹带着清爽的呼吸，从葱茏伟岸高山与广阔温和的大海吹来，让炎热的地带变成清凉世界；它也从温暖的海洋与炙热的大地上带来热量，让严寒地区可以沐浴到融融暖意。它还支撑着人类横跨海洋的商业贸易，并通过水的运载能力促进了人类的制造业发展。它将植物的种子散播到四方的旷野，并将人类城市的浊气驱散，让我们可以呼吸到更新鲜的空气。
+
+72．地球自转对风的影响
+
+
+
+
+
+实验80：让一个地球仪从左向右旋转起来，在其旋转的过程中，拿一支粉笔轻轻地从北极点向赤道画一条线，然后让地球仪静止下来，观察这条线的形状，它像一根经线吗？在地球仪的低纬度区域，这条线的大致走向是怎样的？
+
+
+
+
+
+地球的自转，会影响到地球表面上所有自由运动物体的运动方向。如果一股气流从北极点开始向南移动，它会在运动过程中向右偏移，当其到达中纬度地区的时候，便不再是向南运动而是向西南方向运动了。为什么会这样呢？当我们仔细考虑过风的运动状况之后，便很容易理解这个现象背后的原因了。
+
+由于地球周长大致为25000英里，完成一圈自转需要24小时，因此赤道表面的物体便以差不多每小时1000英里的速度跟着地球一起自西向东运动，而位于南北极点的物体则几乎是在原地转圈而已。因此，如果我们从极点向赤道运动，沿途的每一点都会逐渐增加一个自西向东的速度。
+
+空气由于没有附着在地球表面，因而其自西向东的速度增加会显得慢一些，但依然还是根源于摩擦力。于是乎，当风从高纬度地区吹向低纬度地区的时候，它自西向东的速度就会慢于地球表面上的物体，这样看起来就好像风在自东向西运动一样。这就好比一个坐在火车上的人，在火车开动的时候他会感觉车窗外的物体在朝相反方向运动一样。气流的南北向与表面上东西向运动的组合，就让其自身有了一个向东北或者向西南的运动方向。
+
+我们可以对这个现象做一个正式归纳：在地球表面自由运动的物体，当其位于北半球时会向右偏移，当其位于南半球时会向左偏移。这个结论也被叫做费雷尔定律。
+
+73．行星风带——由于赤道上的空气所接受到的热量很大，因此它们的温度也会比较高，从而变得很轻，而极地附近的空气则相对寒冷而滞重。因此地球上空气的流动就必然出现这么一个趋势：极地表面的空气会始终向赤道运动，而赤道上层的空气则又会向极地运动，就像前面实验中分别装着水和油的两个罐子里的液体交互对流一样。但这一运动趋势又会受到地球自转以及某些大气状况的影响，因此在赤道两边25°到35°的范围内，会存在一些高气压区，它们在一月和七月的等压线地图上可以明明白白地看出来。
+
+从这些高气压区出发，风会向赤道和极地两个方向运动。但由于地球自转，风的运动方向就不是纯粹的南北向，而是在北半球呈现东北向和西南向。这种风向会出现在所有拥有大气层的行星表面上，因此这种类型的风又被叫做行星风。
+
+由于地球的自转以及赤道空气的受热状况均无法改变，我们这颗行星上就随之出现了同样永久不变的风带。热赤道带，也即是温度最高的风带，会受到太阳南北位移的影响而发生位置改变，进而也会让整个地球的风带都联动性地发生改变。因此受到这类影响的行星风有时也被叫做地面风。
+
+74．地球风带——在炙热的赤道附近，空气总是受热而向上飘升，于是这里的风一般都很平缓，人们便也将这一区域称之为赤道无风带。同时也由于这里的空气始终处于上升和冷却的过程，其自身便有了较强的水分保持能力，因此让这一地区变得多云多雨而且高温，地面大部分地区都是沼泽与湿地，草木极其茂盛，农耕也自然变得困难。
+
+从赤道向南北延伸大约28°纬度的区域内，风总是恒定不变地吹向赤道，以填补那里因气流上升而空出来的空间。在北半球，它们从东北吹向西南；在南半球，它们从东南吹向西北。它们是地球上无论强度还是方向都非常恒定的风，因而被叫做信风。又由于它们是从较冷的地方吹向较暖的地方，因此其保持水分的能力也不断增强，让沿途的天气变得很不稳定，且十分干燥。全球的许多大沙漠就位于这一地带。
+
+紧挨信风带并靠近极地的一侧，就是我们之前提到过的高气压区，它们被叫做马���度区，或者叫做热带无风带，定义有点不那么准确。这一区域的空气处于下降趋势，在靠近地表的地方变得很轻且没有规律性，于是便跟赤道无风带一样，空气运动显得很平静。但它与赤道无风带不同的是，这一地带极为干燥，因为我们在59节中讲了绝热增温的效应，不断下降的空气的温度会逐渐升高，因此其保持水分的能力也会显著增强。于是乎，这一带的空气往往趋向于保持水分而不是释放出来。
+
+在中纬度地带，那里的风没什么规律性，只是大多都从西边吹向东边或者东北边，并在运动过程中不断被一些旋转气流所阻碍，这些旋转气流的运动直径很大，可以达到500至1000英里，它们就是我们常说的气旋或反气旋。由于这一地带的空气总是趋向于向高纬度地区运动，因此它们所形成的西风便也富含大量水分，显得很潮湿。
+
+在反气旋中，空气会从中心慢慢向下向外移动；而在气旋中，则是会向里向上运动。因此反气旋中心是一个晴朗并具有高气压的所在，而气旋中心则乌云密布，但气压会相对较低。故而在反气旋的覆盖地，或者说高压带，风一般干燥、寒冷、轻柔；而在气旋的覆盖地，或者说低压带，风的特点则是强烈而湿润。
+
+75．大陆与海洋风——由于陆地在太阳光热照射下的受热速度远远快于海洋，因此在白天，陆地的温度会高于周围的水域温度。这样，水面的冷空气便会向陆地流动，以取代其受热上升的空气。因此在大的水域周围的陆地上，白天总会吹着从水上而来的风。到了晚上，同样因为陆地散热也更快，所以风便会吹向相反的方向。这类海洋风在很大程度上调节了热带海洋沿岸地区的气候，也让那里成为了令人陶醉的休闲度假胜地。
+
+76．季风——在亚洲大陆的内陆地区，空气在夏天温度很高，于是不断膨胀，重量也随之减轻，这就让海面的冷空气不断吹向内陆。同时，那里的高温还让热赤道远远向北偏离于地球赤道，也让西南季风不断吹向地球赤道。这些不断向右、往赤道北边偏离的气流便形成了西南风，它们无形中极大地加强了内陆地区的受热效应。
+
+从热带海洋吹来、附带着大量水汽的风，在它们登上印度洋沿岸的高地之后，开始向高空飘移，然后开始冷却并形成大量雨水，让亚洲南部地区成为了地球上雨量最大的地方。
+
+在冬季，地球的热赤道逐渐南移，内陆地区也变得寒冷，便导致了空气从陆地向温暖海洋的强烈运动。这样的气流运动，加强了印度洋上东北信风的力量。如此一来，在亚洲南部的海岸地区，便出现了很强的季节风，它们夏天吹向东北，冬天吹向西南，因此被叫做季风。在早期的航海历史中，季风非常重要，夏天人们可以利用季风的力量向印度扬帆，冬天则可以利用季风之便从印度出海航行。也正是托季风的福，欧洲诸国才在印度捞到了不少好处。
+
+77．降雨量及其测量
+
+
+
+
+
+实验81：在空旷的地方放置一个平底盘子，保持边缘水平，大致离地一英尺距离。将盘子固定，让其不会被风吹翻。待一次降雨完毕，测量一下盘子里的积水深度，精确到英寸的最小单位刻度。这便是这次降雨的降雨量大小。
+
+
+
+
+
+在地球的不同地方，每年的降雨量大不相同。一些地方降雨量极其稀少，比如在秘鲁一些地区，平均五年才有一次降雨。但在印度的卡西山，其年降雨量可以达到600英寸；而其24小时的持续的降雨量也可以高达40英寸，这差不多和美国东部地区的年降雨量不相上下了，且这一般都出现在西南季风的季节。
+
+人们仔细测量了全球不同区域的降雨量，并以此绘制了显示各地区平均降雨量的地图。由于农业生产对降雨量以及其各季节雨量分布的依赖很大，因此这类地图便在一定程度上反映了各地区农业出产能力的高低。要适合农业生产需要的话，则该地区的年降雨量不能低于18英寸，且还必须在各个季节有恰当的雨量分布。
+
+通过对降雨量地图的分析，我们可以发现，有大量的地区如果没有人为灌溉的话，便很不适合农业生产。这些区域一般位于干燥风带，以及远离海洋的内陆地区。湿润的海风在内陆不断向上飘升，进而不断降温，这让它们还没到内陆便凝结成了水滴，降落到地面了。
+
+一个地方的降雨量大小会受到如下因素的影响：（1）海拔高度，因为大部分带雨云都处在低海拔地区；（2）地区的风向及风的类型；（3）周围地区的海拔高度。一些大山的向风面往往都雨量充沛，而背风面则大多干燥荒芜。以此类推，我们可以在上一页的地图中找到全球五大沙漠的形成根由。
+
+雨量计是由一个圆筒形容器和一个垂直面板组成，它便是用来测量降雨量大小的仪器。它被放置在远离树木和建筑物的空旷区域，每次降雨之后，降雨量便得到了测量。雪在融化之后也可以如此进行测量降雪量，就一般规律而言，8到10英寸的降雪量差不多就等于1英寸的降雨量。
+
+如果温度在0摄氏度以下，水便会结冰，空气中的水分也会形成绚丽多姿的六瓣雪花，它们会顺着空气的流动随风飘舞，给茫茫大地覆盖上一层厚厚的积雪。虽然雪自身的温度很寒冷，但它却能为它所覆盖的地表保存热量，因此在严寒地区，有白雪覆盖的土壤便比没有白雪覆盖的土壤冻结得浅。所以在气温差不多的情况下，在降雪量充沛的生活区，为了防止水管冻结，便没有必要将水管埋得跟降雪量较少的地方一样深。
+
+如果雨滴在空气中结冰了，便会以冰雹的形式降落到地面。冰雹在夏季较为常见，主要是由于含有雨滴的上升气流，到达一定高空后遇冷而结冰，并且一般会在降落之前夹杂着一些雪花。有时冰雹的个头很大，直径可达半英寸，因此偶尔会对农作物和建筑物玻璃窗造成严重损害。
+
+雨夹雪就是雪花和雨滴混合在一起从天空降落的天气现象。
+
+78．美国的降雨量——从下页所附的美国雨量图上，我们可以看到全美国降雨量的分布，并可按降雨量的大小将其分成四个不同的雨量带。尽管不同雨量带之间的划分都是渐变过程，但总体依然清晰，它们也常常被分别叫做：北太平洋斜坡、南太平洋斜坡、西部内陆区和东部地带。
+
+在北太平洋沿岸地区，挟持着西风的暴风雨十分常见，尤其在冬天，这一地带的西风狂暴而凛冽，其年降雨量大致在70英寸左右。
+
+从加利福尼亚中部往南，降雨量呈逐渐递减的趋势，在其雨量最少的区域，有时整个夏天滴雨不下，全年降雨量也不过15英寸。再参照一下之前附页的等压线地图可以发现，干燥的热带无风带的高气压区，会在夏季向北部大范围移动，这就让这一地区不能受到夏季湿润的西风吹拂，从而变成了恒久干旱少雨的区域。
+
+从喀斯喀特山脉以及内华达山脉一直延伸到第100经度的西部内陆区域，大部分地区都很干燥，因为湿润的西风在翻越山脉时已经释放了自身所富含的水分。而在山地和高原上，雨量往往很充沛，因为空气流经这里时，会由于温度降低而致使水蒸气凝结，进而形成降雨。因此在南加利福尼亚一类的干旱地带，为保证农业丰收，灌溉也就成为必须。而山地与高原的充沛降雨则形成了河流，这便为灌溉提供了非常充分的水资源，因此这一区域内的大部分地区，会在不久的将来被充分开发，以供人类生产生活所需。政府目前也正在加紧建设这一地区的灌溉设施。
+
+从第100经度到大西洋沿岸的区域内，降雨量变化较大，不同的雨量地带对应着不同的农作物生长范围，其变化正好彰显了不同作物对雨量的需求规律。在这一地带，降雨量向东逐渐递增，原因是从墨西哥湾和大西洋吹来的南风和西风为这一地带提供大量的水汽，让这里常年风调雨顺，年平均降雨量大致在30－60英寸。
+
+79．电
+
+
+
+
+
+实验82：在桌面上放一些小纸屑或者小海绵颗粒，然后拿一根玻璃棒在绸布上摩擦后，靠近这些小颗粒。然后再用一根蜡皮棒或者橡胶棒在法兰绒或者动物皮毛上摩擦后，做同样的实验，注意观察这些小颗粒的反应。
+
+实验83：将一根玻璃棒在绸布上快速摩擦后，悬挂在一根类似于实验12中所用的钢丝吊索上，再拿一根用绸布摩擦过的玻璃棒，将其一端靠近悬挂玻璃棒的一端，它们会相互吸引还是相互排斥？又拿一根用皮毛摩擦过的橡胶棒，将其一端靠近悬挂玻璃棒的一端，这次是相互吸引还是相互排斥呢？
+
+实验84：在环架上用丝线悬吊一个小海绵球，拿一根用绸布摩擦过的玻璃棒靠近它，但不让二者接触，观察小海绵球的反应。再用该玻璃棒接触小球，它还是之前的反应吗？又拿一根用皮毛摩擦过的橡胶棒靠近小球，它跟刚才用玻璃棒靠近时反应一样吗？用该橡胶棒轻轻接触小球，它又有什么反应？现在再拿一根用绸布摩擦过的玻璃棒，靠近刚才与橡胶棒接触过的小球，玻璃棒会吸引还是排斥小球呢？
+
+实验85：在环架上用铜丝悬挂一个小海绵球，铜丝不能粗过一般的线绳，并将铜丝在小球上随便缠上几圈，再拿一根用绸布摩擦过的玻璃棒靠近小球，它的反应跟刚才用丝线悬吊时一样吗？再让小球与玻璃棒接触，其反应也跟刚才用丝线悬吊时一样吗？���用橡胶棒做同样的实验。
+
+
+
+
+
+古希腊人早就发现，有些物体比如琥珀在摩擦之后会吸引轻小物体。这一特性后来就被叫做摩擦起电，英语里面的“电”这个词就来源于希腊语“琥珀”这个词。在刚才的试验中我们已经看到，用丝绸摩擦过的玻璃棒和用皮毛摩擦过的橡胶棒，各自带有不同性质的电，且同性相斥，异性相吸。这两种电就是正电和负电。
+
+是否只存在这两种性质的电，似乎还没有确定性的结论，但就电的日常现象来看，它们都只表现出两种电性，因此通常人们都基本认同这个结论[3]。在实验84中我们发现，用丝线悬吊的小海绵球与带电物体接触后也会带电。而实验85则告诉我们，如果小球被铜丝悬吊，则不会发生这个现象，因为铜丝会将小球所带的电完全导走。像铜这样能够导电的物体叫做导体，而那些像丝绸一类不能导电的物体便叫做绝缘体。
+
+
+
+
+
+实验86：将一台静电机开动起来，拉动把手，离火花远一点，在过程中观察火花的状态。它是沿着直线运动的吗？在把手之间放置一张硬纸板，让其边缘正好不挡住它们的相互运作。在火花运动的方向上，硬纸板上会发生什么现象？再将硬纸板完全遮住其中一个把手，现在电火花会穿过纸板吗？拿一根有尖头的金属丝绕在每个把手上，并在把手上伸长两到三英寸，再开动机器，现在火花会在把手间跳跃吗？一些房子为什么要安装避雷针？
+
+
+
+
+
+在18世纪中叶，本杰明·富兰克林通过他著名的风筝实验，证明了闪电其实就是云层与大地或者不同云层之间的放电现象，这个放电过程跟静电机上的放电其实很类似。云层所带的电荷会将大地表面的另一种电荷吸引过来，并在很高的物体上不断积累。如果吸引力足够大，云层和高物体之间就会放电，我们就说该物体被雷击中了。
+
+如果高物体有一个尖点，比如避雷针尖，电量便会向这个尖点上积累，之后它便会让电量逐渐地释放，以免形成大电流造成破坏。当然了，避雷针必须用导体构造，而且必须与地面接通。
+
+80．雷雨——在闷热的夏季午后，常常可见浓重的云层不断翻涌，最终遮蔽整个天空。不一会儿，云层所覆盖的地方狂风大作，云层则纷纷朝着风的反方向运动。当带雨云不断靠近，一种我们叫做雷阵风的强力狂风也开始在暴风雨之前呼啸，然后闪电烈烈，雷声隐隐，暴风雨已经越来越近。先是雨滴开始落下，一般半小时后便成了倾盆大雨。渐渐地，云层又开始慢慢移开，天空又清晰明朗起来，还有可能出现一架彩虹挂在天边，增加了这风雨天地的澄明之美。
+
+雷雨是由湿热空气上升形成气流，并不断吸入周围空气而导致。上升空气所含的水分不断凝结，形成了厚重的云层，并带有电荷。当电荷不断累积，放电时便形成了闪电。放电一定会沿着电阻最小的路线，因此闪电非常没有规则，并且总是分叉很多。由于高物体很容易为云层放电提供路径，故而在雷雨天，远离树木和高大建筑物最安全。
+
+闪电会极其强烈地搅动空气，这个过程就形成了我们听见的隆隆雷声，就像开枪时空气被激烈搅动而形成枪声一样。由于声音的传播速度为每秒5英里，而闪光几乎是瞬时的，因此不同地方的云层放电所形成的雷声，会在不同时间到达我们的耳朵，云层和周围山地还会让雷声形成隐隐回声，在大地上沉吟回荡。也正因如此，我们才总是在看见闪电之后才听到雷声，且时间间隔一般在5秒钟左右。
+
+在傍晚频繁出现的闪电叫做热闪电，一般它们会出现在地平线上，它们一般是地平线以下的地区的闪电被云层反射而形成。雷雨也会发生在冬天，这在热带地区比较常见。
+
+81．电力通讯
+
+
+
+
+
+实验87：将一根铜丝线的一端接在干电池的一极上，另一端接在一台电报机的接线柱上，在其另一个接线柱上也接一根铜丝线，另一端与电报按键的接线柱相连，再将电报按键的另一个接线柱与刚才的干电池的另一极相连。当按键被按下时，电路就接通了，电报机也就嘀嘀作响。如果能找到一个继电器的话，可将电报机移除，将继电器按照同样的方式连接起来。
+
+将继电器的一个接线柱与电报机的一个接线柱相连，继电器另一个接线柱则与干电池的一极相连，然后再将干电池的另一极与电报机的空余接线柱相连。当继电器让回路接通后，电报机便会发出声音，这就是在电报局简单的办公室里面常有的装置。实验中前半部分用到的电报机也可以换成电铃，当按键按下时，电铃会发出响声，这其实就是一个简单的门铃���置。
+
+
+
+
+
+除了摩擦可以产生电，一些化学反应也可以产生电，人们因此还发明了许多种类的电池。其中最简单的，是将一块铜片和一块锌片分开相对放置在一个盛有稀硫酸的器皿中，不过这样的电池提供的电流非常微弱，因此干电池现在已经成为了最常用的电池组部件。
+
+关于原电池与电流的发现历史十分有趣也非常重要，但是说来话长，这里就不详细探讨了。在1832年，一个名叫塞缪尔·F·B·莫尔斯的美国人发明了商业电报，这是远途信息传输方面的一次巨大进步。它所使用的设备就是刚才实验中我们接触到的按键和发报器，发报器就类似于将我们在实验14中用到的电磁铁装置，用弹簧与一块熟铁片连接而成。当这块铁片与磁铁接触后，便会敲击另一块铁片并发出声音，同时输出图文信息，只要回路接通，发报器就会运作，因此其发出的敲击声音也会有长有短。莫尔斯将这些长短不同的敲击声，按不同组合编排，让其分别代表不同的字母，这样我们就可以在不同地域之间传输文字信息了。实验87就演示了电报的传递原理。
+
+自从莫尔斯首次在华盛顿和巴尔的摩之间成功传送了第一份电报文件，人们已经对电报传输系统做出了很多方面的改进，但发报器和按键一直沿用至今。自1832年开始，陆地上各个地方都架设了电报线缆，海洋也环绕铺设了同样的线路网络，这就让地球上一发生什么大事马上便会传遍全球，让地球人都知道。再伴随着电话、无线电报、无线电话等电力通讯方式的加入，地球上各地区的距离被大大拉近了，以至于现在每个地方都知道地球上其他地方正在发生的一切。直到今天，人们都还没有找到另外一种形式的能量，可以像电能一样被如此广泛地利用于人类生活的方方面面。
+
+82．龙卷风与龙吸水——有些时候，引起雷雨的气象因素过于强烈，以致除了暴风雨之外，它们还可以形成猛烈的内向气流，并伴随着狂暴的涡流运动，我们把这样的激烈暴风雨就叫做龙卷风。在它的内部，温暖潮湿的空气快速上升并延展为一个漏斗状云层，其顶点倒悬于大地表面。在漩涡的中心，空气压力极小，因此涡流的卷流速度非常巨大，在其所经之途，几乎无坚不摧。
+
+一次龙卷风的覆盖范围一般是三十到四十英里长、四分之一英里宽的区域，其前进速度在密西西比河流域大约为每小时20-50英里，且通常为东北风。它们经常被错误地称为气旋。当这种类型的风暴发生在海面上的时候，便会在漏斗气旋涡流中形成一根巨大的水柱，这就是我们所称的龙吸水。
+
+83．气旋——人们发现西风带常常会出现大面积的暴风雨，这就是我们常说的气旋。在气旋中心，大气压力比外部低很多，因此在气象地图上，这一区域往往被标上“低压”标记。在这个低气压中心，四面八方的空气不断涌入，但由于地球自转引起的偏差，持续的气流并不是直接吹向中心，而是产生了一个巨大的涡流，致使气流在其中螺旋上升。
+
+在涡流中，不同位置的气流的运动速度虽各不相同，但都处于高速状态。在北半球，漩涡会按照逆时针方向运转，在南半球则是顺时针方向。空气上升的区域一般都是暴风雨的多发区，但其激烈程度在不同地方也大不一样，这主要取决于上升气流来自哪个方向，这一点可以在附页的地图上看得很清楚。
+
+从大陆内陆地区而来的气流一般都很干燥，从海洋而来的空气则含有大量水分，它们中的一部分在上升过程中便会凝结成水滴。而这些大致向东移动的涡流虽然速度各异，但总体在每小时20到30英里。这就是中纬度地区下雨的主要气象成因。我们还在第74节中提到过，在高气压区空气会从中心向下向外运动，这就是反气旋，这些地区一般都相对干燥，以晴朗天气为主。
+
+84．横跨美国的气旋轨迹——后面的附页地图显示了横跨全美国的气旋风暴的运行路径，看得出来，尽管这些轨迹线条错综复杂，但基本都呈现出东偏北的方向。气旋的运动会在很大程度上受到中纬度地区盛行风风向的影响。
+
+在夏季，气旋在大陆上的运动速度大约为每天500英里，在冬季则是800英里。夏天气旋的风速也远远小于冬天，因为在那个时候，高低气压区的气压差大大减小了。冬天暴风雨来临的时候，气流温度的改变要比夏天更剧烈，也是因为这时南风和北风都会吹向低气压中心，而这些地区的温度也会在这一过程中发生很大变化。
+
+85．突然的天气改变——冬天在中纬度地区经常发生几个小时的温度骤变，温差可达20度以上。拿美国来说，如果温度在一天24小时之内下降20度以上，并且在北方最终低于32°F、南部低于40°F的话，这就是专业术语所讲的寒潮了。每当此际，气象局便会升起如插图所示的一面特别的旗子，向人们显示大降温将至。
+
+当这种极大的气温波动向南部大范围蔓延的时候，它对橘树和农作物纤维的破坏力就体现出来了，这就是我们所知的“冰冻”灾害。曾经在1886年发生的大冰冻灾害，让佛罗里达州大范围的橘树都遭受到了灭顶之灾。这次灾害对当地工农产业打击之大，以致多年后人们都依然将这一年作为年份标记。
+
+如果寒潮还夹带着西北风并伴随着降雪，这就是我们所称的暴风雪了，它往往会席卷整个平原和草原地区，破坏力极其巨大，有时甚至会造成人和家畜的频繁死亡。在欧洲南部，最寒冷的风是来自西伯利亚平原的东北风，而美国最冷的地区却在西南部，这里也是气旋轨迹的后方区域，而欧洲的气旋后方区，却在其北部前沿。
+
+上面已经讲到，当强劲的西北风在寒冷的季节吹入气旋后方区，便会形成寒潮。同样的，当极度高温的空气从南方大范围地向低气压区前沿涌动，并且这时正处在较热的月份的话，“夏季热浪”也就随之而来了。这时空气会极度闷热，并伴随着异常的高温，让人难以忍受，中暑便也时常发生。德克萨斯、堪萨斯以及加利福尼亚的圣安娜地区的“热风”和意大利南部的西罗科风，都是其显著的例子。从这里我们也看得出来，以上所有的天气极端变化，都与低气压地区的大气运动状况有关。
+
+86．天气预报——为了预报天气而必需的数据，每天都会通过电报传送给气象局，并会在气象地图上留下记录。这些积累在气象地图上的观察记录，为气象预报员判断天气变化提供了极其有用的信息。我们已经说过，影响我们天气变化的一个显著因素，便是气旋与反气旋的向东运动。
+
+如果它们的运动速度和方向能被确定下来，那受它们影响地区的天气变化，就可以精确预报了。如果在密西西比河谷地区覆盖着一个气旋中心，而其西部还存在着一个反气旋中心，我们便可以预见，密西西比河的东部及东南部地区，吹着南风和东南风的风雨天气将会逐渐转晴，并且西风会带来更多寒意，这都是因为反气旋会慢慢向东取代气旋。
+
+天气变化的频繁程度，其实主要取决于高低气压区的运动速度。风向的改变，在许多地方也是很大程度上取决于高气压区的中心位置分布。因此，气旋区域的运动速度与运动方向，便成了预报天气必须关注的两个主要因素。也正是这个原因，越靠近大西洋，暴风雨的强度也才会越来越大。
+
+87．气候——一年四季纷繁多样的天气变化，经过较长时间之后，慢慢会形成一些规律，这便形成了气候的概念。因此，如果一个地方一年到头的平均温度很高、雨量充沛且分布均匀的话，我们就可以说这个地方拥有高温湿润的气候。气候只是天气状况的总体性描述，所以即使两个地方的年平均温度相同，它们依然可能拥有不同的气候，比如可能一个地方一年中气温变化不大，而另一个地方的气温则可能在不同季节变化显著。因此，是很多因素决定了气候的各个具体侧面。
+
+88．气候对动植物的影响——植物受气候的影响非常明显。比如棕榈树和橘子树，如果在北方生长，一到冬天就需要被小心呵护，以免受冻。而要是在南方，则大可不必，它们会像苹果树、梨树在北方那样，生长得枝繁叶茂。玉米和小麦是美国北方地区的主要农作物，而棉花、水稻和橘子则是南方的主要农作物。
+
+要让植物繁育茂盛，则必须有适应自身的气温及水分供应，以便植物的种子能不断成熟，并在后续的生长中保持顽强的生命力。像仙人掌和丝兰棕榈一类的植物，可以在新墨西哥州和亚利桑那州的干旱地区长势良好，但是要是在路易斯安那州的潮湿气候下，则会很快凋零。
+
+由于动物需要靠植物或其他动物维持生存，因此植物必须要给草食动物提供营养，并因此也向以草食动物为食的肉食动物提供养分。这样，植物的分布便会在很大程度上影响动物的生存与生长状况。吃草的动物自然就不可能在沙漠生存，以坚果为食的动物当然也不可能在大草原生存，因为那里没有产生坚果类种子的植物。
+
+跟植物一样，动物也会受到气温和湿度的影响，不过动物可以为了躲避炎炎夏日而入水纳凉，植物却是不行的。北极熊有一身浓厚的皮毛以抵御严寒，但要是将它转移到非洲热带雨林地区，那就马上斯人独憔悴了。而在那里，狮子却生活得十分惬意。对沙漠里的骆驼而言，绿草茵茵的大草原无疑是枯燥的荒漠和葱茏的墓地而已。当一个地方的动植物在当地气候条件下已经彼此适应，如果再将它们移换至其他气候条件下，二者最终都会枯萎凋零。
+
+89．气候对人的影响——由于人类能够根据需要改变着装，也能更好地长时间储备所需食物，同时还能通过人工办法改变所处环境的温度，因此气候对人类的影响，就不像对一般动植物那样明显。
+
+除此之外，每个人都还能够在极地区域和热带区域生活一段时间，而且在长达几个世纪的生息繁衍之后，人类几乎可以适应地球上任何地方的气候。比如拉普兰人和南太平洋的岛民，在他们的移居地上都生活得很好。不过，由于地域限制，他们在那里也的确没能发挥出人类自身所能达到的最大发展潜力。气候的严酷可以消耗人的能量，而它的温柔舒适却也能够消磨我们的野心与意志。
+
+在中纬度的温带地区，有些地方到冬季植物便不能生长，而且也很难寻找到食物，有些地方由于清爽的寒冷而让人们血液循环更加活跃，还有些地方气候多变四季分明，让人们更富有生命的激情。在这一类的地方，人们对自身潜力的拓展都取得了很高的成就，人们为了生存，不断与物质气候条件斗争，但生命力不仅没有因此而歇绝，反而越挫越勇。大自然的恩赐同样没有让人类忘记奋发图强，艰苦环境往往塑造了人们战胜困苦的决心。如此说来，气候对人类的影响依旧是伟大的。到底有多伟大呢？不好说，人与自然的关系，很复杂。
+
+
+
+
+
+总结——大气层与能量、光热、水分与大陆一样，对地球上的生命非常重要。空气中含有氧气，我们的身体必须依靠它来获取热量和能量，还有二氧化碳气体，它是植物组织体的构成原料，以及氮气，它正好稀释前两种重要气体的浓度。
+
+空气的重量往往不易被人们察觉，这是因为它们在各个方向上的压力都是一致的。空气受热会膨胀，因此1立方尺的热空气比同体积的冷空气要轻得多。热空气还比冷空气更能保持水分。
+
+由空气自身重量而产生的气压，可以通过气压计进行测量。这个仪器还可以用来测量山的高度，因为不同的高度会对应着不同的气压值。风是由气压变化所引起，而风向还会受到地球自转的影响。在所有行星表面都会存在行星风，它们在地球上自然会受到地球地理及气象条件影响，因而被叫做地面风。它们中有些风会有着恒定的方向，并且有助于人类的交通贸易，这就是信风。在南亚地区，还有季风。
+
+当潮湿的空气开始冷却，它便不能再保持之前的水分。于是，这些水分将会以雨雪冰雹的形式降落地面。各地区的降雨量差别很大，有很多地方一年到头滴雨不下，而有的地方则可以超过50英尺。在美国的北太平洋斜坡地区，年降雨量大致在70英寸左右，在南太平洋斜坡则只有15英寸。东部的岩石地带相对干燥一些，但密西西比河流域及其以东地区，还是有30到60英寸的年降雨量。
+
+伴有雷电的暴风雨叫做雷雨，雷电是由不同云层所带的不同电量而引起的。高电量云层会向低电量云层放电，这就形成了划过天空的闪电，以及随后的响彻天宇的雷声。
+
+当风力极其强大，并且具有严重的破坏力所向披靡的时候，这样的风就是龙卷风了。这通常被人们说成气旋，其实准确地说，气旋是巨大的空气涡流，而且一般并不具有破坏力。
+
+所有的气流风暴都会影响天气，天气是可以预报的。一个地方长时间显现出来的天气状况变化规律，就被叫做气候。任何地方的气候都对该地区的动植物和人类产生了非常大的影响。
+
+
+
+
+
+思考题
+
+
+大气中三种主要气体各有什么特性以及主要用途？
+
+怎样才能显示出空气也有重量和压力？
+
+加热会对空气的重量和体积产生什么影响？
+
+加压会对空气的重量和体积产生什么影响？
+
+两种不同的气压计在构造上有什么区别？
+
+你在什么地方见到过绝热升温现象？
+
+你有过什么样的生活经验能体现热空气比冷空气可以保持更多水分？
+
+太阳的光热辐射会怎样地影响大气层？
+
+列举并逐一解释一下影响一个地方气温的诸多要素。
+
+风是什么原因引起的？如何测量它的速度？
+
+风会受到地球自转怎样的影响？
+
+当一只帆船从波士顿出发去往合恩角，它会穿越哪些风带？这些不同风带的风会对它的行进路线产生怎样的影响？一路上它可能会经历哪些天气状况？
+
+轮船一般在什么季节从苏伊士运河去往印度最合适？为什么？
+
+一个地方的降雨量主要取决于什么因素？
+
+美国的降雨量是如何分布的？
+
+闪电和电之间有什么关系？雷的本质是什么？
+
+你对什么电器设备比较熟悉？
+
+龙卷风和气旋之间有哪些本质上的不同？
+
+气候对动植物都有哪些重要影响？
+
+描述一下什么样的气候状况对人类的发展最为有利？
+
+
+
+
+
+译 注
+
+
+[1]在今天看来，人类所达到的高度已经远远不止这个数据了。1969年7月16日，3名美国宇航员尼尔·阿姆斯特朗、巴兹·奥尔德林、迈克尔·柯林斯与“阿波罗11号”宇宙飞船一起跨过了38万公里的征程，承载着全人类的梦想踏上了月球表面。正如阿姆斯特朗的名言，这确实是一个人的小一步，但却是整个人类迈出的一大步。他们的脚印至今还原封不动地留在月球表面。
+
+[2]中国唐朝时的著名历算学家李淳风（602－670）是世界上第一个给风定级的人。他在其著名的天文历算著作《乙巳占》中，根据树木受风影响而带来的变化和损坏程度，创制了八级风力标准，即：“动叶，鸣条，摇枝，堕叶，折小枝，折大枝，折木飞砂石，拔大树和根。”一千多年后，英国人蒲福（Francis Beaufort，1774-1857）于1805年才把风力定为12级。以后又几经修改，风力等级自1946年以来已增加到18级。
+
+[3]今天来看，这已经是确凿无疑的结论。正电根源于质子，负电根源于电子。在基本粒子的标准模型中，正负电荷是由组成质子和电子的夸克的不同组合导致，它们的组合只能导致两种不同电荷，没有第三种。
+
+
+
+
+
+CHAPTER 6
+
+THE LIVE PART OF THE EARTH
+
+地球上的生命
+
+
+90. Plants and Animals. —Plants and animals are combinations of the earth's elements endowed with life. By means of the sun's energy they are able, the plants directly and the animals indirectly, to do both internal and external work which results in growth, reproduction and other activities. Since plants and animals are entirely dependent upon the earth and sun for their existence, they, like other earth and sun phenomena, should be studied in this course.
+
+91. Plants. —Although in their lower microscopical forms it is very difficult to distinguish between plants and animals, yet the forms ordinarily seen differ greatly. Most plants are fixed and consist of root, stem and leaves, while most animals are movable and possess a variety of different parts. But some plants, like the seaweeds, appear to have no roots; some, like the dandelion, no plant stem, and some, like the cactus, no leaves.
+
+
+
+THE GRIZZLY GIANT.
+
+The monarch of all plants, 93 feet around at the base. Notice the cavalry at the foot.
+
+
+
+If we dig around the base of a tree, we find in the soil a network of roots holding firmly erect a pillar-like stem with branches bearing a profusion of leaves. If we examine these divisions carefully, we shall find that each has a distinct part to play in the life work of the tree. We shall also find (1) that plants as well as animals need air, water and other kinds of food, (2) that plants, like animals, take in, digest and assimilate food, and (3) that each in the higher forms has parts which are particularly adapted for doing these different kinds of work.
+
+
+
+ATYPICAL PLANT.
+
+Showing root, stem, leaf and flower.
+
+
+
+92. Plant Roots. —Plant roots not only usually secure the plant to the ground so that the stem may be supported, but they take up food from the soil and pass it on to the rest of the plant. In most plants all the foods except carbon and oxygen are taken in by the roots. The soil elements that the plants must have are nitrogen, potassium, calcium, magnesium, phosphorus, sulphur and iron. Water is composed of hydrogen and oxygen, while carbon, the other necessary element, is taken from the air. The soil elements must be in soluble chemical combinations, such as nitrates, phosphates, sulphates and so on.
+
+
+
+
+
+Experiment 88. —Fill three 2-quart fruit jars each about half full of distilled water. Add to the water in the first of these 1/2 gram of potassium nitrate, 1/4 gram iron phosphate, 12/100 gram calcium sulphate and 12/100 gram magnesium sulphate. Add to the water in the second jar the same ingredients with the exception of the potassium nitrate. Replace this by potassium chloride. Put the three jars where they will receive plenty of sunlight and warmth and place in each a slip of Wandering Jew about 10 inches long. Note which slip grows the most thriftily. In the third jar there is no mineral food, in the first all of this food which is necessary and in the second all the necessary food except nitrogen.
+
+
+
+
+
+In Experiment 88, it was found that in the distilled water the plant made but little growth. It did not thrive when the nitrogen was lacking, but grew very well when all the necessary elements were present. All plant foods must be in dilute solution before plants can appropriate them.
+
+
+
+
+
+Experiment 89. —In another fruit jar make a strong solution of potassium nitrate or, as it is commonly called, saltpeter. Place in this a slip of Wandering Jew as was done in the previous experiment. Does the slip grow well? It has a great abundance of nitrogen, which was found so important. Place in a similar strong solution a growing beet or radish freshly removed from the ground. Notice how it shrivels up. Place a similar beet or radish in water. It is not similarly affected. What is the effect of strong solutions on plants?
+
+
+
+
+
+If the solution is too strong, as seen in Experiment 89, the plant cannot use it. This is the reason many alkali soils will not support plants. The alkali salts are so readily soluble that the soil water becomes a solution stronger than the plants can use.
+
+
+
+
+
+Experiment 90. —Place three or four thicknesses of colored blotting paper on the bottom of a beaker. Thoroughly wet the paper and scatter upon it several radish or other seeds. Cover the beaker with a piece of window glass and put in a warm place. Allow it to stand for several days, being sure to keep the blotting paper moist all the time. When the seeds have sprouted, examine the rootlets, with a magnifying glass or low power microscope, for the root hairs which look like fuzzy white threads. Touch the root hairs with the point of a pencil. They cannot, like the rest of the root, stand being disturbed. On what part of the root do the root hairs grow? As the blotting paper dries, what happens to the root hairs?
+
+
+
+
+
+Plant roots are prepared particularly by the little root hairs, which were examined in Experiment 90, to take the film of water which surrounds the soil particles and carry this water to the stem and, through it, to the leaves. The water which the roots take from the soil is a dilute solution containing the plant food substances. Not only do roots absorb the water from the soil, but they secrete weak acids which aid in dissolving the mineral substances which the plants need. This can be seen where plant roots have grown in contact with polished surfaces, such as marble. These surfaces are found to be etched.
+
+
+
+
+
+Experiment 91. —Cut a potato in two. Dig out one of the halves into the shape of a cup and scrape off the outside skin. Fill the potato cup about 2/3 full of a strong solution of sugar. Mark the height of the sugar solution by sticking a pin into the inside of the cup. Place the cup in a dish of water. The water should stand a bit lower than the sugar solution in the potato cup. After the cup has stood in the water for some time, notice the change in the height of the denser sugar solution.
+
+
+
+
+
+Experiment 92. —Bore a 3/4-inch hole 3 or 4 inches deep in the top of a carrot. Scrape off the outside skin and bind several strips of cloth around to keep the carrot from splitting open. Fit the hole with a one-hole rubber stopper having a glass tube about 1 meter long extending through it. Fill the hole in the carrot with a strong sugar solution colored with a little eosin and strongly press and tie in the stopper. The sugar solution will be forced a short distance up the tube by the insertion of the stopper. Mark with a rubber band the height at which it stands. Submerge the carrot in water and allow it to stand for a few hours. Mark occasionally the height of the column in the tube. Taste the water in which the carrot was submerged. There has been an interchange of liquids within and without the carrot.
+
+
+
+Fig. 83.
+
+
+
+The plant root takes up its water in the same way the water was taken into the sugar solution of the potato cup or of the carrot. The water or sap within the substance of the root is denser than the soil water, just as the sugar solution was denser than the water outside. It has been found that whenever two liquids or gases are separated by an animal or plant membrane, there is an interchange of the liquids or gases, the less dense liquid or gas passing through more rapidly. This is called osmosis and is of the greatest importance to both plants and animals.
+
+All animals and plants are made up of exceedingly minute parts, called cells. Fig. 84 shows the cells in a leaf and the leaf hairs greatly magnified. The higher plants and animals are composed of vast numbers of these cells. The cell usually has a thin cell wall, which in living and growing cells incloses a colorless semifluid substance called protoplasm. This protoplasm is the living part of the plant. It is found in all the cells where growth is taking place, where plant substances are being made, or where energy is being transformed. It has the power of dividing and forming new cells, and it is in this way that the plants grow.
+
+
+
+Fig. 84.
+
+
+
+The little root hairs are one kind of plant cells. They consist of a thin cell wall within which is protoplasm and cell sap, a solution of different plant foods. Since the protoplasm and cell sap are denser than the soil water, more liquid moves into the cell than from it. A little of the cell solution does move out, however, and it is this which helps to dissolve the soil particles. The protoplasm in the cell regulates to some extent the interchange of liquids.
+
+
+
+
+
+Experiment 93. —Cut off the stem of a thrifty geranium, begonia, or other plant an inch or two above the soil. Join the plant stem by a rubber tube to a glass tube a meter long, of about the same diameter as the stem. See that the rubber tube clings strongly to both glass tube and stem. It may be best to tie it tightly to these. Support the glass tube in a vertical position above the stem and pour into it sufficient water to rise above the rubber tube. Note the position of the water column. Thoroughly water the soil about the plant. Watch the height of the water column, marking it every few hours.
+
+
+
+Fig. 85.
+
+
+
+The water taken in by the roots passes on from cell to cell by osmotic action and rises in the stem in the same way that the water rose in the tube attached to the stem of the growing plant in Experiment 93. The root pressure, together with capillarity, as seen in Experiment 54, will account for the rise of the sap in lowly plants, but the cause of the rise of the sap to the top of lofty trees is difficult to understand.
+
+Roots extend themselves through the soil by growing at the tips. Here the cells are rapidly dividing, forming new cells and building root tissue. As water is so essential, they are always seeking it and extending themselves in the direction where it is to be found. This causes them to extend broadly and to sink deeply (Fig. 86). A single oat plant has been found to have an entire root extension of over 150 feet. This seeking of the roots for water sometimes causes the roots of trees to grow into drain pipes and stop them up. For this reason the planting of certain trees near sewer pipes is often prohibited.
+
+
+
+
+
+Fig. 86.
+
+
+
+Experiment 94. —Boil some water so as to drive out the air and after it has become cool fill a 2-quart fruit jar half full. Dissolve in this all the necessary plant food as was done in Experiment 88, making the solution the same strength. Place in this a slip of Wandering Jew. Pour over the surface of the water a layer of castor oil or sweet oil. Place this jar alongside the slip in the other complete food solution, Experiment 88. Both slips have the same conditions except that the oil keeps out the air from the roots of one of them. Does the absence of air affect the growth of the slip?
+
+
+
+
+
+As the tips of the roots are delicate, it can be readily seen that if they are to grow readily the soil around them must be mellow. It was also seen in Experiment 94, that if roots are to grow they must have air, another reason for keeping the soil mellow.
+
+Roots are, however, not simply absorbers of water and dissolved food. Some of them act as storehouses for the food that the plant has prepared for future use. Beets, carrots, parsnips, turnips and sweet potatoes are examples of roots which store food ready for the rapid growth of the next year's plant.
+
+
+
+
+
+93. Stems.
+
+
+
+
+
+Experiment 95. —Examine a corn stalk. Notice how and where the leaves are attached to the stem. Do the alternate leaves come from the same side of the stem? Cut a cross section of the stalk. Notice the outside hard rind, the soft pithy material and the small firmer points scattered about in the pith. Cut a section lengthwise of the stalk and notice how these small firmer points are related to the lengthwise structure of the stem.
+
+Cut off a young growing corn stalk and place the cut end in water colored by eosin or red ink. Allow it to stand for some time and then cut the stalk off an inch or two above the surface of the water. How have "the firmer points" been affected? If possible, make the same observations and experiments on the stem of a small seedling palm tree.
+
+
+
+
+
+Experiment 96. —Examine a piece of the growing young stem of a willow, apple tree or other woody stem that shows several leaf scars. Is the arrangement of the leaves the same as in the corn stalk? Cut a cross section of this stem and examine it. Does it resemble the cross section of the corn stalk? Strip off a piece of the bark and compare it with the rind of the corn stalk. Examine carefully the smooth, slippery surface of the wood just beneath the bark. This is the cambium layer.
+
+Examine the firm wood beneath this layer. Where is the pith in this stem? With a lens you may be able to see lines radiating from the pith to the circumference of the stem. These are called the pith rays. Cut a lengthwise section of the stem and examine it. Are there any fiber-like bundles as in the corn stalk? Cut off a piece of the stem already examined having the bark on it, or a piece of sunflower stem, and place the end of it in colored water. Allow it to remain for some time and then cut a cross section above the point where it was in the water. Has the water risen and colored this cross section as it did the cross section of the corn stalk?
+
+
+
+
+
+A PINE TREE.
+
+Notice the erect position of the stem.
+
+
+
+Stems vary greatly in the positions they assume. Some rise firmly erect from the root, like the oak and the pine; some cling to supports, like the grape and the ivy; some twine around supports, like the bean; some creep upon the ground, like the strawberry; some grow in the form of a thickened bulb like the onion (Fig. 87); some, like the cacti, assume a fleshy leaflike, though leafless form; some, like the nut grass, Johnson grass and witch grass, grow underground and send up shoots, and some stems store up food underground in tubers, like the potato (Fig. 88), from which the next year's plant may grow.
+
+
+
+AN IVY BRANCH.
+
+Notice the tiny root-like appendages by which it clings to its support.
+
+
+
+
+
+Fig. 87.
+
+
+
+
+
+Fig. 88.
+
+
+
+Notwithstanding all the diversity shown by the stem, its principal functions are to support the leaves, so that they will best be exposed to the light, and to conduct the food solutions from the root to the leaves. The part of the stem through which the cell sap flows was seen in Experiments 95 and 96.
+
+There are two great types of stems, one represented by the corn stalk and palm and the other by the willow, sunflower and bean. On account of the structure of the seeds these are called, respectively, monocotyledonous (one seed leaf) and dicotyledonous (two seed leaves). That these differ greatly in their appearance was seen in Experiments 95 and 96, where the two kinds of stems were compared. It was also found in these experiments that, in the first, the red colored water that took the place of the sap rose in the fibrous bundles scattered through the pith, while in the second it rose through the woody tissue within the bark.
+
+
+
+
+
+Experiment 97. —Examine a cross section of a hardwood tree several years old, and if possible of a palm. Notice the ring-like arrangement of the layers in one and the absence of all such arrangement in the other.
+
+
+
+
+
+In Experiment 97, when the cross section of a dicotyledonous tree was examined, it was found to be composed of circular rings, but no such rings are found in the cross section of the monocotyledonous tree. When later we examine the seeds of corn and bean, we shall find that they also differ very much.
+
+
+
+A BANYAN TREE.
+
+Some of its branches descend, and take root in the ground, and so appear like stems.
+
+
+
+When the bark is removed from a stem, like the willow or apple, the soft smooth layer underneath is found to be composed of living cells. This is called the cambium layer. During the season of growth, these cells are continually subdividing and forming new cells, thus adding to the thickness of the stem. The age of a tree can be determined by counting these rings. No such layer is found in the monocotyledonous stems. Grafting (Fig. 89) and budding (Figs. 90 and 91) are processes of bringing the cambium layers of two trees of similar kinds in contact and keeping them protected so that they will grow together. In this way, many of our finest species of fruit are propagated.
+
+
+
+
+
+Fig. 89.
+
+
+
+
+
+Fig. 90.
+
+
+
+
+
+Fig. 91.
+
+
+
+Experiment 98. —Examine several growing stems or twigs which have buds upon them and notice how the buds are arranged. Is the arrangement the same in all? If these buds grew into twigs or leaves, would they shade each other? Is there a bud at the end of the twig or stem?
+
+
+
+
+
+If we examine the tip of a growing stem or twig, we shall find a bud. In most of the trees and shrubs of temperate regions a terminal bud is formed at the close of the growing season, and from this the shoot continues to grow the following season. Buds are also found along the length of the stem and branches, as was seen in Experiment 98. These are lateral buds and, since they are usually found in the axis of the leaf, at the angle formed by the leaf and stem, they are called axillary. In some trees the terminal buds die at the end of the growing season, and the next year's growth is due to one of the axillary buds.
+
+
+
+DIFFERENT FORMS WHICH LEAVES ASSUME
+
+
+
+94. Leaves. —If we examine the arrangement of the leaves on a plant or tree, we shall see that they do not lie one directly above the other, but that they are so arranged as not to shade each other. Their position generally is such that the broad upper surface of the leaf receives the strong light rays perpendicularly upon it. To accomplish this, the leaves in many trees are arranged spirally around the stem.
+
+The stem of the leaf itself, in some parts of the tree, often grows long and twists about, in order to push the leaf out to the light and yet not let it be wrenched away by the wind. The horse-chestnut is such a leaf. In some plants, like the sunflower, the younger leaves follow the sun all day. In other plants the rays of the sun seem to be too bright in the middle of the day and the leaves are then held edgewise to the light.
+
+A striking example of this is the compass plant, the leaves of which arrange themselves so that the sun's rays strike the broad surface of the leaves at night and morning when the rays are not very strong, but at noon the edge of the leaf is toward the sun, the leaf thus maintaining a nearly vertical position all day, with its greatest length extending in a nearly north and south line. It is the effort to regulate the amount of light falling on the leaf, and not any magnetic influence, which causes the leaf to point in the direction of the compass needle.
+
+
+
+Fig. 92.
+
+
+
+The shapes of the leaves vary greatly in different plants. Sometimes they assume very singular forms, as in the pitcher plant (Fig. 92) and Jack-in-the-pulpit. Sometimes they even become carnivorous, as in the sundew and Venus flytrap.
+
+Around the margin of the sundew leaf and on the inner surface are a number of short bristles each having at the end a knob which secretes a sticky liquid. As soon as an insect touches one of these knobs, it sticks to the knob and the other bristles begin to close in upon the insect and hold it fast. Soon the insect dies and the leaf secretes a juice which digests its soluble parts. In the Venus flytrap (Fig. 93) the leaf terminates in a portion which is hinged at the middle and has on the inside of each half three short hairs, while the outside is fringed by stiff bristles. As soon as an insect touches the hairs, the trap closes rapidly upon it and stays closed until it is digested, when the trap again opens. Carnivorous plants of this kind usually grow in places where it is difficult to get nitrogenous foods, and may have adopted this way to supply the need.
+
+
+
+Fig. 93.
+
+
+
+Some leaves extend themselves into spiny points, like the thistle (Fig. 94), in order to keep animals from destroying the plant, or they may develop a sharp cutting edge, like some grasses, or emit a bad odor, or have a repugnant bitter taste.
+
+
+
+Fig. 94.
+
+
+
+The veins or little ridges extending through the leaf from the leaf stem vary (Fig. 95.) Sometimes these veins extend parallel to each other through the leaf, as in the corn and palm. This is generally characteristic of monocotyledonous leaves. In other leaves, the veins form a network, as in the maple and apple. This is characteristic of dicotyledonous plants.
+
+
+
+Fig. 95.
+
+
+
+Experiment 99. —Place the freshly cut stem of a white rose, white carnation, variegated geranium leaf, or any thrifty leaf which is somewhat transparent, in a beaker containing slightly warmed water strongly colored with eosin. Allow it to remain for some time. The coloring matter can be seen to have passed up the stem and spread through the leaf or flower.
+
+
+
+
+
+The great function of the leaf is to manufacture plant foods. The leaf is so constructed that air can enter it and come in contact with its living cells, as does the water coming up from its roots. The circulation of water in the leaf was seen in Experiment 99. There is in the living cell of the leaf a green substance called chlorophyll. This has the power to utilize the energy of sunlight and to combine carbon dioxide, a gas which makes up a small part of the air, with water from the roots, forming a substance which probably at first is grape sugar, but which in many leaves is changed at once into starch.
+
+
+
+
+
+Experiment 100. —Boil a few fresh bean or geranium leaves for a few minutes in a beaker of water. Pour off the water and pour on enough alcohol to cover the leaves. Warm the alcohol by putting the beaker in a dish of hot water. When the leaves have become colorless, remove from the alcohol and wash. Place the leaves in another beaker and pour on a solution of iodine. (This solution can be made by dissolving in 500 cc. of water, 2 grams of potassium iodide and 1/2 gram of iodine. The solution should be bottled and kept.) If the leaves turn dark blue or blackish, starch is present.
+
+
+
+
+
+Experiment 101. —Place a thrifty geranium or other green plant in darkness for two or three days and then treat the leaves as was done in Experiment 100. Do they show the presence of starch? The direct Presence of the sun's energy in the form of light is necessary for the formation of starch in the leaves.
+
+
+
+
+
+It was found in Experiment 100 that leaves exposed to the sun contained starch, and in Experiment 101 that leaves which had been deprived of sunlight did not have starch. The starch disappeared while the plant was in darkness. Carbon dioxide is composed of carbon and oxygen; and water of hydrogen and oxygen. In the manufacture of starch by the chlorophyll some of the oxygen is not used and becomes a waste product which the leaves throw off. This is seen in Experiment 102.
+
+
+
+
+
+Experiment 102. —Under an inverted funnel in a battery jar, place some pond scum or hornwort. Fill the jar with fresh water and over the neck of the funnel place an inverted test tube filled with water. When placed in the sunlight, bubbles of oxygen will rise into the test tube and collect. The oxygen can be tested by turning the test tube right side up and quickly inserting a glowing splinter. If the splinter bursts into a flame, oxygen is present. (A freshly picked leaf covered with water and put in the sunlight will be seen to give off these bubbles.) After a small amount of gas has been collected in the test tube, mark the height of the water column and place the battery jar in the dark, allowing it to remain there for ten or twelve hours. No oxygen is given off in the dark. Place the jar in the light again. Oxygen is given off. Is the sun's energy needed to enable the plant to give off oxygen?
+
+
+
+Fig. 96.
+
+
+
+The starch manufactured is insoluble in water and is stored in the leaf during the day. But at night, when the leaf is not manufacturing starch, it is able to digest the starch by means of a special substance, leaf diastase, which it forms. This changes it into sugar, which is soluble and which flows to other parts of the plant. Compounds such as starch and sugar, in which there is only carbon, hydrogen and oxygen, are called carbohydrates.
+
+The cells in the leaf and in other parts of the plant have the power to change the sugar and combine it with other substances contained in the sap, thus forming more complex chemical compounds. These contain nitrogen and sulphur, besides the elements of the sugar. Such compounds are called proteins. They are essential to the formation of plant protoplasm and are very important as animal foods.
+
+
+
+A FOREST OF PINES.
+
+From the sap of these, turpentine and resin are made.
+
+
+
+The digested and soluble substances which are prepared by the leaves are transported to other parts of the plant, where they are combined by the protoplasm of the living cell with other substances contained in the cell sap. Thus the protoplasm itself is able to increase and form new cells as well as other substances, such as woody tissue and oils and resins. In forming these substances the plant uses oxygen just as animals do. If air is kept from the roots of certain plants, as was seen in Experiment 94, the plants cannot live.
+
+These food substances which plants make by using the energy supplied by the sun are the bases of all plant and animal life. The sun's energy stored up in the green leaf is the source of all plant and animal energy. If it were not for the leaf manufactory run by the sun's power, life, as we know it, would cease. Even white plants, like the mushroom, must live on the food manufactured by the chlorophyll of the green plants.
+
+
+
+
+
+Experiment 103. —Procure a small thrifty plant growing in a flower pot. Take two straightedged pieces of cardboard sufficiently large to cover the top of the flower pot and notch the centers of the edges so that they can be slipped over the stem of the plant and thus entirely cover the top of the flower pot. Fasten the edges of the cardboard together by pasting on a strip of paper. The top of the pot will now be entirely covered by the cardboard but the stem of the plant will extend up through the notches of the edges. Cover the plant with a bell jar. No moisture can get into the bell jar from the soil in the pot as it is entirely covered. Set the plant thus arranged in a warm sunny place. Moisture will collect on the inside of the bell jar. This must have been given out by the plant leaves.
+
+
+
+Fig. 97.
+
+
+
+Since all the processes of forming new material by the plant require large amounts of water, it can readily be seen why water is so essential to plant development. The water from which the food materials have been taken is thrown off by the leaves, as seen in Experiment 103. The amount of water thus thrown off by plants is very great. A single sunflower plant about six feet tall gives from its leaves about a quart of water in a day, and an acre of lawn in dry hot weather gives off probably six tons of water every twenty-four hours.
+
+
+
+A SUNFLOWER PLANT.
+
+
+
+If the water passes out of a plant too rapidly so that there is not enough left to provide for the making and transporting of the food, the work of the plant cannot be carried on, and the plant dies. It is on account of this that many plants are especially prepared to retain their water supply. In almost all plants the stomata, or little pores in the leaf through which the water passes out, close up when too much water is being lost.
+
+In some plants, like the corn, when the root cannot supply sufficient moisture, the leaves curl up and thus present less surface for evaporation. In trees like the eucalyptus the leaves hang vertically when the sun gets too bright and present their edges to the sun's rays. Some leaves, like the sage, are especially prepared to conserve their moisture by having their surfaces covered with hairs. Others have a waxy covering, as the cabbage and the rubber tree. In some plants the leaves are very small and have few pores, as the greasewood of the desert, and some have done away with leaves altogether, as the cactus. It is because the roots cannot supply sufficient moisture where the ground freezes in the winter that trees having large leaves shed them, and only trees like the pine whose needle-like, waxy leaves give off almost no moisture can retain theirs.
+
+
+
+EUCALYPTUS LEAVES.
+
+
+
+95. Flowers. —The stem not only bears leaves but, in the higher kinds of plants, it bears flowers. The function of the flower is to produce seeds and provide for the continued existence of its kind. If the flower of a buttercup, quince, cassia, or geranium is examined, it will be found to be made up of four distinct kinds of structures.
+
+Around the outside is a cluster of greenish leaves. This is called the calyx. Within the calyx is the corolla, a cluster of leaves which in many plants are colored. Within the corolla are a number of parts consisting of a rather slender stalk with an enlarged tip. This tip is called the anther, and the stalk and anther together, the stamen.
+
+
+
+FLOWER, SHOWING COROLLA, STAMEN AND PISTIL.
+
+
+
+In the center of the flower are the pistils. At the top of a pistil is generally a somewhat enlarged portion, the stigma, which is sticky or rough; and at the bottom there is an enlarged hollow portion, the seed-bearing part, called the ovary. These two parts are connected by the stalk-like style. The stamens and pistils are the essential parts of the flower, the calyx and corolla being simply for protection or assistance. All flowers do not have these four parts, but every flower has either stamen or pistils or both.
+
+
+
+PINK GENTIAN.
+
+Showing the anthers which are covered with pollen.
+
+
+
+The anther produces a large number of little granular bodies, called pollen grains, each of which consists of a free cell containing protoplasm. When the pollen grains are ripe, the anther opens and exposes them. If a pollen grain of the right kind falls upon a stigma, it grows and sends down a tiny tube through the style into the ovary, where a little protoplasmic cell, called the egg cell, has been produced. The essential parts of these two different kinds of protoplasms unite and a new cell is formed.
+
+This new cell grows and divides into more cells, thus forming the young embryo of a new plant. This embryo is the living part of the seed and around it usually a great deal of plant food is stored, so that when it begins to grow it will have plenty of nourishment until it is able to develop the roots and leaves necessary to prepare its own food.
+
+Embryos cannot be produced unless pollen grains and egg cells unite, so it is absolutely essential that the right kind of pollen grains be brought to the stigma. Some stigmas are able to use the pollen grains produced by the anthers of their own flowers, but others can only use pollen from other flowers and other plants. It is therefore necessary that these pollen grains be carried about from flower to flower if fertile seeds are to be produced.
+
+
+
+MINT FLOWER.
+
+
+
+In some cases the pollen is borne about by the wind, as in the case of corn. In this way an exceedingly large number of pollen grains are wasted, as can be seen by the great amount of yellow pollen scattered over the ground of a cornfield when the corn is in bloom. In the corn each one of the corn silks is a pistil and a seed is produced at its base if a pollen grain lights upon the stigma at its upper extremity. The flowers of walnut and apple trees are fertilized by wind-blown pollen.
+
+The pollen of very many plants, however, is carried about by humming birds, bees and other insects. As the bee crawls into the flower to get the nectar at the bottom, it brushes against the anther and some of the pollen grains become attached to it. These, later, are rubbed off by the rough or sticky stigma of another flower which the bee has entered and thus the flower is fertilized. The humming bird, by reaching its long slender beak down into the long narrow tube formed by the corolla of the "wild honeysuckle" (Fig. 98), brushes upon the stigma the pollen grains it has obtained from another flower and thus distributes pollen from flower to flower. In no other way could these plants be fertilized.
+
+
+
+Fig. 98.
+
+
+
+The beautiful colors of flowers and the sweet nectars that many of them secrete are the adaptations of the plant for enticing insects to enter them and bring to their stigma the pollen from other flowers, or take from their anthers pollen needed to fertilize another similar plant.
+
+Some flowers are so constructed that only certain insects can fertilize them, the wild honeysuckle requires the humming bird, the red clover the bumble-bee (Fig. 99) and other plants, other kinds of insects. Flowers of some varieties of plants cannot be fertilized by flowers of a like variety. Certain varieties of strawberries, for example, need to have other varieties planted near them, if they are to prosper. Some plants need not only to have other varieties planted near, but they also require the presence of special insects.
+
+
+
+Fig. 99.
+
+
+
+One of the most striking examples of this is the Smyrna fig. For many years attempts were made to introduce this fig into California. The trees grew all right but the fruit did not mature. It was then observed that in the regions where this fig was successfully grown a species of wild fig was abundant and that the natives were accustomed to hang branches of the wild fig in the Smyrna fig trees at the time they were in flower. These wild fig trees were brought to California and grown near the Smyrna fig trees, but still figs did not mature. Upon further examination it was observed that at the time of flowering a small insect issued from the wild figs and visited the flowers of the Smyrna figs. This insect was brought to California and now it is possible to grow figs. The flower of the Smyrna fig has no stamen and it is necessary for the wild fig to furnish the pollen which is only successfully carried to the stigmas of the edible fig by the small fig-fertilizing insect.
+
+
+
+YUCCA OR SPANISH BAYONET.
+
+
+
+A somewhat similar case is that of the yucca found in the dry region of southwestern United States. This flower can only be fertilized by the aid of a small moth which flies about at night from flower to flower. It enters the flower, descends to the bottom, stings one of the ovaries, deposits an egg, then ascends and crowds some pollen on the stigma. The grub, when it hatches from the egg, feeds on the seeds in the ovary, but as there are many seeds in the flower which have been fertilized and the grubs eat only a few of these, the moth has made it possible for the yucca to produce seeds sufficient for its continued propagation, which would be impossible if it were not for the moth.
+
+These are only a few of the vast number of cases which show the close relationship existing between plants and animals and the dependence of the one upon the other.
+
+96. Seed Dispersal. —Not only must flowers produce fertile seeds, if the plants are to continue to exist, but these seeds must be scattered. To do this the seed pods of some plants suddenly snap open and spread their seeds. The touch-me-not and pea are examples of this. In some plants, like the maple, the seeds are winged (Fig. 100) and float for some distance in the air. Others, like the thistle and the dandelion, have light hairlike appendages which enable them to float away. In the case of the tumble-weed (Fig. 101) the plant itself is blown about, scattering the seeds over the fields as it bumps along from place to place.
+
+
+
+Fig. 100.
+
+
+
+
+
+Fig. 101.
+
+
+
+Some seeds are provided with hooks or barbs, like the beggar's ticks (Fig. 100), which attach the seeds to animals so that they are carried to a distance. Seeds having an edible fruit cover, such as the cherry, blackberry and plum, are eaten by birds and animals and the undigested seed deposited far away from the place where the seed grew. Seeds like the acorn are carried about by squirrels and other animals. Many seeds are able to float in water for a considerable time without being injured and are borne about by currents. Shores of streams and islands receive many of their plant seeds in this way. The cocoanut palm is a notable seed of this kind and is found widely scattered over tropical islands.
+
+
+
+
+
+SCRUB OAK BRANCH.
+
+Showing the acorns.
+
+
+
+97. Seeds and their Germination.
+
+
+
+
+
+Experiment 104. —Take two common dinner plates and place in the bottom of one of them two or three layers of blotting paper and thoroughly wet it. Place some wheat or other kinds of seeds upon this. Now invert the other plate over the first, being careful to have the edges touch evenly. This makes a moist chamber and gives the most favorable conditions for germination. Do all the seeds germinate at the same time? Does the position of the seed make any difference? What takes place first in the process of germination? What appears first, the leaf or the root? Why does the seed shrivel up?
+
+
+
+
+
+Experiment 105. —Cut open several seeds, such as pumpkin, squash, bean, corn, and drop on to the inside of each a few drops of the iodine solution made in Experiment 100. Do the seeds show the presence of starch?
+
+
+
+
+
+Experiment 106. —Soak some beans for about twenty-four hours. Rub off the skin from two or three and examine their different parts carefully. Plant the beans in a box of damp sawdust. Put the box in a warm place. Plant some corn that has been soaked for two or three days in the same box. After the seeds have been planted several days, carefully remove a bean and a grain of corn and examine. Make a sketch of each of the seeds.
+
+
+
+
+
+After a few days more remove another seed of each and examine and sketch. Continue to do this until the little plants have become quite well grown. Do the two seeds develop alike? Which of the seeds has two similar parts? These two parts are called cotyledons. What appears to be the use of these parts to the sprout? Consult the results of Experiment 104. Note the root development in each seed and the stem development. The sprouts get their food from the seed.
+
+When we examined the different seeds in Experiment 105, we found that they each contained starch. When the seeds were soaked and planted, we found that a part of the seeds began to grow, forming a sprout. This part is the embryo already described. We also saw that the bean seed divided into two like parts which gradually withered and shrank, as the sprout grew, while the corn had only one such part.
+
+These parts are called cotyledons, or seed leaves (Fig. 102). The bean seed is a dicotyledon (two seed leaves) and the corn a monocotyledon (one seed leaf). These cotyledons are the food storehouses for the germinating seed. As the sprout grew, the root, with its root hairs, developed, and the stem with its leaves. When these had grown strong enough, the cotyledons, having performed their part, dropped off. The plant was now ready to prepare its own food by the aid of the sunlight.
+
+
+
+
+
+Fig. 102.
+
+
+
+Experiment 107. —Place several beans in a tumbler of damp sawdust and put it in a warm, light place. Keep the sawdust moistened. After the beans are well sprouted, with a sharp knife cut one of the half beans or cotyledons off from a sprout. Cut both cotyledons off another sprout. Put the sprouts back on the sawdust. Do the sprouts grow as well as those of the other beans?
+
+
+
+
+
+Experiment 108. —Fill a 16-ounce wide-mouth bottle about one third full of peas or beans. Pour in water enough to more than cover them. Tightly cork the bottle and put in a warm sunny place. Put another similar corked empty bottle beside it. Allow the bottles to stand for several days until the peas have sprouted. Remove the cork from the bottle containing the peas and insert a burning splinter. Do the same to the empty bottle. Why does not the splinter burn as well in each? If on being placed in either bottle the splinter is smothered out, it shows the presence of carbon dioxide.
+
+
+
+
+
+Experiment 109. —Fill two 8-ounce wide-mouth bottles each about one third full of coarse sawdust and fill the remaining part with peas which have been soaked for a day. Pour in sufficient water to cover the sawdust. Cork one of the bottles tightly, leaving the other open. Put the two bottles in a warm sunny place. Whenever necessary, pour on sufficient water to keep the sawdust in the open bottle wet. In which bottle do the seeds sprout the better? Does air appear to be necessary for the growth of seeds? As determined by the previous experiment, what part of the air is used?
+
+
+
+
+
+We found in Experiment 107 that if the cotyledons were cut off before the sprout had become sufficiently mature, it could not continue its growth. In Experiment 108 we found that the sprouting seeds took up oxygen from the air and gave out carbon dioxide just as animals do. Energy was needed and this energy was obtained by combining the carbon in the seed with the oxygen in the air, as it is when wood is burned. We found in Experiment 109 that the seeds could not sprout well unless sufficient air was supplied. That was because there was not enough oxygen supplied to furnish the necessary energy.
+
+
+
+
+
+Experiment 110. —Place several sprouted seeds in each of two tumblers nearly filled with damp sawdust. Put these tumblers side by side in a warm light place. Cover one of the tumblers with a box painted black so as to exclude the light. In which do the seeds grow the better?
+
+
+
+
+
+After the seeds were sprouted and had begun to prepare their own food, it was found in Experiment 110 that they were not able to do this unless exposed to the light of the sun. The parent plant had stored, in a latent form in the seed, energy which it had received from the sun. This potential energy the sprout was able to change into the kinetic form by the aid of oxygen, and to use in the work of growing. After this latent energy had been expended, it had to fall back upon the direct energy of the sun which came to it in the form of sunlight.
+
+
+
+
+
+98. Fungi.
+
+
+
+
+
+Experiment 111. —Expose a piece of moist bread to the air for a short time and then put it into a covered dish so as to retain the moisture. Does any change take place in the bread? Examine with a magnifying glass the mold which appears.
+
+
+
+
+
+Experiment 112. —(1) Bruise a sound apple and place the bruised part in contact with a thoroughly rotten apple. Wrap the two up together in a wet cloth and put in a fruit jar. Seal the jar to prevent the water from evaporating. (2) Plunge a pin repeatedly first into a rotten apple and then into a sound one. Wrap the sound apple in a wet cloth and seal in a fruit jar. (3) Place a lemon which has developed a green, spongy, rotten place in it in contact with a perfect lemon and keep them where they will be moist. What happens to the sound fruits?
+
+
+
+
+
+The plants that we have so far studied are green plants and contain chlorophyll. They are able to prepare their food from the air and soil by the aid of the sun's energy. There is, however, another great group of plants which have no chlorophyll and which are obliged to live upon the food that green plants have prepared. They find this food either in the living or in the dead parts of plants or animals, the animals having digested it from plants or other animals who originally obtained it from plants.
+
+Plants that have no chlorophyll and live upon the food green plants have prepared are called fungi. The bacteria belong to this group. If plants live upon living plants or animals, they are called parasites, if upon dead ones, saprophytes. Plants of this kind are exceedingly important, although many of them can be seen only with the microscope. Without them the earth would soon become uninhabitable. Some of them are injurious to plants and animals, but a large number are most beneficial.
+
+
+
+MISTLETOE GROWING ON AN OAK.
+
+An interesting parasitic plant.
+
+
+
+These plants cause the decay of dead animal and vegetable matter. If it were not for them, all the plants and animals that die upon the earth would encumber its surface indefinitely with their bodies, and none of the material that they have taken from the soil would return to fertilize it. These plants make possible the manufacture of vinegar, some cheeses and a great many other things which we use daily.
+
+On the other hand, the decay in fruit, the mold on bread, the corn smut, the smut on oats and barley, the potato blight, the scabs of apples and potatoes, the rusts on grains and many other common plant diseases are simply fungus plant growths. The wheat rust alone costs the United States many millions of dollars each year. Thousands of feet of timber are destroyed yearly by the wood-destroying fungi. Dry rot of timber, as it is called, is due to a fungus growth. The fight against these harmful fungi costs millions of dollars each year.
+
+But some fungi are exceedingly useful. The fungus most commonly made use of is the yeast plant. In bread making, yeast which contains the little yeast plants is mixed thoroughly into the material which is to compose the bread, and the bread is then put into a warm place to rise or, more exactly, to grow yeast plants. If the materials and the temperature are right, the yeast plants grow very rapidly, feeding upon the material of the dough and changing the sugar into carbon dioxide and alcohol. Little bubbles of gas are developed throughout the dough, making it slightly porous.
+
+The bread is then kneaded to mix the greatly increased number of yeast plants still more thoroughly and is allowed "to rise" again. The plants are by this time very uniformly scattered through the dough and they develop little bubbles of carbon dioxide throughout the mass so that a light sponge results. When this is heated in the oven, the tiny bubbles of gas expand, making a more porous sponge, the alcohol evaporates, and the dough hardens, thus forming light bread. Although the study of these minute fungi is very interesting, it must be done by aid of the microscope and will not be attempted here.
+
+We are most of us familiar with some of the larger fungi such as the mushrooms (Fig. 103) and toadstools. Mushrooms are widely used as a delicacy and their growth is an important industry in some sections. They are grown in soils very rich in humus and generally in dark, cellar-like places. The mushrooms that grow wild in the woods are abundant in some localities but should not be used for food unless most carefully examined by some one who is expert in determining the different species. There are several species of mushrooms which are exceedingly poisonous. For one of these there is no known antidote. The general structure of these larger fungi can be seen by examining a mushroom obtained from the market.
+
+
+
+Fig. 103.
+
+
+
+Experiment 113. —Place a slice of freshly boiled potato in each of five clean 4-ounce wide-mouth bottles. Close the mouths of the bottles with loose wads of absorbent cotton. Place four of these bottles in a sterilizer and sterilize for half an hour. Allow one bottle to remain unsterilized. (A sterilizer can be made by taking a covered tin Pail and putting into the bottom of it a bent piece of tin with holes punched in it to act as a shelf on which to put the bottles. A shallow tin dish with holes in it is good for the shelf. There must be holes so that the steam will not get under the shelf and upset it. Fill the sterilizer with water to the top of the shelf and place the bottles on the shelf. Keep the water boiling.) A reliable inexpensive sterilizer is the pressure cooker shown in Figure 104.
+
+
+
+Fig. 104.
+
+
+
+Take the bottles out and allow them to cool. Remove the cotton from one of them for several minutes and then replace. Run a hat pin two or three times through the flame of a Bunsen burner to sterilize it and place it in the water of a vase which has had flowers in it for some time. Carefully pulling aside the edge of the absorbent-cotton stopper in the second bottle, insert the pin and place a drop of the vase water on the surface of the piece of potato. After having sterilized the pin again, rub it several times over the moistened palm of the hand and then, using the same precautions as before, scratch the potato in the third bottle. Keep the fourth bottle just as it was taken from the sterilizer, as an indicator, that is, to see whether the bottles were thoroughly sterilized. Put all of the bottles away in a warm place and observe them each day for several days. The spots appearing on the pieces of potato are bacteria colonies.
+
+
+
+
+
+99. Bacteria. —The nitrogen-fixing bacteria were considered, to some extent, under soil, but, as these soil bacteria are but few in com parison with the great number of species found existing almost everywhere upon the earth's surface, bacteria will be further considered here. In Experiment 113 we found that if substances are left exposed to the air they soon undergo certain changes, which they are free from when properly protected. These changes are due to bacteria.
+
+The bacterium is a single-cell plant, probably the simplest of all plants; it can only be seen with a high-power microscope. Bacteria are rod shaped, thread-shaped, screw-shaped or have various other forms (Fig. 105). The protoplasm in the cell of bacteria has the power to assimilate food and build more protoplasm. When the cell has grown sufficiently, it divides into two cells.
+
+
+
+Fig. 105.
+
+
+
+A healthy bacterium grows fast enough to be ready to divide about once an hour. If it divided once an hour and each division continued to divide once an hour, in the course of twenty-four hours there would be nearly seventeen million bacteria produced. If this were kept up for some weeks, the mass of bacteria would be as large as the earth. Of course, this would mean that each bacterium had plenty of room to live in and plenty of food to live on and nothing to injure it. These conditions are not found, and each bacterium has to struggle for existence just as every other plant does. As it is, however, bacteria are numberless.
+
+Since bacteria and fungi cause the "spoiling" of food, it is necessary to find means of stopping their growth. It has been found that thoroughly smoking fish and meat preserves it; that salt acts as a preservative; that if fruit is heated to a boiling temperature and tightly sealed in cans it will keep, and that fruits do not spoil if placed in strong sugar sirups.
+
+These and many other methods are used to keep bacteria away from food and to prepare the food in such a way that bacteria cannot live in it. It is found that bacteria do not thrive as well if placed where it is cold, so foods are kept in cold places. Many bacteria cannot stand the sunlight; that is one of the reasons why it is so much more healthful to live in sunny rooms.
+
+
+
+PREPARING SMOKED FISH AT GLOUCESTER.
+
+
+
+Steam is sufficient to kill bacteria as they usually exist. Under some conditions they can, however, withstand a greater temperature than that required to boil water. We found that they did not pass through absorbent cotton. It has been discovered that certain substances, like formaldehyde and hydrogen peroxide, prevent their growth. These substances are called disinfectants.
+
+Certain bacteria thrive in the living flesh; it is therefore necessary to disinfect cuts or else blood poisoning, which is a bacterial disease, may set in. Sometimes when a rusty nail is run into the hand or foot, if the wound is not properly disinfected and cared for, lockjaw, another bacterial disease, is developed. After a wound is disinfected, it is usually dressed with absorbent cotton in order to keep out the bacteria.
+
+Bacteria are the cause of many diseases, such as pneumonia, tuberculosis, smallpox, typhoid fever and others. People having diseases of these kinds throw off great quantities of bacteria, usually called germs. If such germs are breathed into the lungs or swallowed into the stomach and intestines of other people, they give them these diseases. It is necessary, therefore, in diseases of this kind to take every precaution that the germs shall not be scattered abroad.
+
+Tuberculous patients should be exceedingly careful to use individual dishes, to cover their mouths with cloths when sneezing or coughing, otherwise they will scatter vast numbers of disease germs and become a menace to society. Although thousands are afflicted each year with tuberculosis, largely through the carelessness of those having the disease, it is a readily preventable and curable disease. The vile and dangerous habit of spitting should be abolished everywhere and public drinking cups and towels should be abolished.
+
+When diseases are very virulent, like smallpox or diphtheria, the patients are usually kept by themselves, quarantined, their rooms kept disinfected and every precaution taken that people who are susceptible to the disease shall not be exposed to the germs.
+
+When disease bacteria get established in the system, they secrete a poison called toxin, which is absorbed by the blood and carried throughout the body, thus poisoning many other parts beside those immediately attacked by the bacteria. The cells of the body at once begin to secrete a substance to counteract this poison, an antitoxin. If the vitality of the patient is great enough, sufficient antitoxin will be secreted to neutralize the effect of the toxin and the disease will be overcome.
+
+Of late years it has been found that these antitoxins can be artificially supplied or caused to develop. Thus the system may be aided in neutralizing the effect of the toxin, and in warding off the disease. By injecting these antitoxins or stimulating their development, people are now protected against smallpox, diphtheria and other diseases.
+
+Disease bacteria are not only found in the air, but also in water and milk and other kinds of food. We must therefore be very careful to keep these germs from our water, milk and food supply. Many cases of typhoid fever have been directly traced to the milk supplied by a dealer in whose family was a case of the fever. Flies (Fig. 106) are great carriers of bacteria and, by crawling over food, spread diseases.
+
+
+
+Fig. 106.
+
+
+
+Germs thrive particularly in sewers, cesspools and unsanitary places, so that these should be especially watched. The best guards against disease, however, are plenty of sunshine and air, wholesome food, sufficient rest and a tranquil mind. With these aids, the body is usually prepared in itself to kill the germs that come into it. Every day each person probably receives into his system thousands of disease germs and usually it is only when the vitality of the body is low that these germs are able to establish themselves. Right living is the great disease preventer.
+
+As has already been stated, however, disease bacteria are only a small portion of the bacteria group of plants and the usefulness of the other members of this group is far greater than their harmfulness. Science each year is becoming more and more able to fight the disease germs, but it is entirely unable to supply the necessary aid given by the useful bacteria to animals and plants and, through them, to man.
+
+100. Animals. —Animals do not take their energy directly from the sunlight, but indirectly from the latent energy stored up in the foods prepared by green plants. These foods may be eaten as stored by the plants, or they may have passed through the medium of other plants and animals. The energy thus stored up is liberated by combining the carbon with oxygen. Carbon dioxide is freed.
+
+The green plants use this carbon dioxide again and, by the aid of the sun's energy, free the oxygen and store up the carbon. Thus the cycle goes on, over and over, the plants freeing oxygen and taking up carbon dioxide, and the animals freeing carbon dioxide and taking up oxygen. The cells of plants which feed upon the food prepared by the chlorophyll of the leaves use oxygen and give out carbon dioxide just as the animal cells do; so also do other plants to some extent, but this is in small quantities.
+
+101. Classification of Animals. —For convenience of study the animal kingdom has been divided into two great classes—the invertebrates (without backbone) and the vertebrates (with backbone). The invertebrate is the much more numerous class as it contains the worms, shellfish, insects and those almost countless forms of animal life which have no internal bony skeleton and backbone. The higher animals, like fishes, amphibia, reptiles, birds and mammals, belong to the class of vertebrates. Man himself is the highest of the vertebrates, and as the purpose of this book is to study the earth and its relation to man, his structure will be studied later.
+
+102. Invertebrates; Protozoa. —The very lowest form of animal life, the protozoa, are single-celled animals. In some species they are very difficult to distinguish from plants. They are microscopic in size and most of them live in water. Our chief interest in them in the present study is that they are the cause of several kinds of disease which can readily be prevented with proper care. Malaria, and the terrible African disease called the sleeping sickness, and probably yellow fever are due to these little animals.
+
+Unlike bacteria, the protozoa do not cause disease by passing directly from one person to another, instead they need to live in some insect between whiles. In malaria and yellow fever the insect in which they live is the mosquito, and in the sleeping sickness they live in a fly called the tsetse. If a mosquito of the right species bites a person afflicted with malaria or yellow fever, some of these little animals, the protozoa, are sucked up with the blood and enter the mosquito. They grow in its body, undergoing several changes, until the animal germs are ready to be injected into their victim, when they pass into the salivary glands of the mosquito. In biting, the mosquito always injects a little saliva into the wound and with this go the germs. These enter the blood, multiply rapidly and cause the disease.
+
+
+
+A DISEASE-BEARING MOSQUITO.
+
+Greatly magnified.
+
+
+
+If mosquitoes can be kept from biting people who have these diseases or if infected mosquitoes can be kept from biting other people, such diseases will not spread. The best way to keep mosquitoes from biting is to exterminate them. Since mosquitoes breed in stagnant water, all old dishes or small pools where water accumulates should be emptied and drained. Larger stagnant pools should be drained or have a film of kerosene spread over their surface by frequently pouring a little of the oil on the water. This will keep the mosquitoes from breeding and prevent the diseases.
+
+
+
+A "MALARIAL" SWAMP.
+
+A breeding place for mosquitoes.
+
+
+
+Thus mosquitoes and flies, the summer pests, are not only exceedingly annoying, but are very likely to spread disease. The Texas fever which has caused such great financial losses to the cattlemen of the United States is caused by a protozoan injected into the cattle by the bite of a tick.
+
+103. Worms. —Another class of invertebrates is the worms. One of these, the earthworm, was found in the study of soilmaking to be very important and should be considered in this place. If an earthworm is examined, it will be seen that the body is made up of segments or rings, and that it moves by successively shortening and elongating its body. Extending through the middle of the body is an alimentary canal consisting of a mouth, gizzard for grinding food, stomach and intestines.
+
+
+
+EARTHWORM.
+
+A great helper of the farmer.
+
+
+
+Near the head is a little nerve center. The whole animal may be regarded as built up by the joining of a number of essentially similar segments. A more minute examination will show that these segments have been materially modified in some portions of the animal, but they have not been in any respect organized, as have the different parts of higher animals. This simple animal, as has already been seen, is an untiring worker in preparing and fertilizing soil for plants, and thus is a most efficient helper to man.
+
+
+
+
+
+104. Insects.
+
+
+
+
+
+Experiment 114. —Procure a grasshopper or honeybee, as a type insect, and inclose it in a small glass-covered box. Into how many parts is the body divided? Describe these parts. To which part are the legs attached? The wings? How many legs are there? How many wings? Notice the largest part into which the body is divided. Notice the eyes and the feelers, or antennoe, on the head. Write a short description of the general characteristics of the bee's body.
+
+
+
+
+
+This class contains more than half the known animal species. They are spread widely over all parts of the earth.
+
+Both good and bad insects abound. Economically, they furnish millions upon millions of dollars worth of produce every year and on the other hand destroy hundreds of millions of dollars worth of crops and trees. It has been estimated that in the United States insects destroy every year crops and trees which have a value of $50,000,000, to say nothing of the countless losses due to diseases spread by flies and mosquitoes. Not many years ago grasshoppers nearly devastated several of the middle western states.
+
+
+
+BUTTERFLIES ON ALFALFA.
+
+
+
+The most productive insects are the silk worms and the bees. Without the silk worm (Fig. 107) there would be no silk produced, and without the bee, no honey. These two products each year run into hundreds of millions of dollars. We have already seen that bees and other insects are needed also for the fertilization of flowers.
+
+
+
+Fig. 107.
+
+
+
+Among the most interesting of the insects and perhaps, everything considered, the most valuable, is the honeybee. This is the great flower fertilizer; it would fertilize about all the plants man really needs except the red clover. In the United States alone there is produced by it about twenty-five million dollars worth of honey and wax each year.
+
+In Experiment 114, it was found that the body of the bee, like other insects, is divided into three parts. These parts are called head, thorax and abdomen. The eyes and the feelers, or antennæ, are on the head. The mouth is a very complex organ, fitted both for biting and for sucking. The six legs and four wings are on the thorax. The hind leg of each working bee is so shaped and fringed with hairs that it forms a pollen basket.
+
+Honeybees live in large colonies and in the colony there are three kinds of bees, the male bees, or drones, the workers and the queen or female bee. The workers are the ones that make all of the honey and wax, do all the work of the hive and feed the grubs on rich food formed in their own stomachs, as well as on pollen mixed with honey. The grubs are the first stage in the development of the bee from the egg. The queen lays all the eggs, sometimes as many as a million. There is but one queen in each swarm. Whenever another queen is ready to be hatched, the old queen takes about half the colony and goes off to form another swarm.
+
+The wax is secreted from glands in the abdomen of the workers and with this the bees build the comb. Each cell is hexagonal in cross section and the comb is so constructed that the least possible amount of wax will inclose the greatest possible amount of honey. The nectar at the bases of flowers supplies the bee with the material from which it makes the honey. It is in seeking for this that the bee visits so many flowers and scrapes the pollen on to the different parts of its body to be borne away to fertilize other flowers which it enters. Such an interesting animal and so exceedingly useful is the bee that hundreds of books have been written about it, more than about any other domestic animal. Some of these should be read for further information concerning this most instructive animal.
+
+
+
+BEEHIVES.
+
+Hundreds of dollars worth of honey are produced here each year.
+
+
+
+105. Vertebrates.
+
+
+
+
+
+Experiment 115. —If possible, secure the skeleton of some vertebrate animal, preferably man. Notice how the bones are fitted to each other and how the joints are arranged to allow movement. Observe how carefully the brain and the spinal cord are protected, and also the thorax, which contains the heart and lungs. If a human skeleton is procured, notice the curving of the spine which enables the body to stand erect.
+
+
+
+
+
+We have just studied briefly some of the invertebrates most closely related to the welfare or injury of man. Man himself belongs to the other great class, vertebrates. The higher animals which furnish him with the greater part of his animal food also belong to this class. Although there are great variations in the structure of vertebrate animals, yet they are alike in having a backbone and an inner supporting skeleton.
+
+
+
+AHUMAN SKELETON.
+
+Notice how the bones are arranged to protect the delicate organs.
+
+
+
+The bony skeleton in the higher forms of animal life consists of a vertebral column, skull, ribs and appendages. The main skeleton protects the most delicate organs and acts as a support for the attachment of the muscles. The appendages, like the legs and arms in man, are jointed to the central part of the skeleton, and it is the action of the muscles in moving these about the joints that makes movement from place to place possible.
+
+In the skull is situated the great nerve center of the animal, the brain, and from this through the vertebral column passes the great nerve distributor, the spinal cord. From the spinal cord, nerves are sent to all the muscles of the body, to the skin and to those organs, like the eye and the ear, which transmit to the brain impressions received from without the body. These nerves give the stimulus which causes the muscles to thicken or contract. In fact, all the voluntary movements of animals are controlled from the brain just as the movements of trains on a railroad are controlled from the despatcher's office.
+
+106. Respiration. —All animals must have a way to breathe, or energy cannot be supplied to carry on the activities of the body. Different animals breathe in different ways, but in the higher vertebrates and in man it is the same. Respiration in man will, therefore, be taken as the type.
+
+
+
+THE NERVOUS SYSTEM OF MAN.
+
+Notice how the nerves are distributed to a11 parts of the body.
+
+
+
+Air enters the body through the nose or mouth, and passes down through the windpipe into the lungs. In order to keep out dust and germs, the opening of the nose is supplied with a large number of hairs projecting from the mucous membrane which lines the whole nasal chamber. These hairs and the secretion from the membrane catch and hold most of the harmful particles. At the back of the mouth the windpipe and the throat come together.
+
+
+
+THE LUNGS.
+
+They are here pulled aside to show the heart.
+
+
+
+When food is being swallowed, the passage into the windpipe must be closed, and this is done by the little valvelike epiglottis. If, in swallowing, the epiglottis is not able to close quickly enough, something may pass into the windpipe and cause choking. The windpipe, at the upper part of the chest, branches into two parts, one branch going to each of the lungs.
+
+The lungs fill the upper part of the chest and enfold the heart. In them the air tubes divide again and again, forming a vast network of tubes which grow smaller and smaller until they end in little air sacks. Interlacing with these air tubes are veins and arteries which carry the blood. The tiniest parts into which the blood vessels are divided, the capillaries, form close networks within the linings of the air sacks. The air and blood are thus separated by an exceedingly thin animal tissue, which allows an exchange of soluble materials. Thus the blood is able to take up the oxygen needed and to rid itself of the carbon dioxide and other waste products which it has accumulated.
+
+The air-tight thoracic cavity in which the heart and lungs are situated is inclosed and protected by the ribs and at the lower part by a dome-shaped muscle called the diaphragm. Air enters the lungs because the muscles of the chest pull the ribs so that they move upward and outward and the muscles of the dome-shaped diaphragm cause it to move downward. These two actions enlarge the thoracic cavity. The air enters in the same way that it enters a hollow rubber ball that has been compressed and then set free. When the ribs move downward and the diaphragm upward, the air is expelled as in the rubber ball when compressed.
+
+There are then two ways in which air can be made to enter the lungs, the "raising of the chest" and the movement of the diaphragm. In the proper kind of breathing these two movements go on together. The lungs are filled throughout and not simply at either the top or bottom. If this is to be accomplished, the body must be free and not restricted by tight clothing about the chest or the lower part of the trunk of the body, the abdomen. Not only is the right kind of breathing necessary for properly supplying the blood with oxygen, but also that the lung tissues themselves may be properly nourished and cared for. We should be particularly careful about this now that infectious diseases of the lungs are so prevalent.
+
+
+
+
+
+107. Circulation.
+
+
+
+
+
+Experiment 116. —If a compound microscope can be procured, tie a string tightly around the end of a clean finger, and when it has become full of blood, prick it quickly with a sterilized needle. Rub the drop of blood that comes out on a glass slide and quickly examine under the microscope. Notice the great number of round disk-like bodies, red corpuscles. Try to find an irregular-shaped body which, while the blood remains fresh, slowly changes its shape, a white corpuscle. These are rather difficult to find, but can be seen if the drop of blood is thoroughly examined quickly enough.
+
+
+
+
+
+In order that all parts of the body may be provided with the materials used in building their cells and in doing the work necessary for continued existence there must be a distributory system. This is necessary wherever diversified work is to be carried on. This necessity has brought into effect the railway and canal systems of the world. The body is a little world by itself, and it has a most complete and wonderfully adapted system for supplying the material needed and for removing the waste. The center and motive power of this system is the heart. The medium of circulation is the blood.
+
+When the blood is examined, it is found to consist of a watery liquid, called the plasma, a great number of little disk-shaped bodies, the red corpuscles, and some irregular whitish bodies, the white corpuscles (Fig. 108). The white corpuscles are protoplasmic cells having various functions and possessing the power of movement and even of working their way out of the blood vessels. The main function of the red corpuscles is to carry oxygen from the lungs to the different living cells of the body. They contain a pigment, hoemoglobin, which carries the oxygen and gives the blood its color. The plasma, an exceedingly complex fluid, is composed largely of water, but contains the nutrient and waste materials supplied by the different organs of the body.
+
+
+
+Fig. 108.
+
+
+
+The blood passes through different kinds of vessels. Those leading from the heart are called arteries, and those returning to the heart are called veins. As the arteries proceed out from the heart they divide continually, becoming smaller and smaller until they terminate in very small thin-walled vessels called capillaries. These capillaries unite and form veins. Thus the blood is continually flowing from the heart through the arteries and capillaries into the veins and back to the heart.
+
+As a rule the arteries are below the surface of the body, where they are protected, but if the finger is placed on the wrist or the side of the face near the ear, an artery can be felt through which the blood is pulsing. The veins can be seen in the back of the hand and a pin piercing the body anywhere will break open some of the capillaries and cause blood to ooze out. The capillaries spread throughout the entire tissue of the body and supply with food and oxygen the different living cells of which the body is composed.
+
+
+
+THE CIRCULATORY SYSTEM.
+
+Notice that the veins (white) are outside of the arteries (black).
+
+
+
+The heart is a muscular force pump composed of four chambers, two auricles and two ventricles. It is shaped somewhat like a pear and is situated almost directly behind the breastbone. The blood coming back from the veins flows into the right auricle, a chamber with rather flabby walls. From here, it passes through a valve into the right ventricle, which is a chamber with very thick muscular walls. From the right ventricle, the blood is driven out through the arteries, capillaries and veins of the lungs, where carbon dioxide is given off and oxygen absorbed by the red corpuscles.
+
+Returning from the lungs, the blood enters the left auricle and when this becomes full, passes through a valve into the left ventricle. This has such powerfully muscular walls that it is able to force the blood throughout the body and back again to the right auricle. As the blood leaves either ventricle, there are valves that close and prevent its return. If the hand is placed a little to the left of the breastbone, the strong contraction of the ventricle can be felt.
+
+
+
+CROSS SECTION OF THE HUMAN HEART.
+
+Showing auricle, ventricle and ventricle valve.
+
+
+
+108. The Senses. —In order that the brain may communicate with the outside world and so be able to protect the animal from destruction and to provide for its wellbeing, animals have become provided with a number of sense organs which communicate with the brain by the nerves. The most conspicuous sensations of the human body are sight, hearing, taste, smell and touch.
+
+The organ of sight, the eye, is an exceedingly sensitive, automatically adjustable camera that records through the nerves. The camera box is the hard bony socket in which it is placed, the eyelid is the shutter, and the iris, the diaphragm. The iris is the membrane in the front of the eye which opens or contracts to let in more or less light. In the center of it is a hole, the pupil.
+
+Back of the shutter, or iris, is a small adjustable lens and beyond this the sensitive plate, the retina. Between the iris and the front of the eye is a watery-like material. the aqueous humor, which keeps the front of the eye extended into its rounded form. Back of the lens is a thick, transparent, jelly-like material, the vitreous humor, which holds the retina extended and keeps the eye from collapsing.
+
+
+
+CROSS SECTION OF THE HUMAN EYE.
+
+The pupil is the opening between the upper and lower parts of the iris as shown in the figure.
+
+
+
+Instead of moving the retina back and forth to focus a picture, as is done with the ground-glass plate in a camera, the eye lens is capable of adjusting itself so as to focus objects which are at different distances. Leading back to the brain from the retina is the optic nerve, which carries the impressions made on the retina to the brain where they are interpreted into the sensation of sight.
+
+This rough comparison is by no means a description of the eye, for it is a most complex and wonderful organ, vastly superior in construction to a camera. A technical description would, however, be out of place here.
+
+
+
+CROSS SECTION OF THE HUMAN EAR.
+
+
+
+The ear, which is the sound transmitter, consists of the outer ear, which is so arranged as to catch the sound waves and converge them upon the ear drum. The ear drum is a thin membrane stretched tightly across a bony opening and vibrating when the air waves strike it, as a drum does when struck by the drumstick. On its inner side the drum is attached to the inner ear by a chain of three bones. The sensitive cells of the inner ear transmit the impressions made by the sound vibrations through the auditory nerve to the brain, where they are interpreted into the sensation of sound.
+
+On the tongue and in the nose are cells which transmit to the brain the impressions produced upon them by different qualities, the one of solutions and the other of gases. The sensations thus produced are called taste and smell.
+
+The sensation of touch originates in the skin and is much more acute in some portions than in others. The tips of the fingers in the blind are often trained to such delicate perception that they, in a great degree, take the place of the lacking sense organ. These sensations, like all others, are carried to the brain by the nerves and there interpreted into the sensation of touch.
+
+
+
+
+
+109. Food.
+
+
+
+
+
+Experiment 117. —Chop a piece of the white of a hard-boiled egg into pieces about as large as the head of a pin and place in a test tube. Chop up another piece much finer than this and place it in a second test tube. Make a mixture of 100 cc. of water, 5 cc. of essence of pepsin and 2 cc. of hydrochloric acid. Pour into each test tube enough of this mixture to cover the white of egg to a considerable depth. Shake thoroughly and put in a place where the temperature can be maintained at 37° C. or 98° F. A fireless cooker or a bucket of warm water is good for this. Allow to stand for several hours, keeping the temperature constant. The white of egg is dissolved, the action being more rapid in the second tube. Try the same experiment using water; using dilute hydrochloric acid. Do these have the same effect as when used with the pepsin? The pepsin solution is an artificial gastric juice.
+
+
+
+
+
+In order that the work of the body may be carried on, food is required. This food may be supplied by either animals or plants. The original source of all animal and plant food, as has been seen, is in the chlorophyll manufactory of the leaf and green stem. After this leaf food has been manufactured, it is simply modified by the plants and animals through which it passes. The food is used (1) in growing new cells, (2) in repairing cells that have been used up or destroyed, (3) in providing energy to carry on the activities of the body and maintain its heat or (4) in doing external work, such as moving the body itself from place to place or moving other bodies.
+
+To furnish any of this energy, the cells must be able to combine food with oxygen. To do this the food must be digested or prepared so that it can pass through animal tissue. In the higher animals, a complicated apparatus is provided to accomplish this. In man, it is briefly as follows: a long continuous tube, the food-tract or the alimentary canal (Fig. 109), extends through the body. Different portions of this tube are adapted to different processes. In the mouth, the teeth grind the food into small bits and mix it with the saliva. This is an exceedingly important part of the process, because if the food is not ground fine, the digestive juices cannot readily get at it, and the whole process of digestion is greatly retarded. Thus much more energy is expended than otherwise would be. The saliva is necessary to digest some of the starch and to aid in the further digestion.
+
+
+
+Fig. 109.
+
+
+
+The food passes from the mouth down the throat and through a valve into the stomach. This is a large pouch which will hold usually from three to four pints. It has muscular walls which enable it to contract and expand, thus keeping the food moving about so that it is thoroughly mixed with the gastric juice. The gastric juice is secreted by little glands thickly imbedded in the lining of the stomach. Artificial gastric juice was made in Experiment 117. Some of the proteins (Experiment 119) are digested in the stomach, although the larger part of digestion takes place in the small intestine.
+
+From the stomach the food passes through a valve into the small intestine. This is a complexly coiled tube which fills the larger part of the abdomen. The inner wall of the tube is lined with glands which secrete digestive juices, and into the intestine are poured the secretions from two large glands, the pancreas and the liver. The small intestine is the great digestive organ of the body. Here the fats and oils (Experiment 120) are digested and the digestion of the starches and proteins is completed. The small intestine opens through a valve into the large intestine, a tube five or six feet long decreasing in size toward the exit to the body. There is little digestion in the large intestine.
+
+The changes that take place in the food as it passes through the alimentary canal are very complex, but during its progress the valuable part of the food is so changed and prepared that it can be absorbed by the blood and transported by it to the different parts of the body where its energy is needed. Absorption takes place all along the alimentary canal wherever the food has been sufficiently prepared.
+
+
+
+
+
+110. Necessary Foods.
+
+
+
+
+
+Experiment 118. —Place in different test tubes small amounts of (1) corn starch, (2) grape sugar, (3) scrapings from a raw potato, (4) flour, and (5) the white of an egg. Pour in a little water and shake thoroughly. Drop into each tube a few drops of the iodine solution prepared in Experiment 100.
+
+
+
+
+
+Experiment 119. —Place in test tubes small quantities of (1) the white of a hard-boiled egg, (2) tallow or lard, (3) grape sugar, and (4) any other food which may be handy. Pour a little concentrated nitric acid into each tube and allow to stand for a minute. Be careful not to get the nitric acid on the clothes or hands. Pour the acid out into a slop jar and wash the substances with a little water. Pour off the wash water and pour on a little strong ammonia. If the substances turn a yellow or orange color, proteins are present. Which substances contain proteins?
+
+
+
+
+
+Experiment 120. —Gasoline vapor is very inflammable, so be sure in this experiment that there is no flame in the room. Place about a spoonful of (1) both the white and the yellow of an egg, (2) flaxseed meal, (3) yellow corn meal, (4) white flour, and (5) other foods it is desired to test in separate evaporating dishes or beakers near an open window. Pour on to these enough gasoline to more than cover them and stir thoroughly. Cover the evaporating dishes and allow to stand for ten or fifteen minutes. Pour the gasoline off into a beaker and set the beaker outside the window until the gasoline has evaporated. If there is anything left it must have been dissolved from the food. If a substance remains, place a drop of it on a piece of paper. Smell of it. Try to mix it with water. Rub it between the fingers. Try any other fat or oil test of which you can think.
+
+
+
+
+
+Experiment 121. —In a place where there is a good draft so that odors will not penetrate the room, burn in an iron spoon over a Bunsen burner (1) small pieces of meat, (2) a little condensed milk or milk powder, (3) part of an egg, and (4) any other food. Is there a residue left after burning? If so, this is mineral matter.
+
+
+
+
+
+In Experiments 118—121 we found that our ordinary foods are of three great groups of chemical compounds, carbohydrates (starches and sugars), proteins, and fats or oils. The common foods that consist largely of proteins are lean meat, cheese, eggs, beans, and peas. Those largely composed of carbohydrates are most cereals, vegetables and fruits. The fats are butter, pork, nuts and chocolate. Milk contains all three of these compounds in approximately the proportion needed by the body.
+
+Careful experiment has shown that the average, full-grown American needs each day two to three ounces of proteins, about four ounces of fats and a pound of carbohydrates. The weight of food eaten, however, is very much greater than this, as all foods are composed largely of water. The proteins are needed for growth and repair, since the living part of the cells, the protoplasm, is composed of proteins. The rest of the food furnishes energy.
+
+
+
+A DATE PALM.
+
+
+
+Until recently, it was thought that a great deal of meat was necessary to furnish the energy needed for hard muscular work. But now investigation has shown that this energy can better be supplied by other foods and that eating too much meat is not only needlessly expensive but bad for the system. The staple food of northern Africa and southwestern Asia is the date palm, which is admirable for hot climates. In cold regions where the body requires great energy to keep up its heat, much fat is eaten and sugar, if procurable. The exact kind of food used must always depend largely on its availability and on the tastes of the individual, but the diet should be so varied as to contain sufficient of each of the three great classes of foods.
+
+Besides the necessary foods, most individuals desire especial additions for relishes and beverages. These commonly consist of spices, tea and coffee and other like materials. When used in moderation, they are usually a benefit, as they stimulate the appetite. But excessive use is harmful.
+
+Alcohol, except possibly in exceedingly small quantities, cannot be considered a food, and as a stimulator for the appetite it should not be used. Many careful experiments have shown that while it may stimulate the body temporarily, it does not enable it to do more work. Instead, those using it cannot do as much work, or withstand as great physical or mental strain, as those not using it. Even if it were not for the ungovernable appetite which its use almost invariably engenders, and for the degrading influences with which its use is usually surrounded, its physiological action is such as to lessen the body's vitality, decrease its resistance to disease, and dull its nervous and mental efficiency.
+
+
+
+A BUNCH OF DATES.
+
+
+
+Careful scientific experiments have also been made upon the effect of tobacco. Although there are differences of opinion about its effect upon fully matured adults, there is no such difference of opinion in regard to its effect upon those who have not stopped growing and are not yet fully matured. Measurements and comparisons made in regard to the physical development, endurance and mental ability of a large number of college men has shown conclusively that those who have not used tobacco, as a rule, have better physiques, are better students and can stand more physical exercise than those who have used it. In the competition for athletic teams it is found that only about half as many of those who have used tobacco make good, as of those who have not used it.
+
+
+
+COFFEE PLANT.
+
+Showing the clusters of beans from which coffee is produced.
+
+
+
+111. Preparation of Foods. —When foods are appetizing, look good, smell good and taste good, both the saliva and the gastric juice are secreted in larger quantities, so that this sort of food, when taken into the system, is more readily digested than food which is not attractive. One of the reasons for cooking food is to render it appetizing, and this should never be lost sight of by the cook. Cooking also softens and loosens the fibers of meats and causes the cell walls of the starch granules to burst, thus rendering it possible for the digestive juices to attack the food more readily. In addition, cooking kills the germs and other parasites that are sometimes found in foods.
+
+To cook food properly is a fine art and requires most careful study and great skill. The science of providing economically the kinds of food necessary and of cooking these properly so that they will be attractive, easily digested and will lose none of their nutritive value, is one that is at present in its infancy. Human beings, like other animals, must have a balanced ration or diet if they are to be most productive economically. They differ from other animals in having a much greater range of food possibilities and in being much more sensitive as to the appearance and taste of food.
+
+
+
+
+
+Summary. —Plants and animals form the live part of the earth. Most plants consist of root, stem and leaves. The root takes in all the plant's food except carbon and oxygen. These are supplied through the leaves. The leaves are the original food manufactories for all plants and animals. They are supported on the stem, of which there are two great classes, monocotyledonous and dicotyledonous.
+
+The stems also support the flower, which usually consists of calyx, corolla, stamen and pistils. The chief function of the flower is to produce the seeds by which the plants are reproduced. The pollen grains which are necessary for the fertilization of the egg cells are carried and spread by the wind and by insects and birds. The seeds are also scattered by the wind and by animals and sometimes by floating down streams.
+
+Besides these green plants there is another class called fungi, Instead of preparing their own food by the help of the sun and the soil, they live upon the food prepared by the green plants.
+
+Probably the simplest plants are the bacteria, single-cell plants, which multiply very rapidly. Bacteria and fungi cause many diseases as well as most of the spoiling of food. Disease bacteria are usually called germs. Their effects may be counteracted by the use of disinfectants and antitoxins.
+
+Animals take their energy indirectly from the foods prepared by green plants or by other animals. They are usually classed as invertebrate and vertebrate. The lowest form of invertebrate is the protozoön. Worms and insects are other forms of invertebrates, the importance of which is seldom realized.
+
+Vertebrates usually have a backbone, skull, ribs and appendages. In the skull is the brain, connected to the various parts of the body by nerves. Vertebrates breathe by "drawing" air through the windpipe into the lungs. This is done by the muscles of the chest and diaphragm. The lungs purify the blood, which circulates from the heart through the arteries and capillaries and back through the veins.
+
+The five senses are sight, hearing, taste, smell and touch. These sensations are carried to the brain by the nerves, and they come from the eye, the ear, the nose, the mouth and the skin, respectively.
+
+For all the activities of body and brain, food is required. As food passes through the alimentary canal, various juices are mixed with it and certain parts of it are digested and absorbed into the body. Foods are of three kinds, proteins, carbohydrates and fats, and we need a certain percentage of each for proper nourishment. Usually foods are most nourishing and most appetizing when properly cooked.
+
+
+
+
+
+QUESTIOTS
+
+
+Why should plants and animals, like other earth phenomena, be studied in this course?
+
+What are the three parts into which most plants can be readily separated? In what three respects are plants and animals alike? What do the plant roots do for the plant? How do they do it? Describe some different kinds of stems that you have seen, and explain their adaptability or lack of adaptability for making the best of the conditions where they were.
+
+What do the leaves do for the plant? How do they do it?
+
+What is the value to the plant of the flower? How are the flowers prepared to carry out their part in the life struggle of the plant? Describe any way in which you know that animals have been of assistance to plants.
+
+How do plants provide for the dispersal of their seeds?
+
+How does the seed develop into a plant?
+
+With what useful or what harmful fungi have you ever had any experience?
+
+In what ways are bacteria helpful and in what ways harmful to mankind? How are harmful bacteria guarded against?
+
+How are plants and animals mutually helpful to each other?
+
+Name and describe some of the invertebrate animals you know.
+
+How have we found that the angleworm benefits the soil?
+
+What is the use to the vertebrate of the skeleton and the nervous system?
+
+Describe how vertebrate animals breathe. Why is it vitally necessary for them to breathe freely?
+
+What is the use of the blood? How does it get around to where it is needed?
+
+Describe the ways in which man becomes aware of what is outside his body.
+
+Why is food needed? How and where is it digested?
+
+What are the three great groups into which foods are divided? Why should you not use alcohol or tobacco?
+
+Why is cookery one of the most useful arts?
+
+
+
+
+
+【中文阅读】
+
+
+90．植物与动物——植物与动物是地球生命要素完美组合的产物。通过接收到的太阳能量，它们能够直接或间接地加以利用，完成自身内外的新陈代谢，让自身得以生长、繁殖并进行其他生命活动。鉴于植物和动物都必须完全依靠太阳和地球才能生存，因此它们便也如其他与地球和太阳有关的现象一样，须放在这个大背景下进行研究。
+
+91．植物——尽管在显微镜下植物与动物不是很好区分，但在生命形式上，二者显然差异甚大。大多数植物都是结构精致而位置固定的，并都由根、茎、叶组成，而大多数动物都是可以自身运动的，并且可以控制自身的不同部分。但是有些植物，比如海藻，似乎就没有根；还有一些像蒲公英之类的植物，没有茎；而仙人掌一类的植物，没有叶。
+
+若我们在一棵树的底部四周挖下去，便会在土壤中看见纵横交错的树根，正是它们让枝叶茂盛的树干部分得以坚固地挺立。如果我们仔细观察研究植物的这些不同的部分，我们一般会发现，各部分都在植物的生命活动中扮演着不同的角色。我们尤其会发现以下几点：（1）植物和动物都需要空气、水及其他养分；（2）植物和动物都能够从外界摄取、消化、吸收食物；（3）越是高级植物和高级动物，它们肢体的部分分工便越明显。
+
+92．植物的根——植物的根，不仅能将植物固定在土壤中，让植物的枝干部分得到支撑，它同时还能从土壤中吸收养分，并输送给植物肢体的其他部分。对大多数植物而言，除了碳和氧以外，其他所有养分都是靠自身的根来吸收的。在土壤成分中，植物必须摄取的养分有氮、钾、钙、镁、磷、硫磺和铁。水由氢和氧组成，因此氢氧可以通过水来获得；另一种必需成分碳，则是从空气中获取。这些土壤中的养分必须存在于可溶的化合物中，比如硝酸盐、磷酸盐、硫酸盐等等。
+
+
+
+
+
+实验88：将3个两夸脱的罐子都装入半罐蒸馏水��在第一个罐子中再加入0.5克硝酸钾、0.25克磷酸铁，0.12克硫酸钙以及0.12克硫酸镁；在第二个罐子中放入的原料，除了用氯化钾取代刚才的硝酸钾，其余跟第一个罐子一样。然后将三个罐子都放到日照充足且温度适宜的地方，并在每个罐子中放一根10英寸长的白花紫露草，持续观察，看哪个罐子里的紫露草长势最好？在第三个罐子中没有任何无机矿物养分，第一个罐子中的所有成分都是植物所需的养分，第二个罐子中只是没有氮，其余养分也都有。
+
+
+
+
+
+在实验88中我们会发现，在没有矿物养分的蒸馏水中，植物的生长极其有限。在没有氮的环境中，长势也不是太好，但是当所有养分都具备时，长势可就非常好了。植物的养分必须存在于稀溶液中，这样才能被植物的根所吸收。
+
+
+
+
+
+实验89：在另一个罐子中装入适量硝酸钾浓溶液，也就是平常所称的硝石，再放入一根与刚才试验中一样的白花紫露草，观察一些时间，它还会长势很好吗？这次溶液中有丰富的氮，而它对植物的生长，是非常重要且是必要的。再在同样的浓溶液中放入一片刚从土地里拔出来的新鲜甜菜或者萝卜，注意观察它们是如何渐渐枯萎的。如果将甜菜和萝卜放入水中，它们则不会如此反应。如此看来，浓溶液对植物会所产生什么作用？
+
+
+
+
+
+如果溶液浓度太强，就像我们在实验89中看到的那样，植物根本无法吸收利用它所含有的养分。这就是为什么许多碱性土壤无法供养植物生长的原因。因为碱性盐一般都非常容易溶解，因此便让土壤中的水分变成了浓溶液，超过了植物所能适应并加以利用的范围。
+
+
+
+
+
+实验90：在一只烧杯下面放上三四张厚一点的彩色吸墨纸，烧杯中放入足量的水以备所需。将纸张彻底打湿，然后在上面撒上几粒萝卜或者其他植物的种子，将烧杯用玻璃片盖住，然后将它们一起放在一个温暖的环境中。如此保持几天，注意必须确保纸张始终是充分湿润的。几天之后，当种子发芽了，注意观察长出的根须，可用放大镜或者低倍显微镜，因为有些根须几乎就是细微的白色绒毛。在触碰这些根须时，最好用铅笔尖轻轻点击，因为它们非常柔弱，不像其他根须那么有韧性。观察下来，这些小根须主要生长在根的哪个部位？当这些吸墨纸干了以后，又会发生什么现象？
+
+
+
+
+
+植物的根上面都长着纤细的根毛，我们在实验90中专门对其进行了仔细观察，正是它们吸收了土壤颗粒中的水膜，并将它们运转到植物的茎干，进而又传输到叶子当中。被植物根须吸收的水分都是含有植物养分的稀溶液，在此过程中，植物的根须不仅从土壤中吸收水分，它们还会分泌出一些弱酸物质，这些弱酸物质正好又可以帮助这些植物所需的矿物养分在水中进一步溶解。如果让植物的根在一个磨光的表面上生长，比如大理石表面，便可以清楚看到这个现象，最后会发现这些光滑表面会被一点点地慢慢腐蚀。
+
+
+
+
+
+实验91：把一个土豆切成两半，将其中半个土豆挖成类似杯子的形状，并削去外皮，在杯形中倒入2/3满的浓糖水，并用一颗大头针在杯形的内壁上记下糖水的液面高度。然后将其放在一盘水中，水的液面略低于土豆杯中的糖水液面。过一段时间之后，注意观察杯中糖水液面高度的变化。
+
+实验92：在一根胡萝卜顶部钻出一个3到4英寸深、孔径大致0.75英寸的小洞，将胡萝卜外皮去掉，然后裹上几圈布条以防止胡萝卜开裂。用一个单孔橡胶塞塞住洞口，从孔中插入一根1米长的玻璃导管，然后通过它向小洞内注入红色的浓糖水，注意将胶塞压紧捆牢，让糖水在外边的玻璃管中露出一部分，然后用一根橡皮筋在液面处做下记号。再将这根胡萝卜连同其导管装置放入水中，保持几个小时。不时给玻璃管中的液面位置做下记号，品尝一下浸泡胡萝卜的水，会发现胡萝卜内外的液体出现了交换现象。
+
+
+
+
+
+植物的根吸收水分的过程，与刚才实验中的土豆杯和胡萝卜里的浓糖水吸收水分的过程是相同的。根须中所含有的水分和细胞液都比土壤让中水分更浓稠，就像实验中浓糖水比外边的水更浓一样。人们发现，如果两种液体或气体被动物或者植物的生物膜隔开，就会发生液体或气体的交换，浓度低的液体或气体会很快地进入到浓度高的液体或气体中。这就是我们所称的渗透现象，它对植物和动物都极其重要。
+
+所有的动物和植物都是由一种很微小的基本单元构成，叫做细胞。插图84就显示了一个树叶细胞的样子，我们可以看见叶毛被放得很大。高大���植物和动物所拥有的细胞数量非常之多。细胞通常都有一层很薄的细胞壁，在生长中的细胞里面，还有一种半流质的无色物质，叫做细胞质，细胞质正是植物的生命物质。人们已经发现，植物的所有生长发育、物质合成、能量传输等生命活动，都发生在细胞中。细胞还能通过分裂形成新的细胞，植物也正是通过这个方式实现生长的。
+
+纤细的根毛也是一种植物细胞，它们由很薄的细胞壁和其中的细胞质、细胞液组成，细胞液就是富含植物所需养分的溶液。由于细胞质和细胞液的浓度比土壤中的水分高出许多，因此进入细胞的液体远比其渗出的液体多。当然也有极少部分的细胞液从细胞中渗出，这在一定程度上也促进了土壤颗粒的溶化分解。细胞质也可以通过浓度变化，来调节内外环境中液体的交换。
+
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+
+
+
+实验93：将一株长势良好的天竺葵、秋海棠或其他植物的茎截断，在地面以上留大致一两英寸长就行。将这段植物的茎用一节橡胶管与一根一米长的玻璃管相连接，玻璃管与植物的茎差不多粗细，并让橡胶管与玻璃管和植物的茎紧密连接，最好用细绳拴紧。然后将玻璃管竖直支撑起来，并注入足量的水，让管中液面高于橡胶管。记下玻璃管中的液面位置，然后不断给植物周围的土壤浇水，注意观察管中液面的位置变化。
+
+
+
+
+
+植物的根吸收的水分，通过细胞间的渗透进行传输，并上升到植物的茎中。这个过程我们在实验93中可以清楚地看见，玻璃管中的水不断上升，其原因就是这样的。根部的压力，以及我们在实验54中看到的毛细现象，都可以解释水分在低矮植物中的上升传输，但是它们是怎样到达高大植物的顶端的，这个问题还有点难以理解。
+
+根须在土壤中不断延伸，是因为根须的尖端在不断地生长。在那里，细胞快速地分裂，不断形成新的细胞并成为新的根须组织。由于水对植物非常重要，所以植物的根总是顽强地寻找水源，并将它们自己不断向水分充足的地方延伸，这就导致它们有时会有偏向延伸，以及能深入地下很深的地方。一株燕麦的整个根须可以延伸到超过150英尺的长度。根的向水性会导致大树的根向着排水管不断生长，有时甚至令排水管堵塞。因此，有些种类的植物是禁止在排水管附近栽种的。
+
+
+
+
+
+实验94：将适量的水煮沸，排尽其中的空气。待冷却后装入一个2品脱的罐子，半罐水就行。像实验88那样，将植物所需矿物质溶解其中，浓度也与之前保持相同。然后在罐中放入一株白花紫露草，再倒入适量蓖麻油或橄榄油，让其在溶液表面形成一层油膜。在旁边再放置一罐含有所有植物所需的矿物溶液的液体——就跟实验88中用过的溶液一样，也放入一株白花紫露草。二者其他条件完全相同，差别仅仅是一个有油膜隔绝了空气，一个没有油膜。观察一段时间后，营养液与空气的隔绝会对植物生长造成影响吗？
+
+
+
+
+
+根须的尖端十分纤弱，因此若要它们能充分自如地生长，周围的土壤就必须足够松软。还有一个条件在实验94中已经看到了，那就是根须的生长还必须需要空气，这也是让土壤保持松软的必要条件。
+
+植物的根还不仅仅只是简单地吸收水分与分解养分，它们同时还是植物养分的储存器，可以将矿物养分存储起来供将来使用。甜菜、胡萝卜、防风草、大头菜和番薯就是很好的例子，它们的根部能储存可供下一年生长所需的大量养分。
+
+93．植物的茎
+
+
+
+
+
+实验95：仔细观察一根玉米秆，看它的叶子是如何与茎的部分相连接的。这些交替的叶子都从茎的同一侧面长出来的吗？截取一段茎秆，观察其横截面，尤其侧重观察一下茎秆中心的柔软髓质，以及散布于其中的坚硬小点。再截取它的纵截面，看这些坚硬小点是如何与茎的纵向组织相联系的。
+
+截取一段生长中的玉米秆，将其立在用曙红颜料或红墨水染红的水中，保持一段时间后，再将水面上一两英寸上方的茎秆截断，会发现这些“坚硬小点”出现了什么反应？如果可能的话，用一段小树苗做相同的观察实验。
+
+实验96：仔细观察一段稚嫩的柳树或苹果树等木质茎，注意观察其中的叶痕。它们的树叶排列跟玉米秆一样吗？截取它的一个横截面，它与玉米秆的横截面相似吗？剥落一些它的树皮细屑下来，跟玉米秆外壳进行比较。仔细观察树皮下面光滑的木质表面，这就是树木的生长层。
+
+再仔细观察生长层下面的部分，茎的核心在什么地方？在放大镜下你会看见，从核心延伸出许多放射状的线条，叫做髓射线。再观察��段茎的纵截面，它里面也有像玉米秆那样的纤维束吗？再将一段附有树皮的茎，或者一段向日葵的茎，立在有颜色的水中。放置一段时间后，在水面上方的部分截取一个横截面，也会像在玉米秆中看见得的那样，有颜色的水也上升了吗？
+
+
+
+
+
+植物的茎的外在形态变化多样，有些从根部直接挺立，向上生长，比如橡树、松树；有的需要支架来支撑，比如葡萄和常春藤；有的会在支架上缠绕生长，比如豆类植物；有的只能在地面蔓延生长，比如草莓；有的会长成一个球状茎，比如洋葱；还有像仙人掌一样肥厚的叶状茎，像香附子、假高粱、茅草一样的地下茎，还有像土豆一样的在地下储存养分的块茎，它所包含的养分可供下一年植物的生长所需。
+
+尽管茎的样式千差万别，但其最重要的功能都是为叶片提供养分与支持。因此它们最好能保有充足的光照，并能将养分从植物根部送达叶片。植物的茎中所包含的传递汁液的细胞，在实验95和96中我们已经进行了仔细的观察。
+
+植物的茎，就大体而言主要有两大类。一类可以玉米秆和棕榈树为代表，另一类可以柳树、向日葵和豆类为代表。根据其各自种子的结构不同，我们把它们分别叫做单子叶植物和双子叶植物。在实验95和96的比较观察中，我们已经充分见识了二者之间的差别。在实验的前半部分我们还发现，红色的水可以通过纤维束上升并分散到茎的核心区域；在实验的后半部分中，则发现红色的水可以通过木质组织进入到了树皮里面。
+
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+
+
+实验97：仔细观察一棵具有几年树龄的硬木树种茎干的横截面，如果可能的话，棕榈树就很合适。注意观察它截然不同的环状排列层。
+
+
+
+
+
+在实验97中，当我们仔细观察双子叶树木的横断面的时候，我们会发现一圈圈的圆环，而在单子叶植物茎干的横断面中却没有。在接下来观察玉米和豆类植物种子的时候，我们也会发现它们大不相同。
+
+当我们将柳树或者苹果树的树皮去掉后，会发现它们下边有一层由活细胞组成的光滑木质层，这就是树木的生长层。为了实现生长，这里的细胞不断分裂形成新的细胞，并因此让茎干慢慢变得粗壮。树木的年龄也可以通过这些环状物的圈数来决定，但单子叶植物的茎干中却没有这些环状排列的物质。人们经常通过嫁接发芽的方式来让两种类型相似的树种一起合并生长，并通过接触与保护的手段来让它们形成生长层。许多优良的水果品种就是通过这个方式加以繁殖栽种的。
+
+
+
+
+
+实验98：仔细观察一些长着树芽的树木的茎干与枝条，看这些树芽是如何排列的。它们的排列顺序全都一样吗？如果它们都长成枝条或者叶片，会相互遮挡吗？在茎干或枝条的顶端有树芽吗？
+
+
+
+
+
+如果我们仔细观察生长中的茎干枝条，便会发现在它们顶端都有一个萌芽。温带地区的大部分树木和灌木在每一季生长结束后，都会在枝条顶端长出一个萌芽。当下一个生长季节来临之后，它们便又会继续生长。在实验98中我们也都已经看到了，萌芽会沿着茎干和枝条均匀分布。不过这些萌芽都是侧芽，因为它们都长在叶轴上，与叶片的生长角度一致，因此它们又被叫做腋芽。有些树种的顶芽会在生长季节结束后死掉，到下一年的生长季便是腋芽接着继续生长。
+
+94．植物的叶——如果仔细观察植物叶片的分布，我们就会发现它们并不是按同一方向排列分布的，而是错落有致，互补遮掩。它们的位置排列，正好让叶面的宽阔部分可以接受到最大限度的阳光直射，因此许多树木的树叶都是沿着枝干呈螺旋状排列。
+
+树木的有些枝干有时会延伸得很长，并连同枝叶的卷曲盘旋，其目的便是让树叶能获得最大光照，并可以防止大风对树木的损坏，七叶树就是这样的典型例子。还有些植物，比如向日葵，它的嫩叶会整日朝向太阳；另一些植物则会在中午太阳光最强烈的时候，将叶片的边缘伸出以接受光照。
+
+其显著的例子就是所谓的指向性植物，其叶片的分布可以让早晚太阳光不强烈的时候，叶面的大部分都可以得到光照。而在中午的时候，叶片的边缘便正好可以接受到太阳光垂直地照射，叶片整体也会整日垂直于光照，并会在南北向上延伸得很长。这是为了调节光照才让植物显出如此特性，而并非是地磁的原因，虽然叶片的这一特点也的确类似于罗盘指针与固定方向的关联。
+
+不同的植物，其叶片的形状也各不相同。有时它们会呈现出非常奇特的样式，比如猪笼草和黄花菖蒲；有时它们还具有肉食性，比如毛毡苔和捕蝇草。
+
+毛毡苔的叶片边缘和内表面上长着大量的刚毛，每根刚毛顶端有一个能够分泌粘液的球状突出物。一旦有昆虫接触到它，刚毛便会很快将昆虫围住并死死控制。不一会儿，昆虫便会死亡，叶片的分泌物会将其分解，进而被植物体消化吸收。
+
+捕蝇草的叶片呈铰片形，两边各有三根短须，外边缘长着浓密的硬刚毛。一旦有昆虫触碰到这些短须，这个陷阱就会迅速收拢并保持关闭，直到昆虫被完全消化，然后有才慢慢打开。这类肉食性植物一般生长在含氮养分难以获取的地方，因此便采取这个办法以满足自身的需求。
+
+有些叶片会像刺一样，比如蓟类植物。其目的是防止动物对它们的伤害，有的还像一些草一样，长着刀刃状的叶片；有的会散发着难闻的气味；有的还会有一种让动物无法忍受的苦味。
+
+叶面上的纹理脉络也形式多样。有些相互平行，比如玉米和棕榈树叶片，一般单子叶植物均有这个特征。还有些页面纹路呈网状，比如枫树和苹果树叶面，这是典型的双子叶植物的特征。
+
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+
+
+
+实验99：摘取一些白玫瑰、白康乃馨、天竺葵或者任何茂盛植物的有点透明的枝叶，放入一个装有温水的烧杯中，并将水用颜料深度染红。如此保持一段时间，有色物质便会在其枝叶和花瓣上明显显现出来。
+
+
+
+
+
+植物叶片的最大功能，还是为植物制造养分。叶片的构造可以让空气能进入其中，并与它的活性细胞相接触，就像水分能进入植物根细胞一样。叶片的水分循环我们在实验99中已经看到了。植物叶片的绿色组织中，有一种物质叫做叶绿素，它能将太阳光的能量与空气中的二氧化碳相结合，再利用植物根部所吸收的水分，形成植物所需的葡萄糖或者淀粉。
+
+
+
+
+
+实验100：用烧杯烹煮几片新鲜的大豆叶片或者天竺葵叶片，让水保持沸腾几分钟。然后将沸水倒出，再倒入足够的乙醇将叶片覆盖，并将烧杯放入一盘热水中对乙醇加热。让叶片慢慢褪去颜色以后，将它们从乙醇中取出，清洗。然后将其放入另一个干净烧杯，并倒入适量碘酒（配制碘酒可将2克碘化钾和0.5克纯碘溶于500cc水中，然后密封保存）。如果叶片变成暗蓝色或者黑色，则证明产生了淀粉。
+
+实验101：将一株茂盛的天竺葵，或者其他绿色植物在黑暗处放置两三天，然后将它们的叶片按照实验100的步骤进行一次实验。其结果还会显示淀粉的存在吗？在叶片中，太阳光能量的直接对应产物便是淀粉。
+
+
+
+
+
+在实验100中我们发现，暴露在太阳光下的植物叶片含有淀粉；在实验101中我们还发现，没有接受太阳光照的叶片便不含有淀粉。当植物一直处于黑暗中时，淀粉便消失了。二氧化碳是由碳和氧气构成的，水是由氢和氧构成。叶绿素在制造淀粉的过程中，一部分氧气会以废弃物的形式排放出来，这个现象我们将会在实验102中见到。
+
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+
+
+实验102：在一个电解槽内倒置一个漏斗。漏斗下面放一些绿藻类植物比如金鱼藻，然后给电解槽倒满水，在漏斗颈上再倒置一支装满水的试管。将整个装置放在太阳下，不一会儿便会有氧气泡慢慢在试管中聚集，从而得到收集后的氧气。验证其是否为氧气很简单，拿起试管，快速插入一根带火星的小木条，如果出现火焰，则证明是氧气。（新摘取的叶片泡在水里暴露在太阳下也会出现类似的气泡）当少量气体在试管中被收集后，给其中的水面高度做下记号，然后将整个装置放在黑暗处，保持十到十二小时。会发现，在黑暗中时，不会再产生氧气。然后再将装置放在光照下，氧气又继续产生。是太阳的能量促使植物不断产生氧气吗？
+
+
+
+
+
+淀粉被制造出来后便不能溶于水，并在白天储存在植物叶片中。但到了晚上，当植物叶片不再继续制造淀粉时，它便会通过一种特殊的物质被植物吸收，这种物质就是叶淀粉酶。它将淀粉转化为糖分，而糖分则可溶于水，并会最终形成植物自身的组成部分。像淀粉和糖分一类的物质，他们只含有碳、氢、氧三种元素，因此也被叫做碳水化合物。
+
+植物的细胞能够将糖分与其他有机物结合，然后将其转化为植物汁液，进而形成更多种类的化合物。这些化合物一般除了糖分以外，还含有氮和硫磺，它们叫做蛋白质。它们对植物细胞质的形成极其关键，同时也是非常重要的动物食物来源。
+
+这些可供植物消化吸收的物质被叶片制造出来后，通过内循环被转运到植物肢体的其他部分，并在那里与其他物质一起被植物的细胞质合成为细胞液。因此细胞质本身能自我制造并形成新的细胞，还能不断形成其他物质，比如木质组织、植物油以及树脂等等。在形成这些物质的过程中，植物跟动物一样，也需要氧气。因此要是植物的根与空气隔绝，就像实验94那样，植物便不能存活。
+
+这些植物利用太阳能合成的养分物质，是动植物的生命基础，储存在植物叶片中的太阳能也是动植物能量的来源。如果没有植物叶片在太阳能量下对有机物的合成制造，生命便会终结。即使白色植物[1]，比如蘑菇，也必须仰赖绿色植物的叶绿素所合成的养分来生存。
+
+
+
+
+
+实验103：将一株生长茂盛的植物放入花盆，再用两张直边纸板覆盖花盆的上边部分，中间开孔让植物的茎能够穿过，然后将两张纸板的合缝处用纸条糊住，这样花盆的土壤部分就被完全遮盖，植物的茎从纸板开孔处伸出来，不受其影响。取一个钟罩将植物盖住，这样土壤和空气中的水分便都不能进入到罩内。将其放在室外温暖的地方，不一会儿便会看见钟罩内壁出现露水，这一定是从植物叶片中释放出来的水分。
+
+
+
+
+
+植物在物质合成的过程中需要大量的水，因此水对植物的生长发育非常重要。在实验103中我们已经看到，植物用来吸收养分的水，是通过植物的叶片排放出来的。植物所排放的水分数量很大，一株六英尺高的向日葵的叶片，在整个白天可以排放一夸脱水，一英亩草地在干燥的天气下，一天可以排放六吨水。
+
+如果植物向外排放水分太快，便会造成自身水分缺失，进而导致无法输送养分，各部分的正常功能也将衰竭丧失，最终走向死亡。因此，许多植物都会通过一些途径来保持自身水分。几乎所有的植物叶片上都有气孔或者很小的孔洞，当水分大量流失的时候，这些气孔便会立即关上以防止水分进一步流失。
+
+有一些植物，比如玉米，当根部不能提供足够的水分时，叶片便会卷曲起来，以减小蒸发面积，进而减少水分的蒸发量。还有一些树种，比如桉树，当阳光强烈的时候，它们便会将叶片垂直耷拉下来，只让叶片边缘接受日照。另一些植物叶片保持水分的方式很特别，比如鼠尾草，它们通过叶片上生长的大量毛发来保持水分。还有些植物，比如卷心菜和橡胶树，其叶片表面像涂了一层蜡一样，对水分也有很好的保存作用。沙漠地区的窄叶灌木的叶片非常小，而且几乎没有气孔；仙人掌则更是差不多连叶片都没有。这是由于冬天结冰后，这些植物根部无法提供足量的水分，而叶片依然不断向外排放水分，因此只有像松树这样的针形叶和其蜡纸表面，能真正起到阻止水分流失的作用。
+
+95．植物的花——植物的茎上不仅仅长着叶子，很多高等植物的茎上还会开出绚丽的花朵。花的主要功能是为植物制造种子，以保持物种的繁衍。如果我们仔细观察一下毛茛类植物、柑橘、肉桂或者天竺葵的花朵，便会发现花一般都是由四种不同的结构组成。
+
+环绕花朵四周有一系列呈绿色的小叶，这就是花萼。在花萼里面的就是花冠，许多植物的花冠都是由有颜色的花瓣簇拥而成。在花冠内部，有许多细长的须子顶着一个球冠顶，这就是花粉囊，它和这些须子一起构成了花的雄蕊。
+
+花朵的正中心便是花的雌蕊。雌蕊的顶部有一个膨大的部分，这便是柱头，它一般都有粘性并显得很粗糙。在其底部，有一个膨大的中空部分，这就是种子的所在地，叫做子房。柱头与子房被一根类似茎干的花柱连接在一起。雄蕊和雌蕊是花朵关键部分，花萼和花冠仅仅起到保护和支撑作用而已。并非所有种类的花都包含以上四个部分，但是雄蕊和雌蕊却是都有的。
+
+花粉囊会制造出大量的颗粒状物质，也即花粉，每一个花粉颗粒都是一个富含细胞质的自由活细胞。当花粉成熟以后，花粉囊便会打开，将花粉散播出去。这时要是一个正好匹配的花粉颗粒落在柱头上，它便会开始生长，并沿着花柱向下伸出一根细管，一直延伸进子房，在那里一个叫做卵细胞的原生质细胞开始孕育。渐渐地，这两部分植物原生质体的精华物质单元，和一个新的细胞就形成了。
+
+这个新的细胞迅速生长，并不断分裂出新的细胞，形成一个初级胚芽。它是植物的种子里面的具有生命的部分，并且在它周围通常储藏着大量的植物养分，因此在它发育成熟长出自己的根和叶来获取食物养分之前，这些储藏的养分也都基本够用了。
+
+如果花粉和卵细胞不能结合，胚芽则不能形成。因此，让匹配的花粉接触柱头就十分必要。有些植物的花朵柱头，可以利用自己花粉囊所制造的花粉，而有些���物却只能运用其他植物的。如此一来，为了增强繁殖，不同植物花朵之间的花粉传递，便显得很有必要了。
+
+有时花粉还会随风飘送，比如玉米就是例子。只不过这种情况下大量的花粉粒会被浪费掉，每当开花期的那一地落黄便是见证。玉米的每一根玉米须都是一个雌蕊，因此只要有花粉落在柱头末端，雌蕊下面就会孕育一颗种子。还有核桃树和苹果树的花，也是通过风中的花粉而孕育受精的。
+
+不过对大多数植物而言，花粉传递更多是靠鸟类、蜜蜂以及其他昆虫。当蜜蜂爬进花朵采蜜的时候，便会接触到花粉囊，这样花粉就会沾到它们身上而被带走。于是，当它们进入到其他花朵中，与具有粘性的粗糙柱头触碰时，便也会让花朵受精。蜂鸟在采食金银花花粉的时候，将长长嘴壳伸进金银花狭长的花冠中，也让花粉在花朵柱头间进行了传递。这类植物，都是通过这类途径进行受精孕育的。
+
+花朵的美丽色彩以及花蜜的甘甜美味，都是植物诱使昆虫进入的手段，因为它们可以为柱头带来花粉，也可让自己的花粉为别的植物受精繁育。
+
+某些花的结构只允许特定的昆虫或鸟类来完成花粉传递，比如金银花对应蜂鸟、红花草对应大黄蜂，其他许多植物也都对应着自己的特定的授粉动物。还有些植物，不能被同类植物受精，比如某些草莓，要想其长势良好，则必须在它们旁边栽种一些其他植物。有的植物甚至不仅需要其他植物靠近它们，还需要特定种类的昆虫参与授粉过程。
+
+其中最显著的例子就是士麦那无花果，为了将其引入加利福尼亚，人们做了多年的努力。可是人们发现它在加利福尼亚虽然生长得很好，可就是不结果。于是人们通过仔细地观察发现，在它生长正常的地区，当地居民会在其开花期，习惯性地将野生无花果的枝条搭在它的枝叶上，可是人们将这种野生无花果也引进到加利福尼亚之后，它还是不能正常结果。于是人们又进一步细心观察缘由，结果发现，在其开花季节还有一种小昆虫在野生无花果上产生，并且会爬到士麦那无花果植株上。现在这种昆虫也被引入加利福尼亚之后，士麦那无花果的生长状况就几乎一切正常了。其实主要原因还是因为士麦那无花果没有雄蕊，因此必须要野生无花果来将花粉传递到它的柱头，而这个途径的最终实现，还须要这种小昆虫来完成。
+
+还有一个较为类似的例子就是美国西南部干燥地区的丝兰，它的授粉过程必须依靠一种晚上在花朵之间飞来飞去的小飞蛾。它们飞入花朵后便会深入底部，刺探到子房，然后产出一个卵子后又向上爬，并将花粉传到柱头上。当幼虫从卵子孵化出来以后，便以子房中的种子为食，但子房中授粉的种子很多，它们只能吃掉其中很少一部分，这样它们便让丝兰为了繁殖而产生大量的种子，而要是没有这种小飞蛾的话，这一切就难以实现了。
+
+以上只是大量类似例子中的个别实例，这些实例无一不显示着动物与植物之间的重要关系，它们有时是如此地相依为命。
+
+96．植物种子的散布——各种类的植物要想在大自然中长久存在，就不仅需要植物的花来制造多产的种子，还需要这些种子的合理散布。为了达到这个目的，一些植物会突然炸裂它们的果实来散布种子，含羞草和豌豆就是典型的例子。还有一些植物则会八仙过海各显神通，枫树会让种子飘散于空中，以便落到别处；蓟类植物和蒲公英的种子拥有很轻便的附体，可以随风而去；风滚草则会卷成一团，在土地上颠簸着向四处流播。
+
+有些植物种子就像乞丐的打狗棒一样，带有弯钩和倒刺，这样它们便可以附着在动物身体上，并被带到远方。种子外面一般都有一层可食用的果肉，比如樱桃、黑莓、李子等等，鸟类和其他动物食用以后，未经消化的种子也便会被带到其他地方。橡子的种子则是被松鼠和其他动物搬运到其他地方，还有些植物种子可以在水里长时间漂浮而不会损坏，以此随波逐流到其他地方，海岸和小岛就是以这种方式接纳了大量的植物种子，椰子树和棕榈树就是这类植物的代表，它们的种子在热带群岛之间散布得十分广泛。
+
+97．种子的发芽过程
+
+
+
+
+
+实验104：取两个普通餐盘，在一个盘子中放上两三张吸水纸，彻底打湿，然后在上面放一些小麦或者其他植物种子，再将另一个餐盘扣到这个餐盘上，保持边缘均匀合缝。这样就为种子发芽制造了一个良好的潮湿空间。经过观察，所有的种子都是同时发芽的吗？种子的不同位置对它们的发芽有影响吗？发芽的第一���会出现什么现象？叶和根哪个最先生长出来？后来种子为什么会慢慢枯萎？
+
+实验105：取几粒植物种子，比如南瓜、西葫芦、大豆、玉米等都行，将它们切开，然后向里面滴上几滴在实验100中用到的碘溶液，种子会出现淀粉的反应吗？
+
+实验106：将几粒大豆种子在水里浸泡24小时后，取两三粒去掉外皮，仔细观察其内部的结构。然后再取几粒将其栽种在一个装满锯末的盒子里，并将其放在温暖处。再取几粒玉米种子，浸泡两三天后也栽种在这个盒子里。过几天之后，小心地取出一粒大豆种子和玉米种子，仔细观察，给它们作一个外形素描。
+
+再过上几天后，又各取出一粒，再次仔细观察并进行素描。然后持续如此，直到它们长出健壮的小苗。这两类种子的生长发育状况很相似吗？哪种种子有两个很相似的部分？这两个相似的部分叫做子叶。从直观上看，这个部分对小苗有什么作用？请参考实验104的结果。同时请注意观察两种种子发芽过程中根与茎的发育，小苗就是靠种子来提供生长所需的食物养分的。
+
+
+
+
+
+当我们在实验105中观察不同植物种子的时候，我们发现它们都含有淀粉。而将种子浸泡栽种后，我们发现种子都会有一个部分生长出一个苗芽，这就是我们之前提到过的胚芽。在实验中我们还看见，大豆的种子分裂成了两个相似的部分，并会随着小苗的生长而渐渐枯萎收缩，而玉米种子则只会有一个这样的部分。
+
+种子的这一部分叫做子叶，或者叫种子的叶片。大豆就是双子叶植物（种子有两个叶片），玉米便是单子叶植物（种子只有一个叶片）。子叶便是种子发芽生长的粮仓，当种子发芽生长，植物的根、茎、叶都会随之发育。而当它们都发育完全并长势良好之后，子叶便也完成了自己的任务，慢慢萎缩了。从这开始，植物便可以通过太阳的光热自己准备食物养分了。
+
+
+
+
+
+实验107：在一个装满潮湿锯末的玻璃杯中放几粒大豆，然后将其放置在日照充足的温暖地方，并让锯末始终保持湿润。等大豆都已经充分发芽后，用小刀将一株小苗的一片子叶切断，再取一株小苗并将其两片子叶都切断，然后再将它们栽回锯末杯中。它们还会生长得跟其他小苗一样吗？
+
+实验108：给一个16盎司的广口瓶装入大约三分之一容量的豌豆或者大豆，然后往里面倒入适量的水，水面盖过豆子就行。然后塞紧瓶子，将其放置在一个温暖向阳的地方，并在旁边放一个也被塞住的同样的空瓶子。几天之后，当种子已完全发芽并长出幼苗后，将其瓶塞拔去，并放入一根燃烧的小纸条，在旁边的空瓶子里也进行同样的操作，为什么两个瓶子里纸条燃烧得不一样呢？如果放入瓶中纸条就熄灭，则表明里面存在大量二氧化碳。
+
+实验109：给两个8盎司的广口瓶都装入大约三分之一容量的粗锯末，再在剩余部分放入用水泡过一整天的豌豆，然后倒入适量的水，让其盖过锯末，然后塞住一个瓶子的瓶口，另一个瓶子不塞，然后将其都放在温暖向阳的地方，注意不时向开口的瓶子中倒一些水，以保持锯末湿润。经过观察，那个杯子里的种子发芽的长势更好？看起来空气对种子的生长是极为必要的吗？根据前一个实验的结论，空气中的什么成分在这里被使用了？
+
+
+
+
+
+实验107告诉我们，如果在植物幼苗充分成熟之前就将其子叶割去，它便不能继续生长。实验108则显示，处于生长发芽阶段的种子会像动物那样，消耗空气中的氧气并排放出二氧化碳。植物种子发芽需要能量的支持，而这能量正是来源于种子里的碳与空气中的氧相结合的过程，就像木头燃烧一样。实验109更说明，没有足够的空气，种子便不能很好地生长发芽，这是因为没有足够的氧气来提供必要的能量。
+
+
+
+
+
+实验110：给两个装满潮湿锯末的玻璃杯都放入几粒发芽的种子，然后将它们并排放在温暖向阳的地方，然后用一个漆黑的罩子罩住其中一个杯子，经过观察，哪个杯子中的种子长势更好？
+
+
+
+
+
+我们在实验110中发现，当种子发芽并开始自己合成养分时，如果没有太阳光的照射，这个过程就不能完成。植物的母体，其实也在种子当中隐含性地储存了来自太阳的能量，这种形式的势能，也可以借助于氧气而在种子发芽的过程中转化为具有动力性的能量，并在其成长过程中加以利用。而当这样的潜在能量用完之后，种子和苗芽便只能直接仰赖太阳光所提供的能量了。
+
+98．真菌
+
+
+
+
+
+实验111：将一片湿面包在空气中暴露一段时间后，放进一个盘子并严严实实地盖住，���防止水分蒸发。过一段时间后，面包上会发生什么变化吗？请用放大镜仔细观察面包上出现的霉斑。
+
+实验112：（1）将一个完好的苹果碰伤一块，并将其碰伤处与一个彻底腐烂的苹果靠在一起，再用布将二者包紧放入一个水果罐中，盖上盖子防止水分蒸发；（2）用一根大头针在一个烂苹果上反复插刺，然后再往一个完好的苹果上插刺，然后用布将这个好苹果包严实放入水果罐，封死；（3）将一个部分腐烂的柠檬与一个完好的柠檬放在一起，并保持环境的湿润。在以上各环节中，完好的那个水果上最后会出现什么现象？
+
+
+
+
+
+到目前为止，我们谈到的植物都是绿色植物，它们都含有叶绿素，都能自己借助于太阳的能量从空气和土壤中获取养分。然而，还有另一大类没有叶绿素的植物，它们只能依靠绿色植物所制造的养分而生存，并且它们不仅可以从活体动植物身上吸取养分，还可以在死去的动植物尸体上获取养分。而动物所含有的它们所需的养分，有的是来自于对植物的消化，有的则是动物直接从植物那里获取到的。
+
+这类没有叶绿色并且仰赖绿色植物而生存的植物类别就是真菌，我们熟悉的细菌也属于这一大类[2]。它们中间，依靠活体动植物生存的叫做寄生菌，而依靠死去的动植物尸体存货的则叫做腐生菌。这一类的植物同样极其重要，虽然它们中有很多个头极小，必须用显微镜才能看到。但要是没有它们，我们的地球将会立刻变得不适宜人类居住。它们中有一些对动植物有害，但是大部分还是有益的。
+
+正是它们导致了动植物的尸体的腐烂，因此要不是它们，地球上死去的动植物将尸横遍野，直至无法堆积，且动植物肢体从土壤中吸取的物质也无法重新回归大地，大地也将不再肥沃。它们还让醋、奶酪以及许许多多我们日常生活中不可或缺的物质得以制成。
+
+但从另一方面说，腐烂的水果、长霉的面包、玉米和麦子的黑穗病、苹果和土豆的疤痕、谷物的锈病以及其他许多普通植物的病变，都与真菌的生长有关。在美国，小麦的锈病所造成的损失，每年都高达数百万美元。数千英尺的原木木料，每年也被有害菌直接毁掉，其原因，就是真菌的生长。同时，为了对抗有害菌，每年的花费也同样高达数百万美元。
+
+不过很多真菌还是非常有用的，其中被利用得最广泛的便是酵母菌。制作面包的时候，含有酵母菌的酵母被充分融合到制造原料中，形成酵面，然后又将酵面放到温暖的地方，让其发面，或者准确地说，让酵母菌进行繁殖。如果原料和温度都很适宜，酵母菌的生长会非常迅速，同时让生面团不断膨胀，并将其中的糖分转化为二氧化碳和酒精。这时面团会产生小的气泡，整体也开始变得疏松透气。
+
+然后酵面团又会被充分揉合，以便让更大数量的酵母菌继续生长，也就是继续“发面”。这之后，酵母菌就一致地均匀分布在面团中，并且会制造出二氧化碳的小气泡，从而让面团进一步绵软透气。这时将它们送进烤炉加热，小气泡就会释放出来，让面团更加松软，同时酒精会蒸发掉，面团会变得脆硬，到这里，面包制作就大功告成了。尽管这个过程很有趣，但真要仔细探究的话，还得借助于显微镜，这里咱们就不多说了。
+
+还有一些我们都非常熟悉的大型真菌，比如蘑菇和伞菌。蘑菇是一种使用广泛的食物原料，在许多地方甚至已经形成重要的产业。它们生长在富含腐殖土的阴暗环境中，类似于地窖的地方往往就是它们的家园。蘑菇会在一些树木繁多的地方大量滋生，但在被有经验的人士辨别其种类之前，一定不能作为食物食用。有些种类的真菌含有剧毒，还有一种至今还没有解药。要研究蘑菇的基本结构是怎样的很简单，在市场买一个回来仔细观察观察即可。
+
+
+
+
+
+实验113：在五只4盎司的广口瓶中各放入一片煮熟的土豆片，瓶口用脱脂棉堵住。将其中四只瓶子放入消毒器进行半个小时的杀菌消毒，剩下一个让其保持未消毒状态。（消毒器可用如下办法构建：找一个有盖子的锡罐，在底部放一片开有孔洞的弯曲锡片，以充当放置瓶子的架子。再有一个也开有孔洞的浅锡盘放在下面就会更好了。这里开孔是为了蒸汽能够逸出，以免冲击到锡片和锡盘本身。然后往里面掺水，水面盖过架子就行，让水始终保持沸腾。）高压锅就是非常理想的消毒器。
+
+将瓶子从消毒器中取出，冷却。去掉一个瓶子的棉花塞，保持瓶口敞开几分钟后又拿新的棉花堵上。拿一根针在火焰上来回炙烤消毒后，放入插有鲜花的花瓶的水里面，保持一段时间。小心地将第二个瓶子的棉花塞拉向一边，将刚才在花瓶中浸泡过的针插入土豆片，再在该土豆片上滴一滴刚才花瓶中的水。将针取出消毒，然后在潮湿的手掌上摩擦几下，再像刚才一样，将第三个瓶子中的土豆片戳伤。第四瓶不动，作为参照，以判断这些瓶子消毒是否彻底。然后将这些瓶子全都放置到一个温暖的地方，观察几日。土豆片上会出现斑点，这些就是细菌菌落。
+
+
+
+
+
+99．细菌——我们知道在土壤中存在着固氮细菌，但和地球上无处不在的整个细菌群落比较起来，它们就太微不足道了。因此，我们接下来就好好聊聊这个群体。在实验113中我们发现，如果物体暴露在空气中，假以时日，它们便会发生一些改变，而如果采取了保护措施则可避免。这些改变的背后原因，便是细菌。
+
+细菌都是单细胞植物，差不多算是结构最简单的植物了，因而只能在高倍显微镜下才能看到它们。细菌的形态多样，有杆状、线状、螺旋状等等许多形状。细菌的细胞质能从外界吸收营养，并不断生长出新的细胞质。且当这样的一个细胞生长到一定程度时，它便会分裂为两个细胞。
+
+一个正常细菌的分裂速度很快，几乎每小时分裂一次。这样一来，每个细胞都不间断地每小时分裂一次的话，经过二十四小时便会产生出一千七百万个细菌。如果这样的分裂持续几周时间，细菌的总量体积几乎可以和地球差不多大小。当然，其前提是每一个细菌都占据足够的存活空间，都有足有的食物赖以生存，并能够避免任何对自身的伤害。不过这样的情况还不曾发现过，因为每个细菌都会和其他细菌一样，为了生存而竞争。不过即使如此，细菌的总体数量依然是无量无边的。
+
+既然细菌和真菌都会导致食物变质，因此想方设法阻止其生长就非常有必要了。人们发现，经过充分熏制的鱼和肉类食物可以免于变质；盐也可以起到防腐剂的作用；水果被沸水煮过后放入罐子密封也能长久保存，要是再加入高浓度蜜糖水的话，同样就不会变质了。
+
+以上这些，还有许多其他办法，都被人们用来防止食物滋生细菌，并以此对食物加以保存。人们还发现，在极低的温度下，细菌也不能繁殖，因此食物也常常通过冷冻的方式进行保存。还有很多细菌无法抵挡阳光的照射，因此人要是住在向阳的屋子里，会健康得多。
+
+蒸汽也能有效地杀灭细菌，不过在有些情况下，某些细菌可以忍受的温度可以远远高于沸水的温度，但它们却不能穿过脱脂棉。人们同时还发现，有些物质也能阻止细菌的生长繁殖，比如甲醛、过氧化氢等等，这类物质我们就叫做消毒剂。
+
+某些细菌可以在肉体中迅速繁殖，因此对外伤伤口的消毒便非常必要，否则一种叫做败血症的细菌性疾病便容易流传。有时如果不小心让生锈的铁钉扎进了手里或者脚底，要是伤口没有被正确消毒并妥善处理的话，便容易患上另一种叫破伤风的细菌性疾病。伤口被消毒过后，一般会包扎上脱脂棉，以隔绝细菌的侵入。
+
+细菌能够导致多种疾病，比如肺炎、肺结核、天花、伤寒症等等，得这些病的人便是感染了大量的这类细菌，因此它们又被叫做病菌。如果这类病菌被人吸入肺中，或者吃到肚子里，人就会得病。因此，针对这些病菌一一采取预防措施以防止其传播就非常有必要。
+
+结核病人应该小心地使用独立餐具，打喷嚏和咳嗽时也应用布遮住嘴巴，否则便很容易传播病菌，成为其他人的健康威胁。每年都有成千上万的人遭受结核病的折磨，其中大多都是由于粗心大意而得病，因为它本身是一个完全可以预防也可以完全治愈的疾病。随地吐痰这样的肮脏危险的习惯，在任何地方都应该被禁止，公共水杯和毛巾也应该被彻底销毁。
+
+如果疾病具有恶性传染能力，比如天花、白喉病之类，患者必须隔离，病房也必须消毒并采取严格预防措施，易感人群也不应靠近有这类细菌存在的环境。
+
+如果致病菌按某种方式结合，它们便会分泌出一种毒素，它可以被血液吸收，并随血液循环进入到身体各个部分，因此除了可以毒害一开始袭击的地方以外，其他所有地方都可能被其毒害。这个时候，身体的细胞便开始分泌出一种物质来抵抗毒素，这就是抗体。如果病人的身体机能足够好，便能分泌出足够的抗体来战胜病毒，最终使身体康复。
+
+近年来人们发现，抗体也可以被人工制造或者合成。因此这样的机制便可以用来抵抗病毒的侵袭，从而让人们免于这种疾病。通过注���这类抗体，或者促进其生长繁殖，人们现在已经攻克了诸如天花、白喉等其他许多疾病。
+
+致病菌不仅存在于空气中，在水、牛奶以及许多食物中也有它们的身影。因此我们必须小心保存我们的饮用水、牛奶以及整个食物供应链。曾经一段时间，许多伤寒发烧的病例在经过追根溯源后发现，皆是缘于一个有家族伤寒病史的牛奶商供应的牛奶。苍蝇也是一种主要的致病菌传播者，它们通过到处攀爬而到处传播疾病。
+
+病菌在下水道、污水坑等一些不卫生的地方繁殖得尤其旺盛，因此这些地方应该尤其注意清洗。战胜疾病的最好武器，是足够的阳光和空气、合理健康的膳食、足够的休息，以及一颗安之若素的平常心。在这些因素的帮助下，身体便能轻易地杀灭入侵的细菌。每一天，我们每个人都会接触到成千上万的病菌，但只有当我们身体的免疫力降低时，这些病菌才能肆虐逞能。因此，健康的生活，永远是疾病的最大死敌。
+
+之前已经提到过，致病菌在整个细菌家族里面只占到很小一部分，而这个家族中的其他成员对我们的益处，也远远大于它们的有害之处。随着科学不断地发展，人类已经越来越能够与病菌作斗争，但是有益菌对动植物与人类的贡献，我们却完全无法取代。
+
+100．动物——动物不是直接从太阳获取能量，而是通过食物间接地从绿色植物所储存的养分中进行获取。这些食物养分有的是直接被动物食用，有的则需要经过一些食物链的中间环节，通过其他植物与动物流转后再传递到动物。这些能量来自于碳氧结合的过程，二氧化碳也是这个过程的产物。
+
+绿色植物又会借助于太阳的能量将二氧化碳加以利用转化，并排放出氧气。如此循环不已，植物排放氧气吸收二氧化碳，动物排放二氧化碳吸收氧气。植物的叶绿素为植物细胞提供了食物养分，它们也会像动物细胞一样，吸收氧气而排放二氧化碳，其他有些植物也是如此，但这样的植物数量非常少。
+
+101．动物的分类——为了便于研究，动物被分为了两个大类——无脊椎动物（没有脊椎骨）和脊椎动物（有脊椎骨）。无脊椎动物的种类极其繁多，它包括了蠕虫类、贝类、昆虫类以及数不尽的各种体内没有骨架的动物。相对高级一些的动物，比如鱼类、两栖类、爬行类、鸟类和哺乳类动物，都属于脊椎动物。人类便是最高级的脊椎动物，由于本书的主旨是阐述地球与人类之间的关系，因此关于人的自身结构，后面还有专文叙述。
+
+102．无脊椎动物及原生动物——世界上最低级的动物要算原生动物，因为它们都是单细胞动物。它们中有些种类由于结构过于简单，几乎很难与植物区分开来。它们非常微小，且一般生活在水中。我们之所以在这里要谈一谈它们，是因为它们是好几种疾病的罪魁祸首，我们必须加以注意。疟疾，以及一种在非洲流行的可怕疾病昏睡病，还有黄热病，都很可能是由它们造成的。
+
+和细菌不一样，原生动物导致疾病并不是通过直接在不同人之间流转传播，而是因为它们还在一些昆虫体内停留，进而通过昆虫进行传播。传播疟疾和黄热病的昆虫就是蚊子，传播昏睡病的昆虫叫做舌蝇。如果一只蚊子叮咬了一个患有疟疾或者黄热病的病人后，一些这类致病的原生动物就会通过血液被吸入蚊子的血液中，然后在蚊子体内生长发育，经过一系列变化，病毒便慢慢形成，进入到蚊子的唾液腺体中。在蚊子叮咬其他人的时候，它会向叮咬处注入少许唾液，这样，病毒就随之进入了人体血液，经过迅速繁殖而导致疾病的产生。
+
+如果能够防止蚊子叮咬人群，或者让已感染病原体的蚊子不再叮咬其他人，则这类疾病就不会传染流行。防止蚊子叮咬人的最好办法，就是彻底研究它们。既然蚊子是在污水中生长繁殖，因此那些沉积脏水和污垢的旧碗碟和小水池就应该被彻底清洗干净，比较大的水塘应该将水排掉，或者向其中倒适量的油，在水面上形成一层油膜，这样便可以阻止蚊子的生长繁殖进而导致疾病[3]。
+
+因此，蚊子苍蝇这类夏天的讨厌鬼，不仅仅是让人觉得厌烦，而且更能导致疾病流行。曾给美国牧民造成重大经济损失的德克萨斯州热病，就是由一种虱子，将某类原生动物注入被它们叮咬过的牲口体内引起的。
+
+103．蠕虫——另有一类无脊椎动物叫做蠕虫，其中之一，便是我们熟悉的蚯蚓。由于它们在土壤形成的过程中扮演了重要角色，因此有必要在此聊聊它们。如果我们仔细观察一条蚯蚓，便会发现其身体是由一环一环的片���构成，蚯蚓的移动便是依靠这些环段相继不断地收缩伸展。贯穿其身体中间的，是一条简单的消化道，由嘴、磨碎食物的砂囊和肠胃分段组成。
+
+靠近其头部的位置，有一个小小的神经中枢，整个动物体则是由许多不同环节的片段连接构成。更加详细的研究观察表明，这些不同的身体环节已经有一些侧重性的分工，但还并没有像高等动物一样，身体各部分有鲜明的分别。我们也都看到了，蚯蚓这种动物虽然简单低级，但它们却永不疲劳地在土地上耕耘着，让土壤肥沃疏松，适宜植物生长，从而默默无私地为人类奉献着毕生精力。
+
+104．昆虫
+
+
+
+
+
+实验114：将一只具有代表性的昆虫，比如蚱蜢或者蜜蜂，装进一个有盖子的玻璃盒中。仔细观察，它的身体可以分为几个部分？请描述一下各部分的特点。它的腿长在身体的哪个部分？翅膀呢？它一共有几条腿？有几扇翅膀？尤其注意观察它身体中最大的那个部分，再观察它头上的眼睛和触角，准确地说，它的触须。写一段话描述一下蜜蜂身体的大致特征。
+
+
+
+
+
+昆虫是世界上极为重要的动物种类，且种类繁多。它几乎涵盖了地球上已知动物种类数的半数以上，并广泛地分布于地球的每个角落。
+
+有益的昆虫和有害的昆虫都大量地存在于我们周围，从经济效益上说，它们每年可以为我们提供价值数以亿计的农副产品；但另一方面，它们每年也毁掉了价值数千万的庄稼和树木。据估计，昆虫每年在美国造成的农业和林业损失已高达50000000美元，这还不算苍蝇蚊子一类昆虫由于传播疾病而造成的损失。就在不久之前，蝗虫都还差点毁灭了中西部地区好几个州的整个农牧产业。
+
+最有用的昆虫应该算是蚕和蜜蜂了，没有蚕，就没有华丽的丝绸；没有蜜蜂，就没有甘甜的蜂蜜。就这两项农产品，每年可以创造数亿美元的经济效益。我们之前已经提到，蜜蜂和其他一些昆虫在花的授粉过程中也非常重要。
+
+在所有最有趣的昆虫当中，如果我们把全部因素都考虑进去的话，最有价值的昆虫，应该非蜜蜂莫数了。它们是最伟大的园丁，除了红三叶草，它们可以为全世界所有植物的花授粉。在美国，每年年产的蜂蜜和蜂蜡的经济价值就达到两千五百万美元。
+
+在实验114中我们已经看到，跟其他昆虫一样，蜜蜂的身体可以分成三个部分，分别是头部、胸部和腹部。眼睛和触角，或者叫触须，都长在头上。它们的嘴是一个很复杂的器官，既要能够咬刺，又要能够吮吸。六条腿和四面翅膀都长在胸部，且工蜂的后腿一般更加细长，并长有毛刺，这在采花粉的时候正好成了天然的花粉筐。
+
+蜜蜂都是大量群居的，在其群落内部，有三种类型不同的蜜蜂，分别是雄蜂、工蜂和蜂王，也叫蜂后。工蜂就是制造蜂蜜和蜂蜡的蜜蜂，并且建造蜂房，还用它们胃里的养分与花粉蜂蜜混合后喂养幼虫。幼虫是蜜蜂从卵中孵化后的第一形态阶段，蜂后负责所有的产卵工作，它们产卵的数量巨大，有时可达百万，但每个蜜蜂群落只有一个蜂后。只要有新的蜂后被孵化出来，老的蜂后便会带走一半群落部队，另行打造新的独立王国。
+
+蜂蜡是其腹部腺体的分泌物，也是蜜蜂打造蜂巢的原料。蜂巢的横截面都是一个一个的正六边形孔洞，整个蜂巢就是如此累积扩展而成，这样的结构无形中保证了最少的蜂蜡用量和最大的蜂蜜产量。花朵内部的花粉是蜜蜂制造蜂蜜的基本原料，正是为了四处搜寻花粉，蜜蜂才在不同的花朵间穿梭往来，并将花粉蹭在身上，又带进其他花朵当中，以此完成花的授粉过程。这样一个极其有趣且对人类和大自然十分有用的小昆虫，在许多书本里都有它们的故事，人们对它的讴歌与关注，超过了任何一种家畜动物。如果想多了解一些关于它们的信息，此类书不妨读读。
+
+105．脊椎动物
+
+
+
+
+
+实验115：如果有条件的话，找一些脊椎动物的骨骼标本，当然能找到人体骨骼标本最好。注意观察骨骼之间是如何相互连接的，这些关节又是如何运动的。还有大脑和脊髓是如何被骨骼保护的，以及胸部骨骼是如何保护心脏和肺部的。如果能够找到人体骨骼标本的话，尤其注意观察脊柱是如何通过弯曲变形以支撑我们的身躯能直立行走的。
+
+
+
+
+
+先前我们简略讨论了与人类利害关系比较大的几类无脊椎动物，而我们人类本身，则属于另一个大类：脊椎动物。被人类用来当作食物食用的大部分高等动物也属于这个类别，尽管这一类别的动物种类繁多结构复杂，但都有一个共同点，就是它���体内都有完整的骨架和支撑全身的背脊骨。
+
+高等脊椎动物的骨骼由以下几部分构成：脊柱、头盖骨、肋骨以及附件骨，骨骼的主体部分主要是保护柔弱的器官，并且支撑肌肉的运动。我们的腿和手臂的骨骼则连接在主体骨架上，同样也是肌肉让关节得以运动，进而带动骨骼发生运动。
+
+大脑作为我们的神经中枢，存在于头盖骨中。从这里脊椎开始延伸，脊椎里面就是神经传导物质，脊髓。从脊髓延伸出去的神经遍布于全身的肌肉，其末梢更到达每一寸皮肤和我们的所有器官，比如眼睛、耳朵，它们会在我们接收外界刺激后，立刻通过神经传输到大脑，而大脑则通过刺激信号让肌肉运动。事实上，动物身体的所有运动都受控于大脑，就像火车的运动都是受控于调度室一样。
+
+106．呼吸——所有的动物都必须呼吸，否则身体就得不到足够能量供应。不同的动物有不同的呼吸方式，但高等脊椎动物和人的呼吸方式是一样的，我们人类的呼吸方式也是非常有代表性的。
+
+空气通过口鼻进入到我们身体里面，经过气管到达肺部。为了防止灰尘和细菌的进入，我们鼻子的整个鼻腔内壁上都长有鼻毛，上面布有粘液形成粘膜，这些鼻毛和粘膜可以阻挡绝大部分的有害物质微粒，防止它们进入我们的呼吸道。在口腔后面，紧接着就是器官与喉咙。
+
+当我们吞咽食物的时候，气管的通道必须关闭，这就是由像阀门一样的喉头盖来完成的。如果在吞咽食物时它没有及时盖上，食物就有可能卡在气管里，造成人的窒息死亡。气管位于胸腔的上端，往下分成两支，各自与两边的肺叶相连通。
+
+肺充塞在胸腔里，并包裹着我们的心脏。在肺叶中，气管不断分枝开杈，从一根粗管分成越来越小的细管，最终进入微小的空气囊。在这些气管束之间穿插的是输送血液的静脉与动脉血管，它们也逐渐分岔成为微小的毛细血管，周密地分布在整个肺叶中。空气和血液在那里被一层很薄的生物膜隔开，但其中的可溶物质则可以通过生物膜相互交换。这样，血液便可以带走所需的氧气，并过滤掉积累在其中的二氧化碳及其他废弃物。
+
+心脏和肺叶都充塞在密闭的胸腔里面，并被肋骨环绕保护着。在胸腔底部，有一个穹庐形的肌肉组织，叫做膈膜。空气能够进入到肺里面，就是因为胸部的肌肉拉紧肋骨，让它们向上向外挤压，膈膜的肌肉则将它向下挤压。这两个方面的共同作用，让胸腔空间变大，这样空气就像进入到一个先被压扁然后又释放开来的橡皮球一样。当肋骨向下运动，膈膜向上运动时，空气又像被压缩的球往外排气一样，从胸腔中被挤压了出来。
+
+因此，空气进入肺部便有两种方式，即胸腔的扩张与膈膜的运动。在自然状态下，这两种呼吸方式是同时进行的，也只有这样才能保证整个肺部都能充分充气，而不只是上半部或者下半部。当这个过程完成后，身体便会放松下来，整个胸腔和躯干下半部的腹腔便不会再处于紧张状态。这样的呼吸方式不仅让血液获得了它们所需要的氧气，也让我们的肺得到了滋养与呵护。不过目前肺部的传染病已经越来愈流行，我们对此还需特别注意。
+
+107．血液循环
+
+
+
+
+
+实验116：准备一台显微镜。然后将一根橡皮筋紧紧缠绕在一根消毒过的手指尖上，等到指尖血液胀满时，用一根消过毒的针迅速小心地刺一下，将挤出的血液涂到载玻片上，迅速放置到显微镜下观察，注意看其中数量繁多的像小盘子一样的物体，这就是红血球。然后试着找到一种形状不规则的物体，即白血球。它相对比较难找，但是只要血液足够新鲜，而我们的取样动作很快的话，还是可以找到的。
+
+
+
+
+
+为了让我们身体的每一部分都得到滋养，让每一个细胞能持续不断地正常工作，就必须有一个复杂多样的循环系统来加以维持，它对维持我们身体中多样化的机能运作也极为必要，这就好比铁路交通和水路运河对我们人类世界的作用一样。我们的身体也是一个小小世界，因而它也有一个完整的新陈代谢系统来为各个地方输送原料并转运垃圾。这个系统的核心和力量源泉就是我们的心脏，而物资赖以循环的媒介与载体就是血液。
+
+人们在细致研究血液的时候发现，其主要由三种成分构成。它的主体是一种清淡的液体，叫做血浆；其中含有大量的小盘子形状的物体，叫红血球；还有一些形状不规则的白色物体，叫白血球。白血球是具有多种功能的原生质细胞，并且能够游离到血管外面的人体组织中去。红血球的主要功能则是从肺��吸取氧气，再将它们输送到全身的每一个细胞当中去。它们自身含有一种叫做血红素的色素，这种物质可以携带氧气并决定了我们身体的外观颜色。血浆一种成分很复杂的液体，大部分是水，但也包含着许多营养成分以及我们身体各部分器官的废物垃圾。
+
+经过我们身体各处的血管也不尽相同，从心脏延伸出去的血管叫做动脉，回到心脏的血管叫做静脉。动脉血管从心脏延伸出来后，不断地分叉变得越来越细微，到最后变成了只有一层薄壁的毛细血管。毛细血管又渐渐地与静脉血管相连，这样血液就不断地从心脏输出，经过动脉流向毛细血管，再慢慢地流进静脉，最后又返回到心脏。
+
+动脉血管全都分布在我们身体表面以下较深的地方，这样也便于得到保护，但当我们将手指放在手腕的脉门上，或者脸腮后的耳根处，我们也能感受到动脉血管的跳动。静脉血管在手背上就可以看见，拿一根针在我们皮肤表面任何地方轻轻地刺一下，都会伤及那里的毛细血管，然后慢慢渗出血液。毛细血管也分布在我们身体的所有组织当中，为组成我们身体各部分的细胞输送营养和氧气。
+
+心脏是一个由肌肉构成的动力泵，它有四个空腔，两个心室，两个心房。它的外形就像一个梨子，位置就在胸骨的背后。回流的血液会首先流进右心房，这是一个内壁松弛的空腔。从这里经过一个瓣膜，血液又流进右心室，这里的内壁就很厚了。血液又从这里流进肺动脉，经过肺部的毛细血管在流入肺静脉，红血球便在这里释放掉二氧化碳，并获得新鲜氧气。
+
+从肺部流回来后，血液先进入左心房，充盈之后经过瓣膜再进入左心室。强有力的心肌组织不停地将血液压入动脉进而流经全身，最后又再一次回到右心房。当血液流进心室之后，心脏瓣膜便立刻关闭，以阻止血液回流。将手掌放在胸部偏左的地方，便能很容易地感受到心脏的收缩与跳动。
+
+108．感官——为了确保动物的自身安全和拥有良好的生存状态，大脑就必须对外部世界的信息做出反应，同时我们的身体还必须拥有一定数量的感觉器官，通过神经系统与大脑联系。就我们人类而言，对外部世界最显著的感知方式有视觉、听觉、味觉、嗅觉和触觉。
+
+眼睛作为我们的视觉器官，对光线非常敏感，并像一台智能照相机一样，能通过神经中枢自动调节。这台照相机的外盒就是我们骨骼上的眼窝，眼睑就是快门，虹膜就是光圈。其实虹膜就是眼球前面的一层薄膜，能够通过张开与收缩让不同强度的光线进入。在它的中心有一个小孔，这就是瞳孔。
+
+在虹膜后面的是一个可以收缩调节的晶状体，在它之后的感光板，就是视网膜了。在虹膜前面是一种像水一样的物质，叫眼房水，它可以保持眼睛的前部始终像圆球一样撑开。在晶状体后面是一层很厚的透明胶状物，叫玻璃体，它支撑了视网膜，并确保了眼睛不会塌陷。
+
+在照相机里面都有一片磨砂玻璃，通过前后移动来给物体对焦，眼睛里面当然不可能这样，在那里是晶状体通过自我收缩来对不同距离的物体对焦。从视网膜连接到大脑的视觉神经，将视网膜上的影像传给大脑，并在那里转化成画面信息。
+
+这粗浅的比喻其实还并没有对眼睛做出清晰的描述，因为眼睛非常复杂精巧，比照相机的构造高级到不知哪里去了。对它的专业描述已经超出本书的范围，这里就不深入探究了。
+
+耳朵是我们的声音接收器，它的外面部分叫做外耳，主要是将声波获取后将它们聚集在鼓膜上。鼓膜是一层紧绷在耳骨上的薄膜，当声波触及它时，便发生振动，就像鼓槌击鼓时，鼓面的振动一样。鼓膜后面由一个有三片小骨头组成的链条与内耳部分相连，内耳的敏感细胞将振动形成的信号通过听觉神经传给大脑，并在那里转化为声音信息。
+
+舌头上和鼻子中的细胞会把它们接触到的信息传递给大脑，前者通过液体接触，后者通过气体接触，分别形成了我们吃出的味道和闻到的气味。
+
+我们的皮肤会产生触觉，而且有的地方会比其他地方更敏感。盲人的手指触觉一般都非常灵敏，几乎在很大程度上取代了他们所缺少的感觉器官。触觉也是通过神经传递给大脑，并在那里转化为我们感受到的感觉。
+
+109．食物
+
+
+
+
+
+实验117：将煮熟的鸡蛋白剁碎，碎渣的大小跟钉子头一般大就行，然后取适量放入试管。再将另一些剁得更细的鸡蛋白放入另一支试管，然后将100cc水、5cc胃蛋白酶和2cc盐酸制成混合溶液，分别加入到这两支试管中，充分淹没蛋白碎渣，摇匀���放置在温度恒为37°C或者98°F的环境中。无焰灶或者温水桶就是比较理想的恒温设备。让整个装置保持几个小时，会发现蛋白溶解掉了，而且第二支试管里面的蛋白溶解得更彻底。如果用水做同样的实验，或者只用盐酸做同样的实验，会产生同样的现象吗？实验中所用到的胃蛋白酶溶液就是一种人造胃液。
+
+
+
+
+
+为了让身体获得运动的能量，食物就成了必需。动物和植物都可以给我们提供食物，如前所述，作为食物的植物，它们的养分来源于叶片和茎干中的叶绿素。养分在叶绿素中被制造出来以后，会在经过食物链中动植物各个环节的过程中不断转化与整合。食物可以被用来：（1）促进新细胞的生长；（2）修复已经衰败或者毁坏的 ；（3）为身体内部运转提供能量并为其保持热量；（4）为其外部运动提供能量，比如身体的移动和搬运物体。
+
+为了提供这些能量，细胞就必须获得食物与氧气，因此食物就必须得到充分的消化吸收，然后被输送到身体各部分组织。高等动物都有一系列复杂的器官来完成这个过程，拿我们人类来说，其过程概况如下：一条细长的消化道贯穿我们的身体，它的不同部分分别对应着不同的消化过程。在嘴里，食物被牙齿磨碎，变成小颗粒后被混合在唾液中。这是一个极为重要的步骤，因为要是食物没有被充分细化，消化液便不能与它充分反应，导致消化的时间变得非常缓慢，同时也会导致额外能量的大量消耗。唾液对消化淀粉非常关键，同时其本身对食物的整个消化过程都很有帮助。
+
+食物在嘴里被磨碎之后，经过喉咙，穿过一个孔口进入到胃里，这是一个可以容纳三到四品脱食物的袋状器官，它的肌肉壁可以收缩和伸展，这样可以通过蠕动让食物在里面与胃液充分接触并发生反应。胃液是由胃的内壁中很小的腺体分泌的，人造胃液就是我们在实验117中用到的混合溶液。部分蛋白质会在胃里直接消化，更大部分的消化过程则会在小肠里进行。
+
+食物从胃里出来再经过一个孔口就到了小肠，这是一个错综盘绕的管状器官，并占据了大部分腹腔的空间。它的内壁分布着能够分泌消化液的腺体，两外还有两个为小肠分泌消化液的腺体，它们便是胰腺和肝脏。小肠是我们身体中最强大的消化器官，在这里，脂肪和油类开始被消化，淀粉和蛋白质则在这里彻底完成消化吸收。小肠通过一个管口与大肠相连接，大肠只有五到六英尺长，并朝着身体的出口慢慢变细，大肠中的消化过程就很少了。
+
+食物在消化道各环节所发生的变化非常复杂，但在此过程中，有营养的部分则可以被血液吸收，并能被转运到需要能量供应的身体各个部分。在消化道的每一阶段，只要食物被充分消化，吸收也就随时进行了。
+
+110．必需食物
+
+
+
+
+
+实验118：在几支不同的试管中分别放入少量的（1）玉米淀粉（2）葡萄糖（3）土豆泥（4）面粉（5）鸡蛋白。再放入适量的水，并充分摇匀。再向每支试管中滴入几滴实验100中用到的碘溶液。
+
+实验119：在几支不同的试管中分别放入少量的（1）鸡蛋白（2）牛油或猪油（3）葡萄糖（4）其他一种比较常见的食物。向每支试管中滴入几滴浓硝酸并保持一分钟。注意不要将硝酸弄到衣服上或手上。然后将酸液倒掉，用适量清水清晰试管中的物质，然后再将水也倒掉，再滴入几滴浓氨液。如果里面的物质颜色变成黄色或橙色，则表明蛋白质产生了。其中哪种物质含有蛋白质呢？
+
+实验120：由于汽油属于易燃物，因此必须确保做此实验时，房间里面没有任何火源。在几个不同的蒸发皿或者烧杯中分别放入一汤匙（1）煮熟的蛋白以及蛋黄碎末（2）亚麻粉（3）玉米粉（4）面粉（5）其他一种比较常见的食物，然后将它们放在窗台上。然后向其中都倒入足量的汽油，并充分搅拌。再用盖子将它们盖住，保持十到十五分钟。然后倒掉汽油，并将废弃的汽油置于户外，确保其彻底蒸发。如果还有东西剩下，那一定是食物的溶解物。取一滴剩下的物质，滴到一张纸上，闻闻它什么气味？并在手指上搓捻一下，看有什么特性？可以再用其他油脂类物质测试一下你想拿来做实验的物质。
+
+实验121：找一个空气流通的房间，以免屋里满是气味。在房间里面的本生灯[4]上用一个小铁勺燃烧（1）一小片肉（2）浓缩奶或者奶粉（3）一部分煮熟的鸡蛋（4）其他一种比较常见的食物。燃完之后会有残留物吗？如果有的话，这些就是矿物质。
+
+
+
+
+
+从实验118-121中我们发现，我们常见的所有食物都由三大类化合物构成，碳水化合物、蛋白质和脂肪。在普通的食物中，含有大量蛋白质的有瘦肉、奶酪、鸡蛋、黄豆和豌豆，由碳水化合物构成的有谷类食物以及蔬菜和水果，属于脂肪的有黄油、猪肉、坚果以及巧克力。牛奶则含有全部以上三种成分，并且其各占比例与身体所需也大致相当。
+
+科学研究表明，一个成年美国人平均每天要消耗两到3盎司的蛋白质、4盎司的脂肪和1磅碳水化合物。但我们每天吃下去的食物重量可比这要多，这是因为所有食物都还含有大部分的水的缘故。蛋白质主要用于细胞的生长与修复，因为细胞的细胞质就是由蛋白质构成的。其他两种食物成分则用于提供能量。
+
+直到最近以前，人们一直认为肉类食物是我们肌肉运动的必要能量来源，但现在经过研究发现，其他食物反而更能为肌肉提供能量，并且大量吃肉不仅是毫无必要的金钱破费，而且对我们的膳食结构没有好处。北非和东南亚一带的居民一般以椰枣为主食，这是热带气候条件下一种非常理想的食物。在寒冷地区，身体需要大量的能量以保持热量，有条件的话，人们会食用大量的脂肪和糖类。具体的食物样式往往在很大程度上决定于它的实用性和独特的味道，但从合理膳食的角度而言，丰富多样的食物类别才能有效地提供我们所必需的这三种必需食物成分。
+
+除了必需食物以外，大多数人都还会吃些别的零食和喝一些其他饮料。这些一般都含有香精、茶碱、咖啡因等类似物质，少吃有益，可以促进食欲，但多吃就有害了。
+
+酒精除了在它极少量的状态下可以作为食物而外，其他情况下都不能直接食用或者用来促进食欲。深入的实验证明，它能暂时性地对身体产生刺激，但并不能让身体机能变得更好。而且，考虑到饮用它而造成的机能损失和增加的身体或精神负担，这跟不沾染它相比，根本没得到什么好处。即使不是由于一贯的嗜好，或者出于逢迎所需，酒精的使用也能从生理上降低我们身体的活力，损害身体的免疫力，并让神经系统和精神心智都反应迟钝。
+
+人们对烟草也做了详细的实验研究，尽管在它对成年人的影响方面存在不同观点，但人们在它对未成年人的负面影响方面却是没有异议的。经过对大量中学生身体发育状况、体能耐力以及心智能力等多方面的测量与比较，结果清晰地显示，那些没有吸烟的学生的体格健康状况、学习成绩以及锻炼强度，都远远好于那些吸烟的学生。在田径比赛中也发现，吸烟的学生的成绩，往往只有不吸烟的学生的成绩的一半。
+
+111．食物的备制——当食物菜肴色香味俱全的时候，唾液和胃液就会被大量分泌，这样当食物进入我们的消化系统后，就会很容易地被消化吸收。我们烹饪食物的理由之一就是要让它们变得美味可口，这应该是食物烹饪过程中不能背离的原则之一。烹煮可以软化肉类食物的纤维束，让淀粉颗粒爆裂，进而让消化液能与食物充分接触。除此以外，烹煮还能杀灭细菌或寄生虫。
+
+精心烹饪食物可是个精细活儿，需要细致地学习研究，还得具备高超的技巧。如何经济便利地备制必需食物，并且要它们秀色可餐，还要便于消化而不失营养，已经成为一门专门的学科，现在才处于起步阶段。人类和动物一样，要想保持足够的活力，则必须有一个合理平衡的膳食体系。我们与动物最大的不同在于，我们的食物面更广，而且对食物的色香味更敏感，也更挑剔。
+
+
+
+
+
+总结——植物与动物构成了地球的生命世界。大多数植物都由根、茎、叶组成，根部吸收了除了碳和氧以外的其他所有植物所需的养分，这两种养分则专门由植物的叶片负责供给。植物的叶也是所有动植物原初的食物加工制造基地，它们被植物的茎干支撑着。植物可分为两大类：单子叶植物和双子叶植物。
+
+植物的茎上还有花朵，它们一般由花萼、花冠、雄蕊和雌蕊组成。花的主要功能是为植物自身繁殖培育种子，其中用于让卵细胞受精的花粉颗粒会被大自然的风，以及昆虫和鸟类带走，进而向四处传播。植物的种子也会被风和动物带向广漠大地，有时还会随着水流漂向远方。
+
+除了绿色植物，还有一类植物叫做真菌。它们不像绿色植物那样，借助于阳光和土壤中的养分自己制造食物，而是完全依赖于绿色植物合成的食物。
+
+最简单的植物可能算是细菌了，它们是一种单细胞植物，并且繁殖非常迅速。细菌和真菌导致了许多疾病的产生与流行，更是使食物变质的罪魁祸首。导致疾病的细菌叫做病菌，我们���以用消毒剂和抗毒素来抑制它们的生长。
+
+动物直接从植物和其他种类动物身上获取能量，它们通常分为两大类：无脊椎动物和脊椎动物，最低等的无脊椎动物是原生动物。蠕虫和昆虫以及其他种类的无脊椎动物对人类和大自然都具有极大的重要性，目前我们对这一点的认识还远远不够。
+
+脊椎动物都拥有脊柱、头盖骨、肋骨以及附件骨这四大部分的骨骼。在头盖骨中的就是我们的大脑，它通过神经系统与我们身体的每一部分相连。脊椎动物的呼吸主要是将空气通过呼吸道“抽入”肺部，这一过程是靠胸部肌肉和膈膜完成的。血液在肺部得到净化，然后通过血管在心脏的心室与心房和全身各处之间循环。
+
+人体的五类感觉分别为视觉、听觉、味觉、嗅觉和触觉，感觉信号都是由神经系统传递给大脑，并依次分别来自于眼睛、耳朵、嘴巴、鼻子和身体的皮肤。
+
+要让我们的身体和大脑能正常工作，食物自然必不可少。食物在消化道中会与各种消化液充分混合，并在消化道的不同部分被消化器官吸收进我们的身体。食物在本质上可以分为三大类，即碳水化合物、蛋白质和脂肪，为了营养的合理平衡，我们的身体对它们的需求也各有固定的比例。通常烹饪后的食物才最有营养，也才最美味可口。
+
+
+
+
+
+思考题
+
+
+在这一章里面，我们为什么要像研究其他地球现象一样，专门研究动物和植物？
+
+大多数植物从自身结构形态上可以分成哪三个部分？动物和植物存在哪三个方面的相似性？植物的根对植物有什么作用？这些作用是如何体现并完成的？描述一下你见过的一些不同种类的植物的茎，并解释一下它们是如何适应其所处环境的。
+
+植物的叶对植物有什么作用？这些作用是如何体现并完成的？
+
+植物的花对植物有什么价值？它们在植物的生存斗争中扮演了什么角色？描述一些你知道的动物为植物的生存充当助手的例子。
+
+植物是如何散布它们的种子的？
+
+种子是如何长成植物的？
+
+在你的生活经历中，你发现真菌有哪些好处又有哪些坏处？
+
+细菌在什么情况下对人类有益？又在什么情况下对人类有害？我们可以用什么办法对付有害细菌？
+
+植物与动物是如何相互帮助对方的？
+
+列举一些你所知道的无脊椎动物。
+
+我们发现蚯蚓对土壤有哪些益处？
+
+骨骼和神经系统对脊椎动物有什么作用？
+
+描述一下脊椎动物的呼吸过程。为什么自由呼吸对脊椎动物而言极其重要？
+
+血液对身体有什么作用？它们是如何在体内循环并到达它们要去的地方的？
+
+描述一下人类是如何感知外部世界的。
+
+我们为什么需要食物？它们是在哪里被消化的？又是怎样被消化的？
+
+食物可以分为哪三大类？为什么你不应该抽烟酗酒？
+
+为什么说烹饪是世界上最有用的一门艺术？
+
+
+
+
+
+译注
+
+
+[1]此概念并不严谨，蘑菇属于菌类生物，尽管菌类属不属于植物这一问题在生物学界一直争论不已，但白色植物的提法早已过时，读者勿受此影响。关于菌类生物的话题，后文还会专门论及。
+
+[2]真菌究竟是植物还是动物，抑或是有别于动植物的另一个单独生物类别呢？迄今为止，科学家们对这个问题依然众说纷纭。在1975年召开的第12届国际生物学会议上，动植物学家们又进行了广泛深入地讨论，试图确定真菌在生物界中的地位，但遗憾的是，最后还是没有得出统一的结论。不过大部分的真菌学家认为，由于真菌具有一系列特殊的形态特征和生理性状，它还是应该属于与动物界、植物界并列的新的生物分类界——真菌界。
+
+[3]这不是一个好办法，不过却是那个时代应急的办法之一，比如有瘟疫大爆发，且有些池塘的水又无法及时排走，此法应急可减少蚊蝇滋生，以传播急性瘟疫。
+
+[4]Burson，本生（1811-1899），德国著名化学家，他发明了实验煤气灯。此处本生灯即以其名字命名。
+
+
+
+
+
+CHAPTER 7
+
+LIFE OF THE EARTH AS RELATED TO PHYSICAL CONDITIONS
+
+环境对地球生命的影响
+
+
+112. Ancient Life History. —As the rock layers of the earth are explored, fossils of different kinds of plants and animals are discovered. The fossils of the more recent rock layers correspond very closely to the plants and animals that are found upon the earth to-day, but the older the layers, the less they correspond. There seems to have been a gradual development in life forms through the past ages, a fragmentary record of which is engraved upon certain of the sedimentary rocks. Rocks which were formed under different conditions contain different species of life forms, showing that throughout all time the geographic condition has had a marked influence upon plants and animals.
+
+
+
+PETRIFIED TREES.
+
+Found near Holbrook, Arizona.
+
+
+
+The rocks and fossils show that the geographical conditions of certain areas also have varied greatly. Some regions have been below and above the sea several times. Regions now cold have been warm, and those now dry have been wet, and vice versa. Thus the life in certain areas has suffered great changes by the geographical accidents to which the region has been subjected. The petrified forests near Holbrook, Arizona, show some of the most remarkable tree fossils ever found and indicate that the region has been subjected to remarkable geographical changes.
+
+113. Distribution of Life. —Plants and animals are found wher ever the conditions are suitable for their existence. In ice-covered ar eas, like the interior of Greenland, and in exceedingly dry regions, like the Sahara and certain parts of southwestern United States, there is little life of any kind. With a few such exceptions, however, the surface of the earth is a universal battlefield of plants and animals struggling to exist and to increase. They extend themselves wherever attainable space is opened. But barriers may oppose their spread and geographical accidents may drive them from areas which they had heretofore held. The retreat of the sea may cause a change in the position of shore life. In the water a land barrier or an expanse of deep water may prevent the spread of shore forms. On the land a mountain uplift, a desert area, or a water barrier may limit the space occupied by animal and vegetable species.
+
+
+
+GILA MONSTERS.
+
+The most poisonous reptiles of the southwestern American desert.
+
+
+
+Certain plants and animals are much more widely distributed than others. Plants like the dandelion and thistle, whose seeds are easily blown about by the wind, spread rapidly, while trees like the oak and chestnut spread slowly. As plants have not the power to move about, they cannot distribute themselves as easily as animals. Certain birds which are strong of flight are found widely distributed over regions separated by barriers impassable to other animals.
+
+Some of the present barriers to life distribution have come into existence in comparatively recent geological time. There is good reason to believe that the British Isles and Europe were formerly connected, and that in very ancient times Australia was joined to Asia. It is also believed that for long ages North and South America were separated by a water barrier and that even after they were once connected, the Isthmus of Panama was again submerged.
+
+These are but a few illustrations of the changes in the earth's surface which have affected the distribution of animals and plants. Climatic changes like that which brought about the great ice advance of the Glacial Period have affected in a marked degree the distribution of life. It is thus found that when a study is made of the present distribution of life, careful attention must be given to the present and past geographical conditions of the region.
+
+
+
+CANADA THISILE
+
+One of the most widely distributed of plants.
+
+
+
+114. Adaptability of Life. —There is hardly a place on the earth's surface not adapted to some form of life. Even upon the ice-bound interior of Greenland a microscopical plant and a tiny worm have found a home. The dry desert regions have a few plants with small leaves or, like the cactus, with no true leaves. This prevents the evaporation of the water from their surfaces and so protects them from drought. To protect them from animals, many of these plants are armed with thorns.
+
+
+
+A RATTLESNAKE COILED READY TO SPRING.
+
+The color of these reptiles makes them hardly distinguishable from the surrounding desert.
+
+
+
+
+
+CACTI.
+
+These are adapted to desert life because they have no leaves from which water can evaporate.
+
+
+
+Another example of adaptability is the fact that the small animals of the desert are generally of a sandy color, which makes them hardly distinguishable from their desert surroundings. The large ones are swift strong runners, like the antelope and ostrich, or, like the camel, are able to travel for long distances without water.
+
+
+
+A HERD OF REINDEER.
+
+This animal is of invaluable service to man in polar regions.
+
+
+
+In the colder regions the plants have the power of rapid growth and germination during the short season when the snow has melted away. Then, during the long winter, they lie dormant but unharmed under the snow and ice. The animals are either able, like the reindeer, to live upon the dry mosses, lichen and stunted bushes, or else upon other animals. Their color, like that of the polar bear, often blends with their surroundings.
+
+Some animals have a wide range of adaptability, like the tiger, which is found from the equator to Siberia. But usually the range of an animal species is much more restricted, since it is seldom able to adapt itself to widely differing conditions. The surrounding region, the elevation, the temperature, the amount of moisture, the soil, the kinds of winds and their force, all have a marked effect upon the fauna (animals) and flora (plants) of a country.
+
+
+
+TIGER.
+
+One of the most widely distributed of animals.
+
+
+
+The species that thrive in a region must have adapted themselves to the existing conditions, yet other animals and plants may be as well adapted for certain regions as those now inhabiting them. Striking examples of this are the English sparrow and the gipsy moth, which have spread with such tremendous rapidity since their introduction into this country. The rabbit in Australia and southern California is another striking example. The adaptability of plants to a new region is also illustrated by the Russian thistle which was introduced into this country in 1873 and which has now become a national pest.
+
+
+
+A CALIFORNIA RABBIT DRIVE.
+
+In some communities rabbits become such a pest that the inhabitants turn out in a body and drive them into enclosures.
+
+
+
+115. Life of the Sea. —The plants living in the sea are nearly all of a low order. The mangrove trees which border some tropical shores represent their highest type. The most abundant of sea plants, the seaweeds, have no flower or seed or true root, although most of them have an anchoring device by which they are attached to the bottom. Their food is absorbed from the surrounding water. They have developed little supporting tissue, but instead have bladder-like air cavities or floats, which enable them to maintain an erect position or to float freely in the water. Usually they abound near the shore where the water is shallow.
+
+
+
+DIFFERENT KINDS OF SEAWEED.
+
+
+
+The vast surface of the open sea supports few plants except the minute one-celled plants, the diatoms, of which there are many species and an almost infinite number of individuals. These furnish about the only food for the animals of the open sea except that obtained by preying upon each other.
+
+A great quantity of detached seaweed (Sargassum), filled with multitudes of small marine animals and the fishes which prey upon them, covers the surface of the middle Atlantic, the center of the oceanic eddy. Through this Columbus sailed from the 16th of September to the 8th of October, 1492, greatly to his own astonishment and to the terror of his crew, who had never before heard of these "oceanic meadows."
+
+The animals of the sea vary in size from the microscopic globigerina, whose tiny shells blanket the beds of the deeper seas, to the whale, that huge giant of the deep, in comparison with which the largest land animals are but pigmies. Although monarch of all the finny tribe, it is not a fish at all, but a mammal which became infatuated with a salt-water life and so through countless ages has more and more assumed the finny aspect. It is obliged to rise to the surface to breathe. It cares for its young like other mammals.
+
+
+
+A SMALL SHARK.
+
+Photographed under water.
+
+
+
+Here, too, are found the jellyfish, the Portuguese man-of-war (Fig. 110), some fishes, many crustaceans, a few insects, turtles, snakes and mammals. Most of these animals are lightly built and are well equipped for floating and swimming. Some sea animals, like the oyster, barnacle and coral polyp, are fixed, and rely upon the currents of the water to bring them their food, while others, like the crab, the lobster and the fish, move from place to place in search of prey.
+
+
+
+Fig. 110.
+
+
+
+In the warmer seas the surface water is often filled with minute microscopical animals which have the power, when disturbed, of emitting light, so that when a boat glides through these waters, a trail of sparkling silver seems to follow in the wake.
+
+Between the surface and the bottom of the deep ocean there seems to be a vast depth of water almost devoid of life. This region, like the bottom of the ocean, has been little explored and there may be life here which has not been discovered. From the bottom of the sea the dredge has brought up some very curious forms of life. Here under tremendous pressure and in profound darkness have been developed species of carnivorous fishes.
+
+Some of these have large, peculiarly well-developed eyes and others have not even the rudiments of eyes. As the light of the sun never penetrates to these depths, it would seem at first that eyes could be of no use to animals, but it has been found that some of the animals of the ocean bottom have the power of emitting light in some such way as the glow worm and firefly do, and it is probable that it is to see this phosphorescent light that the eyes of the animals are used. There are no plants here and the life is much less abundant and less varied than near the surface.
+
+
+
+FLYING HSH.
+
+Notice how the front fins have become wiag-like.
+
+
+
+
+
+SELAS.
+
+Origianlly land animals.
+
+
+
+There is but little variation in the conditions surrounding the animals of the sea, so the organs corresponding to these conditions are not diverse. Living in a buoyant medium dense enough to support their bodies, and of almost unvarying temperature, the sea animals have never required or developed varied organs for locomotion, like the wing, the hoof and the paw, or for protection from cold, like the feather, the hair, or wool. It is true that certain sea dwellers, like the seal, are covered with hair, but these air breathers were probably originally a land type and have acquired the habit of living in the water. The highest traits of animal life, such as are found in land animals, have not been required or acquired by the sea animals, and although the number of species and kinds is very great, there is not found among them the same grade of intelligence or power of adaptability, as among the land animals.
+
+116. Life of the Land. —The highest development of both plant and animal life is found upon the land. Here at the meeting place of the solid earth and its gaseous envelope, subjected to great variations in amount of sunlight, moisture, temperature and soil, the plants and animals have acquired a marvelous variety of forms and structures to adapt them for their varied surroundings, and enable them to secure a living.
+
+Some plants lift their strong arms high into the air to intercept the sunbeams before they strike the earth, while others clothe the surface with a dress of varied green. In some plants, odor, nectar or juicy berries attract the animals whose aid is needed for fertilizing and scattering their seeds, while in others, noxious odors, prickles, thorns and acrid secretions warn away animals destructive to their welfare. The highest perfection of beauty, utility and productiveness among plants has been reached by those of the land.
+
+
+
+PRICKLY PHLOX.
+
+Notice the thorns by which it protects itself.
+
+
+
+The animals of the land, surrounded by the air, which bears no food solutions to inert mouths, must be well endowed with the power of motion in order to procure their food. They must either crawl over the surface or be provided with appendages to support their weight against gravity. There is no floating supinely in the air as in the water. Movement, exertion, search are the requisites of this realm. The eggs and young, as a rule, cannot be cast adrift to hatch and care for themselves; the nest, the burrow, the den must be provided. This is the realm of homes.
+
+
+
+BIRD'S NEST.
+
+A simple home.
+
+
+
+The land animals are also the most intelligent. Birds long ago solved the problem of flight for a body heavier than air, which is now being successfully solved by man after years of effort. Certain animals, like the bee, the ant and the squirrel, have the provident habit of storing up food in the summer against a day of need. Other animals, like the birds, have learned to migrate to a warmer clime when winter comes. The beaver is probably the pioneer in hydraulic engineering. When he feels the need of a water reservoir, he builds a dam and makes it. Today many a swamp in the northern states owes its origin to him. Wonderful indeed is the intelligence of many of the land animals, due in large part to their development amid varied geographical conditions.
+
+
+
+A BEAVER DAM.
+
+Notice the two beavers on top of the dam.
+
+
+
+117. Distribution of Animals. —An examination of a globe shows (1) that the land is massed around the north pole, (2) that the three continental masses to the south are separated from each other by wide seas, and (3) that while two of these are connected by narrow strips of land to northern continents, the third is entirely separated from all other land.
+
+But slight changes in elevation would connect the northern continents with each other. As they are so closely related to each other, it might be expected that the animals of these continents would resemble each other, particularly in the more northern parts. This is true. Bears, wolves, foxes, elk, deer and sheep of nearly related species are found distributed over the northern continents.
+
+
+
+OSTRICHES.
+
+The largest of all birds.
+
+
+
+The animals of the southern continents are much less nearly related. The ostrich, giraffe, zebra and hippopotamus are among the characteristic animals of Africa which are not found elsewhere. In South America the tapir, great ant eater, armadillo and llama are among the animals not represented elsewhere. Both of these continents, however, have animals closely related to those of other great divisions, showing that their present isolation has not continued far back in geological time.
+
+
+
+OPOSSUM.
+
+Many opossums have no pouch but carry their young on their backs.
+
+
+
+The animals of Australia differ greatly from those of the other continents. The quadrupeds here are marsupials, animals which usually carry their young in a pouch. The only members of the family existing at present elsewhere are the American opossums. The largest of the marsupials is the great kangaroo which measures between seven and eight feet from its nose to the tip of its tail. Although it has four feet, yet it runs by making extraordinary leaps with its strong hind feet. Here is also found one of the most singular of all living animals, the duckbill, the lowest of all quadrupeds, which in its characteristics resembles both quadrupeds and birds.
+
+All this seems to show that the distribution and development of the animals of the different continents have been largely dependent upon the former geographical relations of the land masses. The native animals of a region are not necessarily the only ones suited to it; animals from other places may be even better adapted, but they have been kept out by some natural barrier. This is particularly evident in the case of Australia, where the weak native animals would have been readily displaced by the stronger animals of Asia could these have reached that isolated continent.
+
+
+
+A KANGAROO FEEDING.
+
+Notice the peculiar position it is forced to take because of its short front legs.
+
+
+
+
+
+TIMBER LINE ON A HIGH MOUNTAIN.
+
+
+
+118. Life as Affected by Climate. —Climate has had a great effect upon the distribution and development of life. But the life on the earth cannot be grouped into climatic belts, as certain animals and plants are able to endure a wide range of climatic conditions. Moisture, sunlight and temperature are the chief factors which determine the growth and development of plants. If the temperature is too high or too low, they cannot exist. If sunlight or moisture is wanting, they cannot build their tissues.
+
+In regions where the temperature is constantly below freezing, there can be no plant life. Where the temperature is above freezing for only a short time in the year, plant growth is slight and whatever plants there are must be small and stunted. Only where there is a long growing period can large plants exist. That is why there is almost no plant and therefore almost no animal life on the upper parts of lofty mountains, while somewhat farther down the life is stunted, and still lower down life flourishes. Changes in latitude have the same effect.
+
+
+
+MESQUIT BEANS.
+
+From this desert plant some Indian tribes made their bread.
+
+
+
+
+
+AN OASIS IN THE MOJAVE DESERT.
+
+
+
+Where moisture is lacking, no plants can grow. Where there is but little moisture, only those plants can grow which are able to make the best possible use of the available moisture. In dry regions the plants are few and so constructed that little moisture can be evaporated from them. In dry desert regions, except where water finds its way to the surface in considerable quantity, forming oases, there can be but little forage for animals, and what there is, is scattered. The desert animal must therefore be a wanderer, able to subsist upon meager fare.
+
+This is true of the human inhabitants of the desert as well. They must rove about in small bands living in tents, picking up a precarious living for themselves and their animals, and they must be hardy and capable of withstanding privation. They must move rapidly and carefully over the long distances separating the patches where food can be found. They must therefore be fine horsemen, like the Arabs of Arabia and the Sahara, or strong and swift runners, like the Indians of the southwestern United States.
+
+
+
+A WATER HOLE IN THE DESERT.
+
+
+
+
+
+A DESERT AND OASIS.
+
+Notice the oasis at the foot of the gully where a spring comes to the surface.
+
+
+
+The life of man on the oases, although much less miserable than that on the desert, is subject to great disadvantages. These spots are of limited extent and frequently of limited moisture. They are often separated by almost untraversable areas from the other inhabitants of the world. There can be but little commerce or intercourse with the rest of mankind. Although the oasis may appeal with a poetic charm to the dweller in the desert, yet to the inhabitant of a fertile country it is but a sorry place.
+
+In regions where there is plenty of moisture, sunlight and heat, the growth of plants and animals is abundant and is only limited by space and food. Here life is at its best.
+
+119. Life on Islands. —Islands which rise from the continental shelves were probably at one time connected with the continents, but have since been separated by the submergence of the intervening lowland. The animals and plants of such islands are similar to those of the adjacent large land masses. But oceanic islands possess only those types of plants and animals which originally were able to float or fly to them over the surrounding water expanse. Indigenous mammals, except certain species of bat, are wanting. Birds are abundant.
+
+On the tropical islands the cocoanut palm is the main supply of vegetable food, clothing and building material. Many of the species of both plants and animals are different from those of the nearest continent and even of the adjacent islands. So complete has been the isolation of the life of these islands for so long a time that it has been possible for great differences in species to develop. Large unwieldy birds unable to fly or run rapidly have been found on some oceanic islands, the dodo of Mauritius, now extinct, being one of the most notable.
+
+
+
+THE DODO.
+
+Although the dodo is extinct, sufficient remains have been found to enable scientists to tell how it looked.
+
+
+
+The absence of predatory animals has probably made the development of such forms possible. The great species of tortoise from the Galapagos Islands perhaps owes its development to the same cause. Nowhere else have such huge tortoises been found. The remarkable fauna and flora found on oceanic islands may be regarded as due to their geographical isolation.
+
+120. Life as Affected by Man. —Wherever man has established himself, he has become a dominant factor in the distribution and ex- istence of plants and animals. Forests are cut down, swamps are drained and streams dammed. Shade-loving plants suddenly find themselves exposed to the full glare of the sun, plants which need much water find themselves in a dry soil, and other plants which need a dry soil are flooded by the impeded streams. They cannot stand these sudden changes of environment and die out. The plow overturns the sod and the fields are sown with seeds the natural home of which may have been thousands of miles away across the sea.
+
+
+
+ORIGINAL FLORA SUPPLANTED BY NEW PLANTS.
+
+
+
+In the course of years the original flora of the region is represented by only a few species inhabiting places man has not deemed it worth his while to cultivate. New plants suited to man's wants have taken the place of those which through thousands of years of struggle have shown themselves the best adapted to the geographical conditions in the region.
+
+The animals share the same fate as the plants. Domestic animals replace the wild denizens of the country. Only in inaccessible and waste places are a few lone remnants of the native fauna left. If the area which man enters is limited and bounded by impassable barriers, as in the case of islands, certain animals may be entirely exterminated, as wolves and bears have been in England. If the animal is a unique species, its extermination from its native island may mean its total destruction, as in the case of the dodo of Mauritius.
+
+121. Forestry. —One of the notable ways in which modern man has affected the life and industries of the earth is in his treatment of the forests. In North America, before the coming of the white man, there were probably extensive areas where the growth of forests had been checked by fires set by the Indians. The prairie regions were probably much enlarged by the annual grass fires. Tree-covered areas, too, were often burned over, and the growth of the trees checked, in order to make hunting less difficult. The greater part of the country was, however, covered with thrifty forests.
+
+
+
+BAD FORESTRY.
+
+The débris was left to feed the forest fires, and all the standing timber was ruined.
+
+
+
+In recent years the demand for lumber and wood pulp and the careless and wasteful way in which the forests have been handled by the lumbermen has greatly reduced the forests of the United States. It has been authoritatively stated that if the present waste of our forest land continues, the timber supply of the country will be exhausted before 1940. Not only are the forests being recklessly cut down, but forest fires are each year destroying millions of dollars worth of timber. When the importance of lumber to all kinds of industries is considered, the rapid exhausting of our forest supplies becomes almost appalling.
+
+
+
+BAD FORESTRY.
+
+The forest was razed, leaving no small trees for future growth.
+
+
+
+But not only is the destruction of the forests a menace to the industries in which lumber is necessary, but the effects are far reaching in many other directions. Slopes from which the forests have been removed become an easy prey to the forces of erosion, and the soil which for thousands of years has been accumulating may be swept away by the rainfall of a few seasons, leaving the slopes bare of soil and devoid of vegetable life. Thus the sites of valuable forests, which by proper care might have been continual wealth producers, are rendered nearly profitless deserts.
+
+
+
+BAD FORESTRY.
+
+The hillside was stripped, leaving it a prey to erosion.
+
+
+
+The harmfulness, however, does not stop here. The rain that falls upon these slopes, and which was formerly retained by the roots and vegetation, so that it slowly crept downward into the valleys and streams, now runs off quickly, flooding the rivers and doing damage to regions at a distance. Streams which formerly varied but little in their volume during the entire year, now become subject to great extremes of high and low water. This renders them less useful for manufacturing, commerce and water supply to say nothing of the frightful damage done each year by floods.
+
+The destruction of the forests tends also to exterminate the wild animals and deprives man of a chance to get away from his artificial surroundings and obtain a knowledge and an enjoyment of life and nature which has been unaffected by his own dominant influence.
+
+
+
+GOOD FORESTRY.
+
+Notice how carefully the underbrush has been removed to guard against fire.
+
+
+
+
+
+GOOD FORESTRY.
+
+Notice how the underbrush and small limbs have been cleaned up.
+
+
+
+In many European countries the forests have become a national care and not only is the cutting of trees, except under certain restrictions, prohibited, but the greatest care is maintained to guard against fires. In our own country the government has recently established a number of forest preserves which are carefully patrolled, and here the destruction from forest fires is rigidly guarded against. Great care of all forests should be taken by hunters, campers and all others who visit them, and also by the railways passing through them. Loggers and lumbermen should see that it is to their interest to maintain growing forests and not wantonly to destroy them.
+
+When the native forests are destroyed, trees of other kinds may in time replace those removed, but frequently these are of less commercial value. Thus, when the conifer forests of the northern states are cut off, birches and poplars replace them. If only the larger trees had been cut, leaving the smaller and younger trees to hold the ground, the more valuable forests might have been retained.
+
+122. Flora of the United States. —The United States has such a great range of climate, soil, elevation and other geographical conditions that it possesses a large variety of plants. About five hundred different kinds of native trees have been listed, and there are many different kinds of shrubs and smaller plants. To these native plants must be added many useful as well as ornamental plants and not a few noxious weeds, which have been imported from other countries and have found an agreeable home here.
+
+
+
+A CALIFORNIA BIG TREE.
+
+Notice the size at the base.
+
+
+
+The largest and perhaps most remarkable trees in the United States are the Big Trees of California, a species of redwood. Trees of this species have been measured which were 325 ft. high, more than 100 ft. higher than Bunker Hill Monument, and others which were more than 90 ft. in circumference. A plant which has probably appropriated more territory than any other plant in the world is the "sage brush" of the arid western plains. These low grayish shrubs cover hundreds of thousands of square miles. They are useless except for fuel. In the well-watered part of the country, plants are abundant and varied, ranging from the subtropical palms of the Gulf coast to the semi-arctic types of the northern border.
+
+123. Fauna of the United States. —As the wide treeless plains and prairies of the central part of the country contained few coverts for skulking animals of prey, they were admirably adapted to the wants of gregarious grazing animals. Here were found the countless herds of antelopes and buffaloes which in the early days of transcontinental travel swarmed over the territory crossed by the railroads and not infrequently forced the trains to stop and wait until they had crossed the track. To-day they are almost exterminated.
+
+The forests of the northern regions with their grassy glades and meadows, once the home of great herds of caribou and great numbers of moose, have too frequently resounded to the sound of ax and gun still to contain many of these noble creatures.
+
+Among the mountains with their rough surfaces and rugged fastnesses the black, brown, and grizzly bear once roamed supreme, but now they find security only in the most inaccessible places. Wolves once skulked in bands far and wide over almost the entire country, ready to pull down and devour weaker animals, but now both they and their prey have almost vanished.
+
+
+
+COYOTE.
+
+The prairie wolf of the western plains.
+
+
+
+
+
+PRAIRIE DOG.
+
+
+
+The fur-bearing animals which inhabited the streams and dales and whose valuable pelts tempted the early hunters to explore the unknown regions of the country and mark out the trails afterward followed by the pioneers, have almost entirely disappeared. Small animals, like the fox, the prairie dog, the skunk, the woodchuck, still remain, the puny survivors of a once varied fauna. Plain and valley, hill and mountain are at present nearly devoid of their native inhabitants. The horse, the cow, the mule are the chief members of the present fauna.
+
+
+
+
+
+Summary. —Physical conditions have a great effect on the distribution of life upon the earth. It is hard for living things to cross high mountains, broad oceans or vast deserts. When confined to certain climates and areas, plants and animals naturally adjust themselves to these.
+
+Life in the sea is so simple that plants and animals there are not forced to become as highly developed as are those of the land. On land there are greater ranges of climate and other physical conditions, so that plants and animals have been forced to a high development in order to survive. Probably the two greatest forces affecting land life are climate and man. Man transplants and transports animals and plants according to his desires. The physical conditions decide whether or not they shall live.
+
+By his treatment of forests man can also have a great effect upon the wealth and beauty of the country, upon the safety of its rivers and the reliability of its water power. Much more money has been lost in the United States through the floods and fires caused by bad forestry than has been gained by the people who cut the trees.
+
+Because of the wide range of climate and the variety of physical conditions, there are a great many different kinds of plants and animals in the United States, but the wild animals have been steadily killed off as man has needed their haunts for his farms and dwellings.
+
+
+
+
+
+QUESTIONS
+
+
+What do the rock layers show in regard to the history of life?
+
+Give several reasons why the same kind of plants and animals are not found all over the earth.
+
+What animals do you know that are peculiarly adapted to the conditions under which they live?
+
+If you could take a trip in a submarine capable of traveling over and through the sea, what kind of animals would you expect to find? Under what different conditions would these animals be living?
+
+How are land animals prepared to meet the conditions about them?
+
+Describe how the distribution of animals has been influenced by geographical conditions.
+
+How are animals affected by climate?
+
+What influence have oceanic islands had on life?
+
+How has man in many places changed the life of the land?
+
+Why is the proper care of forests so important to the well being of man?
+
+To what plants and animals is the United States the "home land "?
+
+
+
+
+
+【中文阅读】
+
+
+112．远古生命的故事——人们在勘探岩层的时候，经常发现不同种类动植物的化石。而且化石所处的岩层年代越近，它与今天的动植物形态越符合；反之越是久远的岩层，其化石与今天的动植物形态的符合度越差。这表明生命的形态在漫长的历史长河中有逐渐发展变化的迹象，一些零碎的生命印记就埋藏在一些沉积岩层中。在不同条件下形成的岩石所包藏的不同物种形态，显示出地理环境一直对动植物有着巨大影响。
+
+岩石与化石也说明不同地区的地理环境的差异也非常明显。一些地方已经经历过数次沧海桑田的变换，寒冷的地方也曾温暖过，干旱的地方也曾滋润过，反之亦然。因此，生命自然也不可能在这样变幻无常的地理条件下置身事外，只能与它们共同进退。亚利桑那州霍布鲁克附近的石化森林，就展示着被人们所发现的极不寻常的树木化石，并表明该地区曾遭受过剧烈的地质改变。
+
+113．生物的分布——植物和动物总是存在于适合自己生存的地方，在千里冰封的格陵兰内陆地区，或者异常干旱的撒哈拉沙漠和美国西南部一些地方，生命便极其稀少。除了这些极少数的例外，整个地球表面都是动物和植物物竞天择适者生存的战场与舞台。只要有适合它们生存的地盘，它们便一寸也不会放过。但环境的边界往往会限制它们的流布传播，地质条件的改变也可能会让它们的生活乐园发生迁移。比如海洋的退却就会让岸边的生命重新寻找家园，而水下的陆地屏障，或者广阔的深水区，也可以成为海岸不同形式的延伸。在陆地上，山脉的抬升、沙漠或者水域的阻隔，都可能限制动植物的生存空间。
+
+某些种类的动植物会比其他种类的动植物有更加广泛的分布，比如蒲公英和蓟草，由于它们的种子极易随风飘流，因此传播得也非常迅速，而一些像橡树和栗树一类的树种，传播就很慢了。植物不能自行移动，因此便不能像动物那样可以四海为家。一些飞行能力很强的鸟类，便不像其他大部分动物那样受到地理及环境的限制，而能在千差万别的不同的地域之间广泛分布。
+
+目前我们看到的各类生物分布状况的边界，都差不多形成于晚近的地质年代。而且也有理由相信，不列颠群岛和欧洲大陆曾经是一个整体，远古时代的澳大利亚与亚洲大陆也是连在一起的。人们还相信，南北美洲大陆在极其漫长的岁月中曾被大洋隔开，即使它们连接在一起之后，巴拿马海峡还是会陆沉于大海之中。
+
+这些只是关于地表改变对生物分布的影响的一些粗浅例证，详细的例子当然还有很多很多。气候的改变对生物的影响也颇为显著，比如冰河时期大量冰的出现就是如此。因此当我们在做关于现代生物分布严肃研究的时候，更多的注意力应该放在各地区过去和现在的地理环境的差异上来。
+
+114．生命的适应性——在地球上几乎找不到一个地方没有与之相适应的生命形态，即使在冰雪覆盖的格陵兰内陆，真菌和微小的蠕虫类生物依然能够在那里安居。在干旱的沙漠地区，一些小叶植物同样能够生存，比如仙人掌，它们几乎没有真实的叶。这可以防止其体内的水分蒸发，让它们可以在一定程度上适应周遭的干旱的环境。为了防止被动物侵害，许多植物还通过长刺而让周身全副武装。
+
+适应��的另一个例子就是，沙漠里的小动物都长着沙地的颜色的外壳或皮肤，让它们与环境很难区别开来。还有那些能够快速奔跑的大型动物，比如羚羊、鸵鸟和骆驼，都可以在不饮水的情况下行进很长的距离。
+
+在严寒地区，植物能够在冰雪消融的短暂时间内迅速生长发芽，而在漫长的冬季，则处于冬眠状态，在冰雪之下依然生机勃勃。像驯鹿这样的动物，在干苔藓、青苔和小灌木密布的地区也能自如地生存，还有些动物能依靠其他种类动物而存活，而且它们的周身颜色也很特殊，比如北极熊，常常与周围环境的颜色融为一体。
+
+有些动物更具有极强的适应能力，比如老虎，从赤道地区到西伯利亚都有它们的分布。但一般而言，动物的分布往往更受限制，因为它们对复杂外界环境的适应能力相对要弱一些。环境状况、海拔高度、温度、湿度、土壤、风的类型和强度，都会对一个国家的动物群落和植物群落产生显著影响。
+
+在一个地区扎根生存并欣欣向荣生长发育的动植物，必然已经适应了那里的自然条件，其他动植物当然也有自己所适应的归属地与安乐窝。比较显著的例子有英格兰麻雀和吉普赛飞蛾，自从它们被引入美国，其繁衍速度极其惊人。澳大利亚和南加利福尼亚的兔子也是另外一个显著的例子。植物对新环境的适应性也有例子，俄国蓟草在1873年引入美国后，现在已经成了全国之害了。
+
+115．海洋中的生命——海洋中的植物几乎都是低等植物，靠近一些热带海岸的红树就算是它们中的最高级植物了。数量最大的海洋植物——海藻，便没有花和种子，也没有根，大部分都是靠一个锚固装置附着在海底。它们的食物则通过从周围的海水中吸收，同时它们也没有支撑组织，但有一个囊状的气腔，或者叫浮子，来保持直立并在水中自由漂浮。通常它们一般分布于靠近海岸的浅水地区。
+
+广阔的大洋海面上，几乎没有什么植物能在此生存，除了一种单细胞植物，硅藻。它们种类繁多，而且数量几乎无穷无尽。这也为大洋中的动物在扑食其他动物之外又提供了一种食物。
+
+往往数量巨大的海藻（马尾藻）群，连同生存于其中的微小海生动物，以及在此赖以生存的鱼类，会充塞在一处，覆盖中大西洋的大面积海域，包括海洋涡流的中心。哥伦布的船队在从1492年9月16日到10月8日的航行中，他和他的全体船员就曾对此海藻群惊恐万分，因为他们之前从未见识过这样的“海洋草原”。
+
+海洋动物的个体大小差别巨大，小到需要显微镜才能看见的球房虫，深海的海床就是被它们的甲壳所覆盖的；大到体型巨大的鲸，最大的陆地生物和它比起来也成了侏儒。尽管它们是有鳍类生物家族的王者，但它们不属于鱼类，而是一种哺乳动物。由于曾经误打误撞地进入了海洋的咸水生活环境，经过漫长时期的进化，它们便越来越体现出了有鳍类生物的特征。它们必须将头伸出水面才能呼吸，而且它们照顾小鲸的方式跟其他哺乳动物也是一样的。
+
+海洋中还有其他许许多多的动物，比如水母（下图为葡萄牙的僧帽水母）、鱼类以及一些甲壳动物、少量种类的昆虫，还有龟类、蛇类和其他一些哺乳动物。所有这些动物都体态轻柔，其身体结构也便于漂浮和游动。有些海洋动物，比如牡蛎、藤壶以及珊瑚虫则是固定而不能移动的，它们只能依靠流经它们的洋流海水中的食物生存，而其他动物比如螃蟹、龙虾和鱼类，则可以到处捕食。
+
+
+
+Fig. 110.
+
+
+
+在温暖的海域，水面上常常聚集着一种在显微镜下才能看见的动物，它们能够在被扰乱时发出微光，因此当一艘船驶过水面时，后面便会出现一道银光闪闪的涟漪，在水面慢慢荡漾开去。
+
+在洋面和深海之间，有一个巨大的区间似乎完全没有生命的痕迹。某些深海的海底，我们还没有探究到那里，也可能存在我们尚未发现的生命。有时从海底打捞上来的物质中，就含有一些稀奇古怪的生命样式。在那里由于水压巨大而且黑暗无边，也让一些肉食性鱼类在此不断进化。
+
+这些鱼有的长着极为发达的眼睛，有的却几乎根本没有眼睛。因为太阳的光线完全无法到达这里，因此感觉上可以认为它们的眼睛在这里毫无用处，但后来又发现有些海底动物能像萤火虫那样发光，这样它们的眼睛也很可能是用来搜寻这类的磷光性的光线。不过这里没有植物，生物的种类也远远少于临近水面的区域。
+
+海洋动物所处环境的多样性与复杂性也并不强，因此它们身体器官与外在环境的匹配度也几乎相同。生活��那样一个自由自在的水世界中，而且几乎没有大的温度变化，海洋动物便完全没有必要四处迁徙，自然也就没有进化出这类器官，比如翅膀、脚和爪子；也没有一些御寒的体貌，比如羽毛、须发和绒毛。有一些海洋动物，比如海豹，周身也有毛发，但是它们作为哺乳动物，最初很可能是生活在陆地上，只是获得了在水中生存的能力而已。一些陆地动物所具有的高等动物特性，在海洋动物那里则完全没有必要。而且尽管海洋动物的种类也极其繁多，但它们却完全没有陆地动物所具有的智商程度和适应能力。
+
+116．陆地上的生命——最高等级的植物与动物都生活在陆地上。在土壤与空气交界的地球表面，阳光充沛、温度湿度适宜、土质良好，因此动植物在这里体现出了丰富多彩的生命样式，并以不同的结构形式来适应它们各自的周遭环境，从而得以繁衍生息。
+
+有些植物在稳稳扎根土地之前，便高高地伸开手臂拥抱阳光，有的则用深浅不一的绿色衣裙妆点大地；有些植物还以自身的芳香气味、甘甜花蜜和果浆来吸引一些小动物靠近，而这些小动物又可以帮助它们为花朵授粉以及散播它们的种子；有的则用有毒的气味、锋利的刺芒、苦涩的味道来保护自己不受动物的侵害。对自身的美丽、功用和多产特性具有最高保护能力的动植物，都存在于陆地上。
+
+生存在陆地上的动物，周围并没有现成的食物，因此必须被赋予行动的能力以到处觅食。它们要么能够爬行走动，要么能够用其他器官支撑自身重量以抵抗引力。它们可不能像海洋动物悠闲地漂浮在水中那样也漂浮在空气中，四处移动、努力搜寻食物就成为它们必备的生存能力。对它们的卵和幼子，都不能放任不管让它们自生自灭，因此它们必须有自己的巢穴，这就是它们的家了。
+
+陆地动物也是最聪明的动物。鸟类在很久很久之前就解决了如何克服自身重力的问题，而人类则是在多年努力之后的今天，才解决了这个问题。有些动物比如蜜蜂、蚂蚁和松鼠，具有先见之明，总是习惯性地在夏天储存食物，以防他日的不时之需。其他一些动物比如鸟类，更是具有在冬天来临之前向温暖的地方迁徙的能力。海狸恐怕是水利工程建设的先驱了，当它们需要蓄水时，便会以筑建堤坝来达到目的。今天美国北部省份一些沼泽地就归功于它们。许多陆地动物体现出的无穷智慧的确精彩万分，这在很大程度上是丰富多彩的自然地理状况导致的。
+
+117．动物的分布——通过对世界地理状况的研究，我们可以得出以下结论：（1）陆地都相对向北极集中靠拢；（2）南半球的三个大陆都被广阔的海洋隔开；并且（3）其中两个大陆通过一个狭窄的陆地区域与北半球大陆相连，第三个大陆板块则完全被海洋与所有陆地区域隔开。
+
+但只要陆地的海拔高度稍作变化，它们很可能又重新连接在一起。因为它们相互的联系本来就十分紧密，故而它们各自地域上的动物也应该比较类似。这个判断，是正确的。熊、狼、狐狸、麋鹿、鹿子还有山羊等等动物，在整个北半球的大陆上，就都有分布。
+
+南半球大陆上的动物可就没有这么明显的关联性了。鸵鸟、长颈鹿、斑马、河马一类的非洲动物，在其他地方就没有。南美洲的貘、食蚁、犰狳以及美洲骆驼，也在其他地方不曾出现。但即便如此，两个半球的大陆的动物从大类上依然有很强的相关性。这也表明，它们在今天所显现的分布地域的孤立性，在遥远的地质年代其实并非如此。
+
+澳洲的动物和其他大陆上的动物就格外不同，这里的四足动物都是有袋类动物，通常用一个袋囊携带它们的幼子，这个家族中唯一在其他大陆存在的成员是美国的负鼠。澳洲最大的四足动物便是大袋鼠，从鼻端到尾巴尖差不多有七八尺长。虽然它们也有四只脚，但跑动时主要是靠它们强有力的后腿完成距离很大的跳跃。这里还有一种极其独特的稀有动物，鸭嘴兽，算是很低等的四足动物了，它们身上还保留着四足动物和鸟类的共同特征。
+
+这些证据似乎都表明，不同大陆上的动物分布与进化状况，在很大程度上受制于前地质时代的大陆板块结构。一个地区的本土动物也不一定就是唯一能适应这里的动物，其他地区的动物有可能还更适应这里，只不过被自然地理条件所阻隔罢了。在这一点上，澳洲大陆就能提供证据，只要亚洲大陆上的强大动物能转移到那里的话，便能很轻易地取代那里的相对弱小的本土动物。
+
+118．气候对生命的影响——气候对生物的分布与进化影响极其显著。但是地球上的生物却并不是严格按照气候带来对应分布，因为很多动植物能够忍耐复杂多样的气候状况。水分、阳光、温度，是决定植物生长发育状况的三个主要因素。如果温度过高或者过低，植物都不能生存。如果缺少阳光和水分，它们也难以构建自身组织。
+
+在温度常年低于零下的地区，一般也不会有植物存在。而在那些一年之中只有很短暂的时间温度处于零上的地方，植物的生长发育也十分有限，大多长得非常矮小。只有在能提供较长生长期的地方，植物才会郁郁葱葱地大量存在。这就是为什么在一些高海拔山脉的顶峰往往没有动植物生存，而越往下动植物的种类会越来越多的原因。地区纬度的变化往往也有类似的效应。
+
+在缺乏水分的地方，植物难以生长。而在水分虽有但却并不充足的地方，也只有那些能够极力利用各种途径保持水分的植物能够生长。在干旱地区，植物就非常稀少，而且那里仅有的植物都能通过自身的独特结构防止水分蒸发。在干燥的沙漠地带，除了那些由于地下水大量渗出地表而形成的绿洲，其他地区几乎没有动物可以赖以生存的草料，即使有也极其分散而稀少。因此，沙漠里的动物注定都是流浪者，必须具备在食物匮乏的境况下生存过活的本领。
+
+这对生活在沙漠地区的人来说，也是如此。他们必须成群结队四处漂泊，一路住帐篷，和他们的动物伙伴们一起过着不稳定的游牧生活，在物资贫乏的生活中书写流浪的人生。他们必须经常迁徙，并且小心地穿越漫漫长路，去寻找赖以生存的食物来源。因此，他们必须成为骁勇的骑士，就像阿拉伯半岛和撒哈拉的阿拉伯人一样；或者成为强壮的奔跑者，美国西南部的印第安人就是如此。
+
+生活在绿洲地区的人们，虽然比那些生活在完全干旱的沙漠里的人们要稍微好点，但也同样忍受着极大的不便。最主要的就是生活范围极受限制，同时也照样经常缺水。他们被几乎不可穿越的广漠荒原与世隔绝，与外部世界的商业贸易与人文交流也非常稀少。尽管绿洲常常在诗人的笔下散发着浓浓的诗意，但对于生活在肥沃土地上的人们而言，那里并不是一个真正令人向往的地方。
+
+在阳光雨露充足的地方，动植物的生息繁衍都欣欣向荣，只会受到食物养分和生长空间的限制。在那里，生命才尽情呈现着它的最佳面目。
+
+119．岛屿上的生命——位于大陆架上的岛屿曾经很可能与大陆板块直接相连，后来中间的低洼地带沉入海底，它们变孤零零地被水域与大陆分隔开了。这类岛屿上的动植物与它们临近大陆上的动植物一般都很类似。但是在海岛上，却只有那些能够从大陆上漂浮到那里或者飞到那里的动物，除了蝙蝠以外，它们几乎没有本土的哺乳动物，鸟类倒是种类繁多。
+
+在热带岛屿上，可可树成了人们的食物、衣料以及建筑材料的主要来源。那里的动植物，与距离它们最近的大陆上的，甚至与它们临近的岛屿上的动植物也差别很大。这源于长时间的孤绝隔离，让这里的物种发生了完全不同的进化。一些体型硕大、行动笨拙进而既不能飞又不能跑的鸟类常常就生活着这里。毛里求斯的渡渡鸟就是个例子，但值得注意的是，现在它们已经灭绝了。
+
+没有凶猛的捕食动物存在，可能是让它们有此形态的原因，加拉帕格斯群岛的巨龟也很可能是基于同样的进化环境，因为在其他地方从未发现类似的巨龟。因此，海岛上的动植物种群，和它们所处的孤立地理环境有很大关系。
+
+120．人对生物的影响——人一旦在一个地方繁衍生息，他们便会成为影响那里动植物分布与存在形态的决定性因素。森林被砍伐、沼泽被开发、溪流被阻断，喜阴植物会突然发现它们被暴露于刺眼的阳光下，需要大量水分的植物会突然发现它们自己扎根于干燥的土壤中，而那些本该在干燥土壤生存的植物却被阻断的溪流泡在水里。它们无法忍受这些突如其来的生长环境的改变，纷纷凋零殆尽。草地被犁头耕耘，田野中也被播撒了种子，而这些种子本来的家，在可能在千里之外的海的那一边。
+
+过不了多少年，这个地区的本土植物群落，就只会孤零零地存在于因人们瞧不上眼而尚未开垦的地方。满足人们需求的植物，会迅速取代曾经在漫长岁月中不断挣扎奋斗，最后终于适应这里地理条件的那些本土植物。
+
+这里的动物也和植物同命相连，家畜会取代野生动物，只有在一些人们难以到达的或者荒废的地方，还会看见极少本土动物群落的孤单身影。如果人类进入的地区本身就很有限制，或者有自然的明确边界的话，某些动物可能就要遭受灭顶之灾了，英格兰地区的狼和熊就是前车之鉴。如果这些动物本身就是稀有动物的话，它们的消亡就成了物种的灭绝了，就像毛里求斯的渡渡鸟一样。
+
+121．森林资源——当今的人类构建现代生活与工业的一个显著途径，就是它们对森林的采伐利用。在北美洲，当白种人还没有来到这里的时候，可能已经有广阔茂密的森林区域被印第安人放火烧掉了一部分，以拓展他们的生活空间。草原牧场也可能被一年一度的草火而大大扩展。林木植被也一样，被一次又一次地烧掉，树木的生长也被加以限制，这样会让狩猎少一些困难和阻碍。但即便如此，当时大部分地区依然覆盖着茂盛的森林。
+
+最近一些年，由于木料的需求大增以及一些人为的浪费，森林被木材商大面积砍伐，使得美国的森林覆盖面积急剧减少。据权威消息，如果我们的森林像这样继续毫无限制地砍伐下去的话，在1940年之前，木材资源就可能彻底耗尽[1]。不光砍伐会减少森林的面积，每年的森林火灾也会毁掉价值数百万美元的木材。考虑到木材供应对其他各行业的重要性，森林的快速砍伐与耗用真的相当可怕。
+
+更重要的是，森林的毁灭不仅对木材依赖很强的行业是一个巨大威胁，它的严重后果还会体现在其他很多方面。原来覆盖着森林的山坡在森林消失之后很容易被外力侵蚀，堆积数千年的土壤可能在数年之内被雨水冲刷殆尽，只留下光秃秃的岩土，寸草不生。因此只要我们给予森林适当的关心，它的价值会继续惠泽于我们；而如果我们放弃这个立场，等待我们的将是荒芜的沙漠。
+
+问题，还不止这些呢。当雨水降临的时候，以前有树木的根和其他植物可以大量吸收水分，因此雨水只会缓慢地浸润而下，流向山谷，形成一路欢歌的潺潺溪流。而现在，则是奔流而下，形成山洪，给周围很大范围的地区造成危害。以前溪流会随着季节变换而涨落有致，现在则变得非常极端，水量猛增猛减。这让它们本来对工农业、商业与水资源供应的价值大打折扣，每年的洪水造成的毁坏更是让人不愿提及。
+
+森林的毁坏还会殃及野生动物，更会让我们的生活失去色彩。本来人们可以逃离都市的人造环境，到森林里闲庭信步，呼吸一下清新的空气，享受一下大自然的阳光雨露，增加一些在人迹喧嚣的地方不曾有过的生命体验。而这，在森林消失后，便完全不可能了。
+
+在一些欧洲国家，政府非常重视森林的保护，不仅砍树被严格禁止，当然必要时除外，而且对森林防火给予了极大的关注。在美国，政府最近也采取了一系列措施来加强对森林的保护，比如严格的巡逻制度，以及严格防止森林火灾等等。对森林的保护其实还得靠一些猎人、户外旅行者以及所有造访它们的人，包括穿行于其中的铁路干线，只有他们都做到了对森林的爱护，实际效果才能产生。伐木工人和木材商也会慢慢尝到保护森林的甜头，而不应该只是一味杀鸡取卵式的破坏它们。
+
+同时，当本土森林一旦被毁坏，其他树种便会及时取代那些消失的树木，但多数情况下，这些树木都没什么经济价值。举个例子，美国北部的针叶林被砍伐之后，桦树和白杨便取代了原来的树种。或者换一种方式，如果我们在砍伐时只砍伐其成年的高大树木，留下其幼苗来保持水土，则更有价值的森林资源就被保留了下来。
+
+122．美国的植物群落——美国陆地上的气候、土壤、海拔以及其他地理因素纷繁复杂，因此也造就了植物大家庭的丰富多彩。树木就有五百多种，还有其他更多种类的灌木以及矮小植物。在这些本土植物之外，还有许多种其他国家的有益植物、观赏植物，乃至不少有毒害的野草，也被输入到这片土地上，并在这里扎根繁育了。
+
+在美国，最大的树种可能也是最不寻常的树种，就要算加利福尼亚的巨杉了，它是一种红杉树。它可以高达325英尺，比邦克山纪念碑还高出100多英尺，树干周长也可超过90英尺。世界上占地面积最广的植物要算美国西部荒原上的“艾灌丛”，这种低矮植物可以覆盖成百上千平方英里的土地。但它们除了当柴烧以外，没什么别的用处。在水分充足的地区，植物生长繁茂且种类多样，从墨西哥海岸的亚热带棕榈树，到北部边境的亚寒带树种，这里应有尽有。
+
+123．美国的动物群落——由于美国中部的大部分地区都是荒芜的平原，很少有树丛给肉食动物捕食提供屏障，这就为群居的食草动物提供到了极���的生存环境。在那里有成群结队的羚羊和水牛，早些年的时候，常常看见它们大量地结伴而行，穿越大陆，来到新的领地，有时还会穿过铁路，让火车停下来等它们悠闲地越过铁轨又才继续开动。然而今天，它们几乎已经灭绝了。
+
+在北部地区的森林里，遍布着沼泽和草地，这里曾是大量驯鹿和驼鹿的温馨家园。但后来，这里由于大量的砍伐和狩猎，让这些优雅高贵的动物慢慢消失了踪影。
+
+在山区的乱石岗和峡谷中，我们曾经可以看见许多黑熊、棕熊和灰熊在那里悠然踱步，而现在，只在一些险要的悬崖边上才能看见它们了，因为那里才是它们安全的地方。狼过去在全国范围都有分布，常常潜伏捕食弱小的动物，但是现在它们连同它们的猎物都消失得无影无踪了。
+
+还有那些栖居在河流与山谷中的皮毛动物，由于其皮毛颇具价值，早年的猎人们不惜冒着危险，在拓荒者行进的路径上做上标记，深入险境搜寻它们，致使它们如今也已全部灭绝了。还有一些小动物，比如狐狸、土拨鼠、臭鼬还依然存在，它们算是幸存下来的弱小动物种群了。如今，平原、峡谷、丘陵、山地，都已经没有了它们原来的动物居民。马、奶牛、骡子已成为现在动物种群的主要成员了。
+
+
+
+
+
+总结——环境地质状况对生命在地球上的分布具有非常大的影响，因为生命很难跨越高山大海与沙漠荒原。当被固定于一定的气候条件下，动物和植物都会很自然地慢慢适应。
+
+海洋中的生活简单而单一，因此海洋中的动物和植物便没有陆地上的动植物那么高的发达程度。陆地上的气候与地理条件千差万别，这就迫使动植物必须具有相对较高的发达程度，才能在陆地上生存。影响陆地生物最主要的两个因素，可能就是气候与我们人类了。人们会根据自己的意愿，在不同地区之间，移植或者转运不同种类的动植物，而当地的环境条件与地理状况，最终会决定它们能否生存下来。
+
+一个国家的人们如何对待他们的森林，将会在很大程度上决定这个国家自然环境的潜在价值与自然之美的保有程度，还会影响到这里的河流是否经常发大水，以及是否能将它们变成人们可以依赖的水资源。由于对森林的滥砍滥伐导致的洪灾与森林大火所造成的损失，已经远远大于我们通过对它的砍伐而取得的经济效益。
+
+由于复杂多变的气候状况和地理环境条件，美国的动植物的种类极其繁多，但许多野生动物都快被捕杀殆尽了，因为人们需要占领它们的领地来安居乐业。
+
+
+
+
+
+思考题
+
+
+地下的岩层为我们显示了哪些生命历程的片段与碎影？
+
+陈述几个理由说明一下为什么没有任何一种植物或动物在全球任何地方都能找到。
+
+就你所知，哪种动物特别能够适应它自己所处的环境？
+
+如果你能乘坐潜艇穿越海洋的话，你最想见到哪种动物？它们生活在什么样的特殊环境下？
+
+陆地上的动物是如何来适应外部环境的？
+
+描述一下地理环境是如何影响动物的分布的。
+
+动物会受到气候怎样的影响？
+
+海岛的地域特点会如何影响岛上的生物？
+
+人类对很多不同地方的生物产生了怎样的影响？
+
+为什么说保护森林对人类的健康发展极其重要？
+
+美国是哪些动植物的故乡？
+
+
+
+
+
+译 注
+
+
+[1]当然作者在这里危言耸听了一点。直到今天，美国木材资源不仅没有“彻底耗尽”，反而其森林资源依然位居世界前列。据近年数据，美国森林总面积为2.98亿公顷，森林覆盖率33%，人均拥有森林面积1.33公顷。另附目前世界各主要国家的森林覆盖率数据排名：日本67%，韩国64%，挪威60%上下，瑞典54%，巴西50-60%，俄罗斯45%，加拿大44%，美国33%，德国30%，法国27%，印度23%，中国16.5%。
+
+
+
+
+
+CHAPTER 8
+
+THE SEA
+
+海 洋
+
+
+124. The Sea. —On looking at a map of the world or at a globe, one is immediately impressed by the predominance of the sea. The whole area of the globe is about 196,940,000 square miles, 72% of which is water. The larger part of the land is in the northern hemisphere, and of the water in the southern hemisphere. A comparatively small land area extends below 40° south latitude, about the latitude of Philadel-phia in the northern hemisphere. Most of the maps of the world do not represent the southern hemisphere below latitude 60°, which is about the latitude of Petrograd (St. Petersburg) in the northern hemisphere. Thus the equator is usually considerably below the center of the map.
+
+
+
+THE OCEAN.
+
+From a photograph taken in mid-Atlantic.
+
+
+
+125. Divisions of the Sea. —Although most of the surface water of the earth is connected, yet for many purposes it is better to put this water area into somewhat arbitrary divisions. We thus speak of the Atlantic, the Pacific, the Indian ocean, each of which may be divided by the equator into a northern and a southern part, and the Arctic and Antarctic oceans which surround either pole. Sometimes a division is made from the parallel 40° south and this great body of water, almost without land boundaries is called the Southern Ocean.
+
+The boundaries of these oceans are irregular in shape, but with the exception of the great Southern Ocean and of the Arctic Ocean, which is really an inclosed sea, they narrow toward the north. They have a number of partially landlocked seas connected with them. In some instances these penetrate far into the land, as in the cases of the Mediterranean Sea and Gulf of Mexico. The surface of the sea is level, unstable, easily moved and always rising and falling in rapid and changeful undulations.
+
+126. Continental Shelf. —Around the border of the continents and of those islands which are near the continents, there extends, in some cases to a distance of two or three hundred miles, a gradually deepen ing ocean floor. This gradually deepening border is called the continen tal shelf. When this floor has reached the depth of about 600 feet, the gradual slant suddenly changes into a quick descent to the depths of the ocean, two or three miles.
+
+
+
+CONTINENTAL SHELF.
+
+A model showing the sea floor off the coast of Southern California.
+
+
+
+Upon this shelf lie the great continental islands, like the British Isles and the East Indies. It is this that furnishes the great fishing banks of the earth, such as the Grand Banks of Newfoundland and those around Iceland and the Lofoten Islands, where fishermen for ages have obtained vast supplies of fish. There is no equal area of the earth where the life is so varied and the struggle for existence so great as on these shallow continental borders.
+
+Here the mud and sand brought down by the rivers is spread out and the sedimentary rocks formed. It is the elevation of this shelf which has formed the low-lying coastal plains which border many of the continents. There is good reason to believe that the deep floors of the sea have never been raised into dry land, and that the vast extent of sedimentary rocks which make up the larger portion of the land has almost all been laid down in regions which were at the time continental shelves.
+
+
+
+
+
+127. Composition of Ocean Waters.
+
+
+
+
+
+Experiment 122. —Into a dish of fresh water put a demonstration hydrometer or stick such as was used in Experiment 35. Mark the depth to which it sinks. Place the hydrometer now in sea water and mark the depth to which it sinks. If sea water cannot be obtained, dissolve in a pint of fresh water about 15 g., or half an ounce, of salt. This will give the water about the same amount of dissolved solid material as sea water would have. About how much more of its length does the hydrometer sink in fresh water than in sea water? Will a piece of ice project more out of salt water than it would out of fresh water?
+
+
+
+
+
+Experiment 123. —If ocean water can be obtained, boil down about a pint of it in an open dish. Taste of the residue which is left. What is the principal constituent of this residue?
+
+
+
+
+
+There is probably no water on the surface of the earth which is absolutely pure. Water is the greatest solvent known, and it dissolves to a greater or less extent almost all substances with which it comes in contact. When the rivers run into the sea, they carry with them whatever their water has dissolved from the land, and when the sun evaporates the water, and it is borne away again to fall upon the land, the dissolved material is left behind in the ocean.
+
+Thus the sea has for all time been receiving the soluble contributions from the land. It is easy to prove that it contains salt, for we can taste it. It must contain lime, for corals and shell animals depend upon this for the hard parts of their bodies. There must be organic food material in it, or else fixed animals like oysters could not get food. It contains air, for without air fishes could not breathe. These are the principal substances which we need to consider in the study of the ocean water, but the chemist can find many other substances dissolved in it. On account of the materials dissolved, sea water weighs more than fresh water, or has a greater specific gravity. A cubic foot of fresh water weighs about 62.5 lbs. whereas a cubic foot of sea water weighs over 64.25 lbs.
+
+128. Ocean Depths. —The greatest depth thus far found in the ocean is nearly six miles. This was found in the Pacific Ocean near the Ladrone Islands. The greatest depth in the Atlantic Ocean thus far dis covered is a little over five miles at a point north of Puerto Rico. The average depth of the sea is probably about two and one half miles.
+
+Although the pressure at the bottom of the ocean must be tremendous, yet so incompressible is water that a cubic foot of it weighs but little more at the bottom of the sea than it does at the top. Thus a body which sinks will in time reach the bottom no matter what the depth may be. At a depth of two miles the pressure is over 300 times as much as at the surface of the water and here, as we have already found, it is about 15 pounds to the square inch.
+
+If a bag of air which had a volume of 300 cubic inches at the surface were sunk in the ocean to a depth of two miles, it would have a volume of less than a cubic inch, and the pressure upon it would be several tons. It thus happens that deep sea fishes when brought to the surface have the air in their swimming bladders so expanded that the bladder is often blown out of their mouths.
+
+129. Condition of the Ocean Floor. —The ocean floor is a vast, monotonous, nearly level expanse whose dreary, slimy and almost life-less surface is enveloped in neverending night and is pressed upon by a vast weight of stagnant, frigid water. Here and there volcanoes rise upon it with gradually sloping, featureless cones, and sometimes a broad wavelike swell reaches within a mile or so of the surface. Such a swell extends along the center of the Atlantic Ocean through Ascension Island and the Azores.
+
+There are no hills and vales, no mountain ranges having sharp peaks and deep valleys. Gradually rising ridges and volcanoes, sometimes topped with coral islands, alone vary the monotony. It is the nether world of gloom and unaltering sameness. Here the derelicts of ages past, after their fierce buffeting with wind and wave, have found a quiet, changeless haven where they may lie undisturbed until absorbed into the substance of the all-enfolding water. Some animal species which lived in the light of former geological ages have here found a resting place where the strife of progress is stilled and the laggard in the race of development may live in peace.
+
+
+
+CRINOID.
+
+An animal now found only at considerable depths in the ocean.
+
+
+
+130. The Carpet of the Ocean Floor. —Near the shore, the floor of the ocean is covered with sand and mud derived from the waste of the land. In the deeper sea the covering is a fine-textured material of animal origin called ooze. It is composed of the shells of minute ani mals that live near the surface. The most abundant shell is that of a minute animal called the globigerina, hence the deposit is often called the globigerina ooze.
+
+At a depth of about 3000 fathoms (18,000 feet) these shells disappear and a reddish clay appears. This clay is believed to be due to meteoric and volcanic dust and to the insoluble parts that remain after the calcareous (limelike) material of the minute shells has been dissolved in sinking through the deep water. No layers of this kind have ever been found on the land, and this is one of the reasons for believing that the depths of the sea have never been elevated into dry land, but that what is now deep ocean has throughout all time been deep ocean.
+
+
+
+
+
+GLOBIGERINA. (Greatly magnified.)
+
+
+
+131. Waves.
+
+
+
+
+
+Experiment 124. —Take a long flexible rubber band or tube and having fastened one end, stretch it somewhat. Now strike down on it near one end with a small stick. A wavelike motion will be seen to travel from end to end of the band. It is evident that the particles of rubber do not enter into the lateral movement, but that they simply move up and down, whereas the wave movement proceeds along the band. A piece of paper folded and placed lightly upon the band will move up and down but not along the band. Thus wave motion does not necessitate lateral movement of the particles taking part in the wave.
+
+
+
+
+
+When the wind blows over water, it throws the surface into motion and produces waves. The highest part of the wave is called the crest and the lowest part the trough. Trough and crest move along rapidly over the surface of the water. The particles of the water themselves, however, move somewhat like those in the rubber band. That the water itself does not move with the wave can be seen when a floating bottle is observed. It moves up and down but does not move forward. If the water moved along with the waves, it would be next to impossible to propel a boat against the direction of the wave movement.
+
+That it is possible to generate wave movement without the particles themselves moving along with the wave is seen when a field of grain is bending before a gentle wind. The troughs and crests move one after the other across the field but the heads of grain simply vibrate back and forth. The crest of a water wave, however, is often blown forward by the wind and thus a drift in the direction of the wind is established at the surface.
+
+
+
+OCEAN WAVES.
+
+
+
+When great waves are raised by the wind at sea, there is danger that the mighty crests may be blown forward and engulf a ship. To calm the waves ships sometimes pour "oil on the troubled waters." The oil spreads out in a thin film over the water and forms a "slick" which prevents the wind from getting sufficient hold upon the water to topple over the crests, and thus the danger of being swamped is averted. It has been found that oil will spread out even in the direction of the severest wind.
+
+Although sometimes waves are spoken of as "mountain high," it rarely happens that the height from trough to crest is over 50 ft. The length of these great ocean waves, or the distance from crest to crest or from trough to trough varies from 300 to 1500 ft. or more. The velocity is sometimes as great as 60 miles per hour, but usually not more than half of this. The movement of the waves stirs up the water and enables it to absorb the air which is so necessary for the existence of water animals.
+
+Earthquakes occurring under the sea sometimes generate great waves which sweep in over the land destroying coast towns and shipping. These waves sometimes rise to a height of even a hundred feet above sea level. Ships have been carried by them a long distance inland and left high and dry. These waves, wrongly called tidal waves , have no connection with the wind.
+
+
+
+
+
+132. Temperature of Ocean Waters.
+
+
+
+
+
+Experiment 125. —Fill a flask of about 500 cc. with water. Press into the mouth of the flask a cork through which a glass tube about 30 cm. long extends. The tube should be open at both ends and should not extend into the flask below the bottom of the cork. When the cork is pressed in, the water will be forced up into the tube for several centimeters. See that the cork is tight and that there are no bubbles of air in the flask or tube.
+
+Now place the flask for fifteen or twenty minutes in a mixture of ice and water and carefully mark with a rubber band the point at which the water in the tube comes to rest. Take the flask out of the freezing mixture and notice immediately whether the water in the tube rises or falls. Continue for five or ten minutes to notice the action of the water in the tube. The volume of the water is not the least when it is at the temperature of melting ice, 32° F., but when it is a little above this temperature.
+
+
+
+
+
+Unlike fresh water, which is densest at a little above freezing, sea water continues to decrease in volume and grows denser as it is cooled, until it reaches its freezing point at 28° F. Hence the cold water near the poles gradually sinks and creeps under the warmer water of lower latitudes maintaining a temperature of 32° to 35° on the bottom, even at the equator. This steady creep of cooled surface water along the bottom supplies the animals of the deep ocean floor with the air which they must have. Without it the water at great depths would have its air exhausted and all life would be destroyed.
+
+
+
+
+
+At the surface of the ocean the temperature of the water varies in a general way with the latitude; it is over 80° at the tropics and about the freezing point at the poles. Near the poles and near the equator there is very little variation in the temperature of the surface water during the year, but in the intermediate latitudes the annual variation is considerable. Below the surface the effect of solar heat rapidly diminishes and at a depth of 300 ft. it is probable that the annual variation in temperature is nowhere more than 2° F. Below 600 ft. there is probably no annual change in temperature.
+
+On the surface the daily average range of temperature is not more than 1° F. and the annual range does not exceed fifteen degrees, except where the same surface is washed at different seasons by currents of different character, and near the shore, where the heat of the land affects it. This contrast in temperature conditions between the ocean and the land is most marked. The life conditions in one are uniform and unvaried while in the other they are most changeable and are subject to extremes of temperature. That is why the land animals must be much more highly organized than those of the sea in order to survive these changeable conditions.
+
+133. The Best-known Ocean Currents. —The ocean is a region of never-ceasing motion. At considerable depths its motion is very slow, but near the surface, where the prevailing winds can affect it, the movement is considerable. Circulating around each ocean there is a continuous drift of surface water extending to a depth of from 300 to 600 feet and vary ing in rate from a few miles up to fifty or more miles a day. In fact these rotating currents are the chief natural basis for the division of the oce anic area into six oceans, as our geographies generally divide them.
+
+These currents circulate in the northern hemisphere in the direction in which the hands of a watch move and in the southern hemisphere in the opposite direction. In the centers of these rotating areas the water is nearly motionless and here are often found great masses of floating seaweed filled with a great variety of small animals. These accumulations of seaweed are called sargasso seas.
+
+That these surface drifts have a definite direction of movement is indicated by observations made on the courses taken by a great number of wrecks. The direction of these movements has also been determined by throwing from ships in different parts of the ocean thousands of bottles in which had been placed the date and a record of the latitude and longitude of the ship. The places where the bottles came ashore showed the direction of the currents.
+
+If the movement of the water is slow, ten or fifteen miles a day, it is called a drift; if faster, a current. The principal currents have been given names and have been most carefully charted. The warm current that flows northeastward off the southeast coast of North America is called the Gulf Stream. That off the east shore of Asia, which also flows northeast, is called the Kuro Siwo or Japan Current. The cold current off the east coast of Labrador flowing southeast is called the Labrador Current; and the cold current which flows northward off the west coast of South America is called the Humboldt Current. Other names are given to different parts of the ocean movement, but those mentioned here are the most important.
+
+Where the ocean currents are unimpeded by the land, they flow in the direction of the prevailing winds. It has been found that the currents change their directions with a change in the direction of the prevailing wind, such as occurs in the Indian Ocean when the heat equator is farthest north of the earth's equator. It appears that ocean currents are primarily a result of the wind circulation.
+
+134. Effects of Ocean Currents. —A knowledge of ocean currents is of importance to mariners, as the course and speed of a vessel may be considerably affected by them. They also greatly affect the condi tions of animal life in different parts of the sea. Not only do these cur rents bring food to animals which have not the power of motion but they determine the area in which certain animals may live.
+
+The Bermudas, 32° north of the equator, are coral reef formations, while the Galapagos, almost on the equator, are surrounded by too cold water to have any such reefs. At 68° north, near the Lofoten Islands, are the great cod fisheries of Europe. On the western side of the Atlantic these fisheries are on the Grand Banks, latitude 45° north. Many other similar illustrations of the effects of these currents on the distribution of animal life might be cited.
+
+
+
+CORAL FORMATIONS IN THE BERMUDAS
+
+
+
+The temperature of winds blowing from the sea is modified by these currents and greatly affects the habitability of the earth for man. Hammerfest at 71° north is a flourishing seaport, but there are no important settlements above 50° on the western side of the Atlantic. Alaska, the prevailing winds of which are warmed by blowing over the warm ocean, is a region which promises much for human habitation, while the region on the opposite side of the Pacific must remain almost destitute of human inhabitants. It should be noted that the effect of the warm ocean waters would be slight, except along the coast, were it not for the air movements.
+
+135. Tides. —Probably the first thing that impresses us on visit ing the seashore is the regular rising and falling of the water each day. These movements of the water are called tides. If we observe the tides for a few days, we find that there are two high and two low tides each day. As the tidal current comes in from the open ocean and the water rises, it is called flood tide, and as it runs out or falls, ebb tide. When the tides change from flood to ebb or ebb to flood, there is a brief pe riod of "slack water."
+
+If we observe closely, we shall see that the corresponding tides are nearly an hour later each day than they were the day before, and that the time required for the completion of two high and two low tides is nearly 25 hours. Continued observation will show, as Julius Cæsar stated many centuries ago, that there is apparently a relation between the phases of the moon and the height of the tides. The greatest rise and fall of the water will be found to occur about the time of full and new moon.
+
+
+
+HIGH TIDE IN THE BAY OF FUNDY.
+
+
+
+The amount of rise and fall of the tides is not the same in different places. Upon shores of oceanic islands the difference between high and low water, or the range, as it is called, is not more than two or three feet. In funnelshaped bays where the tidal current is compressed as it moves in toward the bay head, the range is very much greater. In the Bay of Fundy the range is sometimes as great as seventy feet.
+
+
+
+LOW TIDE AT THE SAME PLACE.
+
+
+
+Sometimes these compressed tidal currents are formed at the mouths of rivers and move up the rivers like a wall of surf several feet in height, endangering all vessels which are not securely moored. Such tidal surfs are called bores and are found in the Seine, Amazon and other rivers.
+
+The tidal current as it sweeps between islands often forms eddies and whirlpools which make navigation very dangerous. An example of this is found at Hell Gate, N. Y., and at the famous Maelstrom off the coast of Norway. On the other hand in flat countries where the rivers are shallow, ports which could not otherwise be reached are made accessible to ships of considerable burden at the time of high tide. At these places the time of leaving or making port changes each day with the time of high tide. A striking example of this is the port of Antwerp.
+
+
+
+AN ATOLL IN THE MID-PACIFIC.
+
+
+
+The tidal currents are also continually changing the water in bays and harbors and thus keeping them from becoming stagnant and foul. They also bring food to many forms of shore life which have but little or no power of movement, such as clams and other shellfish, and by exposing some of these at the ebb give man a chance to acquire them readily for food.
+
+
+
+THE VATERLAND.
+
+One of the largest vessels afloat.
+
+
+
+It has been found that the position of the sun, as well as that of the moon, affects the height of the tide. If the earth, moon and sun lie in nearly the same line, the tidal range is greatest. This is called spring tide. When the sun and moon act at right angles upon the earth, the tidal range is least and this is called neap tide. The tidal undulations have been proved by astronomers to be due to the rotation of the earth and the gravitational attraction of the sun and moon upon its water envelop. The moon is much the more effective because it is nearer.
+
+In the open sea the rise and the fall of the tides are of equal duration but at the head of bays the tide rises more rapidly than it falls so that low tide does not occur midway between two successive high tides.
+
+136. Corals and Coral Islands. —In warm, clear, shallow areas of the sea are found small animals called corals. Great colonies of these are able by united action to build barriers capable of withstanding the waves and of raising submarine lands into islands. These reefbuilding corals cannot live at a greater depth than 20 fathoms (120 feet), or where the mean temperature is lower than 68° F., or where the discharge of rivers makes the water fresh or muddy. As they are fixed animals and must get their food from the surrounding water, they flourish best where the warm currents flow past continually, bringing a fresh supply of food. In tropical seas many islands are fringed by reefs built by these animals.
+
+
+
+PANAMA CANAL.
+
+An example of man's domination over nature.
+
+
+
+
+
+Two great oceans artificially united.
+
+
+
+In the tropical Pacific circular coral reefs are found nearly inclosing large shallow lagoons. Soundings outside of these reefs show that the sea rapidly sinks to great depths. Islands of this kind are called atolls. How these atolls were formed is still an open question. As reefbuilding corals cannot live below 20 fathoms, it is evident that corals could not have built up the reefs from the bottom of the deep sea.
+
+137. Man and the Ocean. —At first thought it would seem better for the life of the world if the proportion of land and water were reversed. Yet when we consider that almost barren wastes constitute many continental interiors and that plenty of rainfall is necessary to make land habitable, the utility of the great water surfaces becomes ap parent. From the evaporation of the ocean surface comes nearly all the water which supplies man, land animals and plants.
+
+It is not only true that all streams eventually run to the sea but it is also true that all their water comes from the sea. Other things being equal, the smaller the surface for evaporation the less the water supplied to the land. Besides supplying the land with water, the ocean has a great effect on its climate. The animals of the sea also furnish food for thousands. A large part of the earth's population is now, and always has been, located not far from the shore of the ocean.
+
+In early times before the advent of railways almost all commerce was carried on over the sea. Even now this is the cheaper way of transportation. Modern methods of conveyance have enabled man to live with comfort at a considerable distance from the ocean, but the dry interiors of continents still remain sparsely inhabited. All commercial nations must have an outlet to the sea and to obtain it much blood and treasure have often been spent. Man for his commerce has even broken through the barriers by which nature has separatedseas and oceans, as in the case of the Suez and Panama canals.
+
+
+
+
+
+Summary. —The sea occupies nearly three quarters of the area of the globe. It is usually spoken of as divided into oceans, the Atlantic, Pacific, Indian, Arctic and Antarctic. Although some parts of the ocean are nearly six miles deep, the most interesting and most extensively inhabited part of it is that above the continental shelf. This shelf is the gradual slope from the edge of the different continents to a depth of about six hundred feet. Beyond this the descent becomes rapid.
+
+On the surface of the sea are waves varying in height from a few inches to fifty feet. These stir up the water and enable it to absorb more air, which is so necessary to the living things in the sea. Not only is the surface of the sea constantly in motion, but there are also great currents, such as the Gulf Stream, which carry the water from one latitude to another. These currents are caused almost entirely by the winds.
+
+The tides are due primarily to the action upon each other of the earth, moon and sun. The range of tides varies from two or three feet on oceanic islands to about seventy feet at some places in the Bay of Fundy. The tides help drainage and assist vessels over bars that would otherwise be impassable.
+
+
+
+
+
+QUESTIONS
+
+
+What portion of the earth's surface is sea? Into what divisions is the sea usually divided?
+
+What is the continental shelf? Why is it of especial interest to man?
+
+Of what is the sea water composed? How does its composition affect animals?
+
+If you were able to take a trip from the nearest beach over the ocean bed to another continent, describe what you would probably find along the way.
+
+What are waves? What causes them?
+
+What is the temperature of the ocean water in different parts of its surface and at different depths?
+
+Describe the best known ocean currents. What is the cause of these? Why is a knowledge of ocean currents important to mariners and also to those who would explain the climates of different places?
+
+What are tides? What causes them? What are their effects?
+
+How are coral reefs formed? What are atolls?
+
+In what ways is the ocean useful to man?
+
+
+
+
+
+【中文阅读】
+
+
+124．茫茫大海——只要拿一张世界地图或者地球仪看上一眼，上面大面积的海洋会给我们留下深深的印象。地球表面积大约为196940000平方英里，其中72%是海洋。大部分的陆地都处在北半球，南半球的大部分面积都是大海。在南纬40°以下就只有很小的一部分陆地面积了，对应在北半球的话，差不多就是费城的纬度。因此大部分世界地图都不会将南纬60°以下的部分画出来，对应在北半球差不多是彼得格勒（圣彼得堡）的纬度。这也是在一些地图上，赤道会偏离图的中心的原因。
+
+125．海洋的分区——尽管地球上的海洋都是相互连通的，但是为了某些需要，将它们分成不同的部分会更好一些。因此，纵贯地球南北的海域被分成三大洋：大西洋、太平洋和印度洋，赤道把它们都分成了南北两半；处于极地周围的分别被叫做北冰洋和南冰洋。有时人们也将南纬40°以外的整体海域叫做南大洋。
+
+这些不同大洋的边界都没有规则形状，但除了有封闭边界的南大洋和北冰洋，其他三大洋都向北逐渐变窄。还有一些被陆地包围的海域也与它们连通，在有些地方部分海域还会深入到内陆，比如地中海和墨西哥湾。海洋的表面本身是水平面，但不稳定，会随着起伏不定的波涛升降起落。
+
+126．大陆架——围绕着大陆板块的边界，或者在靠近大陆的岛屿周围，往外延伸最远两三百英里，海底逐渐加深，这一个区域，就叫做大陆架。当海底达到600英尺左右的深度的时候，这种逐渐加深的趋势会突然变成向着深海的陡然沉降，达到两三英里的深度。
+
+在大陆架上散布着许多大陆岛屿，比如不列颠群岛和东印度群岛。这一区域，也是地球上最大的渔场，例如纽芬兰的大浅滩海域，以及冰岛和罗浮敦岛周围的地区，渔民们世世代代在这里丰收了巨量的渔产。地球上再也找不到第二个地方像大陆架的浅滩这样，生命的形态如此多样，生物之间的生存竞争如此激烈。
+
+河流会将泥沙带入海洋，它们慢慢沉淀下来，形成了沉积岩。大陆周围也有低洼的海岸平原，这是由大陆架上的一些高地包围而成的。有理由相信，深海的海底从未抬升到洋面成为陆地，但形成大陆板块主体部分的广袤沉积岩层，却都曾在海底沉睡过，那时它们正是大陆架的一部分。
+
+127．海水的成分
+
+
+
+
+
+实验122:在一碗干净的水中放入一个用于演示的液体比重计，或者放一根在实验35中用到过的木条，给它下沉的位置做上记号。再将比重计放入一碗海水中，也给它下沉的位置做上记号。如果没办法取得海水，就在一品脱水中溶解15克或者半盎司的盐，这就让水中的溶解物和海水差不多了。比较一下，比重计在淡水中会比在海水中多下沉多少？冰块在盐水中会比在淡水中漂浮得更明显吗？
+
+实验123：如果能取得海水的话，取其一品脱放入锅里熬干，尝一下残留物的味道，它的主要成分是什么？
+
+
+
+
+
+地球表面几乎没有百分之百的天然纯净水，不过，水也是我们所知道的最好的一种溶剂。任何一种接触到水的物质，都会或多或少地被水溶解掉一部分。当河流汇入海洋，它们也将一路所溶解的物质转运到了海水当中。在阳光的照耀下，海水又不断蒸发，再被空气携带到陆地上变成雨露降落下来，而之前的溶解物便被留在了海洋当中。
+
+因此，海洋无时无刻不在接收从陆地上转运而来的可溶物增益。非常容易证明海水中还有盐分，我们一尝就知道。海水中还一定含有石灰，因为珊���和贝壳类动物需要它来形成壳体的硬质部分。海水还含有一些有机食物成分，不然的话，像牡蛎这样不能移动的动物便得不到食物了。它还含有空气，否则鱼类便不能呼吸。这些都只是在探讨海水成分时我们能考虑到的主要物质，当然，化学家们还可以在海水中发现许多种溶解于其中的其他物质。由于这些溶解物的存在，海水也就比淡水更重，或者说具有更大的比重。1立方英尺的淡水重量为62.5磅，而海水则为64.25磅。
+
+128．海洋的深度——海洋已发现的最大深度已接近6英里[1]，位于太平洋的强盗岛附近。大西洋的最深处位于波多黎各北部地区，深度5英里多一点。海洋的整体平均深度大约为1.5到2英里。
+
+尽管海底的海水压力极其巨大，但由于水本身的不可压缩性，每立方英尺的水的重量，在海底与跟在海面差不了多少。因此，一个物体不管它处在哪个深度，只要一旦下沉，便会直接沉到海底。在两英里的深处的压力是水面附近处压力的300倍，我们之前也已说到，那里的压强大约为每平方英寸15磅。
+
+如果一袋体积为300立方英寸的空气从海面下沉到两英里的深度，其体积会变的还不到1立方英寸，其所承受的压力差不多有七吨重。因此当我们把深海鱼打捞上来暴露在空气中时，它们鱼鳔中的空气会急剧膨胀，常常从腹内鼓到嘴巴外面来了。
+
+129．海底的面貌——海洋的底部广阔浩淼，却也贫乏单调。泥泞粘滑的地面延绵万里却毫无变化，并被无边的黑暗包裹着，处在阴冷且似乎凝滞的巨量海水的重压之下。不时出现的火山会抬升地面，形成斜坡以及一个个毫无特色的锥形体，有时还会产生波浪形的隆起，让海水深度陡然变得还不到1英里。大西洋中心的复活岛到亚速尔群岛附近之间的海底，便延伸着这样的一个隆起地带。
+
+海底没有山丘、没有沟壑，没有延绵的高山与顶峰，也没有深深的峡谷。在广阔无尽的单调中，只有隆起的山脊和锥形火山，珊瑚岛往往就是它们形成的。这里差不多是阴暗乏味到极点的地狱，漫长岁月的残留物经过历史的风霜与大海的波浪激烈冲刷之后，在这里找到了死寂的安息之地，最终与周围无孔不入的海水慢慢地融为一体。一些早前地质年代的古老动物物种，在这里也找到了它们可以寂静长眠的地方，物种进化的动力完全放缓，因此这些相对低等落后的物种便得以在此默默生存。
+
+130．海底的覆盖物——在靠近海滨的地方，海底一般都覆盖着砂土，以及由陆地废弃物衍生而成的淤泥。在更深一些海底，覆盖物则是一层具有细密结构物质，动物就起源于此，学名叫做软泥，它是由生活在海洋表面附近的小动物的壳体形成的。其中数量最多的，是一种叫做球房虫的小动物的贝壳，因此这种沉淀物也被叫做球房虫软泥。
+
+当深度达到3000英寻[2]（18000英尺）之后，贝壳沉淀物就消失了，取而代之的一种红粘土层出现了。据估计，这一粘土层是由大气的和火山的灰尘，以及沉入深海的小贝壳成分中的不溶于水的钙质部分构成的。陆地上从未发现过类似的土层，这也是我们相信深海海底从未抬升成为陆地的原因之一。直到现在，深海，依然是，深海。
+
+131．海浪
+
+
+
+
+
+实验124：将一根橡皮筋或者橡胶管的一端固定，然后伸开拉平。拿一根小棍在它上面敲击一下，会看见一个波浪形的运动出现在整条橡皮筋上。但也非常容易看到，橡皮筋自身的每一部分并没有向侧面移动，而只是简单的上下运动。在橡皮筋上折叠一片小纸片放在上面，更容易看出来，在波动中，纸片完全没有参与波的传播而产生侧向移动。
+
+
+
+
+
+风吹向水面的时候，会让水面产生运动并由此形成波浪。波浪的最高的部分叫做波峰，最低的部分叫做波谷。波峰和波谷会沿着水面迅速移动，而水面的每一部分，自身并没有移动，跟刚才橡皮筋的运动一样。当有一个瓶子浮在水波中时，我们也能清晰发现这一点，水面只是在做上下移动而已，并没有向侧面运动。如果水面自身也会产生横向运动的话，要想推动逆波而行的船，就完全不可能了。
+
+传递波的介质本身不会沿着波动的方向移动的例子，还有田野的麦浪，当清风吹过，波峰波谷在田地里交替呈现，整体看去就像水上的波浪一样，而麦穗一个个都只是点头起伏而已，并没有横向移动。不过水波的波峰，由于被风吹动，也会在风向上产生一些漂移。
+
+当巨大的海浪被海风形成后，那可就危险了，因为它的破坏力非常强大，可将船只轻易吞没。为了让海浪平静下来，船上的人们有时会“往汹涌的波涛里倒油”。这些油液会在水面上形成一层大面积的“浮油”，也可以减弱海风与水面的接触，进而难以吹动海水形成波峰，这样，船只就躲避了被吞噬的危险。但是在风力极强的情况下，人们发现浮油也会被海风吹散。
+
+尽管人们常说“山一样的巨浪”，但波峰到波谷的距离很少有超过50英尺的。最大海浪的波长，也就是相邻两个波峰或者波谷的距离，从300英尺到1500英尺不等，甚至更长。其传播速度有时可以达到每小时60英里，但是一般情况下还不足这个的一半。海浪的翻滚搅动，同时也让海水能充分地吸收空气，这对生活在海洋的动物非常必要。
+
+在海底发生的地震有时会形成恐怖的海浪，它可以冲向海岸将城市和船只全部摧毁。这样的海浪高度有时可以超过100英尺，它的巨大力量常常将船只搬运到陆地上，甚至架到很高的地方晾着。这种现象叫做海啸，与海风并没有联系。
+
+132．海洋的温度
+
+
+
+
+
+实验125：给一个500cc的烧瓶装满水，瓶口用软木塞塞住，在瓶塞中插入一根30cm长的玻璃管，玻璃管必须两端开口，也不要延伸到瓶塞以下。当瓶塞被塞进瓶口时，少量的水会被挤压到玻璃管中，形成几厘米的水柱。保持瓶塞紧密，并确保管中和瓶中均无气泡。
+
+
+
+
+
+将此烧瓶放入冰水混合物中，保持十五到二十分钟，然后用橡皮筋给玻璃管中的水柱高度做下记号。再将烧瓶从冷却液中取出，观察玻璃管中的水面是否立刻发生升降变化。持续关注玻璃管中的水面五到十分钟，会发现水的体积并不是在冰的熔点32°F的时候最小，而是在这个温度以上一点点的地方。
+
+纯净水的密度是在高于冰点温度一点点的时候达到最大，海水则不是如此，它的密度会随着温度降低而持续增大，体积则会持续减小，直到达到海水的凝固点28°F。因此地球两极附近的寒冷海水会慢慢下沉，并会在海底渐渐朝低纬度甚至向赤道的温暖海域缓缓流动，那里的温度大致处在32°F和35°F之间。这种海面寒冷海水的稳定潜流，为生活在深海海底的动物们提供了它们所必需的空气，不然的话，深海的海水中的空气就会彻底耗尽，那里的所有生命也都会全部毁灭。
+
+海面的海水温度也随着纬度的不同而具有很大差异，在热带地区可以达到80°F,而在极地附近则差不多是冰点温度。极地附近和赤道附近的海水，一年到头温度都没什么变化，但是中间纬度地区的海水温度在一年中的变化则比较显著。在海面之下，太阳光的热量会随着深度增加急剧减小，在差不多300英尺深的地方，海水在一年中的温度变化不会超过2°F，在600英尺以下，海水温度几乎就不会有任何变化了。
+
+在海洋表面，日均温度变化不会超过1°F，年均温度变化也不会超过15°F，当然那些在不同季节会遭遇不同洋流的海面，以及靠近海岸而被陆地热量加热的海域不在此列。陆地和海洋各自温度状况之间的巨大差异，真可谓判若云泥。这也造就了其各自生命世界的遽然不同，一个比较单一，没什么变化；另一个则纷繁复杂，常常需要经受极端温度的考验。这也便是陆地生物比海洋生物更高一等的原因，因为它们必须经受住环境变换带来的考验，才能生存下来。
+
+133．我们熟知的洋流——海洋是一个永远喧嚣欢腾的运动场，海水较深部位的运动比较缓慢，但是在靠近海面的地方。由于海面的海风可以影响到这里，这一区域的运动便非常明显而剧烈。各大洋之间也存在着海水的持续循环，影响深度大致在300至600英尺之间，水流的速度从非常微弱到每天50英里不等，当然有时甚至更大。实际上，海洋中的不同洋流才是区分海洋区域的天然判据，远比我们地理上的六大洋划分要科学得多。
+
+在北半球，洋流是按顺时针方向循环的，在南半球则是逆时针方向。在洋流循环圈的中心区域，那里的海水几乎是不流动的，并且常常聚集着大量的海藻和各种各样的小动物。这些不断生长积累的海藻，就是人们常说的马尾藻。
+
+洋流的这一固定运动方向，是通过对很多沉船事故的观察分析后得出的。洋流的运动状况，也被从船上抛入不同海域的数以千计的瓶子所证实。这些瓶子中都装有记录其经纬度的数据收集仪器，它们的位置变化以及最终流向，便清楚显示了洋流的运动方向。
+
+如果海水的流动速度很慢，每天只有10到15英里，便被叫做漂流，比这更快的，叫做涌流。人们给流向趋势明显的几个主要洋流都分别起了名字，并且很细致地画出了它们的路线。离开北美洲东南��海朝着东北方向流动的暖流，叫做墨西哥湾流。离开亚洲大陆东岸，朝东北向流动的洋流，叫做黑潮或者日本洋流。离开拉布拉多东岸，流向东南的寒流叫做拉布拉多洋流。离开南美洲西海岸，向北流动的寒流叫做秘鲁寒流。还有一些其他名字的洋流，但上面提到这些基本上是最重要的了。
+
+当洋流不被大陆阻碍的时候，它们会顺着盛行风的风向流动。人们发现，洋流的方向改变，都伴随着盛行风的方向的改变，比如当热赤道偏离地球赤道最远的时候，印度洋上就经常发生这一幕。直观地看起来，洋流本身也是大自然风循环的一个作用结果。
+
+134．洋流的影响——关于洋流的知识，对水手们来说是非常重要的，因为洋流的过程与速度对航行的船只会产生非常显著的影响。洋流还给生活在海洋中的各类动物的生存环境带来了不小的影响，它们不仅能给不能移动的动物带去食物，而且更能决定动物的生存地域。
+
+北纬32度的百慕大群岛，就是由珊瑚礁积聚而成，而位于赤道之上的加拉帕格斯群岛周围，却由于海水太冷而没有这样的珊瑚礁可以生存。在北纬68度靠近罗浮敦群岛的海域，是欧洲最大的鳕鱼渔场；北纬45度附近的大西洋西海岸，便是大名鼎鼎的大浅滩渔场。如此种种不一而足，而且类似的例子还有很多，它们都能说明洋流对动物分布的影响。
+
+从海洋吹来的风的温度，也被这些洋流加以影响，并进一步深深影响着我们人类。位于北纬71度的哈默菲斯特，是一个非常繁荣的港口，但是北纬50度附近的大西洋西海岸却没什么人口集中的聚居区。阿拉斯加州，由于长年充斥着从温暖洋面吹来的暖风，为人类提供了非常良好的居所，而在其太平洋对岸的地方，则很少有人类的聚居区。我们应该注意到一个现象，就是除了在海洋沿岸以外，温暖洋面的海水对洋流的影响其实很有限，因为它们并不能对空气运动本身施加影响。
+
+135．潮汐——当我们在海边漫步的时候，给我们印象最深的，恐怕就是那日夜不停涨落的潮水了。海水的这种运动就叫做潮汐。如果我们对它观察几天，便会发现，每天都存在两次高潮和两次低潮。当潮水从海洋涌来，海面升高时，叫做涨潮；当潮水退却，海面下降时，叫做退潮。在二者之间，还有一个短暂的周期，叫做“平潮”。
+
+如果我们进一步观察，会发现每天同样的潮汐现象都比前一天要晚一个小时，完成整个两次高低潮汐一共需要25小时。持续的观察还会表明，正如恺撒大帝在很多世纪以前就已指出的那样，月相的周期与潮水的高度之间存在着一定关联。潮水最高和最低的时候，正是月亮分别处于满月和新月的时候。
+
+潮水涨落的高度，在不同海域也并不相同。在海洋岛屿的海岸，海水涨落的高度差，或者叫幅度，一般不会超过两三英尺。而在漏斗形的海湾，由于潮水流进时，容纳海水的空间会被压缩，潮头的幅度就会变得很大了。在芬迪湾，潮头的幅度有时可以达到70英尺之高。
+
+有时候这种被压缩的潮汐流，还会在河流的入海口处形成，像一堵数尺高的浪墙一样涌向河流，危及所有尚未安全停靠的船只。这种冲浪般的潮水也被叫做死鬼潮，在塞纳河、亚马逊河以及其他许多河流都能碰上。当狂潮在岛屿之间肆虐的时候，很容易形成巨大的漩涡，而这对航海的船只而言，极其危险。纽约附近的地狱之门和挪威海岸的著名大漩涡，就是其中典型。但从另一方面说，在一些地势平坦的国家，河流比较浅，因此要不是涨潮来临，那里的港口根本无法通过大型轮船，故而在这些地方，每天港口进出的时间都随着涨潮时间不断变化，其显著的例子便是安特卫普港口。
+
+潮水还不停地改变着海湾与港口附近海域的水质，让那里的海水不会成为一潭死水进而变得污浊。它们还为不能移动的各类海岸动物带来了食物，比如蛤蚌以及许多贝壳类动物，而且在退潮的时候，人们也可以到海滩上寻找需要的食物。
+
+人们已经发现，太阳与月亮的位置，会对潮水的高度产生影响。当太阳、月亮、地球三者处于一条直线上时，潮水会达到最大，这时也叫大潮；当三者呈直角时，潮水会变得最小，这是也叫小潮。天文学家的观测也证实了这一点，由于地球的自转，以及太阳、月亮对海水的引力，综合形成了我们所看到的起伏涨落的潮水，其中月亮的影响更大一点，因为它离我们更近。
+
+在辽阔的海面上，潮汐的升降比较一致，但在海湾地区，涨潮比退潮表现地更加激烈，因此在两次涨潮之间，那里没有明显的退潮阶段。
+
+136．珊瑚和珊瑚岛——在温暖清澈的浅水海域，我们常常能见到一种叫做珊瑚的小动物[3]。它们的大量聚集会形成一道天然的屏障，不仅可以阻挡潮水，还能够慢慢形成小岛。这些造礁珊瑚在20英寻（120英尺）的水下，或者在68°F 以下的水中都不能存活，若有河流将海水污染的话，它们也不能生存。由于它们不能移动，便只能从周围的海水中获取食物，因此那些具有温暖且持续的洋流，能够给它们带来大量新鲜食物的海域，便成了它们最为钟爱的生长乐园。在热带海域，许多岛屿的外围都是这种小动物搭建起来的礁石。
+
+在热带的太平洋海域，会发现许多环形珊瑚礁围绕着一个个很大的浅湖，而珊瑚礁外围的海面似乎在迅速下沉。这类岛屿叫做环礁，它们到底是怎样形成的，目前还是个悬而未决的问题。由于造礁珊瑚不能生活在20英寻以下的水中，这一证据说明，珊瑚不可能从深海海底就开始构建礁石。
+
+137．人类与海洋——一开始我们往往会想当然地认为，要是地球的陆地与海洋面积的比例能颠倒过来该多好。但当我们考虑到许多大陆的内陆地区至今依然还是荒漠，以及必须有足够的雨水才能让一个地方变得适合居住的话，海洋巨大面积的功用，立马就显而易见了。海洋表面的水分蒸发，几乎为人类以及所有动植物提供了全部所需的水分。
+
+有句话没错，所有的溪流最终都流向大海；而所有溪流的水也全部来自海洋，这句话同样正确。很多事，其实都很公平，蒸发的面积越小，提供给陆地的水分就越少。除了给陆地提供水分，海洋对陆地上的气候也影响卓著。另外海洋动物也为人类提供了丰富的食物，从过去到现在，人类的聚集区从来都不曾远离大海。
+
+在很久以前还没有通铁路的时候，几乎所有的商业贸易都依靠海运，即使今日，海运也依然是比较便捷的物流方式之一。借助现代的交通工具，人类已经可以很方便地生活在远离海洋的地方。但在那些干旱的内陆地区，依然人烟稀少。所有的商业国家都保留有自己的出海口，这些利来利往的商业通道，对他们而言就是经济命脉。有些时候人们为了商业利益，还会想方设法突破大自然在他们和海洋之间设置的障碍，苏伊士运河和巴拿马运河就是代表性的例子。
+
+
+
+
+
+总结——海洋几乎占据了四分之三的地球表面积。人们常常把海洋分为大西洋、太平洋、印度洋、北冰洋和南冰洋。尽管海洋有些区域深达6英里，但它最为丰富多彩，也是延伸最广的区域，还是在大陆架上。它是从大陆海岸一直呈斜坡状，向下延伸到600英尺深的海底的这一区域。跨过这一区域，海底的沉降就陡然加剧了。
+
+在海洋的洋面上，波涛滚滚，浪高可以小到几英寸，也可以大到50英尺。翻滚的海水可以吸收更多的空气，这对生活在海洋中的生命，极为重要。不仅海洋的洋面在不停地运动，而且存在很多洋流，比如墨西哥暖流，将海水转运到不同的纬度地区。这些洋流都是由海风形成的。
+
+潮汐是由于地球、太阳、月亮三者之间的相互作用而形成。潮水涨落的幅度也从海岛上的两三英尺，到个别地方比如芬迪湾的70英尺不等。潮汐还有助于海湾附近的水循环，有时船只还必须借助它们才能通过一些浅水港口。
+
+
+
+
+
+思考题
+
+
+地球表面积中海洋占有多少比例？海洋一般分为哪几大洋？
+
+什么是大陆架？为什么说它对人类有特别的贡献？
+
+海水是由什么成分组成的？它的成分对动物有什么影响？
+
+如果你能进行一次跨越海洋的航海旅行，请描述一下你沿途可能会发现的事物。
+
+什么是海浪？它们是怎样形成的？
+
+在不同的海域以及不同的深度，海水会呈现什么样的温度？
+
+描述一下我们熟知的一些洋流，是什么导致了洋流的形成？为什么关于洋流的知识对船员们的航行安全，以及对于解释各地气候差异非常重要？
+
+什么是潮汐？它们是如何产生的？它们有什么作用？
+
+珊瑚礁是怎样形成的？什么是环礁？
+
+海洋对人类的贡献体现在哪些方面？
+
+
+
+
+
+译注
+
+
+[1]目前探测到的最深海洋深度已超过此数据。马里亚纳海沟的斐查兹海渊，深度为海平面下11034米，已经接近7英里。
+
+[2]1英寻＝1.8288米。
+
+[3]珊瑚本身不是动物，而是珊瑚虫的分泌物经过堆积后形成的。它的生物活性完全是由珊瑚虫体现出来的。
+
+
+
+
+
+CHAPTER 9
+
+COAST LINES
+
+海岸线
+
+
+138. Coast Lines. —On examining a map or chart of an extended coast one cannot fail to be impressed with its irregularity. Although it may extend for long distances in an almost unbroken straight line, as along the southeastern part of the Bay of Biscay, yet if we follow even this coast far enough in either direction, it becomes markedly irregular. Islands composed of the same material as the coast itself are often found near, separated from it only by shallow arms of the sea. These are evidently a part of the adjacent mainland with a submerged portion between.
+
+
+
+POSITANO.
+
+A precipitous and densely populated coast.
+
+
+
+Sometimes in warm latitudes where conditions are suitable, the coast is bordered for a long distance by coral reefs. The northeast coast of Australia affords a marked example. Here a barrier reef extends for a thousand miles along the coast, protecting it and leaving a smooth water channel 10 to 30 miles broad for coastwise navigation.
+
+
+
+NORTH CAPE.
+
+A famous promontory which defies the waves of the North Atlantic.
+
+
+
+A coast in warm regions may also be protected by the growth of dense thickets of mangroves. This tree grows in shallow water and sends down from its branches roots upon which crabs and oysters live and among which mud and débris accumulate. Thus low shores are slowly being built out into the sea. Shore borders of this kind are found extending many miles along the coasts of southern Florida and Texas and in many other places.
+
+In extreme northern and southern latitudes the coast may be fringed for long distances with ice. In middle latitudes, however, the coast is generally composed of rock, sand or mud and is sometimes protected by a thick seaweed mantle. Here, besides the usual forces of erosion and deposition found active on the land, are the forces of the waves and currents.
+
+
+
+FINGAL'S CAVE.
+
+
+
+139. Wave Cutting. —Wherever the waves strike on an unprotected shore, they wear it away. The rapidity of the cutting and the forms carved depend upon the strength of the waves and the kind of shore. Wherever there is a point of weakness along the shore, there the waves cut back more rapidly. The harder parts stand out sharply as points and promontories. In some cases the waves cut back so rapidly on lofty coasts that high cliffs are formed.
+
+If the material of the coast does not readily break off when undercut by the waves, a sea cave may be formed. Such is the well-known Fingal's cave on an island off the coast of Scotland where the structure of one of the igneous rock layers allows the waves to quarry it comparatively easily. Spouting holes and caves are usually due to an easily eroded place in the shore where the waves are able to cut back a somewhat horizontal tunnel and by their impact upon the end of the excavation form an opening to the surface through which spray is ejected. The hole may be at some little distance from the shore.
+
+
+
+AN ELEVATED ROCK BENCH.
+
+
+
+Since waves have the power of cutting only to a small depth, it may happen that an exposed rocky coast will have a bench cut along it, under the surface, backed by a sea cliff against which the waves are still cutting. If such a coast becomes elevated, the rock bench will appear with a cliff terminating it on the landward side. If a coast stays at the same elevation long enough, or if its material is easily eroded, large areas of what was formerly dry land may be cut away and brought under the sea.
+
+In 1399 Henry of Lancaster, afterward Henry IV of England, returned from his exile and landed at Ravenspur, an important town in Yorkshire, to begin his fight for the crown. A person disembarking at the same place to-day would be so far from shore that he would need to be a sturdy swimmer to reach the beach. The entire area of the ancient town has been cut away by the waves and now lies under the sea. This is an example of what has occurred in many sea coast regions.
+
+140. Beaches and Bars. —Unless the material pillaged from the land by the waves falls into too deep water, it is buffeted about by them and broken and worn into small pieces. These are then borne along by the shore currents until they find lodgment in some protected place where they can accumulate. When sufficient material has been accumulated, the storm waves and the wind sweep some of it above sea level and fringe the water's edge with a border of waterworn sand and pebbles. These accumulations of shore drift are called beaches.
+
+
+
+A LAKE BEACH FORMED BY STREAM AND WAVE ACTION.
+
+A year after this picture was taken a landslide formed a wave which swept away the entire beach and village.
+
+
+
+The incoming waves are constantly sweeping in material from the shallow bottom against which they strike, and the returning undertow bears its load seaward. Except in time of great storms the accumulation of material along a beach is at least equal to the wearing away.
+
+
+
+NAHANT, MASSACHUSETTS.
+
+A land-tied island.
+
+
+
+The currents moving the loose material with them sometimes form it into bars which inclose or tie islands to the mainland or extend into the sea free ends, forming what are called spits. A famous example of a land-tied island is that of the great English fortress at Gibraltar; although now a promontory, it was once an island detached from the coast of Spain. Shifting sand bars, especially if covered with water, are exceedingly dangerous to vessels, and coasts where these are abundant need especial protection by lighthouses and lifesaving stations. The greatest Mediterranean port of France during the thirteenth century, Aigues-Mortes, has been closed in by sand bars so that there is no longer access to the sea and only the relics of the former great city now exist. Thus have the moving sea sands overthrown the plans of men.
+
+
+
+A SAND SPIT.
+
+Formed by the waves and currents.
+
+
+
+141. Final Effect of Wave and Current Action. —Whether the coast is shallow so that the storm waves spend their force off shore, or deep so that they batter the shore with their full strength, the tendency is to straighten the coast line. In the former case sand reefs with gently flowing outlines are built, and in the latter case the headlands are cut away by the waves and the material moved along by the coast currents to fill the protected bays and coves. As aërial erosion is constantly tending to smooth off the irregularities of the land surface, so the waves and currents of the ocean are constantly straightening out the water margins.
+
+Uniformity seems to be the goal for the erosive forces. But when the results of surface erosion are brought to the sea by the rivers, deltas are formed which interfere with the straightening of the coast. If the material brought by the rivers is sufficient, a delta is built out into the sea in spite of the action of the waves and currents and the coast line becomes increasingly irregular. Lake shores are acted upon in a similar way and with similar results, only the forces here are less powerful than those that act upon the sea coasts.
+
+
+
+A BEACH AT CATALINA ISLAND.
+
+Notice how the water is smoothing out the irregularities of the land.
+
+
+
+
+
+INLAND SEA CAVE AND BEACH.
+
+This coast has been recently elevated.
+
+
+
+142. Instability of Sea Coasts. —It often happens that in making excavations at a considerable distance from the sea, shells are dug up very similar to those now found on the shore. Some inland strips of land are found composed of sand and pebbles like a beach and having the beach slope. Sometimes tree trunks standing with their roots in the ground just as they do on the dry land are seen at a considerable depth in the sea. It can be proved that an old temple near Naples, Italy, has stood above and then in the sea more than once since it was built.
+
+Facts like these show that the sea coast is not stable, but subject to upward and downward movements, some of which are slight and some great. If the land near the sea rises, the coast line is moved into the area which was formerly covered by the water and if the land sinks, a new coast line is formed where before the hills and valleys of the land appeared. Changes of this kind have a marked effect upon the outline of the coast and upon the industries of its future inhabitants. The coastal plain which borders a large part of our Atlantic coast shows the results of an upward movement. The fiorded coast of Alaska affords an example of a downward movement.
+
+
+
+
+
+TEMPLE OF JUPITER NEAR NAPLES.
+
+Although it can be proved that this coast has been elevated and depressed several times, so gradual has been the movement that the pillars have not been overturned.
+
+
+
+143. Elevated Coast.
+
+
+
+
+
+Experiment 126. —Tack enough sheet lead to a very rough board so that it will remain submerged when placed in water. Place the board in a shallow dish of water, lead side down. Taking the board by one edge, gradually lift this edge above the water surface. What kind of a line does the water form where it meets the board? In what way would this line be changed if the board were smoother? If it were rougher? If the edge of the board is lifted higher, does the position of the water line change? Does its form materially alter?
+
+
+
+
+
+The main characteristics of a coast which has been elevated, that is, of one in which the shore line has moved seaward, will readily suggest themselves to any one who considers what has taken place. Soundings show that the continental shelf has a comparatively smooth surface. Thus the water will meet the land in an almost straight line and the deepening of the water off shore will be gradual. The material forming the shore both below and above the water line will be easily eroded, since it has been recently deposited and has not had time to consolidate.
+
+Waves rolling in upon the shore will strike the bottom at a considerable distance off shore. Thus the water rapidly loses its velocity and its power to carry eroded material shoreward, so it builds at a distance from the shore a sand reef inclosing a lagoon. The currents caused by the prevailing winds and the tide flowing along the outside of this barrier give it for long distances a smooth outline, sometimes almost straight and sometimes gently curving. Dunes are formed upon these bars. In time landward-blown sand, together with the silt brought by the streams from the mainland, may fill up the lagoon.
+
+
+
+SAND DUNES PORMED UPON A SAND BAR.
+
+
+
+The filling of these lagoons, both naturally and artificially, has considerably increased the habitable land of the earth. The inclosed waterway back of the sand reefs has in some places rendered coastwise traffic safe and easy. It is proposed artificially to extend and develop certain of these inclosed water areas along the eastern coast of the United States so as to form a protected waterway from New England to the southern ports. At present the low, almost featureless shore of this region, with its shifting sand bars and capes, makes coastwise navigation dangerous, although it is protected by many lighthouses and lifesaving stations.
+
+The sand reefs along the southern Atlantic and Gulf coasts have in some places attained sufficient width and height for considerable settlements. The tidal inlet, the sea-beach resort, and the commercial city with reef-protected harbor are natural results of receding shore lines. In time the waves, by their own erosive action, will deepen the bottom off shore from the reef enough to enable them to attack its front. Thus they will drive it back over the inclosed lagoon, destroying their own work and attacking the shore, which for a time they had shielded against their own rapacity.
+
+Where the range of tide is considerable, the reefs are frequently broken by inlets. Through these the water of the mainland streams finds access to the sea. The shapes and depths of these inlets are in some cases so rapidly altered by the tidal currents that it is impossible to foretell for any length of time where vessels can find the best channel. Thus the inlets must be left uncharted and local pilots relied upon. Bars which cannot be crossed except at high tide often make moon time, not sun time, the determining factor in the sailing schedules of vessels leaving and entering port. The general set of the shore currents has an effect upon the position and shape of these inlets, deflecting the openings in the direction of their flow. They may also singularly modify the outlines of the reefs themselves as in the formation of the three much dreaded capes off the coast of North Carolina.
+
+
+
+
+
+144. Depressed Coast.
+
+
+
+
+
+Experiment 127. —Cover a small board with a piece of thin oilcloth which has been most irregularly crumpled. Take the board by one edge and inclining it slightly gradually submerge it in a dish of water. What kind of a line does the water form where it meets the oilcloth? In what way would this line change if the oilcloth were more crumpled? If it were less crumpled? If the board is more submerged, does the position of the water line change? Why does its form materially alter?
+
+
+
+
+
+Along a coast which has been depressed, the shore line has moved landward and a surface rendered irregular by erosion is lapped by the inflowing water. All the irregularities which lie below the water level are filled with water and the shore line bends seaward around the projecting elevations and landward into the gullies and valleys. Isolated elevations surrounded by land low enough to be covered by the inflowing water have now become islands.
+
+
+
+PART OF THE COAST OF MAINE.
+
+A fine example of depressed coast.
+
+
+
+The river valleys which crossed the region now submerged reveal themselves only to the sounding line, their landward extensions forming estuaries up which the tide sweeps far into the land. Their unsubmerged portions contain fresh-water streams, the size of which seems insignificant when compared to the size of the estuary. Sheltered coves and harbors abound, affording protection to all kinds of crafts and fitting these coasts to be of great commercial importance.
+
+
+
+A NORWAY FIORD.
+
+
+
+The harvest of the sea replaces what might have been the harvest of the land. The distance along the coast between two points is much longer than the straight line distance over the sea. The boat, not the wagon, becomes the important vehicle of travel. Many submerged coasts, such as those of Maine, Alaska and Norway, have been modified by ice action. Their valleys have been smoothed and rounded.
+
+
+
+A NORWAY FIORD.
+
+Showing large vessels anchored near the shore.
+
+
+
+
+
+A NORWAY VILLAGE.
+
+At the head of a fiord.
+
+
+
+In Norway the deep fiords conduct the sea from the island-studded coast far into the interior. Their sides rise steeply, sometimes for several thousand feet from the water's edge and descend so steeply below it that large vessels can be moored close to the shore. Generally there is not sufficient level land along the sides of the fiord for building roads. The villages are usually situated where a side stream has built a little delta, or at the heads of the fiords where the unsubmerged portion of the valley begins.
+
+These U-shaped valleys with their small streams extend back to the interior uplands, sometimes blocked toward their heads by descending glaciers. They often have hanging valleys which enter far up along their sides, the streams of which descend by abrupt falls and adorn the dark rock walls with bands of silver spray.
+
+It was such a coast as this which bred the ancient Northmen, to whom the Sea of Darkness, as they called the Atlantic, was terrorless. While less favored and hardy sailors were dodging from bay to bay along the shore always in sight of land, they were pushing boldly west, guided only by the beacons of the sky, and discovering Iceland, Greenland and the American continent.
+
+145. Harbors. —The importance to mankind of good harbors cannot be overestimated. No civilized country by its own products can fill all the wants of its inhabitants. Since earliest times man has been a barterer of goods. The sea offers him an unrestricted highway for his traffic. Harbors he must have to load and unload his wares safely.
+
+Although many of the best harbors of the world are found along depressed coasts, such as the harbors of New York, San Francisco, London, Liverpool and Bergen, yet there are several other sorts of harbors. The delta of a great river may afford a good harbor, as those of New Orleans and Calcutta. Harbors may be formed by sand reefs and spits, like those of Galveston, Provincetown and San Diego. The atolls of the mid-Pacific and even the submerged craters of volcanic islands afford safe resting places where ships may ride out the storms.
+
+146. The Safeguarding of Coasts. —As nations advance in civilization and their commerce develops, they realize the necessity of safeguarding in every way possible the ships bearing their citizens and their wealth. Thus a great system of weather signals, of lighthouses and of lifesaving stations has been established. From these mariners may know when it is safe to leave port, may be warned off from dangerous shores, and, when in spite of all precaution their vessels become wrecked, may be rescued from a watery grave.
+
+The lighthouses have lights of different kinds and colors, some fixed, some flashing, so that when unable to make out the coast itself, the mariner can still know his position by the kind of light seen. Indeed many wireless telegraph stations are being equipped to communicate with vessels at sea and to inform them of their position, the condition of the shore and the expected weather. Even the kinds of material which form the sea floor near the shore, and which may be brought up by the mariner's sounding apparatus, are charted for him, so that in this way he may be helped to ascertain his position.
+
+
+
+SAN FRANCISCO HARBOR.
+
+A harbor due to depression of the coast.
+
+
+
+147. The Coast of the United States. —The coast of the United States presents a great variety of features. It has long stretches of uplifted, depressed and fiorded, dune-bordered, rock-bound, coral-and mangrove-fringed coasts. Along part of its sea border, harbors are abundant while in other portions, harbors are almost entirely lacking. Its recently uplifted western coast has still more recently been slightly depressed, thus forming the harbors of San Francisco, Portland and Seattle. This coast and the depressed and fiorded region of Alaska are paralleled, but in no way duplicated, by the elevated south Atlantic coast which has more recently been depressed in the Chesapeake Bay region, and by the depressed and slightly fiord coast of northern New England. On the eastern side of the United States a broad coastal plain has been formed which has no counterpart on the west.
+
+
+
+One of the finest harbors in the world.
+
+
+
+148. Influence of Coast Conditions upon Inhabitants. —All natural features have a greater or less influence upon the inhabitants of the earth, but perhaps none has so directly and obviously influenced man's activities as has the kind of coast on which he lives. Europe, with its harborfull and Africa with its almost har borless coasts are in striking contrast to each other. This difference between the inducements to travel and commerce which the two continents afford is one of the factors in producing the marked difference in progress attained by the people of the two continents. They stand to-day as types on the one hand of economic progress and on the other of stagnation.
+
+
+
+MINOT'S LEDGE LIGHTHOUSE.
+
+
+
+The Phœnicians, the Carthaginians, the Greeks, the English and the other great nations of the world have felt the enticing allurement of a captive sea waiting in their harbors like a steed for them to mount and ride away in quest of the world's best. Thus they have extended their conquest and influence far beyond the homeland. From the time of Peter the Great, the efforts of Russia to gain suitable outlet to the sea show the importance placed by progressive communities upon ocean traffic. The struggle of all the great world powers to strengthen their navies, no matter what the cost, shows with what jealousy the products of their ports are guarded.
+
+Coasts with harbors give their people the facilities and inducements for seeking the unknown, while the harborless coasts confine the aspirations of their inhabitants to the products immediately around them. A glance at the coast line and harbors of Greece shows one cause of its ancient civilization and a reason why the Greeks were "always seeking some new thing."
+
+
+
+
+
+Summary. —A glance at a map shows the great variety of coast lines. The waves and tides are constantly cutting and wearing the coast. The material thus cut away is ground fine and spread out by the currents in bars and beaches, filling up coves and straightening the coast. Thus one of the results of these erosive forces is to make the coastline more regular.
+
+Coastlines are affected not only by wind and water erosion, but by elevation and depression. When the shore rises, or is elevated, part of the comparatively even level of the continental shelf becomes the coastline. Thus elevated coasts are usually regular. But when the shore is depressed, the irregularities caused by the surface erosion on land make bays and estuaries. Thus depressed coasts are usually irregular and offer the best harbors.
+
+Wherever there are good harbors we find seafaring people. To make ocean traffic safe, seas are charted, coasts mapped, lighthouses built and lifesaving stations maintained. Water transportation is the cheapest, so good harbors are needed by all commercial nations. Nations with seacoast and harbors are usually more progressive and more civilized than inland nations.
+
+
+
+
+
+QUESTIONS
+
+
+Of what do the borders of different coasts consist?
+
+What do the waves do to the coast?
+
+How are beaches and bars made?
+
+What form do the waves and currents tend to give to the coast?
+
+How can it be shown that coasts are subject to changes in position in respect to the surface of the sea?
+
+What are the characteristics of an elevated coast?
+
+What are the characteristics of a depressed coast?
+
+How are harbors formed?
+
+How are coasts safeguarded?
+
+What kinds of coasts are found in the United States?
+
+How have coast conditions influenced their inhabitants?
+
+
+
+
+
+【中文阅读】
+
+
+138．长长海岸线——当我们注视地图或航海图上长长蜿蜒的海岸线时，它不规则的形状会给我们留下深深的印象。尽管它有时也会呈一条直线延伸很长距离，比如比斯开湾南部海岸，但若沿其两端再作一些延展，则又会看见它变得毫无规则。那些离大陆很近，只由一湾浅水与大陆隔开的海岛，跟海岸的地质结构很类似，它们其实也算是大陆的一部分，只是连接它们的陆地处在水下而已。
+
+在一些温带海洋地区，由于气候温润适宜，沿着那里的海岸线都分布着一个珊瑚礁带，澳大利亚东北沿岸就是很典型的例子，在那里珊瑚礁可以沿着海岸线延伸上千英里的距离。这样的珊瑚礁层，无形中对海岸线形成了一道保护屏障，同时也在它与海岸之间构筑了一条10到30英里宽的缓流水域，为沿岸交通提供了很大便利。
+
+在温暖海域的海岸边，常常生长着一种叫海榄雌[1]的植物，它们也能起到保护海岸的作用。它们像灌木丛一样密集地生长在海岸边的浅水区中，枝条和根茎都向下延伸，螃蟹和牡蛎就在这里生存，海水中的泥沙和残渣也在这里被过滤积累。由此，一些地势较低的海滩渐渐也被侵蚀成为大海。佛罗里达州和田纳西州南岸及其他许多地方，都有这类延伸得很长的海滩。
+
+在南北两极的附近，海岸线上都镶着一道冰圈，而在中纬度地区，海岸一般由岩石和泥沙构成，有时还被一层厚厚的海藻类覆盖物保护着。在这一地带，大自然的力量除了在陆地上存在的侵蚀与沉积之外，海浪和洋流也对这里施加着不小的影响。
+
+139．海浪的鬼斧神工——当海浪不断冲刷没有保护层的海岸之后，会慢慢将它们磨损啃噬。磨损的速度和形式，则取决于海浪的力量强度和海岸的类型。如果海岸存在脆弱点，则这些地方会首当其冲，很快被海浪侵蚀。而那些坚硬的部分，则会渐渐形成挺立的海岬。有些情况下，海浪也会快速地侵蚀一些高耸的海岸，最后形成陡峭的悬崖。
+
+如果海岸的地质构造成分不易在海浪侵蚀下分崩离析的话，便可能形成海蚀洞。比如我们熟悉的苏格兰沿岸岛屿上的芬戈尔大海洞，那里的地质成分主要为火成岩，因此海浪的侵蚀作用会相对更明显一些。岸边的一些喷水的孔洞，则是由于一些容易被侵蚀的地方，被海浪磨损成了一个水平方向的通道，并在其末端形成了通向地表的洞口，这样水一灌进去，就会喷涌出来。这类洞穴有时还会出现在离岸边较远一点的地方。
+
+由于海浪的侵蚀力只能作用于一个有限的深度，因此那些由裸露岩石组成的海岸，常常被冲刷成为梯坎形状。而在水下，陡峭的反向岩石崖岸依然经受着海浪的冲击。如果这种类型的海岸抬升起来的话，岩石梯坎就会变成面朝大陆的悬崖。如果海岸长时���地保持同一个高度，或者它的地质构造很容易被侵蚀的话，它之前的陆地则很可能有一大部分被海浪啃噬掉，最终变成大海。
+
+公元1399年，兰卡斯特王朝的亨利国王，也就是后来的英格兰亨利四世，从他的流放地归来，在约克郡的要塞拉马斯普镇登陆，开始了他的王位之战。要是今天有人也在同样的地方登陆的话，由于其与海岸已经隔得很远，因此他必须先成为一名游泳健将才行。这个古镇今天已经彻底被海浪吞噬，沉到了海底。且这样的例子在许多海岸上都存在，不胜枚举。
+
+140．浅水海滩——海浪持续地拍打着海岸，不让其粉身碎骨誓不罢休。长此以往，海岸岩石的碎片逐渐被海浪卷走，在一些宁静的地方又慢慢沉积下来。当它们沉积到一定数量，巨浪与海风又会将它们翻腾到海面，并卷裹着它们四处漂散，最终给海岸边铺上一道细砂带，这就是我们常说的海滩。
+
+不断来袭的海浪，持续冲刷着浅滩下的砂砾，回潮也反复对它们施加着冲击力。在如此天长日久的作用下，除非有暴风潮，海滩便会呈现出均匀的消磨形态。
+
+洋流有时还会夹带着这些细软的沙粒沉积到一些浅滩，比如海岛与陆地相连接的地带，或者到一些自由海域，形成沙嘴。大英帝国的直布罗陀要塞就是连陆岛的典型例子，尽管今天它已经是一个凸起的海角，但在过去却是完全与西班牙海岸隔离开来的独立岛屿。那些移沙海滩，尤其是覆盖着海水的浅浅沙滩，对船只而言是非常危险的，因此这些地域常常需要灯塔和救助站来保障安全。在13世纪，法国最大的地中海港口艾格莫尔特，就曾因砂砾堆积而关闭，由此阻断了这个城市的出海口，致使今天我们只能看到这个城市的遗迹了。因此我们也可以说，沙滩有时也会干扰人类的生活。
+
+141．海浪与洋流的最终影响——在比较浅的海岸，巨浪会把力量均匀地作用在整个滨海海滩上；而在一些较深的海岸，这一强大的力量将被全部作用在岸边礁石上，并最终慢慢地将海岸雕刻成一条直线。之前的例子中，我们已经提到慢慢构筑轮廓的沙礁的形成过程，也提到了渐渐被海浪侵蚀消磨的海岬的形成过程，以及它们的残片又被洋流卷裹着，填充到一些合围的海湾和海底洼地。总体看来，就像空气总是不断地侵蚀又维护着地球不规则表面一样，海浪与洋流也总是持续地雕镂着我们的海岸线。
+
+长久的冲刷侵蚀最终似乎都会磨平所有的棱角，让参差变成单一。而当陆地表面的侵蚀物被河流带入海洋后，在出海口便会形成三角洲，让平直的海岸又重新变得曲折。如果河流带来的沉积物非常多的话，三角洲还会延伸到海中，与海浪和洋流的破坏力持续地对抗着，同时让海岸变得更加不规则。湖泊滨岸的形成过程和海岸也比较类似，只是它所面对的力量比起海岸来，可就小多了。
+
+142．海岸的不稳定性——有时候会发生这样的情况，人们在离海洋距离很远的地方向地下进行挖掘时，挖出来的贝壳和海边沙滩上的几乎差不多，还发现一些内陆狭长地带的石质土质，居然与海岸沙滩非常类似，而且其地质构造也存在和海滩相似的斜坡。在海洋深处，我们有时还能看见一些树木像在陆地上那样，扎根于土地，挺立在海底。意大利那不勒斯的一处废旧庙宇也能说明类似的问题，其建成之后已经多次在海洋中浮浮沉沉。
+
+这些事实都说明，海岸并不是非常稳定一成不变的，而是会出现上升和下沉的运动，并且有时候很轻微，有时候却很剧烈。如果近海的陆地抬升，之前被水淹没的区域就会变成新的海岸，而如果陆地沉降，则是之前陆地上的山丘峡谷成为海岸。这种地质改变对海岸的轮廓，以及人类的产业发展，都产生了十分显著的影响。与美国大西洋沿岸接壤的海岸平原就是海岸抬升的结果，阿拉斯加州的峡湾型海岸则是海岸沉降的例子。
+
+143．海岸抬升
+
+
+
+
+
+实验126：用大头针将一定数量的白纸钉在一张粗糙木板上，让其在水中不会漂浮起来。然后将其放入一盆水中，有纸张的一面朝下。然后握住木板的一边，慢慢将此边缘抬升出水面，注意观察，水与木板的接触的浸水线会呈现什么形状？如果木板光滑一些，这条线会有什么样的改变？如果再粗糙一些呢？如果木板的边缘再抬高一些，浸水线的位置会变化吗？形状也会变化吗？
+
+
+
+
+
+抬升的海岸的一个主要特征就是，海滩的边缘会慢慢向大海的方向推进，这一点任何人应该一想就明白。从海岸的水深状况我们可以得知，大陆架拥有相对较为光滑的陆地表面，因此，海水与陆地的交界线也几乎呈一条直线，且海水的深度也是渐渐加深的。海滩上不论在水面以上，还是在水面以下的砂石，都很容易被海水侵蚀，因为它们大多都是刚被冲到这里不久，还并没有真正稳固沉积下来。
+
+滚滚而来的海浪会对海滩产生巨大的冲击力，并会作用到其底部很深的距离。通过这样的能量释放，形成浪涛的海水会逐渐减缓力量，因此便不会有很强的携带能力向海滩堆积泥沙，这样最终便会在远离海滩的地方形成一个由沙礁围成的环礁湖。由盛行风带动的洋流，以及沿着环礁湖边缘澎湃不息的潮汐海浪，会让环礁湖产生一个光滑的外沿，大多数时候都几乎是直线，偶尔也会有一些轻微的弯曲。沙丘也会形成在这些滩涂上，当海风吹向大陆的时候，这些流沙以及大陆河流所夹带的泥沙，便会慢慢填充到这个环礁湖中。
+
+对此类环礁湖的填充，不管是自然形成，还是人工而为，都大大增加了地球上可供人类居住的陆地面积。沙礁内侧的水域，在很多地方也为人类提供了安全便捷的航道。又有人提议过，将美国东部沿海的这类水道进行人工开挖和延伸，以便从新英格兰到南方港口就有了专门的安全航路。不过现在这一地区的低浅且毫无特色的海滨，已遍布各类随时流变的沙洲以及暗礁，让这一带的航行变得非常危险，虽然周边有许多灯塔和救护站也照样无济于事。
+
+在南大西洋以及墨西哥海湾沿岸，那里的许多沙洲由于面积足够大，且海拔高度也很突出，因此可以考虑作为人类的居住地。那里的海潮通道、度假海滩、受沙礁环绕保护的商业城镇，以及位于沙礁外的港口，都是海岸抬升的天然产物。当海浪来袭，它们的冲蚀能力会加深海滩底部的深度，以保持海浪始终正面冲击海岸。因此海浪也会将海水慢慢从环礁湖内带出来，抵消之前的积累并形成对海滩的磨蚀，到最后渐渐地将沙礁彻底隔离开来。
+
+在潮汐海浪越大的地方，沙礁被入海口的海水侵蚀得也就越厉害。在这里，河流汇入大海，入海口的宽度与深度在一些地方也由于潮汐而变化频繁，有时甚至很长时间内都不能预判船只该从哪儿出海。因此这些出海口一般不会被固定下来，而是通过当地的引航设施来引导船只出海。因此除了在涨潮的阶段，这些礁岸地区都只会采用阴历时间，而不会采用阳历时间，这是确定船只进出港时间安排的决定性因素。
+
+海滩上长年的水流状况，对出海口的形状与位置都会产生影响，并会让河流融汇处的水流流向产生变化。当然它们也可能会不断侵蚀沙礁的外轮廓，北卡罗来纳州海岸的三个奇形怪状的海角就是它们的杰作。
+
+144．海岸沉降
+
+
+
+
+
+实验127：将一张被彻底弄皱的薄油布覆盖在一张木板上，握住木板的一边，让其慢慢倾斜地淹没于一盆水中。注意观察，水与油布接触的浸水线会呈现什么形状？如果油布更皱一些，这条线会有什么样的改变？如果再平整一些呢？如果木板的边缘再多淹没一些，浸水线的位置会变化吗？为什么它的形状也会变化了？
+
+
+
+
+
+沿着沉降的海岸，海滩的边缘会慢慢向陆地爬升，漫延的海水也会渐渐侵蚀之前的陆地表面，让其变得更加不规则。沉入水下的地表上的坑坑洼洼，会完全被海水填得满满的。在一些原先的高地旁，海岸线会像大海突出出去，而在之前的沟谷地带，海岸线则会凹向陆地。而那些周围地势很低的孤立高地，在海水漫过来之后就成了海上的岛屿。
+
+曾经横穿这些地区的河谷现在也沉到了水底，只有测水深的铅锤偶尔还能一探它们往日的容颜了。它们朝向陆地延伸的峡谷，现在就形成了河口，每日涨落的潮水正是通过这里流经河道。那些没有被淹的部分，如今则汇集了无数涓涓细流，但比起河口的大小，它们就很微不足道了。被海滩环抱的河湾与海湾，无形中也为航行于此的各类船只提供了保护，让这一地域的海岸渐渐产生了浓厚的商业气息。
+
+沿岸沉降以后，海产慢慢替代了之前陆地上的物产。海岸线上两个地点之间的沿线距离，当然远远长于它们在海面上的直线距离，于是乎，船只就成了比马车更重要的交通工具，诸如美国缅因州、阿拉斯加州以及挪威的部分沉降海岸，还被冰盖改变了形状，它们的河谷与沟岸已经被海洋打磨成了光滑的圆弧形。
+
+在挪威一些布满岛屿的海岸，小小的峡湾常常将海水深深地引入内陆。崖岸也非常陡峭，有些地方高达数千英尺，而湾内海底也沉降得非常迅速，这让大型船只也能停靠在岸边，且��一带的海岸上也几乎找不到一块平地来修筑平坦的道路。因此当地居民大多都居住在海水支流冲击出的三角洲上，或者峡湾最前端的凹谷底部。
+
+在这样一个U形峡谷内部，有许多源自内陆高地的溪流，常常在流经的途中被下沉的冰川堵塞，并沿着水道形成悬谷。冰盖还不时突然地垮塌而下，不仅妆点了黯淡的岩石，还激起朵朵银白的浪花。
+
+就是这样的海岸，养育了北欧人的先民，对他们而言，眼前的黑暗之海——他们给大西洋起的名字，并无一丝让人恐惧的地方。而那些常常劳其筋骨饿其体肤的水手们，却凭着自身的勇毅与果敢，没有陶醉在这些靠近大陆的温柔海湾，而是在天空日月星辰的指引下，奋力向西漫漫求索，最终发现了冰岛、格陵兰岛以及美洲大陆。
+
+145．海港——优良的海港对人类文明的发展太重要了。没有任何一个国家能出产人们所需的一切商品物资。从远古时代开始，人们就自发地开始了商业贸易活动，大海便为人们提供了一个天然高效的交通途径。海港也就自然成了人们安全装卸货物的必需之地。
+
+尽管我们所知的一些著名海港都位于一些沉降海岸，比如纽约、旧金山、伦敦、利物浦以及卑尔根等城市的港口就是如此，但一些其他类型港口也依然存在。在河流入海口的三角洲附近，往往也会形成良好的海港，比如新奥尔良和加尔各答地区的港口就是例子；沙洲和沙嘴附近也能形成海港，比如加尔维斯顿、普罗温斯顿以及圣地亚哥的许多港口就属此类。还有中太平洋地区的环礁，以及一些火山口被淹没的火山岛，也能为航行的船只提供安全的临时避风港。
+
+146．海岸的防护——随着商业文明的发展，每个国家都会尽力保障航海船只上人们的生命财产安全，因此，一个为灯塔与救助站进行气象预报的庞大信号体系就随之应运而生了。通过这些信号的指示，船员们便能知道什么时候出港会更安全，哪里的海滩可能比较危险。且一旦有意外发生，他们也能获得急救而免于溺死。
+
+灯塔的光信号有很多种类型，有稳定的光，也有闪烁的光。这样当船员们在黑暗茫茫的海上无法辨别海岸的时候，通过这些灯光也能大致确定自己的位置。如今在海岸边上已经修建了许多无线电通讯站，它们可以告知船只所处的具体位置、沿岸状况以及天气变化情况，甚至还可以告知船员们沿岸海底的地质构造，以及船上的水深探测仪可能打探到的一些信息。通过这些综合信息，船员们便也可以知道自己所处的精确位置。
+
+147．美国的海岸状况——美国的海岸状况非常复杂多样，并且各类海岸都延伸得很长。有抬升的海岸，有沉降的海岸；还有海湾型海岸、沙丘型海岸、岩石型海岸、珊瑚型海岸以及红树林环绕型海岸等等。沿着漫长的海岸线，有的地区集中分布着许多港口，而有的地方却寥寥无几。曾经抬升迅速的西海岸在经过轻微的沉降之后，慢慢形成了今天的旧金山、波特兰以及西雅图的大部分港口。这些地区的海岸，与阿拉斯加州的沉降海岸以及海湾沿岸地区，几乎呈平行状态。不过这样的情形其实是很特殊的，根本不能被复制。南大西洋也有一些曾经的抬升海岸，有的近来发生沉降，比如切萨皮克湾；有的还形成了峡岸，比如新英格兰沿岸。在美国东海岸，还有一个宽阔的海岸平原地带，但在西海岸却没有这样的地质形态。
+
+148．海岸状况对沿途居民的影响——自然环境的状况都会或多或少地影响我们人类，但几乎再也找不出第二个像海岸这样深深地影响我们的自然环境要素了。遍布海港的欧洲与港口缺乏的亚洲就形成了鲜明对比，二者之间的交通与商业发展诱因不同，也早已成为这两个大陆生产力发展状况出现巨大差别的原因之一。今天，它们一个成了经济贸易持续繁荣发展的典型，另一个则依然是一副停滞不前、萎靡不振的样子。
+
+面对浩瀚的海洋，腓尼基人、迦太基人、希腊人、英格兰人，以及世界上其他许多民族的先民们都对它的诱惑难以抗拒。每当他们在海边扬帆起航，便像跨上骏马去广阔的天地之间策马奔腾，似乎在遥远的地方，有一个美丽新世界在冥冥之中召唤着他们。伴随着这样的情结，他们因此征服并深深影响了大片远方的土地。彼得大帝时期的俄国，一些进步团体开始鼓吹海洋交通对国家的重要性，于是政府便随之加大力度获取出海口。渐渐地，世界各个大国都开始不计成本加强海军力量建设，并加强对港口的管制，以确保物资的流通。
+
+那些港口分布密集的海岸，给沿��居民提供给了探索未知世界的先天条件；而那些缺乏港口的海岸，则让人们只能着眼于当下，对身边事物倾注更多的关心。当我们看一眼地球仪上希腊的海岸及其港口分布时，我们便会多少明白它们古代文明之所以光辉灿烂的一个原因，以及为什么说希腊人“总是向着新事物进军”。
+
+
+
+
+
+总结——在地图上我们很容易一眼就看到形态各异的海岸线。潮汐和海浪都会不断地侵蚀海岸，组成海岸的土地被啃噬成细微颗粒，并被洋流卷走，沉积到海湾里和沙滩上，还会填充到一些低洼凹谷中，将海岸整体上修剪成直线。因此，海洋的自然侵蚀力量的结果之一，便是让海岸线渐渐变得有规则了。
+
+海岸线不仅受到风浪的影响，还受到自身的抬升与沉降运动的影响。当海滩抬升的时候，部分比较平整的大陆架海底就变成了新的海滩，因此，抬升的海岸一般都比较规整。而当海滩沉降的时候，陆地上受到侵蚀风化的地表则会形成新的海湾和河口，让新的海岸显得极不规则，但是往往却拥有最好的海港。
+
+在有良好港口的海岸地区，我们都能看见常常出海的水上居民。为了确保航海安全，海洋都被标定了区域与坐标，也被绘制成了航海地图。水上交通的成本很低，因此所有的商业国家都非常需要良好的海港。那些拥有长长海岸线且港口密布的国家，一般都比内陆国家更加发达，文明程度也更高。
+
+
+
+
+
+思考题
+
+
+海岸是由什么构成的？
+
+海浪对海岸会施加什么影响？
+
+海滩和海湾是怎样形成的？
+
+洋流与海浪会以什么样的形式对海岸产生影响？
+
+怎样才能说明海岸也会相对于海平面发生位置变化？
+
+抬升的海岸有什么特点？
+
+沉降的海岸有什么特点？
+
+海港是如何形成的？
+
+海岸是如何被加以防护的？
+
+美国有哪些种类的海岸？
+
+海岸的形态是如何影响沿岸居民的？
+
+
+
+
+
+译注
+
+
+[1]原文mangrores直译为“红树林”，而红村林是许多种红树科树种的混生群落，但是原文紧接着“this tree grows...”行文用的是单数，因此逻辑上这里应该是红树林里的一种树。红树林里最为常见的是海榄雌，故此处认“海榄雌”翻译。
+
+
+
+
+
+CHAPTER 10
+
+WATER SCULPTURE
+
+水之妙手
+
+
+149. Rainfall. —The water of the earth's surface is constantly evaporating, rising into the air and being distributed by the winds. Much of this water is blown over the land, where it is condensed and falls as rain. Some portions of the land receive much and some little of this aërial water circulation. When winds from warm seas, where the evaporation is great, strike lofty mountain ranges, the land upon the windward side has a large rainfall, but that upon the lee side comparatively little. This is particularly well shown in the northwestern part of the United States. Since continents, as a rule, have their mountains near their borders it happens that most continental interiors are comparatively dry.
+
+
+
+A HOT SPRING IN THE YELLOWSTONE.
+
+
+
+Regions over which the prevailing winds blow from a colder to a warmer latitude have little rainfall, as the air is continually having its capacity to hold moisture increased by its rise in temperature. This is well illustrated in the Sahara region. Accordingly, the amount of rainfall in different parts of the earth varies greatly. In eastern India south of the Khasi Hills a "record" fall of over 50 feet was recorded in one year, while in desert regions a year may go by without any fall of rain.
+
+
+
+Fig. 111.
+
+
+
+In the United States the greatest rainfall, over 80 inches, is found along the northwest coast and the least in the Basin Region of Utah, Arizona and Nevada. Whether rain falls in large or small quantities, its effect is always marked. Without it the surface of the ground is a parched and barren waste of dust and rock, with it, a green and varied expanse of never failing beauty.
+
+150. The Sphere of Activity of Rain. —When rain falls upon the ground, it may do one of three things. It may evaporate immediately from the surface and return to the air; or it may run rapidly off the surface and quickly join the streams and rivers which bear it to its final goal, the sea; or it may sink into the ground. In this last case part of it returns gradually through capillary action to the surface where it is again evaporated; part finds its way into springs; and part sinks deep into the soil and rock.
+
+Which of these courses the greatest part of the rainfall will take depends entirely upon the condition of its fall and the kind of surface upon which it falls. If the rainfall comes down rapidly, the larger part of it will immediately run off; if it comes down gently, much of it will sink into the ground. If it falls in forest regions or where there is much verdure, its flow will be impeded by the plants and roots. If the surface upon which it falls is hard-packed and impervious, most of it will run off, but if it is loose and easily penetrated, much of it will sink into the soil. Even in the dry parched sands of the desert, however, the rain falls sometimes in such cloudburst torrents that it runs off in rushing streams.
+
+
+
+FLOWING ARTESIAN WELL.
+
+
+
+151. Sub-Surface Water or Ground Water. —The rain that sinks into the ground descends slowly along the little cracks or between the particles of soil until it reaches a point where it can sink no further, or until it finds an opening through which it can flow out to the surface at a point lower than where it entered. Here it may ooze slowly out, or it may be concentrated in a spring.
+
+
+
+A LIMESTONE CAVE.
+
+
+
+If the water which comes to the spring has penetrated below the surface far enough to get away from the heating effect of the sun, it will be comparatively cool when it again emerges, and it will form a cold spring. If, however, in the region where the spring occurs the rocks are hot at the depth to which the water penetrated before it found a crack through which it could come to the surface of the land, then it will become heated and will form a hot spring.
+
+As the crust of the earth is in many places composed of rocks in layers, the rain often falls upon the top of a folded porous rock layer, below which is a rock through which it cannot penetrate. The water will then accumulate throughout the porous rock. If this rock layer in another part of its extent is overlaid by an impermeable layer, its water is held in by the impermeable rocks above and below, and so is under hydraulic pressure. When a hole is made in the upper rock layer (Fig. 111), the water will flow to the surface and if the pressure is sufficient, it may gush out of the hole.
+
+
+
+MONTEZUMA'S WELL.
+
+This famous water hole is due to the dissolving of the underlying rock layer.
+
+
+
+
+
+SINK-HOLE IN TENNESSEE LIMESTONE.
+
+
+
+
+
+GREAT NATURAL BRIDGE, UTAH.
+
+
+
+Borings of this kind form what are called artesian wells. These are of great importance in many regions where it is difficult to obtain sufficient surface water. In some of our western states the water from artesian wells has been obtained in sufficient quantity for extensive irrigation. Although this water often contains minerals in solution, it is free from surface contamination and is therefore usually healthful for drinking.
+
+
+
+NATURAL BRIDGE, SAXONY.
+
+
+
+In some places the surface water penetrates into layers of rock which it can dissolve, such as salt or limestone. Here it forms caves and caverns, the solid material which occupied the place of the cave having been carried away in solution by the water. There are thousands of caves of this kind but perhaps the most noted in this country are Mammoth Cave with its nearly 200 miles of underground avenues and grotesquely sculptured halls, and Luray Cave with its wonderful stalactite and stalagmite decorations. Sometimes the top of one of these caves is nearly eroded away, leaving a part of its old roof standing as a natural bridge, such as the natural bridge of Virginia or of Utah. Sometimes the top falls in, leaving a sink-hole.
+
+
+
+
+
+152. Geysers.
+
+
+
+
+
+Experiment 128. —Fit a 250 cc. glass flask with a two-hole rubber stopper. Through one hole extend a glass tube (a) almost to the bottom of the flask and through the other hole a tube (b), 5 or 6 cm. longer than the height of the flask, to within about 1 or 2 cm. of the bottom of the flask. This last tube should be slightly drawn out at the end and bent at the top so that it slants away from the flask. Arrange the flask on a ring stand so that it can be heated by a Bunsen burner. Connect to the tube (a) a rubber tube long enough to reach into a water reservoir placed higher than the top of the flask and to one side. Fill the reservoir with water.
+
+
+
+Fig. 112.
+
+
+
+Through the tube (b) suck the air out of the flask until the water from the reservoir begins to run into the flask. A siphon will be formed which, when there is no internal pressure, will keep the water in the flask slightly above the bottom of the tube (b). Now heat the flask. When steam begins to form, hot water will be thrown out of the tube (b) until its lower end becomes uncovered and the pressure of the steam relieved. Water from the reservoir will then run in again slightly covering the end of the tube. As soon as more steam is formed, hot water will be ejected as before. Thus a spray of hot water is intermittently ejected from the flask as long as heating continues. We have here an action which resembles that of a geyser.
+
+
+
+
+
+In the north island of New Zealand, in Yellowstone National Park and in Iceland, remarkable spouting springs called geysers are found. These places have had recent volcanic activity. The eruption of a large geyser is a most picturesque and startling phenomenon. Almost without warning there is thrown into the air a column of hot water from which the steam escapes in rolling clouds. It rises in some cases to a height of a hundred feet or more and is maintained at nearly this height by the ceaseless outrushing of the water for a time varying from a few minutes to between one and two hours. Then it gradually quiets down and dies away into a bubbling spring of hot water.
+
+
+
+GIANT GEYSER IN ERUPTION.
+
+
+
+The time at which most geysers will erupt is uncertain, but there is one, Old Faithful, in Yellowstone Park, which is almost as regular as a clock, the time between its eruptions being a little over an hour. This geyser plays to the height of about 150 ft. and maintains the column of water for about four minutes. The Giant Geyser of the same region throws a large column of water to a height of 250 ft. It plays from one to two hours.
+
+The outpouring hot water brings up with it dissolved rock and as the spray falls back and cools, this is deposited, forming craters of singular shape and grotesque beauty. On looking into these craters a smoothly lined, irregular, crooked, tubelike opening is seen to extend down into the ground. It is through this that the water finds its way to the surface. How long these tubes are nobody knows, but they must reach to a point where the heat is sufficient to raise water to its boiling point. This heat is probably due to hot sheets of lava.
+
+
+
+CONE OF THE BEEHIVE GEYSER.
+
+Built from the dissolved material brought up by the hot water.
+
+
+
+When the water in the tube is heated enough to make it boil under the pressure to which it is subjected, steam forms and some of the water is pushed out over the surface. This escape of water relieves some of the pressure, and more of the water far down in the tube expands into steam thus throwing more water out. Huge indeed must be the reservoir to which the tube in a geyser like the Giant leads, to be able to pour out such a vast quantity of water.
+
+153. Run-off. —The rain that falls upon the land and neither evaporates nor sinks into the surface runs off as fast as it can toward the sea. It is joined sooner or later by the water from the springs and by the rest of the underground drainage. Sometimes the journey is long and there are many stops and delays in lakes and pools; sometimes the course is quite direct and quickly traveled. The run-off most profoundly affects the earth's surface. Gullies and valleys are cut, depressions are filled; in fact, running water is the chief tool which has carved the features of the earth. It has had a long time to act and it has kept unremittingly busy, so that the results of its action appear now in our varied landscape.
+
+154. Pools and Lakes. —The water which runs off the surface first fills the depressions. As soon as these are filled, it runs over the lowest part of their rims and starts again on its course to the greatest of all depressions, the sea. If depressions of considerable size become filled with water, we call them lakes. As with mountains, the term lake gives no definite idea as to size. In some localities a water surface of a few acres is called a lake, while in other localities, the area must be several square miles to merit this name. As a rule, when the area covered by water is small, it is called a pool or a pond.
+
+
+
+AN UNDRAINED UPLAND.
+
+
+
+The streams that flow into lakes are continually bringing down the sand and mud they have gathered in their course, and are thus filling up the lakes. Lake Geneva in Switzerland has had its narrow eastern end filled, for a distance of fifteen or more miles, with the coarse sediment brought down by the Rhone. The whole basin of the lake has been covered to an unknown depth by the finer sediment. The outlet to a lake tends to wear away its bed, but it does this slowly, as it has little sediment with which to scour. Thus lakes are being constantly both filled and drained, and so are comparatively short-lived features of the earth. Rivers which have lakes along their courses must be young as otherwise they would have filled or drained the lakes.
+
+
+
+SUNSET ON GREAT SALT LAKE.
+
+
+
+Lakes are very important features to man. They filter river water so that rivers emerging from lakes are clear. Where the Rhone enters Lake Geneva, it is turbid and full of silt, but when it emerges, it is clear and without sediment. Lakes also act as reservoirs for the water that pours into them at the time of freshets. Rivers emerging from lakes of considerable size vary little in the height of their water at different seasons of the year. They are without floods. The St. Lawrence illustrates this. On the other hand the Ohio with its frequent and terribly destructive floods shows the effect of unrestrained run-off.
+
+
+
+THE DEAD SEA.
+
+
+
+Lakes often form most valuable internal waterways, as in the ease of the Great Lakes and the Caspian Sea. Lakes are also most beautiful objects on the landscape and their rippling waters give joy and pleasure to thousands.
+
+In some regions the rainfall is so small that the depressions never fill up sufficiently to overflow their rims. The water is evaporated from the surface as fast as it runs into the lake. Thus all the salt and other soluble substances which have been extracted from the land and brought into the lake by the rivers remain there, since only pure water is evaporated. In this way lakes without outlet become salt. Great Salt Lake in Utah is an example of this. Some salt lakes, like the Caspian Sea, were probably once a part of the ocean, so that they have always been salt.
+
+
+
+LAKE DRUMMOND.
+
+A lake in Dismal Swamp, Virginia, which is being filled by vegetable growth.
+
+
+
+As time goes on, more salt is brought to these lakes without outlets, and they become more and more salty. Great Salt Lake has something like 14 or 15% of solid material in its water and the Dead Sea about 25%. An effort to swim in these waters gives one an exceedingly queer sensation. The buoyancy is so great that a large part of the body is out of water, and one finds oneself bobbing around like a cork.
+
+Depressions that are very shallow and are largely filled with vegetable growths are called swamps.
+
+155. The Work of Running Water. —Running water has the power of carrying solid materials. If it is moving slowly, this power is not great; if moving swiftly and in great volume, it is tremendous. The carrying power of a stream increases very rapidly if its velocity is increased. A stream having its velocity doubled will carry several times as much material as before. Thus it happens that water running over a surface sweeps loose material with it, the amount varying with the rapidity and volume of the flowing water.
+
+As this loose material sweeps over solid surfaces, it cuts them down. Thus flowing water is continually wearing down and sweeping away the surface over which it moves. This sort of work is called water erosion.
+
+
+
+GULLIES BEING CUT BY RUNNING WATER.
+
+
+
+When running water is concentrated into a stream, the work of erosion is also concentrated and the wearing down of the stream bed becomes comparatively rapid. This cutting down goes on irregularly, being greatest at time of flood and least when the flow is slight. It is estimated that the solid material carried by the Mississippi River from its basin lowers the basin about one foot in 5000 years; but the material which is dissolved increases the amount carried away, so that the basin is lowered a foot in from 3000 to 4000 years.
+
+
+
+THE BAD LANDS OF DAKOTA.
+
+Running water has so dissected this land as to render it valueless.
+
+
+
+The channels of some of the streams in this basin are cutting down with far greater rapidity than this. We see gullies cutting down little troughs for themselves several inches deep in one rainstorm. The rapidity of cutting depends upon the material, the slope and the quantity of water. That "the waters wear the stones" was noted even in Job's time.
+
+When rain falls upon a sloping surface of fine textured, easily eroded material not covered thickly with vegetation, this will be deeply and fantastically sculptured. An excellent example of this kind of sculpturing is found in the Bad Lands of Dakota. Here travel is exceedingly difficult. It was in these natural fastnesses that the Sioux Indians made their last ineffectual stand against the white man's civilization.
+
+
+
+DIVIDES BETWEEN STREAMS.
+
+The ridge in the center of the picture separates two streams flowing in opposite directions.
+
+
+
+156. Divides. —If we carefully observe the drainage of a region, we find that the areas from which different streams gather their water are usually so distinctly separated from one another that a line could be drawn so that wherever water falls the rivulets on one side would flow into one stream and on the other side into another. Such a line of the highest land between the drainage areas of neighboring streams is called a divide. The line may be very distinctly marked, as on mountain ridges, or it may be difficult to determine, as in a flat country, but if the drainage is well established, it will be apparent.
+
+If the drainage is not well established, areas may be found which at one time drain in one direction and at another time in another. A singular example of the shifting flow of a drainage area is found in Yellowstone Park where Two-ocean Creek shifts from one side to the other of a fan it has built, and at one time delivers its drainage into the Atlantic Ocean and at another time into the Pacific.
+
+Near the dividing line between two drainage areas, swamps sometimes occur, which have streams flowing from them in two directions so that part of their water goes to one stream and part to another. But as these swamps become better and better drained, each stream will carry off its definite part of the water. Divides are irregular in their height, so that roads and railways in passing from one drainage basin to another usually seek out the lowest part of the divide. In mountain regions these low places are called passes.
+
+Divides do not always remain in the same place, as the river on one side may from some cause become able to carry off the drainage more easily than the river on the other side. It will thus push back its headwaters and shift the divide back until the divide becomes adjusted to the abilities of the two rivers.
+
+157. Falls and Rapids. —In many streams the flow of water is interrupted by falls and rapids. Sometimes the course of a stream is crossed by a great break in the earth's crust, one side of which has been raised above the other. This makes a fall, or, if the stream is able to cut down fast enough, a rapid. Falls or rapids of this kind have been produced in the Colorado River.
+
+
+
+YOSEMITE FALL.
+
+One of the most beautiful falls in the world, due to glacial action.
+
+
+
+Sometimes the course of a stream is changed, as was the case with many of the streams in the northern part of North America at the time of the Glacial Period. In its new course the stream may fall over a cliff, as did Niagara River which used to fall into Lake Ontario over the cliff near where Lewiston is now located. Here was developed a great fall which, owing to the kind and position of the rocks over which the river flowed, has moved back, leaving a gorge about seven miles long.
+
+The rock layers are nearly horizontal with a hard layer at the top and softer layers below. As the water strikes the foot of the falls it drives rebounding currents against the rock wall behind it, and wearing away the softer rock undermines the harder rock at the top, which breaks off in great blocks. Thus the falls maintain an almost vertical wall behind them. These falls are about 160 feet high, one of the grandest of nature's wonders and one of the greatest sources of water power in the world.
+
+Falls or rapids may also be formed in a stream where it passes from harder to softer material, as from the old land to the coastal plain. The softer material is worn away faster than the hard material and the stream bed lowered more rapidly, thus forming a precipitous descent. Falls of this kind were also formed where the glacial ice forced the streams to make new channels for themselves across the upturned edges of layers varying in hardness.
+
+The falls in the northern part of the United States were most of them formed by the rearrangement of the drainage lines at the close of the Glacial Period, and those in the southern part of the country by the more rapid wearing away of the softer rocks of the coastal plain. Thus we see that the hum of the spindle and the lathe are often but the modulated whispers of those ancient forces which thousands of years ago sorted the rock materials and built the vast continental ice palaces of the Glacial Period.
+
+Streams which have falls and rapids have not flowed in their present channels a long time, as time is reckoned in considering the earth's history. If they had, the falls and rapids would have been worn back and smoothed out. Thus, falls and rapids are characteristic of young rivers.
+
+158. River Development. —The rain which falls upon a flat country runs off very slowly, a large part of it soaking into the ground. Pools and lakes are formed in the inclosed basins, and sluggish streams with irregular little crooks, which show that the streams have hardly decided where they want to go, wander in the slight depressions down the gentle slopes and unite with other streams here and there until a river of ever increasing size is formed.
+
+
+
+NIAGARA FALLS.
+
+A young river cutting down a layer of hard rock.
+
+
+
+In some places the streams flow through lakes where they deposit their sediment, thus filling the lake basins. Here and there they pass over hard layers of rock which hold them up in falls and rapids. These they at once begin to smooth down. Rivers of this kind may well be called young, as their life work is just beginning. The Red River of the North, with its shallow narrow valley and tortuous course, and the Niagara River, with its lakes and falls, are examples of young rivers.
+
+Where the slope of the newly exposed surface is considerable, the streams flow much more rapidly and develop their courses more quickly. The small irregularities are sooner straightened and the trough deepened, thus forming side slopes down which run little rivulets which in time form side streams. The heads of these, like the heads of the larger streams, are constantly working back into the undissected area. Gradually the side streams develop side streams of their own, and almost the whole surface is covered with a network of streams.
+
+
+
+YELLOWSTONE RIVER.
+
+A young river flowing in a deep trough.
+
+
+
+As the work of erosion goes on and the streams deepen their valleys, only a few imperfectly drained remnants of the former flat surface are left here and there. These lie between the larger streams in places which the side streams have not as yet been able to reach. Almost the entire surface is so intricately carved into drainage lines, that wherever water falls it immediately finds a downward sloping surface. The main stream by this time has probably smoothed out most of its falls and rapids and has developed long, smooth stretches.
+
+
+
+A STREAM WORKING BACK INTO AN UNDISSECTED AREA.
+
+
+
+Here it is no longer cutting down its trough, but has only sufficient slope to enable it to bear along its load of waste. It here deposits upon its valley floor about as much as it takes away. In this part of its course a river is said to be graded. The longer a river flows undisturbed by any deformation of its valley, the fewer falls and rapids it will leave and the longer will be its graded stretches. The Missouri River near Marshall, Missouri, is an excellent example of a graded river.
+
+Sometimes a stream becomes so overloaded with detritus, which it has acquired in a steeper part of its extent, or which has been brought to it by tributaries, that it is continually being forced to deposit some of its load. Thus it silts up its course and flows in a network of interlacing shallow channels. The Platte as it crosses the plains of Nebraska is an example of such an overloaded river.
+
+
+
+RIVER EROSION.
+
+Cutting down the outer side of the curve and depositing on the inner.
+
+
+
+
+
+THE PLATTE RIVER.
+
+
+
+When a stream swings around a curve, the swiftest part of the current is on the outside of the curve and the slowest on the inside. A river that is carrying about all the load that it can, on passing around a curve, is able in its outer part to carry more than before and cuts into the bank, while on its inner part it flows less rapidly and is able to carry less, thus being forced to drop some of its load. As a river flows along its graded stretches, eroding in some places and filling in others, it broadens its valley floor, leaving at the border of its channel a low plain which in time of flood may be covered with water. Such a river-made plain is called a flood plain.
+
+
+
+RIVER PLAIN.
+
+
+
+If a river once begins to swing on its valley floor, it continues to do so, since whenever it strikes the bank, it is reflected toward the other side, and is made to move in the direction of the opposite bank as well as downstream. The windings that it thus assumes on a flat valley floor are roughly S-shaped and are called meanders, from the name of a river in Asia Minor which was, in very ancient time, noted for having such swinging curves. The size of these curves will be proportional to the size of the river.
+
+Great rivers like the Mississippi have a swing of several miles, while a small stream may have a swing of only a few feet or rods. These meanders are continually changing their shape, owing to the cutting and filling. Since they strike the bank with the greatest force on the downstream side of the curve, they also move downstream themselves. This can be seen from maps of the Mississippi taken at different times.
+
+
+
+RIVER MEANDERING IN ITS FLOOD PLAIN.
+
+
+
+The meanders sometimes become so tortuous that the downstream side of one curve approaches the upstream side of another and even cuts into it, thus causing the river to desert its curved path and straighten itself at this point. The old deserted winding looks something like an oxbow, and when filled with water, is called an oxbow lake. Sometimes the meanders are artificially straightened, as has been done in the lower Rhine valley, and much arable land reclaimed.
+
+
+
+THE MISSISSIPPI AND SOME OF ITS ABANDONED MEANDERS.
+
+
+
+In time of flood, when a river spreads over its flood plain, the velocity of the water is checked outside the channel and some of the sediment it carries is deposited. The most sudden check in velocity occurs where it leaves the channel, so more material will be deposited here than elsewhere on the flood plain. The banks of the channel will thus be built up more rapidly, and the flood plain near the river will slope away from the channel instead of toward it.
+
+
+
+LEVEE ALONG THE LOWER MISSISSIPPI.
+
+
+
+This is well shown in the lower Mississippi, where the river is found to be flowing on a natural embankment, the side streams running away from the river instead of into it. In some places the embankment is fifteen or twenty feet above the rest of the flood plain. These natural levees, as they are called, often force the tributary streams to flow for long distances upon the flood plain before they can enter the main river. The Yazoo River is forced to flow along the flood plain some 200 miles before it can enter the Mississippi. Artificial levees are often built to keep rivers from overflowing their flood plains. Such are the high levees along the Lower Mississippi and Sacramento rivers.
+
+
+
+LEVEE OF THE SACRAMENTO.
+
+
+
+Sometimes the flood plain of the main river is built up more rapidly than the tributaries can build theirs, so that they are dammed up as they enter the flood plain of the main stream and form a series of fringing lakes along its border. A fine example of this is found in the lower course of the Red River of Louisiana.
+
+A river is said to be mature when it has reduced its valley to grade and is able to meander freely upon its flood plain, its side streams having appropriated all the undrained upland which they are able to obtain. The river is now carrying off in the easiest and most effective way the drainage which falls upon its drainage basin.
+
+When a river has graded itself and built its flood plain, its own active work consists largely in carrying off the materials brought to it by its side streams. Although these are now able to appropriate no new territory they continue to wear down the country and round off the divides till the whole region, unless reëlevated, is reduced to an almost level plain with its entire drainage system nearly at grade. Most of the material now carried by the river is in solution, and there is but little erosion. The river has accomplished its life work, it has borne to the sea all the burden it has to bear, its labors are ended, it has reached old age. It has reduced its drainage area to a base level of erosion. When a river has thus done all the wearing down of which it is capable it is said to have completed a cycle of erosion.
+
+
+
+AN OLD RIVER.
+
+This river has done its work and has completed a cycle of erosion.
+
+
+
+159. Rivers in Dry Climates. —In a region where the climate is very dry, rivers are often intermittent in their flow. They contain water only after rains. Such rivers may dry up before they reach any other body of water, their water entirely evaporating or sinking into the dry soil. Their development is therefore somewhat irregular.
+
+
+
+RESULTS OF A SUDDEN FLOOD.
+
+
+
+If the slopes are steep and there is little vegetation to protect them and hinder the quick run-off of the water, rivers flood very rapidly, eroding their channels and washing away their banks. Where they descend upon level ground they silt up their old courses and acquire new channels. Thus a river which for the larger part of the year is a mere brook may after a rain become a devastating torrent, bursting its banks and carrying destruction to settlements and farm lands along its course. It may even change its entire lower course.
+
+160. Accidents in River Development. —While a river is developing its drainage area many accidents may happen to it. The competition of other rivers in the same region affect it. The river that has the shortest course to the sea or the most easily eroded bed has the advantage. It lowers its valley more rapidly, thus giving its side streams steeper grades and enabling them to wear back faster into the upland, and thus to gain more than their share of the drainage of the region.
+
+
+
+Fig. 113.
+
+
+
+
+
+Fig. 114.
+
+
+
+As soon as the unappropriated drainage area has been channeled, it begins to push back the divides of its neighbors, thus appropriating some of the run-off they may have acquired. In Figure 113 a case of this kind is shown. The river A reaches the sea by a long course, while the streams B, C, D have short courses. These short streams have steeper grades than A and thus are able to gnaw back and cut down their valleys faster. Thus they push the divide EF farther and farther toward A.
+
+In Figure 114 a stage is shown in which the divide has been pushed back toward A and at one point has approached very near to the upper part of the branch G. In Figure 115 it has been pushed across this branch and the stream B has tapped G and appropriated its headwaters. This is a case of what is called beheading or piracy.
+
+
+
+Fig. 115.
+
+
+
+As A has lost some of its water it erodes its valley even more slowly than before, and a branch of the stream D may take away the headwaters of its branch H. If time enough is allowed, a branch of the stream C may completely behead A, leaving only its lower trunk as an independent river. Cases of river piracy are most interesting phenomena.
+
+A river may by some accident have its supply of sediment greatly increased, causing it for a time to build up its valley floor instead of eroding it, thus forming a filled river valley. When the supply of sediment fails, the river begins cutting down the filled valley, leaving terraces along the sides to mark the successive levels at which it flowed.
+
+River terraces are often very prominent along our northern rivers, since by the melting of the ice at the close of the last Glacial Period these rivers were supplied with a vast amount of sediment which they were unable to carry away and so deposited on their valley floors. When the supply ceased, they eroded their valleys, leaving terraces along the sides.
+
+The region in which a river is situated may be elevated, thus affecting its normal development and beginning a new cycle in its history. The elevating may take place over its whole drainage area or only over a part of it. If the whole region is elevated, the energy of the entire river is revived, and the river may be called a revived river.
+
+
+
+RIVER TERRACES.
+
+The river is now cutting down its former plain, leaving terraces.
+
+
+
+
+
+INCISED MEANDERS.
+
+
+
+The elevation may take place at any time during the history of the river. If it takes place after the river has become old and is meandering on its flood plain, the river will begin afresh to cut down its valley. But as its meandering course has been established, the trench that it now cuts is not like that of a young river, but is a meandering trench, and what are called intrenched meanders are formed. This region will have the steep V-shaped valleys characteristic of a young region and the well-developed drainage and meandering rivers characteristic of a mature region. The Palmyra, Virginia, sheet shows these characteristics. The main rivers meander in steep valleys. A profile shows these valleys to be steep and V-shaped with broad, rounded uplands between, well provided with drainage channels.
+
+
+
+INTRENCHED MEANDERS.
+
+
+
+When the elevation extends beyond the mouth of the river, the river must prolong its course over the emerged land in order to reach the sea. It may happen that rivers which formerly entered the sea at different points, in extending their courses over the emerging continental shelf, join some large stream and all enter the sea through it.
+
+This is what happened to the rivers now forming branches of the lower Mississippi when the coastal plain bordering the Gulf of Mexico was elevated. These formerly entered the extended Gulf by separate mouths, but when the land rose and forced the water of the sea back, their extended courses joined them to the great central river, thus vastly increasing its drainage area and the volume of water it poured through its mouth into the sea. Many of the great river systems of the world have been built in this way. These may be called engrafted rivers.
+
+In Figure 116 the rivers all enter the sea at the old coast line GH by separate mouths. When this region is elevated so that the coast is at IK, the rivers E, D, B find that their easiest course to the sea is by engrafting themselves upon the river C, and thus they all four find their outlet at one point L. The rivers F and A still maintain their independent courses.
+
+
+
+Fig. 116.
+
+
+
+It may be that the elevation takes place over only a part of the river's course. Then the river is dammed back and laked on the landward side of the elevation and obliged to seek a new course for itself, thus becoming a reversed river, or else it is strong enough to cut its bed down as fast as the land rises, and thus maintain its course. Such a river is called an antecedent river, as its course antecedes the uplift which naturally would have determined its course. The Columbia River has maintained its course through uplifts which have reached thousands of feet.
+
+Not only may a river be elevated, but it may be depressed. In this case its rate of erosion is diminished, and the river becomes marshy where the grade is low. Where the river valleys approach the sea they will be submerged or drowned.
+
+These drowned river valleys form some of the finest harbors on the coast. San Francisco Bay, Narragansett Bay and New York harbor are examples of protected harbors due to the submergence of rivers. The mouth of the Hudson was formerly some seventy miles to the east of Long Island, that of the St. Lawrence to the east of Nova Scotia. In fact the Atlantic coast north of the Hudson furnishes innumerable examples of submerged river valleys.
+
+
+
+ALLUVIAL CONES.
+
+Formed at the foot of each mountain gully.
+
+
+
+The tributary streams which enter low down on a river's course, after submergence enter the sea directly in the bays formed by the submerged valley. Such rivers may be called dismembered rivers. Thus a coast region which was formerly well dissected by streams will on submergence become penetrated by a great number of irregular channels and bays.
+
+
+
+FAN FILLED VALLEY.
+
+Notice how the river is forced to wander around the edges of the fans.
+
+
+
+Delaware and Chesapeake bays, where the early settlers each had a nice little sea inlet instead of a rough wagon road as his means of communication with his neighbors, are fine examples of submerged river systems. These drowned river valleys enabled the early settlers to penetrate easily into the country, and determined many of the early settlements, like Philadelphia, New York and Providence.
+
+161. Alluvial Cones and Fans. —When a stream having a steep grade and bearing a heavy load of sediment emerges upon a flat country where the grade is suddenly reduced, it so quickly deposits its sediment as to be continually obstructing its own course and forcing itself to find new channels. It thus builds a fan or cone-shaped deposit pointed toward the place where the stream reached the plain. If the material is coarse, the deposit will have a steep slope; if fine, a gentle slope resembling a spreading fan. Sometimes these fans so overlap each other as to form an irregularly sloping plain.
+
+
+
+LAKE DELTA.
+
+Notice the triangular formation.
+
+
+
+Plains of this kind are found along the base of many mountain regions. If such formations occur in regions of little rainfall, they yield themselves with peculiar facility to irrigation, as ditches can be easily led out from the apex of the cone in all directions. Southern California offers many examples of easy irrigation due to such cones.
+
+162. Deltas. —When a river enters a body of quiet water, its current is gradually checked and it deposits its material in somewhat the same way as on emerging upon a flat country. But here the deposition is more gradual and the slope of the deposited material less steep. The sediment, too, is sorted by the water, and the finer material is carried far out from the river mouth. Formations of this kind are called deltas, from the Greek capital letter Delta (Δ) which has the shape of a trian gle. Few deltas have this ideal shape, but there is a general correspon dence to it.
+
+
+
+CONE-SHAPED DELTA IN LAKE GENEVA.
+
+
+
+If the delta-forming stream descends steeply, it may build a delta with a steep upper surface rising cone-shaped above the water. Many of the deltas in Lake Geneva are of this kind. If the grade is slight, the delta will be simply a continuation of the flood plain of the river. Such is the Mississippi delta. The layers of sediment composing the delta, slope away from the point where the river enters the still water. Here, as in the alluvial cone, the river is continually silting up its outlet and being forced to seek new channels. In large deltas the river generally enters the sea through several channels or distributaries, as they are called. This is seen in the map of the Mississippi delta.
+
+
+
+LAKE BRIENZ, FROM ABOVE INTERLAKEN.
+
+A rapidly eroding stream at the extreme right has built a great delta dividing the ancient lake into two parts.
+
+
+
+Deltas have rich, fine-textured soils and are very fertile. The Nile delta during all history has been noted for its fertility. But they are treacherous places, as they are liable to inundations by the overflowing of the river at time of flood. Because they are pushed out into the sea, they are peculiarly exposed to the sweep of the waves in great storms.
+
+
+
+MOUTHS OF THE MISSISSIPPI RIVER
+
+
+
+The rate at which deltas grow depends upon the amount of material carried by the river and upon the tides and currents at its point of outlet. In seas where the currents and tides are strong, no deltas are formed, except by very large streams such as the Yukon, the Huang-He and the Ganges. In quiet seas deltas are readily built. The delta of the Mississippi is more than 200 miles long and has an area of more than 12,000 square miles. The Po in historic time has built a delta more than 14 miles beyond Adria, a former port which gave its name to the Adriatic Sea.
+
+163. History and Rivers. —From earliest times rivers have played a most important part in the world's history. At first almost all human movement was along river valleys, as they offered the easiest route of travel. Here, too, men found the fertile and easily worked land so necessary in their primitive agriculture. Thus their settlements were usu ally placed upon the banks of rivers. In war the river offered a means of defense, as the Tiber so often did to Rome.
+
+Before the time of railways, rivers and lakes supplied almost the only means of inland commerce. In our own country the hundred and fifty miles of unobstructed riverway stretching from New York to the north was the great road from Canada and the Lakes to the sea, fought for persistently in French and Indian Wars as well as in the Revolution. If in the Revolution the British could have obtained control of the Hudson, they would have effectually separated the colonists in the north from those in the south and would probably have been able to crush each separately.
+
+The Mississippi River served for years as the only artery of transportation from the interior of the country to the sea. When Spain held the mouth of this river and Congress was unable or unwilling to exert itself to obtain the privilege for American boats to pass to the sea, it seemed for a time that the sturdy colonists along the Ohio and Mississippi would either form an independent country and fight for the privilege or else in some way ally themselves with Spain, so vital to them was the need of this waterway. In the Civil War vast amounts of blood and treasure were spent in fighting for the control of this river.
+
+These are but a few examples taken from our own history of the importance of rivers. They could be duplicated in almost every country of the globe.
+
+164. Great Rivers of the United States. —There are four great river basins wholly or partly within the United States: the St. Lawrence, the Mississippi, the Columbia and the Colorado. The first two of these are navigable for great distances and furnish unexcelled interior waterways. Notwithstanding the great development of railways they still exert a vast influence upon the commerce of the country.
+
+
+
+DRAINAGE BASINS OF THE UNITED STATES.
+
+Notice the positions of the divides separating the different drainage areas.
+
+
+
+The St. Lawrence River with the Great Lakes, which are geographically a part of it, is the greatest internal waterway in the world. From the head of Lake Superior to the mouth of the St. Lawrence, a distance of about 2400 miles, by aid of the canals which have been built around the rapids and falls, vessels of 14 feet draft can pass to the sea. More tonnage passes through the "Soo" canal between Lake Superior and Lake Huron than passes through the Suez canal. Here pass the greatest fleets bearing wheat, iron, and lumber that the world has ever seen.
+
+The old river which once drained this region passed through various vicissitudes before the present noble waterway was formed. From Montreal to the sea it has been drowned by a depression of the land. Its upper basin has been enlarged in places by the action of the glacial ice, and in other places it has been dammed, thus causing lakes and falls.
+
+
+
+THE "Soo" CANAL AT SAULT STE. MARIE.
+
+Notice the "whale-backs," a type of boat peculiar to the Great Lakes.
+
+
+
+The Mississippi and its tributaries offer navigable waterways of about 9000 miles. It is the greatest navigable river system in the world. From the Rocky Mountains on the west to the Appalachians; on the east and from the northern border of the country to the Gulf, the spreading arms of its tributaries stretch out ready to bear to the ocean by cheap and easy paths the products of this vast interior basin. By the aid of the Panama canal these varied products may travel directly by water without more than one or two re-shipments from their source in the vast continental interior to the uttermost parts of the earth.
+
+
+
+TIE JETTIES OF THE MISSISSIPPI.
+
+To keep the river from silting up its channel, it is confined between jetties and made to flow swiftly.
+
+
+
+
+
+DELTA LAND OF THE LOWER MISSISSIPPI
+
+
+
+This noble river presents in its eventful history an epitome of the geographical history of our continent. It winds its masterful way over the oldest and youngest rocks. For part of its course it follows a valley built long before the Glacial Period shrouded the northern part of the continent in ice. In the northern part of its course the blanket of débris left by this vanished ice choked its path and forced it to seek a new channel. For the southern part of its course its mighty sediment-laden waters built new lands that it might extend its dominion.
+
+
+
+THE COLUMBIA RIVER AND ITS OLD FLOOD PLAIN.
+
+
+
+At times in its history its might has been greater than now and at times less. But through all its history it has borne to the ocean the ceaseless current flowing from the heart of our continent. To it the modern geographer turns again and again as an inexhaustible record of geographical development. The geographical, political, industrial and commercial history of this continent are closely connected with this, its mightiest artery.
+
+The Columbia River, although navigable for a distance of only 500 or 600 miles, and thus never destined to have the commercial importance of the St. Lawrence and the Mississippi, presents features of great interest. Guided by this stream the first settlers found their way to our northwest territory. Along its depressed mouth the rich and prosperous states of Washington and Oregon were nurtured through their infancy. Over its possession Englishman and American long contended.
+
+This contention of man, however, was but an echo of the long contention of the river itself to hold its course. Flowing in a region of growing mountains, it was forced again and again to cut its way through barriers uplifted across its path. Sometimes for a time it was checked and forced to raise itself into a lake in order to surmount the obstruction placed in its way, but its strength never failed, and so through new-born ridges, through lake beds born of its own struggle, through growing depressions filled by its own labor, it held its course steadfastly to the sea. For part of its way it flows through cañonlike valleys, and its main tributary, the Snake, has built for itself through great beds of horizontal igneous rocks a cañon but little inferior to that of the Colorado.
+
+The fourth great river, the Colorado, has industrially and commercially attained but little usefulness. Although navigable to about 400 miles from its mouth there is little need in the country it traverses for transportation in the direction of its course. But what it lacks in utility, it makes up in scenery. To no other river on the face of the earth has the opportunity been given to show its sculpturing power as to the Colorado.
+
+Flowing as it does through an arid region of nearly horizontal rocks, it has carved a giant trough for itself, leaving upon the lofty sides the uneffaced chisel marks of all its erosive helpers. The rill, the rivulet, the intermittent torrent, the sand blast of the scouring wind, the pull of gravity, the varied resistances of the rock layers, the structure and composition of these layers have each and all left their peculiar impress upon the resulting sculpture. Standing beside this mighty chasm, one is impressed, as nowhere else, with the mighty power of erosive agents.
+
+
+
+THE COLORADO RIVER.
+
+Flowing through a deep-cut, narrow valley.
+
+
+
+And yet here is seen only the beginning of the vast work which these forces have before them. They have built only the narrow trough of what must be developed into a wide and gently sloping valley, and have hewn out here and there a ravine in that great upland which in time they must carve into the mature forms of a thoroughly dissected country. If the region had not been so dry, the work of dissection would have progressed much farther before the river had been able to sink its channel so deep. The water that falls hundreds of miles away is doing a mighty work which the meager rainfall of the region through which it passes cannot supplement. Majestic, awe-inspiring, stupendous, this gigantic trench is but a prank of the river's boisterous youth.
+
+
+
+
+
+Summary. —Just as the waves and ocean currents work upon the coastlines, so the rain and the streams are constantly wearing down the surface of the land. All streams come from rain or melting snow, which condenses in the air after evaporating from water surfaces. The rainfall varies from nothing at all in some places to over fifty feet a year in others, but in the United States the greatest rainfall is about eighty inches a year.
+
+Some of the rain evaporates at once after falling; some flows away on the surface of the land; some sinks into the ground, to return as springs, wells and geysers. The water which flows along the surface has the greatest effect upon the land. It forms the little streams which remove the surface water, the huge rivers which drain the country and form great arteries of trade, and the beautiful lakereservoirs which hold back floods and offer easy transportation to mighty ships.
+
+But most important of all is the erosion caused by flowing water. It wears down the hills and spreads them out in fertile fields, in deep trenches and broad valleys; it fills lakes and builds great deltas. By its falls and rapids it furnishes water power for manufactures.
+
+Rivers that have not yet widened their valleys and still have falls and rapids are called young; an old river is one whose bed has been worn smooth, and which has built for itself a broad level valley, through which it wanders, doing little if any erosive work. Rivers sometimes develop flood plains through which they wander in S-shaped meanders. Sometimes a river cuts back its divide so far that it reaches another river, thus diverting another stream through its channel.
+
+If the region of a river becomes elevated, the river may be revived, and if it is an old river with meanders, intrenched meanders may be formed. Sometimes the elevation of the land causes a river to be laked or reversed; if it maintains its previous course in spite of the elevation, it is called antecedent.
+
+If a river region becomes depressed, the river may be drowned and its branches may enter the sea separately as dismembered rivers. Many rivers build deltas where they empty into still bodies of water and when the slope is steep, they may form fans.
+
+Rivers have always played a great part in history, from the time Egypt was first called the "Gift of the Nile" to the influence of the Mississippi and St. Lawrence on the settlement and development of the United States.
+
+
+
+
+
+QUESTIONS
+
+
+What conditions influence the amount of rainfall of a place?
+
+What determines what will become of the rainfall when it falls upon the ground?
+
+What does the water do which sinks into the ground?
+
+Where are geysers found? What are they?
+
+Trace the probable journey of the water that fell near your home during the last heavy rain until it reaches its journey's end.
+
+What determines whether a lake is fresh or salt? What are the great benefits derived from lakes?
+
+Describe some effects of running water that you have seen.
+
+Why does not all the water that fell in your town during a heavy rain flow by your home? Where is the 'divide'?
+
+What are some of the causes which have formed falls and rapids? Describe the life history of a river.
+
+What peculiarities have rivers of dry climates?
+
+Describe some of the accidents which are liable to happen during a river's history.
+
+How are alluvial cones and fans formed?
+
+Where and how do rivers build deltas?
+
+What have been the effects of rivers upon history?
+
+Describe the four great rivers of the United States.
+
+
+
+
+
+【中文阅读】
+
+
+149．降雨——地球表面的海水不断蒸发，升入空中，又被风吹向四方。大部分的水蒸气被吹向陆地，它们在那里又重新凝聚，变成雨露降落大地。陆地上不同地域受到的这个水循环的惠泽，有的地方多，有的地方少。在温暖的海域，海水的蒸发量会更大，从那里吹来的风，遇到山脉便向上爬升，在迎风面上因此形成大量的降雨，而背风面就没有这么滋润了。这一情形在美国北部地区表现得很明显。就一般规律而言，各个大陆板块上的山脉都很靠近板块边缘，这样便使得所有的内陆地区都相对比较干燥。
+
+盛行风为寒风的地区，雨量也比较稀少，因为风从寒带吹向温带，随着温度的升高，其保持水蒸气的能力也在不断增强。撒哈拉沙漠的气候状况就能凸显这一点。因此，地球上不同地区的降雨量也各不相同。在东印度的卡西山南部地区，有一年的降雨量的最高纪录到达了50英尺，而在地球上���为干旱的地区，一年到头可能滴雨不下。
+
+美国最大的降雨量可达80英寸以上，一般出现在西北沿海以及犹他州、亚利桑那州和内华达州的盆地地区。不论降雨量是大是小，它对一个地域的影响都是不容小视的。没有降雨，地球表面将会变成极度炎热而且寸草不生的茫茫戈壁；而正因为有了降雨，才有了大自然的葱茏翠绿，才有了广阔无边的生命之美。
+
+150．降雨的范围——雨水降落到地面之后，一般有三种去向：可能立刻从地表蒸发，重新进入到空气中；也可能形成涓涓细流，并很快汇入溪流于江河，并最终流向大海；还可能浸入到土地中去。在最后一种情形中，一部分水又可能通过毛细现象回到地面，进而又被重新蒸发；也有一部分水可能汇入泉水；还有的则可能深深浸入到地下的土壤与岩石中。
+
+而最终雨水会选择以上哪个过程，则完全取决于它降落的形式与土地表面的状况。如果降落得非常迅速，则大部分会从地面流走；如果降落得很平缓，大部分则会浸入地下。如果它们是降落在森林地区或者大面积的植被上，植物以及它们的根茎便会阻碍雨水的流动。如果地表是硬质结构且不能渗透的岩石，大部分雨水便会流走；但如果地表很疏松且能够被渗透，大部分雨水便会渗入地下。不过即使在炎热干旱的沙漠地带，暴雨形成的激流也依然会四处奔流。
+
+151．地下水——渗入到地下的雨水，会沿着土壤的微小缝隙或者从土壤的颗粒之间慢慢向下沉降，直到一个不能再向下继续渗透的地层，或者又会重新汇入一股水流流向地面，当然水流在地面的流出口比雨水先前的浸入处要低一些。它会在这里慢慢渗出，或者集中形成泉水。
+
+如果汇入泉水的部分雨水渗透到地表以下足够深，以致太阳热量无法影响到它的话，便会显得非常寒凉，当它再次涌出地面时，就形成了寒泉。但若在泉水涌出的地方，地表岩石很热，乃至于让泉水在找到裂缝到达地面之前，所处的底层岩石也被加热的话，就会形成温泉。
+
+由于地球外壳在许多地方都是由岩石构成，雨水也常常降落在褶皱多孔的岩石层上，而在这个岩层之下，雨水便不能再继续渗透了，这样雨水就会在多孔岩石层中不断聚集。如果这一岩层在其他地域被不能渗透的岩层覆盖，先前岩层中的水分便会慢慢被涵持在这些岩层中，形成自身的水压。若这时在岩层上开一个孔的话，水便会自然涌出，如果水压足够大，还可能直接喷出来。
+
+这种形式开孔后便会形成自流井，它在一些很难找到地表水源的地方非常重要。在美国西部一些省份，人们主要便用井水来进行广泛的农作物灌溉。尽管这些水含有一定矿物质，但由于它们远离地面污染，因此一般都很健康的水源，可直接饮用。
+
+还有一些地方，地表水能够渗透到一些自身可以溶解的岩层中，比如含有盐分和石灰的岩层。这样便在岩石中形成了洞穴和空腔，原先洞穴处的固体物质通过溶解被水带走了。这样的洞穴非常常见，但著名的有两处，一处是地下差不多200英里长的猛犸洞穴，其中遍布各种被雕饰得光怪陆离的小山包；另一处是卢雷洞穴，在那里，各种各样的钟乳石和石笋装饰出了一个梦幻般的世界。有时这些洞穴的某处顶部彻底被侵蚀，便只剩下一个桥梁般的穹顶，弗吉尼亚和犹他州的天然桥架就是如此形成的。有时这些穹顶还会坍塌下去，这就形成了灰岩坑。
+
+152.间歇喷泉
+
+
+
+
+
+实验128：给一只250cc容量的烧瓶塞入一个双孔胶塞，从一个孔中塞入一根玻璃管A，让其差不多接近烧杯的底部，从另一个孔中再插入一根玻璃管B，它比整个烧瓶的高度要长5到6厘米，并让它离底部大概间隔1到2厘米，然后将它轻轻往外拔出一点点，外端弯曲，呈一定侧向的倾斜度。将烧瓶放在环架上以便可以用本生灯对其加热，并将导管A外端用一根足够长的橡胶管与一个装满水的容器连结，容器的位置必须高于烧瓶顶端。
+
+
+
+
+
+通过导管B将烧瓶中的空气抽出，水容器的水便会流进烧瓶中，这就已经构成一个虹吸装置，在内部压力不改变的情况下，水便会一直漫溢至刚刚淹没导管B末端的位置。现在将烧瓶加热，煮沸之后，热水便会从导管B中涌出，直到水面降至其末端以下，这时气压可以通过导管B得以释放，而水容器中的水又会慢慢流入烧瓶，直至淹没导管B末端。而当蒸汽积累到一定程度时，又会像刚才那样，将水从导管B中压出。因此只要持续加热，水便会间歇性地从导管B中喷出。我们这里其实已经建立起了一个类似间��喷泉的模型。
+
+在新西兰北部以及美国黄石国家公园，还有冰岛，都能找到许多类似的间歇喷泉。这些地区都有晚期的火山活动迹象，巨大的喷泉也是这些地方最为生动精彩且让人惊奇的现象。在毫无征兆的情况下，一股热水会从地面喷入空中，并伴随着翻滚的水蒸气形成的白烟。有时候这些水汽可以达到数百英尺甚至更高的高度，而且不断涌出的水也会在这一高度上保持一段时间，有的几分钟，有的长达一两个小时。然后又慢慢平息下来，只剩下汩汩热水在泉眼处依旧冒着泡泡。
+
+大多数间歇喷泉的喷发时间都不是固定的，但是黄石公园著名的“老忠泉”却像时钟一样有规律，每隔一个小时多一点的时间就喷发一次。它能喷到150英尺高，每次喷水大概4分钟左右。不远处的“巨人泉”水量极大，则能喷涌到250英尺高，每次能持续一两个小时。
+
+喷涌而出的热水还将溶解的岩石矿物带离了出来，随着它们落回地面，又会慢慢降温，并通过不断的物质沉积，在喷涌处形成奇形怪状的环形口，给这里增添了一种无形的参差之美。从这些环形口望下去，会看到它们都有光滑的纹路，但是里面却不是很规则，弯弯曲曲，像一根开口的管子伸向地下深处。地下水就是通过这个通道涌出地面的，这些通道有多深无人知晓，但至少它能达到一个高温岩层，那里的温度足以让水沸腾，而这个热量则很可能来自岩浆。
+
+当通道中的水被加热至沸腾后，内部压力便也越来越大，蒸汽大量形成，一部分水便被压出地面。这些涌出的水释放了一部分压力，而进入喷涌通道的水形成的水蒸气越多，被压出的数量也会越大，因此地下的水源也必须有大量的蓄水量，就像巨人泉一样，这样才能保证巨量的水能够持续形成喷泉。
+
+153．径流——降落到地面的雨水中，那些既没有被蒸发掉，又没有浸入土地的部分，则会从地面流走，一路欢歌奔向海洋。它们都会先汇入泉水溪流或者地下排水，然后开始它们的旅程。但在过程中也会有许多停留，比如在湖泊和池塘中；当然也可能一路非常顺畅，直接到达终点。地表径流对地球表面形态产生了非常深刻的影响。它们冲刷出了沟壑峡谷，填满了低洼地带。可以这么说，径流水是刻画地球表面主要形态特征的直接要素。当然它的作用时间极其漫长，并且始终不间断，经过无涯岁月，造就了我们今天看到的大千世界。
+
+154．池塘与湖泊——地表的径流水首先会填充沿途的低洼地带，而当它们一旦将某一洼地填满以后，便会从洼地的最低处流走，直奔需要它们去填充的最大的洼地，大海。如果其过程中填充的洼地非常大，我们往往便把这些地域叫做湖泊。就像大山一样，湖泊的概念并没有一个确定的尺寸，有些地方几英亩的水域也叫湖泊，而在有些地方，起码数平方英里的水域才够得上这个名字。这样，我们便把面积较小的水域叫做池塘或者水塘。
+
+流入池塘的泉水会不断地将泥沙也带进池塘，并渐渐在其内部慢慢沉积。瑞士的日内瓦湖狭窄的东口，只有差不多15英里宽，就被罗纳河的沉积物给填塞满了，而且整个湖水的流域都覆盖了一层深度尚未确知的细微沉积物。在湖水的出口处，水流都会将湖床渐渐冲刷掉，但这个过程很慢，因为在那里的沉积物其实并不多。因此，湖泊其实始终处于填充和排泄的反复过程中，它们的生命周期在地球上相对而言还是算比较短暂的。这样看起来，沿途有不少湖泊的河流，岁数都不会太大，不然的话，河流会将这些湖泊要么吞噬掉，要么早将它们吸干了。
+
+湖泊对人类也非常重要，它们能够对河水沉淀过滤，让河水变得更加清澈。当罗纳河刚流进日内瓦湖的时候，它还是一股充斥着泥沙的浊流，而当它流出来的时候，则变得非常清澈，几乎没有一点杂质了。在有大量的水流经湖泊的时候，它们便体现了蓄水池的作用。从比较大的湖泊流出来的河流，其水量在一年四季中变化都不太大，也几乎不发洪水，圣劳伦斯河就是典型的例子。而犹他州频发的洪水则显示了不受限制的径流水的破坏作用。
+
+湖泊还形成了一些非常有价值的水流通道，比如五大湖区以及里海。湖泊也经常是风景之中最美丽的要素，它们荡漾着涟漪的湖水，连同交相辉映的湖光山色，千百年来给人们带来了无尽的美丽与欢愉。
+
+有的地方降雨量非常少，让它们的洼地几乎从未被雨水填满过，雨水还未流进湖泊就已经被蒸发殆尽了。但蒸发的都是纯净水，所有的盐分和溶解物则被径流雨水从土地上带到了湖泊，并在那里不断积累。因此，没有出水口的湖泊一般都是咸水湖，犹他州的大盐湖便是如此。还有一些咸水湖，比如里海，很可能曾经就是海洋的一部分，所以一直保持着咸水状态。大盐湖的固体溶解物含量为14%到15%，死海大约是25%。在死海里游泳的话，会感觉怪怪的，由于浮力非常大，我们身体的很大一部分都会浮在水面以上，还会感觉我们整个人像一个软木塞一样不停地摇摆。
+
+比较浅且伴有大量植物丛生的蓄水洼地，则被叫做沼泽。
+
+155．水流的作用与影响——流动的水能够携带搬运一些固体物质。如果它们流得慢，其力量还不太明显；但若它们流速快，流量也很大的话，其力量则是非常惊人的。当一条小溪的流速增加后，它的搬运能力也会随之迅速增强。如果它流速加倍，它能携带的固体物质则会变成以前的数倍。因此水流总是不断地将它流经地方的松散物质冲走，其数量则取决于水流的流速和流量。
+
+而这些松散物质，都广泛地分布在固体表面上，水流将它们冲走，便也无形中雕镂刻画了这些固体表面。于是，水流也会持续地对它所流经的表面产生冲击效应，这样的现象就叫做水蚀。
+
+当径流水融汇到河流中之后，水的侵蚀作用就显得更加突出，其对河床的冲击力也会明显增强。它的冲刷效应会显得不太规则，发洪水时效果最明显，平时风平浪静时效果就很微弱了。据研究，密西西比河所携带的固体物质，在漫长的5000年中，让其流域整体下降了大约1英尺，且水中的溶解物无形中又增加了被冲走的固体物质的数量，这样算下来，在最近的3000-4000年中，其流域河床就被削弱了将近1英尺。
+
+而一些河流流域的河道，其被侵蚀状况却远甚如此，比如我们可以在暴雨中看见水流在地面上冲刷出几英寸深的水沟，具体冲刷的速度则取决于被冲刷面的构成物质、倾斜度以及水的流量。“水能让石头搬家”的现象早在约伯[1]时代就被人们注意到了。
+
+当雨水降落到一些表面粗糙的斜坡上，尤其那里易被冲刷侵蚀的物质覆盖得很浅，且伴随大量植被的话，雨水的侵蚀作用就展现得淋漓尽致了。这样的情形可以在达科他州的“恶地”地区找到例子，这一地带的交通因此变得极其困难。也正是这一天堑鸿沟，让印第安人守住了自己的家园，抵抗住了白人文明的影响。
+
+156．分水岭——如果我们注意观察一个地区的水流方向，就会发现不同河流收纳支流的方向都不一样，而且各区域之间的差别也明显，以至于我们几乎可以在它们之间画出一些线条，将一个地区支线水流的不同汇集方向与旁边地区分割开来。这些分割线所处的高地，将具有不同水流流向的区域与旁边区域区别开来，我们叫做分水岭。这些分割线有时很容易标记出来，比如在山脊上，但有时也难以确定，在一些平坦的地方就是如此。但只要水流的流向是确定而明显的，它最终还是会显现出来。
+
+若一个地区的水流流向不是很明确的话，那就意味着水流某些时候流向一个方向，另一些时候则流向另一个方向。这样的双向流域的突出例子便是黄石公园的“两洋溪”，它像个吹风机一样，有规律地在两个方向上来来往往，一段时间流向大西洋，一段时间流向太平洋。
+
+在分水岭附近常常存在着一些沼泽地，这样从它们流出来的溪流常常也会流向两个不同的方向，也让这些沼泽地里的水，一部分流向一个方向，另一部分则会流向另一个方向。随着沼泽地的水流渠道越来越通畅，流向各条溪流的水也就基本趋于固定。分水岭的海拔高度毫无规律，因此在修筑一些穿越分水岭的公路和铁路的时候，我们只能沿着分水岭的最低部分修建，在山区地带，我们把这一低沿线区域叫做走廊。
+
+分水岭也不会一成不变地始终处于同一个地方，比如当分水岭一边的溪流能够更容易地吸纳另一边的溪流的时候，分水岭就会发生变化。这时某一边的溪流源头会倒转方向，分水岭也会随之发生调整，以至找到不同区域间，新的水流流向划分线为止。
+
+157．瀑布与激流——许多河流都会在途中的某一阶段遇上瀑布或者激流。有时候，水流会跨越地壳的巨大断层处，断层的地方一边很高，而另一边很低。这样便会形成瀑布，或者水流若能很快降落下来的话，便会形成激流。这类的瀑布与激流在科罗拉多河流域十分常见。
+
+有时河流的流经过程也会发生改变，北美洲北部的大部分河流在冰河时代就常常如此。在河流新的流经道路上，它们可能从一个悬崖上降落下来，��如落入安大略湖的尼亚加拉河，在跨越路易斯顿附近的一个陡峭悬崖时，便形成了大瀑布。这也主要是由水流流经的岩石的类型和位置所致，在回迁的地质运动中，形成了前面七英里长的大峡谷。
+
+这里的岩石层一般都呈水平状态，顶层坚硬而底层松软。且由于在瀑布的脚下，会激起强烈的水花，不停地击打着它背后的岩壁，这样就会将巨大坚硬岩层之下的松软岩层渐渐冲刷瓦解。因此，瀑布背后的岩壁总是保持着垂直的状态。这些瀑布都有差不多160英尺高，在它们面前，我们能够真切感受到大自然最为宏伟的手笔，以及水流最为磅礴的力量。
+
+当河流从坚硬河床流经松软河床的时候，也会形成瀑布和激流，比如从陆地河床流经海岸平原时就会如此。相对于坚硬的河床，松软的河床的泥沙土质很容易被水更快地冲刷掉，因此会渐渐形成巨大的落差。这类瀑布还会在冰川地带形成，河流从冰川上流过的时候，冰川向上翘的棱角以及各处不同的硬度，会迫使河水重新寻找新的水道，进而让它们在跌跌撞撞中变成瀑布或者激流。
+
+美国北部地区的大部分瀑布，都是在冰河时代晚期，由于河流河道的重新排布而形成的，而南部地区的瀑布，则多是由海岸平原松软岩层被河流的快速冲刷而形成的。这样说来，就像我们能够听见机床工作时转轴发出的嗡嗡声一样，千载之下，我们也能在瀑布边听见大自然的古老力量所发出的调频声调，正是它们在远古时代参与缔造了地球的岩石表面，更在冰河时代构建了遍布大陆的冰川。
+
+从地球表面的地质演化来看，有瀑布和激流的河流，其实在它们目前的河道上存在的时间并不长。因为如果它们存在时间较长的话，便会将嶙峋的石岸渐渐磨平，进而慢慢平缓下来。因此，瀑布与激流其实也是年轻河流的特征之一。
+
+158．奔腾的河流——雨水降落在平坦的地域之后，会在地面流动得较为缓慢，因而其大部分也会被土壤吸收。在低洼地带会由此而形成池塘与湖泊，缓慢的溪流一路艰难地弯弯扭扭，寻找着前进的方向，最终会在低矮的缓坡下与其他水流汇合，不断壮大的河流便由此逐渐形成了。
+
+一些地方流经湖泊的河流，由于它们会在湖中不断积累沉淀物，长此以往便会将湖泊慢慢填充起来。它们还会不断流经各处的坚硬岩层，并很可能在那里形成瀑布与激流。但只要时间一长，河流都会将这类障碍渐渐抹平，这样的河流因此为被称为年轻的幼年河，因为它们的生命才刚刚开始。美国北部河道曲折且遍布浅窄河谷的红河，以及到处是湖泊和瀑布的尼亚加拉河，都是非常典型的幼年河。
+
+如果新形成的地表形态坡度够大的话，水流在这里的流速会急剧加快，并会更加快速地形成河道。渐渐地，这些不太规则的棱角会慢慢消除，河流也变得越来越直，深度也会逐渐加深。这样，奔流而下的小溪也就逐渐形成了较大的水流。这些水流的源头，就像一些大河的源头一样，会离奔腾的水越来越远，河流最终会自食其力，逐渐收纳更多的细流，形成一个水流的网络。
+
+由于水流的侵蚀作用不断持续，河床岸谷也会不断加深，而之前那些水流渠道不畅的平坦河床便被荒弃在那里了。这在那些比较大的河流之间的地带上就可以看到，尤其是河道尚未完全定型的干流附近。由此整个河床地表都被雕镂得面目全非，到处是沟沟坎坎，这也让水流更容易找到低洼地带。这个时候，干流则很可能会让一路上的瀑布与激流慢慢收敛，逐渐形成更长且更加舒缓的河道。
+
+在这样的状态下，河流不会再像以前那样大力冲刷河床，水流中的杂质会逐渐沉降在坡岸上，它们之前冲走的泥沙土石也会在河谷中不断堆积，这一个阶段的河流就叫做均夷河。河流没有被河谷扭曲的平缓流域越长，形成的瀑布和激流就会越少，其均夷河段也会越多。密苏里州马歇尔附近的密苏里河段，就是均夷河的最好例子。
+
+有时河流还会由于携带了过多的杂志而显得负荷过重，这些杂质要么是从陡峭河岸冲积而来，要么是从支流那里吸纳而来，这样就使得河流在沿途不断沉淀杂质。而这又会导致河道的堵塞，进而让河水流向纷乱的分支网络，形成许多交错的浅流。横跨内布拉斯加的普拉特河，就是这样一条负载过重的河流。
+
+当河道的弯道很大时，其流速最快的地方便在弯道外围，最慢的地方则在弯道内侧。这样一来，当河流处于最大流量通过弯道的时候，其外侧流域会比往常拥有更大的承载力，并会对河岸产生更大的冲击力，��其内侧由于流速减缓，承载力也就会变小，因此其所携带的杂质便会在这里慢慢沉积。像这样的河流，在其均夷河段的一边侵蚀河岸，又在另一边沉积泥沙杂质，就使得其河谷逐渐加宽，让沉淀地区的河岸逐渐形成低缓的平原，由此也常常形成洪水。这样的河流冲积而成的平原地带便叫做河滩。
+
+如果河流的流水一旦开始在河谷之间荡漾，它便会一直持续荡漾下去。因为它一向岸边摇摆，河岸的反作用力便会将它推向对岸，这样它便会在往下流的同时也向对岸流动，这样就会在平坦的河谷地带形成S形的弯曲河道，也叫做曲径，此称谓来源于小亚细亚地区一条河流的名字[2]，在远古时代人们就发现了它独具特色的弯曲河道。当然弯道的大小要和河流本身的大小匹配才行。
+
+像密西西比河这样的大河，一条弯道可能长达数英里，而一条小溪沟的弯道则可能只有几英尺长，一竿子就能撑过去。由于水流的侵蚀和沉淀效应的持续，这些弯道也在不断地改变着自身形状。由于水流不断用强大的力量冲击弯道外侧的河岸，它们自己也不断向外侧延拓，这从不同时期密西西比河流域的地图上就能看出来。
+
+有时候由于弯道太过曲折，使得弯道的顺流区几乎已经靠近逆流区，有时甚至直接横冲了过去，等于让河流放弃了弯曲的河道而在这里直接拉直了自身流域。被舍弃的河道看起开就成了个U形弯，当它们再次被水淹没之后就形成了牛轭湖。有时河流弯道也会被人为地拉直，比如在莱茵河谷的低岸区，这样可以增加大量的耕地。
+
+当河流发大水的时候，水流的速度会因河道堤岸的阻挡而降下来，河水所携带的杂质也会在此沉淀。对水流流速最突出的影响其实发生在河水离开河谷的时候，这也让河水在整个洪水的流经区，将更多的泥沙杂质沉积于此。这样一来河岸会构筑得更快，而河道附近泛流着洪水的岸滩也会形成坡岸，离河道越来越远，而不是靠近。
+
+这样的情形在密西西比河下游地区体现得非常明显，在那里干流在天然的堤岸内流淌，而旁侧支流则是从干流中分流出来，而不是合流进去。有些地方的堤岸高出河滩15到20英尺，这些天然的大堤，就像它们名字显示的那样，让这些支流常常在河滩上流淌很长的距离后才汇入干流。比如亚祖河，沿着河滩流淌了差不多200英里，才汇入密西西比河。人们也常常修筑人工堤岸来防止河流向河滩漫溢，比如密西西比河以及萨克拉门托河下游沿岸的高大堤岸就是如此。
+
+通常主干河流的河滩比支流的河滩要构筑得快得多，因此它常常会在支流汇入干流河道，将支流拦蓄在河滩沿岸，并形成一系列星星点点的湖泊。这样的例子可以在路易斯安那州的红河下游地带清楚地看到。
+
+当一条河流将所有的河谷段渐渐变得面目一致，并能在它的冲积平原上悠然流淌，且所有的支流都能稳定地流经沿途拥有各类地貌的高地的时候，我们则可以说这条河流已经成熟了。这时的它们便能将整个流域的水流，以最高效最便捷的方式运送到水流要去的地方。
+
+当河流经过这一系列的缓慢变化，并已构筑起自己的冲积平原的时候，它的主要工作就成了转运从各条支流中吸纳而来的水流与其他物质。虽然它们现在已经不能再去开拓领地，但它们还依然会继续冲刷流经的土地，但还是依附在分水岭两边按部就班地流淌。只要不发生抬升，在整个流域被削弱成为平原并且其支流网络彻底固定之前，河流都会一如既往地“逝者如斯夫，不舍昼夜”。到这个阶段，河水转运的大部分物质都溶解在水中，其侵蚀效应就已经微乎其微了。它也差不多完成了它的使命，将它必须承载的一切都带到了海洋，它的工作结束了，它，也老了。这时，它的整个流域便已成为一个侵蚀基准面。当一条河流像这样完成了它力所能及的所有转运工作的时候，我们就说它已完成一个侵蚀循环。
+
+159．干燥气候下的河流——在气候非常干燥的地区，河流常常断流，往往只在大雨过后才有短暂的水流。这些河流在到达其他水域之前，一般就已经彻底干枯了，它们的全部水流或者蒸发掉，或者渗进到了干燥的土壤中。因此，它们的水文状态常常显得缺乏规律性。
+
+要是它们的坡岸很陡峭，且没有植被覆盖的话，快速流淌的地表径流就得不到制约，发大水也就是经常的事，进而会严重侵蚀沿途河道，并冲击两边的河岸。它们流经地势平坦的地带之后，会慢慢将河道淤塞，并重新寻找新的流淌路径。因此，一年中大部分时间都是涓涓细流的小河，在大雨之后则可能会形成可怕的洪流，摧毁河岸，并给沿途居住区和农业耕地造成极大破坏，甚至会彻底改变它的整个下游区域。
+
+160．河流进程中的流变——河流在形成自己的流域系统的过程中，常常会发生许多地质流变。同一地区的其他河流会与它产生竞争，这对其本身也会产生不小的影响。那些流程最短或者河床最容易被侵蚀的河流，这时就占据了优势。它们的河谷降低得更快，由此能让支流尽快地流入，并能促使其吸纳水流流域尽快地向高地延伸，这样便能更充分地吸纳该地区的水流资源。
+
+一旦这些尚未排泄掉的水源被河流疏浚以后，它们便会让分水岭的走向不断向外围移动，这样也就会进一步吸纳周围的径流水资源。插图113所显示的便是这样的一个例子，河流A的入海流程很长，而BCD三条小支流的流程则很短，它们因此会拥有更陡峭的河道，并且能更快地侵蚀啃噬它们的河谷，这样一来，它们便会将分水岭EF越来越远地推向河流A。
+
+插图114则显示的是在分水岭被推向河流A的某个阶段，短程河流的吸纳效应可能会让分水岭极其靠近某高地G。插图115则显示的是，分水岭被推到了横跨之前主干河流的地域上的情形，河流B盘踞了高地G的，并完全占据吸纳了它的水源。这种情形叫做夺流或者盗流。
+
+由于河流A失去了部分水流，它对河谷的侵蚀就比以前更缓慢了。不仅如此，再往后河流D又可能盗走其支流H的水流，支流C则可能完全对河流A进行夺流，只留下河流A的下游部分变成一条孤苦伶仃的水道。诸如这类的河流夺流现象非常有趣。
+
+河流有时会因为一些突发原因使得自身杂质增多，这便会导致沉积物对河底的构筑效应显著增强，而侵蚀效应则大为削弱，于是逐渐形成一个被填充起来的河谷。而当这些杂质彻底沉淀以后，河水又开始冲刷填充起来的河谷，河岸的层层印记就显示着河流不同时段的河道状况。
+
+河流的阶段印记，在美国北部地区的河流沿岸非常突出。由于冰川时代晚期的冰川融化，让河水的杂质含量突然增大，河水几乎无法将其全部转运，它们因此便沉积到了河床上。当这些杂质渐渐减少之后，河水又开始冲刷侵蚀河谷，在河岸上留下了层层印记。
+
+当一个地区发生地质抬升时，会对此地河流产生明显影响，让它们开始新的生命循环。抬升的区域可能是整个河流流域，也可能是其流域的某一小部分。如果是整个流域的抬升，则河流的能量就会增加，这样的河流便可以被称作再生河。
+
+流域抬升可能发生在河流生命周期的任何一个阶段，如果发生在河流生命流程的晚期，即河流已经在冲积平原上缓缓流淌的时候，河流则又会重新焕发活力，开始侵蚀它的河谷。但由于这时河流蜿蜒平缓的自然状态已经形成，它现在重新开始雕饰的河床已经不再一如从前，只是一些浅浅的沟渠而已，这也便是嵌入曲流的形成原因。在河流年轻的时候，这里的河谷还是陡峭的V字形，而现在，它们则会成为蜿蜒拖沓平缓滩涂状，岁月留痕，一致如斯。弗吉尼亚的帕尔迈拉地区，就显示了这一地貌特征。在那里，河流都在陡峭的河谷里缓缓流淌，从侧面可以看出这些河谷非常陡峭，几乎呈V字形，在各高地之间蜿蜒盘旋，水流的通道也极其丰富。
+
+如果这样的抬升一直延伸到河流的入海口，则河流就只能在上翘的陆地区域拉长流域，以便能到达海洋。这样一来，曾经在其他地方入海的河流，现在则可能由于流域的变化，汇入其他较大的江河，一起流入大海。
+
+这便是现在密西西比河下游支流上发生的情况，因为那里与墨西哥湾接壤的海岸平原就发生了抬升现象。由于陆地抬升，使得海水后退，让那些之前在海湾拥有各自独立入海口的河流的流程加长，最后汇合在一起成为更大的河流，不但流域整体变大，水流量自然也急剧增加，并通过新的单一入海口流入海洋。世界上许多大河都是这样形成的，因而它们也被叫做接流河。
+
+在插图116中，不同的河流在GH沿线上本来都有各自独立的入海口，而当地质抬升发生以后，新的海岸线则成了IK，E，D，B三条河流这时便会在新的流程中都汇合进河流C中，因为这样可能是它们流向大海最捷近的路径，这样四条河流便都在L处流入大海。河流F和河流A依然保持着它们独立的流域。
+
+当然这类抬升现象也可能只在河流的某一小段流域上发生，这时河流会被阻挡回去，朝着反方向形成内陆湖，并被迫寻找新的流通渠道，这样就形成了逆流河。也有可能由于河流对河谷的冲��力量非常强大，以至于在流域地表抬升之时，依然能够快速地侵蚀河道，进而保持其原来故有的流经渠道，这样就形成了先行河，因为它在地表抬升对它的河道改变发生之前就已经先行固化下来了。哥伦比亚河就在它抬升了差不多数千英尺的河床地表上保持住了它原有的河道。
+
+河床不仅可能发生抬升，也可能发生沉降。当沉降发生时，河流的侵蚀速度会减弱下来，在浅表区域则容易形成沼泽地，而靠近海洋的河段则容易淹没浸入大海之中。
+
+这些淹没的河谷一般都会形成自然地理条件最好的海湾港口，比如旧金山海湾、纳拉甘西特海湾，以及纽约港，都是由淹没的河谷形成的环抱港口。哈德逊河的入海口最早就在长岛地区以东差不多70英里的地方，跟圣劳伦斯河与新斯科舍东部地区的距离差不多。实际上，哈德逊河以北的大西洋海岸上就有许多被淹没的河谷的实例。
+
+一些汇入河流下游的支流，会在河道被淹没之后，直接流入由于河道沉降而形成的海湾，这样的河流叫做分离河。因此，曾经被不同河流妆点得错落有致的海岸地区，在沉降效应发生后，则可能被许多不规则的沟壑与海湾所啃噬，重新变得凌乱。
+
+在特拉华海湾和切萨皮克海湾，早期的居民们相互串门时，用的是小船代步，而不是坐马车，因为这里就是典型的淹没河谷流域，形成了错综复杂的水路河道。这些淹没的河谷让早期的移民能够很方便地进入大陆，造就了当时一些著名的聚居区，比如费城、纽约以及普罗维登斯。
+
+161．冲积锥区与冲积扇区——如果一条河流的河道落差很大，而水中的沉积物又非常多，当这样的河流流经落差突然减小的平坦地域时，水中的杂质会迅速沉积下来，拥堵自身的流域通道，迫使水流重新寻找新的河道，这样就会在平原入口处形成扇形或者锥形的堆积区。如果那里的地质成分很粗糙，堆积物就会形成很陡峭的斜坡；而如果很精细，堆积物则会形成一个平滑的扇形。有时这些扇形群还会一层层地重叠在一起，形成不规则的斜坡平原。
+
+这一类的平原，在许多高山的山脚下很容易找到。如果它们存在于降雨量很少的地区，则会形成许多特殊的地貌以便于雨水灌溉，因为沟壑都很容易从椎体顶端向各方向寻找通路。南加利福尼亚就有许多这类很容易得到雨水灌溉的区域，这都是拜这类冲积锥区所赐。
+
+162．三角洲——当河流流入一个静水区后，其水流会逐渐减缓，并像进入平原地带后那样，不断沉淀水中的杂质。但这里的沉淀过程很平缓，而且沉积物形成的斜坡也没那么陡峭。而这些沉积物本身也一样被流水冲刷，漂流到了离入海口很远的地方。通过这样的过程形成的地域就叫做三角洲，其名字缘于希腊字母Δ，它的形状像三角形[3]。当然很少有真实的三角洲有如此理想的形状，不过大体上都呈三角形。
+
+如果形成三角洲的河流落差很大，它所形成的三角洲则出现一个陡峭的上表面，像一个锥体突出到水面上来。日内瓦湖中的许多三角洲就是此类。如果三角洲的坡岸不很陡峭的话，则会与冲积平原看上去差不多，比如密西西比河的三角洲就是如此。构筑三角洲的物质沉积层，会在河流进入平缓水域的交界处，开始逐步倾斜延伸。在那里，河流会像在冲积锥区里那样，不断堵塞它的水流通路，并迫使自己寻找新的流通路线。在一些面积很大的三角洲上，河流一般都会通过几个不同的流通渠道，或者说通过不同的支流流入海洋。从密西西比河的沿岸地图上便可以见到如此情形。
+
+三角洲一般都土地肥沃，物产丰富。尼罗河三角洲在漫长的历史中，一直便以它的繁荣富庶而享有盛名。不过它们也有它们的问题，那就是常常遭遇大洪水。由于它们都延伸到海洋，当大水来袭时，自然也就完全暴露在滚滚波涛面前，被冲被淹就是免不了的事。
+
+三角洲的堆积速度取决于河水中沉积物的数量多少，以及其所在位置的潮汐及水流状况。在波涛汹涌的大海中，便没有三角洲的立足之地，除非是在一些大的河流附近，比如育空河、黄河、恒河等等。在那些波澜壮阔的海域，三角洲也是可以形成的。密西西比河三角洲就长达200英里，面积也超过12000平方英里。波河也曾在历史上构筑起一个超过14英里长的三角洲，与亚德里亚港口相望，旁边的大海就是因此港口而得名的亚得里亚海。
+
+163．历史与河流——从很早的时间开始，河流就在世界历史的进程中一直扮演着非常重要的角色。首先人类的活动都是从河流沿岸逐渐开始的，因为河流为人类提供给了最为便捷的交通渠道。同时那里土壤肥沃，易于耕种，为早期农业的发展提供给了保障。所以人类的聚集区一般都处在河流沿岸。当战争发生时，河流又会成为天然的屏障，罗马城的台伯河就曾多次发挥此项功用。
+
+在铁路发明以前，河流与湖泊几乎提供了内陆商业贸易流通所需的全部物流渠道。拿美国而言，从纽约到北部地区150多英里畅通无阻的水道，成为从加拿大以及五大湖区直达海洋的重要通路。在法印战争及其后来的大革命中，这条水路便成为兵家必争之地。如果在革命中英国人最终控制了哈德逊河，他们便会将殖民地划江隔开，将北岸据为己有，并还很可能会对各区域殖民地各个击破。
+
+作为美国内陆通向海洋的唯一主干河流，密西西比河已经为我们默默服役很多年了。当西班牙人控制着河口，且康格雷斯的当地人又不能或者不愿意给美国船只通行特权的时候，很长一段时间内，俄亥俄州和密西西比河沿途的殖民者们，要么形成一个独立的共同体为此项权利而战，要么通过某种方式与西班牙结盟以获取通行权，河流水道的重要性由此可见一斑。在南北战争中，为了抢夺这条河的控制权，交战双方都付出了极其惨重的代价。
+
+这些仅仅是美国历史上体现河流重要性的一些事例，而这样的例子在全球每一个国家的历史中，都不胜枚举。
+
+164．美国的大河——美国有四条大河：圣劳伦斯河、密西西比河、哥伦比亚河以及科罗拉多河，它们有的完全处于美国境内，有的则只是部分河道在美国境内。其中前两条河的适航性非常好，是长途水运无以伦比的内陆河道。尽管铁路交通已经十分发达了，但它们至今依然对国家的商业贸易施加着不可估量的影响。
+
+圣劳伦斯河以及它所连带的五大湖区，堪称全世界最完美的内陆水道。从苏必略湖源头到河流入海口，河道长达2400英里，借助于在其急流和瀑布旁边开凿的运河，吃水14英尺的船只便可以在它的怀抱中畅行无阻，直达海洋。穿行于苏必略湖与休伦湖之间的“苏”运河的大型船只，比穿行于苏伊士运河上的还多。每天都有全球最大的物流船队运载着大量的小麦、钢铁、木材，从这里进进出出。
+
+这条默默流淌的苍老河流，在形成它今天优雅舒缓的面目之前，也惯看秋月春风，经历了许多坎坎坷坷。从蒙特利尔到入海口的这一段河道，由于陆地的沉降而被淹没在水域之中，处于高地的河谷又被冰河时代的冰川撑开，其他地方又常常被堤岸所阻拦，因此形成了不少湖泊与瀑布。
+
+密西西比河以及它的支流，加起来为我们提供了差不多9000英里长的畅行水道。这也是世界上最大的水运河系。从落基山西面到阿巴拉契山脉东面，以及从北部边疆到海湾地区，这些支流长长延伸的臂弯，为内陆的工农业产品走向海洋提供了大量低成本高效率的运输渠道。借助于巴拿马运河，这些货物便不用再通过沿途转运的反复装卸，而可以直接从原产地运送到远方。
+
+这条富有贵族气质的河流所经历的风雨沧桑，也正是大陆的地质变迁史的一个缩影侧面。它的成熟河道，在新新旧旧的各类岩石层中迂回穿行，其中一段河道位于一个在冰川时代早期就已形成的峡谷内。那时整个大陆的北部都覆盖在冰川之下，因此在其此段河道上，消融的冰川残渣渐渐堵塞了河道，迫使它漫延开来寻找新的流通路径。在其流程河道的南半段，由于水中大量的泥沙沉积，构筑了大片新的滩涂河岸，河流因此也大大拓宽了自己的领地。
+
+在它自身的历史中，和现在相比，其整体时常发生变化，有大有小。但从总体来看，它始终从我们的陆地心脏波涛滚滚地流向海洋。从它身上，当代的地理学家门也记录下了它不断被自己刷新的地理记录，为地理学的发展也提供了有价值的贡献。我们的地理史、政治史、工业和商业的发展史，都无一例外地与它的滚滚浪涛紧密相连。
+
+尽管哥伦比亚河的适航河段只有500到600英里的距离，以致其商业价值远不如圣劳伦斯河与密西西比河，但是它自身的特点也非常有价值。沿着它的指引，最早的一批移民找到了通往西北领地的路径。在它的低洼河口处，繁荣富庶的华盛顿地区以及俄勒冈州，开始了它们早年的蓬勃发育。在那片广袤的土地上，英国人和美国人也曾作了多年的冤家。
+
+而他们争夺的焦点，其实也是河流本身流域改变的一个直接反映。当它流经一个山脉正在形成的地区时，其流域自然常常被不断抬升的障碍所切断，有时还会在很长一��时间内被阻碍在一个固定地区，进而水面不断上升形成湖泊以克服前进路上的障碍，但其延伸并没有停歇下来。因此，穿过新形成的高山，穿过它们自己构筑的湖泊，穿过它们自己填充的洼地，最终还是一路奔腾到达了海洋。它有一段河道经过大峡谷一样的山谷群，它的主要支流斯内克河，还构筑了一个呈水平状的火成岩河床，整体像个大峡谷一样，一点也不逊色于科罗拉多河的支流。
+
+第四条大河，科罗拉多河，它参与了美国工业和商业的发展，但是实际用处并不大。尽管有差不多400英里的适航河段，但在穿越美国大陆的交通运输方面，它的价值也很小。不过虽然利用价值不大，但是风景却异常迷人。地球上几乎找不到第二条河流像科罗拉多河那样，能让我们见识到大自然如此巧夺天工的魔幻力量。
+
+当它流经由水平状的岩石层构成的贫瘠河段时，会雕镂出一个巨大的水槽出来，并在高耸的岸边岩石上，留下所有参与这项工程的成员的印记。潺潺的小溪、弯弯的小河、间断的激流、呼啸的风沙，以及地球引力的拉扯和岩石的顽强抵抗，让这里的岩层形态与结构产生了极其罕见的雕刻面貌。若你站在它的峡口放眼望去，鬼斧神工的场景一定会给你留下深刻印象，这样的景观在整个地球上也绝无仅有。
+
+不过这里也能发现这些综合力量作用之前的一些状况。在宽阔稳定的河谷形成之前，首先必须在岩石上冲刷出一条狭窄的沟槽，进而在那片巨大的高地上产生条条沟壑，并到一定时间让它们可以形成成熟的河道，将大地切割开来。如果该地区的气候不是特别干燥，河道切割效应的显现则要快得多，河流最终在深深的河谷里稳定流淌之前，切割大地的河道早就水到渠成了。那些奔流而下蜿蜒数百英里的河水，正体现着这里稀少雨量所不能提供的浸润之功。它那气势磅礴、令人胆寒的巨大沟槽，也可算是它年少轻狂时的一个恶作剧吧。
+
+
+
+
+
+总结——就像海浪和洋流对海岸的作用一样，雨水与河流也会不断地冲刷陆地的表面。所有的河流都源自雨水与冰雪的融化，而雨雪也是水分蒸发到空气中又凝结而成的。世界各地的降雨量分布很不均匀，有的地方几乎从来不下雨，而有的地方一年可达50多英尺，在美国的最大降雨量差不多为每年80英寸。
+
+部分雨水在降落地表后很快就蒸发了，另一部分则会在地表流走，还有一部分则会浸入到大地中去，又通过泉水、井水与喷涌的形式返回到地面。在地表流动的这部分水对陆地产生了非常大的影响，它们先形成了很小的溪流，带走了地表的径流水，进而形成大江大河，构建了陆地上的河流系统和贸易渠道，也形成了湖泊水域以抵御洪水，更为巨大的轮船提供了方便的交通要道。
+
+但最重要的影响还是径流水对地表的侵蚀作用。它们从高山上冲刷而下，流经肥沃的田野，形成宽广而渊深的河岸，并填充洼地形成湖泊，还不时冲积出三角洲。它的瀑布与激流也为人们的工农业生产提供了水力资源。
+
+河道不是很宽阔且伴有很多瀑布激流的河流，叫做幼年河；而成熟的河流则拥有平滑的河床，以及平坦宽阔的河谷，当它在这样的河道中悠闲地流淌的时候，侵蚀作用就已经不太明显了。河流在它们S形的曲流河段还会形成冲积平原。有时一条河流会将它的分水岭往后推移得很远，以致其本身最后也转移到另外一条河流的流域中，并通过自身河道汇合在一起。
+
+如果河流的流经区域发生了地质抬升，河流则可能变成再生河，而若它本身已经是一条老态龙钟弯弯曲曲的成熟河流，则可能会形成嵌入曲流。流域的抬升有时还会让河流形成湖泊或者完全变成逆流河，如果它能在抬升之后依然保持原来的河道，则就成了先行河。
+
+如果河流的流经区域发生了地质沉降，河道则可能被海水淹没，而其支流则可能夺流入海。许多河流在它们的平缓水域还会形成三角洲，如果其坡岸不是很陡峭的话，则又会形成冲积扇区。
+
+河流在人类的历史进程中一直扮演着重要角色。从人们对埃及美其名曰——“尼罗河的礼物”，到密西西比河与圣劳伦斯河对美国的移民聚居与国家发展的影响，它们的重要性不言而喻。
+
+
+
+
+
+思考题
+
+
+哪些气候状况会影响一个地区的降雨量？
+
+当雨水降落到地面以后，是什么要素决定了它的流变走向？
+
+进入地下的雨水会在后来有什么样的变化？
+
+什么是间歇喷泉？在哪里能够找到它们？
+
+在你家附件的一次大雨中，注意观察一下雨水最终的流向都有哪些。
+
+什么因素能决定一个湖泊是淡水湖还是咸水湖？湖泊对与人类有哪些益处？
+
+描述一些你所看到的径流水的对陆地的影响。
+
+为什么你所在的城市的一场大雨的全部雨水没有都流向你家附近？这些雨水的“分水岭”在哪里？
+
+河流形成瀑布与激流需要哪些因素？
+
+请描述一下河流的生命周期。
+
+在干旱气候地区的河流会有哪些特性？
+
+请描述一下河流的生命历程中会出现哪些意外流变现象？
+
+冲积锥区和冲积扇区是如何形成的？
+
+河流在哪些地方会形成三角洲？以什么样的方式形成？
+
+河流在人类历史上产生了哪些影响？
+
+请描述一下美国的四大河流的相关情况。
+
+
+
+
+
+译　注
+
+
+[1]约伯，《圣经》中的人物。据记载，约伯是上帝的忠实仆人，以虔诚和忍耐著称，其活动时代大约为公元前16到14世纪。《约伯记》是《希伯来圣经》的第18本书、基督教《旧约圣经•诗歌智慧书》的第一卷，也是《圣经》全书中最古老的书籍之一。
+
+[2]指的是比约克弯河（Büyük Meanders River），曲径的英文单词meanders即来源于此。
+
+[3]三角洲的英语单词Delta就是希腊字母Δ的发音。
+
+
+
+
+
+CHAPTER 11
+
+ICE AND WIND SCULPTURES
+
+冰心风吟
+
+
+165. Snow in Winter. —When the temperature of the air falls below the freezing point, its moisture congeals into little flake-like crystals and falls as snow. Where the cold is continuous for a considerable time, the snow may accumulate in deep layers over the ground. If the heat of the summer is not sufficient to melt all the snow which falls in the winter, then the layers of snow will increase from year to year.
+
+
+
+SNOW CRYSTALS.
+
+
+
+To have this occur the temperature for the whole year need not be below the freezing point, but the heat of the summer must not be sufficient to melt the snow which fell in the colder season. Lofty mountains, even in the tropics, have their upper parts snow-covered. In the far north and the far south the line of perpetual snow falls to sea level, inclosing the mighty expanse of the Arctic and the Antarctic snow fields.
+
+166. Glaciers. —Wherever there is not enough heat in the warm season to melt the snow which accumulates during the cold season, a thick covering of snow and ice will in time be formed. The ice is due to the pressure exerted on the lower layers by the weight of the snow above and to the freezing of the percolating water which comes from the summer melting of the upper snow layers.
+
+Although ice in small pieces is brittle, in great masses it acts somewhat like a thick and viscid liquid. It conforms itself to the surface upon which it lies, and under the pull of gravity or pressure from an accumulating mass behind, slowly moves forward, resembling in some ways thick tar creeping down an incline or spreading out when heaped into a pile. The exact manner of glacial movement, however, is not fully understood.
+
+
+
+SNOW FIELD AT THE HEAD OF A GLACIER.
+
+
+
+In mountain regions where the snow holds over through the summer, the wind-drifts and the snow-slides carry great quantities of snow into the upper valleys, until ever accumulating masses of snow and ice, hundreds of feet thick, are formed. The ice then slowly flows down a valley till a point is reached where the melting at the end is equal to the forward movement. An ice stream of this kind is called a valley glacier or an Alpine glacier, because first studied in the Alps.
+
+
+
+THE GORNER GLACIER.
+
+A typical Alpine Glacier.
+
+
+
+Although the moving ice conforms to the bed over which it passes, it does not yield itself to the irregularities as easily as does water. When it passes through a narrows or over a steep and rough descent, it is broken into long, deep cracks called crevasses. These make travel along glaciers sometimes very dangerous. The travelers are usually tied together with ropes, so that if one of the party slips into a crevasse, the others will be able to hold him up and pull him out.
+
+
+
+CREVASSES IN A GLACIER.
+
+The danger points in travel over glaciers.
+
+
+
+A glacier, like a river, is found to flow fastest near the middle and on top, and slowest at the bottom and on the sides. The rate of motion in the Alpine glaciers varies generally somewhere between 50 feet and one third of a mile in a year, being greatest in the summer and least in the winter.
+
+
+
+THE COE GLACIER, MOUNT HOOD.
+
+
+
+Alpine glaciers are found not only, as the name would indicate, in the Alps, but also in Norway, in the Himalayas, among the higher mountains in the western United States, on Mt. Shasta and in fact wherever the snow accumulates in the mountain valleys year after year.
+
+As glaciers creep down the valleys, dirt and rocks fall upon their edges from the upper valley sides and are borne along upon the ice. If two glaciers unite to form a larger one, the débris upon the two sides which come togetherforms a layer of dirt and rocks along the middle of the larger glacier. At the end of the glacier this material which it has borne along is deposited in irregular piles of rocks and dirt.
+
+The accumulations of débris along the sides are called lateral moraines, those in the middle, medial moraines, and those at the end, terminal moraines. Great boulders may be carried along on the ice for long distances without the edges being worn, since they are carried bodily and not rolled as in streams.
+
+
+
+THE DANA GLACIER IN THE HIGH SIERRAS.
+
+
+
+On the under surface of the glacier, rocks are dragged along firmly frozen into the ice. The weight of the glacier above presses them with tremendous force upon the surface over which the glacier passes. In this way scratches or grooves are made in the bed rock underlying the glacier, as well as upon the boulders themselves. Scratches of this kind are called glacial scratches or striations. They are found abundantly in places that have been glaciated. The rubbing of the rocks upon each other wears them away and grinds them into fine powder called glacial flour, which gives a milky color to the streams flowing from glaciers.
+
+
+
+THE FIESCH GLACIER.
+
+Notice the medial moraine
+
+
+
+If a glacier extends over a region where the surface has been weathered into soil, this fine material may be shoved along under the ice for great distances. When a glacier melts, all the material which it has moved along under it, as well as that which it has carried on its surface or frozen into it, is deposited, forming what is called ground moraine. This is the formation which constitutes the soil of many of our northern states.
+
+The melting of glacial ice, whether by the sun's heat on top, by friction on the bottom or from whatever cause, produces streams which flow in the ice-cut channels under the glacier and emerge in front, laden with rock, glacial flour, and silt. Where the amount of material these streams carry is great, it is usually deposited in an alluvial plain near the end of the glacier.
+
+The length of a glacier does not always remain the same, but increases and decreases slowly in conformity with the amount of snow which falls in successive years. Like rivers, only more slowly, they are subject to the changing conditions of atmospheric precipitation.
+
+
+
+A TERMINAL MORAINE.
+
+
+
+
+
+A BOULDER BORNE ALONG ON TOP OF A GLACIER.
+
+Notice its size as compared to the umbrella.
+
+
+
+
+
+A STONE SCRATCHED BY A GLACIER.
+
+
+
+Wherever glaciers are easily approached they form a great attraction for the summer tourist. The glistening white snow fields circled by the green foliage of the lower slopes, with the glaciers descending in long, white arms down the valleys, pouring out turbulent, milky-colored streams from their lower ends, and here and there covered with boulders and long, dark lines of medial moraines, form a picture which once seen is never forgotten, and the enticement of which lures the traveler again and again to revisit the fascinating scene. The exhilaration of a climb over the pathless ice with the bright summer sun shining upon it, the bracing air, and the ever changing novelty of the surroundings make a summer among the glaciers almost like a visit to a land of enchantment.
+
+
+
+ROCKS POLISHED BY A GLACIER.
+
+The glacier in the background recently extended down over these rocks.
+
+
+
+For this reason Switzerland has become the summer playground of Europe and America, and there the tourist crop is the best crop that the natives raise, and the scenery is more productive than the soil. Norway, with the additional beauty of its fiords, is fast becoming another Mecca of the tourist, and this region, denuded and made barren by the ancient glaciers, is now becoming rich and prosperous because of the glacial remnants still left. The high Sierras are each year enticing greater and greater numbers of travelers to enjoy their wonderful beauties and their invigorating climate.
+
+
+
+MOUNT HOOD.
+
+A view taken in the fall when the mountain is covered with snow, although the surrounding country is still green.
+
+
+
+167. Greenland and the Antarctic Ice Fields. —The whole of the island of Greenland is covered with a deep sheet of ice except a narrow border along a portion of the coast and the part of the island north of 82°, which has little precipitation. The extent of the ice sheet is nearly equal to the area of all the states of the United States east of the Mis sissippi and north of the Ohio. The depth of the ice is not known, but probably in some places is at least several thousand feet. Although along the coast mountains rising from 5000 to 8000 feet are not uncommon, yet in the interior the thickness of the ice is so great that no peaks rise above it.
+
+The surface of the inland ice is a smooth snow plain. Extending from this ice field are huge glaciers having at their ends a thickness of from 1000 to 2000 feet. One of these has a rate of motion of nearly 100 feet per day in summer, the highest rate ever observed in a glacier. The average movement throughout the year on the border of the ice sheet is probably not more than two inches a day.
+
+In the Antarctic region an area vastly greater than Greenland is covered with ice probably of a greater thickness. Although little is known about this ice cap, it is thought by some explorers to be nearly as large as Europe and to rest partly on an Antarctic continent and partly on the sea bottom.
+
+
+
+A VIEW OF THE JUNGFRAU.
+
+Showing the snowy mountains and verdant valleys which make Switzerland the delight of the tourist.
+
+
+
+168. Icebergs.
+
+
+
+
+
+Experiment 129. —Fill a beaker so full of ice water that if any more is added it will run over. Put carefully into the beaker a piece of ice, and catch in another beaker the water which runs out. After all the water which readily overflows has been caught in the second beaker, carefully push the ice into the water till it is entirely submerged, and catch in a third beaker the water which overflows. The experiment must be done with considerable quickness, so that the ice will not melt between the two steps.
+
+The water in the second beaker is equal to the volume of the ice submerged when it floats, and that in the third beaker to the volume of the part out of water when the ice floated. The two together are equal to the whole volume of the ice. Measure in a graduate or weigh on a balance these two volumes of water. (A cubic centimeter of water weighs a gram.) Determine the part of a floating block of ice that is out of water. Would the amount of ice out of water be greater or less if the water were salt? This can be demonstrated by dissolving a considerable quantity of salt in the ice water and very rapidly repeating the experiment.
+
+
+
+AN ICEBERG.
+
+
+
+When a glacier extends out into the sea, the water tends to float the ice. If it extends out into deep enough water, the buoyancy of the water will be sufficient to crack the ice, and the end of the glacier will float off as an iceberg. Glacial ice is about eight ninths under water when it floats.
+
+Icebergs may float for long distances before they melt. In the North Atlantic the steamer routes are changed in the summer months for fear of running into floating bergs. Some of the most appalling disasters of the sea have been due to ships colliding with icebergs. As the berg melts, the rocks and gravel or whatever it may have upon it drop into the sea, so that the waste brought down to the sea by the glacier may be spread over the sea bottom far away from the place where it originated. Much of the knowledge of the geology of the Antarctic continent has been gained from the boulders dredged up at sea.
+
+
+
+BOWLDERS AND SAND LEFT BY A RETREATING GLACIER.
+
+
+
+Although icebergs in the northern seas are sometimes very large, those in the Antarctic region are vastly larger. They have been seen extending above the water 200 or more feet with broad flat tops miles in length. They were indeed huge floating islands of ice.
+
+169. Glacial Formations. —In a region which has been glaciated, peculiar deposits are found which occur nowhere else. Sometimes the end of a glacier remains comparatively stationary over an area for a considerable time, owing to the advance of the ice being just balanced by the melting. In this case, the morainic material which has collected on the top is deposited over the surface, forming irregular heaps of bowlders, gravel and sand, with inclosed hollows between. This material is unstratified and without any uniformity in its arrangement.
+
+
+
+A DRUMLIN.
+
+These low, smooth, rounded hills, like that seen in the background, usually extend nearly north and south.
+
+
+
+When the glacier has retreated, ponds and lakes are formed in the depressions, and streams wander about in the low places between the heaps and receive the overflow of some of the lakes and ponds. Others of these lakes and ponds are so fully inclosed and receive the drainage from so small a surface that not enough water enters to overflow the rim. The arrangement of the streams is unsymmetrical and without order. The whole surface is a hodgepodge of glacially dumped material, a terminal moraine country.
+
+Further back from this morainic dumping ground may be found other kinds of glacially deposited material. If a glacier is pushing along under it a mass of material and it meets some obstruction, or if on account of melting or a decrease in the rate of its flow it has not the power to carry its load, it deposits a part of it. The ice slides over the deposited material and rounds it off, but leaves it as a river leaves its sand bars.
+
+
+
+AN ESKER.
+
+
+
+But this material is not stratified, like the material left in water. When the glacier melts away, these rounded deposition heaps are left as hills of greater or less height. Since the material forming them has been continually brought from the direction from which the ice came, they will have their greatest extension in that direction. Such hills have received the name drumlins.
+
+Where there are stream channels in the under surface of the ice, the streams may aggrade or fill up their beds as rivers do when overloaded. When the glaciers retreat, ice walls which bordered the channels melt away, and the sand and gravel which the streams had laid down along their beds are left as long, irregular ridges, at the end of which sometimes an alluvial fan or delta may be found. Such long ridges are called eskers.
+
+Where the glacier has little load, as near its source, the bed rock is stripped bare, smoothed, polished, and scratched by the material which the ice has scraped over it and borne away. Where the rock is soft, it is scooped out, and hollows are formed, afterward making lakes; and where it is hard, rounded ridges are made.
+
+
+
+GLACIAL ROCK LAKE.
+
+
+
+The valleys through which glaciers go are rounded out and left shaped like a U. If side glaciers join the main glacier, they may not be able to wear down their valleys as fast as the main glacier, so the mouths of these U-shaped valleys may be much higher than the bottom of the main valley. These are called hanging valleys. (See section 144.)
+
+
+
+A U-SHAPED VALLEY IN NORWAY.
+
+This has been rounded out by glaciers. The moisture in the atmosphere makes it necessary to hang the hay up to dry, as seen in this picture.
+
+
+
+The bowlders which are borne along by the ice are deposited irregularly over the surface in all kinds of positions when the ice melts. Some of them are very large and are left perched high up on the hillsides where no other known force besides moving ice could have carried them. These irregularly distributed perched bowlders are called erratics.
+
+170. Glaciated Areas. —Over large areas of what are now the most thickly populated regions of North America and Europe are found widespread formations similar to those described in the preceding paragraphs. The soil throughout is not like that of the underlying rock; it must have been transported. Careful examination of all the surface formations has led geologists to believe that at a former period in the earth's history, perhaps not more than a few thousand years ago, the northern part both of North America and Europe was covered with a thick layer of ice, which after several advances and retreats finally disappeared, leaving the country as we now find it.
+
+
+
+A HANGING VALLEY.
+
+
+
+Although the border to which the ice extended and many of the changes which the ice made in the surface of the country have been carefully studied and mapped, yet the cause of this extension of the ice and the exact time at which it occurred have not yet been determined. Many theories have been brought forward to account for it, but none of them explains all the facts.
+
+That the ice was here seems to be sure, but exactly when or why is unknown. This period when the ice was of great extent is called the Glacial Period. Probably during the earth's history there have been several of these periods, but to the last is due the great changes wrought upon the present surface of the country and upon its plants and animals.
+
+
+
+A HUGE PERCHED BOWLDER.
+
+
+
+171. Glacial Lakes. —In northern countries are found ponds and lakes filling the irregular depressions in the deposit left by the retreating ice. Lakes of another kind are also sometimes formed in glaciated regions. The advancing or retreating ice may happen to make a barrier to the escape of the drainage, and thus may form a lake with an ice dam at one end. The lake will continue to exist only as long as the ice obstructs the drainage.
+
+The Märjelen Lake in Switzerland is a well-known example of this. The Aletsch glacier, the greatest of all the Swiss glaciers, obstructs a lateral valley, forming an ice wall about 150 feet in height, behind which the drainage of the side valley accumulates and forms a lake. Pieces of ice from the glacier fall off into it, forming icebergs which float upon its surface.
+
+Sometimes a crevasse opens in the ice wall, and then the lake quickly drains and floods the valley at the end of the glacier. This formerly caused so much damage that a canal has been constructed across the head of the valley, so that now no great quantity of water can accumulate behind the ice dam. When the lake drains, the bottom is left as a comparatively level, dry plain until the crevasse closes and the lake again forms.
+
+Toward the close of the Glacial Period a vast lake of this kind was formed in the northern part of the United States, the region now drained by the Red River of the North. The slope of the land is here toward the north, and as the ice retreated it formed a barrier to the drainage and dammed back a great sheet of water in front of it. When the ice melted, the lake was drained, leaving the flat fertile plain through which the Red River now flows. The ancient glacial lake has received the name of Lake Agassiz in honor of the great scientist who did so much toward the explanation of glacial phenomena. Glacial lake plains of this kind are found not infrequently. They now form fertile areas of great agricultural value.
+
+
+
+MARJELEN LAKE.
+
+
+
+172. Waterfalls Due to Glaciation. —As the ice spread over the country it filled the river valleys in many places with débris. When the ice melted away, some rivers could no longer find their old courses and were forced to seek new ones. It frequently happened that in deepening these new channels the river came upon buried ledges, and in wearing these down, rapids and falls were developed. In this way many of the water powers of New England and the northern states were produced.
+
+
+
+NIAGARA FALLS.
+
+Due to rearrangement of the drainage by the ice of the Glacial Period.
+
+
+
+The Merrimac furnishes a fine example of water power due to glaciation. The great manufacturing cities of Lowell, Lawrence and Haverhill would not exist had not the river been displaced from its previous channel by the glacial ice, and in developing its new valley come upon ledges which it is now trying to reduce to grade. The Niagara is another notable example of vast water power due to the displacement of drainage by the ice. It is probable that in pre-glacial time there was a river which carried off the drainage of the area now drained by the Niagara, but it did not flow where the Niagara now flows.
+
+173. Glacial Period. —Evidences of an ancient ice covering are seen in North America, even as far south as the Ohio River and extending over a vast region which now enjoys a temperate climate. The greatest ice invasion during this period extended from northern Canada across New England into the sea, across the basins of the Great Lakes and the upper Mississippi valley and across a part of the Missouri valley. It wrapped in its icy mantle almost the entire region between the Ohio and Missouri rivers and the Atlantic Ocean.
+
+
+
+AREA COVERED BY THE ICE OF THE GLACIAL PERIOD.
+
+
+
+Another great ice invasion spread out from the highlands of Scandinavia. As in later days the Norsemen, so at that time the glacial ice overspread northern Europe, carrying Scandinavian bowlders across the Baltic and what is now the basin of the North Sea, forerunners of the Scandinavian sword which in later ages carried devastation to these regions.
+
+The thickness of the ice over these central areas was very great, probably approaching a mile. The pressure on the ground below must have been tremendous and the scouring and erosive effect vast indeed. The soil which previously covered the surface was swept away and borne toward the ice margin, leaving the rocks smoothed and bare.
+
+Prehistoric man probably saw the great ice mantle; he may even have been driven from his hunting grounds by its slow encroachment. His rude stone implements are found mingled with the glacial gravels. But like the spreading ice he has left no record from which the time or cause of the Glacial Period can be determined.
+
+174. Effect of the Glacial Period upon Plants and Animals. —All plants and animals were forced either to migrate before the slowly advancing ice or to suffer extermination. Individual plants, of course, could not move, but as the ice spread toward the south with extreme slowness and with many halts, the plants of colder latitudes found conditions suitable for their growth ever opening toward the south. They were thus induced to spread in that direction, so that at the time of the greatest extension of the ice the plants suitable to a cold climate had penetrated far to the south of their former habitat.
+
+As the ice receded, these cold-loving plants were forced to follow its retreat or to climb the mountains in order to obtain the climate they needed. They did both, so that in areas covered by the ice, plants similar to those of far northern regions are found on the tops of the mountains in middle latitudes. What was true of the plants was true also of the animals. Thus the conditions at the time of the Glacial Period explain some of the most difficult problems in Botany and Zoölogy.
+
+175. Man and the Glacial Period. —Although the Glacial Period occurred thousands of years ago, probably before man was widely spread over the earth's surface, yet its influence upon him has been most marked. His manufacturing depends largely for its power upon the falls and rapids due to the rearrangement which the glaciers made in the drainage. Some of the most fertile soil of middle latitudes is due to the pulverized rock left unexhausted by plant life as the glacier retreated. Since the soil was largely brought from the inhospitable northern regions where man cannot easily exist, it has increased the extent of arable land suitable for his cultivation.
+
+
+
+ELECTRIC PLANT AT NIAGARA.
+
+Man's use of the power which the glaciers arranged for him.
+
+
+
+By the mingling of unweathered yet valuable soil-producing rocks over the surface, the permanence of the soil's fertility has been increased, although the difficulty of tillage is greater. The surface has been beautified by innumerable lakes which furnish man excellent water supplies and restrain the rivers from excessive floods. Glacial lake beds of great productiveness have been formed for his cultivation. Hardy plants from the north have been brought to cover the mountain sides in middle latitudes. In fact, man's whole condition in these latitudes has been modified by the ancient ice invasion.
+
+176. Wind Work. —The wind must be considered among the forces affecting the earth in its relation to man. Whenever the wind blows over dry land, particles of dust and sand are blown away and deposited elsewhere. The interiors of our houses often become covered with dust blown from the dry streets. Even on ships at sea, thousands of miles from land, dust has been collected.
+
+In volcanic eruptions great quantities of dust are thrown into the air and spread broadcast over the earth. On the highest and most remote snow fields particles of this dust have been found. In the great eruption of Krakatoa, dust particles made the complete circuit of the earth, remaining in the air and causing a continuance of red sunsets for months.
+
+Sand is not carried as far as dust, but at times of strong wind it is often borne for long distances. Even houses, trees and stones of considerable size may be lifted and moved by a fierce wind storm. The wind-swept detritus has been known even to obstruct and modify the course of streams. Where the wind blows dust constantly in one direction, deposits of great thickness are sometimes made.
+
+In Kansas and Nebraska there are beds of volcanic dust, reaching in some places to a thickness of more than a score of feet and yet there are no known volcanoes either past or present within hundreds of miles. In China there is a deposit of fine dustlike material, in some places a thousand feet thick, which is thought by some to be wind blown. This forms a very fertile and fine-textured soil and supports a great population. Many of the inhabitants of the region live in caves dug in the steep banks of the streams, so firm and fine textured is the material. Wind deposits of this kind are called loess beds.
+
+177. Wind Erosion. —Not only does the wind take up particles of dust and sand and carry them from one place to another, but it uses these particles to cut and erode obstacles in its path. The artificial sand blast is in common use. In it a stream of sand is driven with great velocity upon an object which it is desired to etch. In nature the same kind of etching is done by the wind-blown sand.
+
+
+
+TREE BEING DUG UP BY THE WIND.
+
+
+
+The glasses in the windows of lighthouses along sandy coasts are sometimes so etched as to lose their transparency. Rocks exposed to the winds are carved and polished; the softer parts are worn away more rapidly than the harder parts, just as in all other forms of erosion. In certain regions where the prevailing winds are in one direction, one side of exposed rocks is found to be polished, while the other sides remain rough.
+
+178. Wind Burying and Exhuming. —In exposed sandy regions where there are strong winds, objects which obstruct the movement of the air cause deposition of the transported sand just as obstructions in flowing water cause sediment to be deposited. And just as sand bars may be deposited by a river and then carried away again, owing to a change in the condition of the river's load, so forests and houses in sandy regions are sometimes buried, to be uncovered again perhaps by a change in the load carried by the wind.
+
+179. Sand Dunes. —Sand-laden wind generally deposits its burden in mounds and ridges called sand dunes. When once a deposition pile begins, it acts as a barrier to the wind and thus causes its own further growth. In great deserts where the wind is generally from one direction these sand dunes sometimes grow to a height of several hundred feet, but usually they are not more than 20 or 30 feet high.
+
+
+
+A FOREST ON CAPE COD.
+
+The trees are being engulfed in wind-blown sand.
+
+
+
+They generally have a gentle slope on the windward side and a steep slope on the leeward side. The sand is continually being swept up the windward side over the crest, thus causing the dune to move forward in the direction in which the prevailing wind blows. (Fig. 117.)
+
+
+
+Fig. 117.
+
+
+
+Dunes make travel difficult, as both in climbing and descending the traveler sinks into the yielding sand. Almost no plant life can find lodgment in these shifting sand piles, so the wind continually finds loose sand on which to act, and a dune country is always a region of shifting sands. As the dunes move in the direction of the prevailing wind they sometimes invade a fertile country, so that it becomes necessary if possible to find a way to check their movement. This has been done in some places by planting certain kinds of grasses capable of growing in the sand and thus protecting the sand particles from the action of the wind.
+
+Sand dunes are found along almost all low sandy coasts, and they render difficult the building and maintenance of roads and railroads to many beach towns.
+
+
+
+
+
+Summary. —Besides the sculpturing of waves and rivers, two other agents of erosion are glaciers and winds. Alpine glaciers are formed by huge masses of ice and snow crowding into mountain valleys where the snow never melts entirely. Glaciers are intersected by great cracks called crevasses and they carry accumulations of débris called moraines. Icebergs are the ends of glaciers which have broken off.
+
+The northern part of America was once covered by a huge glacier at a time which we call the Glacial Period. This glaciation has had a great effect upon the region covered. Glaciers smooth out irregularities in the surface, grind rocks, transport soil and bowlders, dam lakes, force rivers to seek new channels and on account of this create waterfalls. Thus the glaciers of the Glacial Period have had a great influence upon the conditions of life.
+
+
+
+QUARRYING A SAND DUNE TO MAKE BRICK.
+
+
+
+The winds not only blow the clouds about over the land, but they bear dust and sand with which they sculpture and erode rocks and cliffs. They also build up sand dunes and by moving them over the surface of the land sometimes destroy forests and fields.
+
+
+
+
+
+QUESTIONS
+
+
+How are glaciers formed? Where are they found? What do they do?
+
+How large and how thick is the Greenland icefield?
+
+How are icebergs formed? Why are they dangerous?
+
+Describe the different kinds of deposits and formations due to glaciers.
+
+How are glacial lakes such as Lake Agassiz formed? Why are they fertile when drained?
+
+Some waterfalls are due to glaciation. Why?
+
+What was the extent of the North American ice sheet during the Glacial Period?
+
+What has been the effect of the glacial period upon plants, animals and man?
+
+In what ways has the wind modified the surface of the earth?
+
+How are sand dunes formed? Why are they destructive to plant life?
+
+
+
+
+
+【中文阅读】
+
+
+165．冬天的雪——当空气的温度降到零度以下，其中的水蒸气就会凝结成小片的冰晶，然后翩然飘落，成为雪花。如果严寒持续的时间较长，地面上便会铺上一层厚厚的积雪。倘若来年夏天的热量还不足以完全将上一个冬天的雪完全融化的话，积雪就会一年一年逐渐增加了。
+
+这样的情况并不需要整年的温度都必须在零度以下，只要夏天的热量不足以将寒冷季节的降雪全部融化成水就行。在一些高山上，即使是热带地区的高山，山顶也长年覆盖着白雪。在地球南北极附近的雪线以内，海上几乎终年飘着白雪。在极地的陆地上，更是一片茫茫雪原。
+
+166．冰川——如果温暖季节的热量不能将寒冷季节的积雪全部融化的话，一个厚厚的冰雪覆盖层便会在地表形成。底层的雪，由于上层的压力所致，渐渐会结成冰。而在夏天，上层的雪融化成水后，渗透到底层也会变成冰。
+
+尽管一小片冰看起来柔弱易碎，但当它们大量聚集的时候，便会显现出厚厚的半流体特性。因此它会在引力与上层积雪的压力作用下，紧贴着地表的形态，贴合地铺漫开来，并慢慢向前流动，类似于粘粘的沥青在斜面上缓缓向下爬行，或者满满的一堆沥青慢慢流淌出来的情形。不过，冰川的准确运动方式，到目前我们知道得还不太确切。
+
+在夏天也覆盖着厚厚白雪的高山上，吹流和雪崩会将大量的积雪带到高地的溪谷中，在那里堆积起厚厚的冰雪层，有时达数百英尺之厚。然后它们又会慢慢顺着山谷向下流动，直到融化的冰雪可以自由向前奔流。这样的冰封溪流也叫做山谷冰川，或者高山冰川，因为最早研究它们就是从阿尔卑斯山开始的。
+
+尽管移动的冰层会紧密贴合它经过的表面，但也并不像水流那样轻松自如地随弯转弯。当它们经过一个狭窄的，或者陡峭的崎岖斜面时，便会爆裂开来，形成一条又深又长的大裂缝，叫做破口。这就让移动中的冰川常常也显得非常危险，因此攀登冰山的人们常常用绳子将他们自己连在一起，这样如果其中一个人掉进了冰川破口，其他人还可以将他拉出来。
+
+我们还发现冰川也会像河流一样，在它中间和顶部流动得最快，而在靠近岸边的地方流动得最慢。高山冰川的移动速度也有快有慢，有的一年只移动50英尺，有的则可以移动三分之一英里，但都是在夏天移动得最快，在冬天最慢。
+
+高山冰川也不仅仅像它的名字一样，只在阿尔卑斯山存在[1]。在挪威、在喜马拉雅山脉、在美国西部的山地、在沙斯塔山，以及所有长年积雪的山上，都有高山冰川的存在。随着冰川滑向溪谷，一些土石也跟着从高处的谷岸边落到冰川边缘，然后被它们带到冰面上。如果两座冰川结合起来形成一个更大的冰川，这些落在冰川边缘的残渣碎片就会在这个大的冰川中间形成一个土石夹层，在这个冰川的演化后期，这些土石杂质便会随着冰川的移动慢慢沉淀并且沿路堆积。
+
+这些沿着冰川谷岸不断堆积的土石叫做冰川侧碛，处在中间地带的则叫做中碛，处于尾段的叫做终碛。巨大的岩石因此也可以被完好无损地在冰川中转运很远的距离，因为它们是被冰川整体移动，而不像在��流里面那样翻滚前行。
+
+在冰川的下表面上，部分岩石也由于冰川的拖拽而被冻结到冰川之中，上面冰川巨大重量的压力也会作用到冰川经过的岩石表面上。这样一来，冰川底部便会对岩床造成许多刮痕，冰川里面的岩石的下表面上一样也会产生类似的刮槽。这样的刮痕叫做冰川刮痕，也叫冰川条纹。有冰川存在的地方，我们都能看见大量的类似刮痕。同时在刮擦的过程中，岩石表面也会自我损耗，最终被强大的碾压力磨成粉末，叫做冰川粉粒，就是它们让那些从冰川区域流出的小河呈现出了乳白色。
+
+如果冰川的延展地表上的岩石风化成了土壤，这些土石颗粒便会在冰川的冰面下堆积很远的距离。当冰川融化的时候，其表面以下的土石颗粒，以及被它卷裹而去或者冻结到冰块中的土石颗粒，就会慢慢沉淀下来，形成我们所称的底碛。美国北部许多省份的土壤都是通过这样形成的。
+
+冰川的融化，不管是由于顶部的太阳照射，还是底部的表面摩擦，或者其他什么原因，都会形成许多溪流，从冰川底部的冰槽中流出来，夹杂着岩石、冰川粉粒、泥沙等等呈现在我们眼前。如果这些杂质的数量很多的话，它们往往会在冰川尽头处的冲积平原上慢慢沉淀下来。
+
+冰川的整体长度并不是一成不变的，它会慢慢随着逐年降雪量的多少而按比例增减。就像河流一样，会随着大气降水量的变化而变化，当然冰川的变化比它来得慢多了。
+
+一些我们很容易到达的分布有大量冰川的地方，到了夏天自然成了人们避暑的胜地。在那里，皑皑白雪被山坡上郁郁葱葱的绿叶簇拥着，冰川则将她雪白如玉的臂弯缓缓延展到山谷脚下，从那里又流出股股乳白色的欢闹溪流，沿岸到处是圆滑的碎石，再映衬着一条长长的暗褐色土石分布带，一幅迷人的画面已然形成。看过一眼，终生难忘。许多人都会屡次三番地来到这里，就为了欣赏那令人难以忘怀的绝美风景。当夏天烈日高照之时，攀行在这人迹罕至的冰川之上，呼吸着沁人心脾的清新空气，沉浸在新奇多变的周遭环境之中，这样的愉快经历，会让你的夏天变得别具一格，似乎来到了一片梦幻般的神秘土地。
+
+正是因为这个原因，瑞士便几乎成了欧洲人和美国人的夏季游乐园，那里的旅游收入也是他们最丰厚的经济收成，天然的美景比土壤更有生产力。而挪威，由于他们还有美丽的海湾与海景，很快也成了旅游者的麦加圣地，这片曾经被远古冰川剥蚀摧残进而变得荒芜贫瘠的土地，如今早已繁荣富庶，其原因正是冰川所留下的宝贵遗产。著名的塞拉斯高地每年正吸引着越来越多的游客到此欣赏美景，并亲身体验那让人激动振奋的攀越经历。
+
+167．格陵兰岛以及南极冰原——在整个格陵兰岛上，除了部分海岸线以及北纬82°附近地带没有冰雪堆积以外，其余所有地区全都覆盖着厚厚的冰雪。其覆盖面积非常大，差不多与美国密西西比河以东到俄亥俄北部之间所有省份的总面积旗鼓相当。而且冰雪的覆盖深度至今也无人知晓[2]，但估计一些地方至少能达到数千英尺。尽管在海岸线上有海拔5000到8000英尺的高山，但在内陆地区，由于冰盖太厚，因此没有任何高地突出于茫茫冰原。
+
+内陆冰盖的表面是一个覆盖着白雪的广袤平原，从这个冰原周围延伸出去的就是巨大冰川群，它们在末端的冰盖厚度也能达到1000到2000英尺。其中有的冰川在夏天可以一天移动100英尺，这是目前观察到的冰川最快运动速度。不过整体冰川在其边缘上体现出来的全年平均运动速度，也就每天一两英寸的样子。
+
+在南极地区，冰盖的覆盖面积和深度都远胜于格陵兰岛，目前人们对南极冰盖的了解也还远远不够[3]。据某些探险家估算，其冰盖面积与欧洲陆地面积差不多大小，当然这些冰盖有些在南极大陆上，有些还存在于周围附近的海中。
+
+168．冰山
+
+
+
+
+
+实验129：在一只烧杯中加满冰水，满到不能再加一滴为止。然后再向里面小心地放入一小块冰块，并将溢出的冰水全部收集到第二只烧杯中。收集完毕后，然将冰块按入水中，使其全部被淹没，并将这时溢出的水全部收集到第三只烧杯中。这个实验必须以非常快的速度完成，以保证在这两个步骤之间冰块不会融化。
+
+第二只烧杯中的水的体积就是冰块浮在水上时，其淹没在水中的部分所占的体积。第三只烧杯中的水的体积就是冰块浮在水上时，暴露在水面以上的部分所占的体积。因此两部分加起来，就是整个冰块的体积。在量杯里或者天平秤上称量这两部分溢出冰水的体积（1立方厘米冰水的质量为1克），并确定浮冰暴露在水面以外部分的体积。如果冰水是盐水，上面的数据会变大还是变小呢？这时可以将一些盐溶解在冰水中重复完成上面的实验。
+
+
+
+
+
+当冰川延伸到海上之后，海水便会将冰体浮在水面上。如果它们延伸到了海水很深的地方，海水的巨大浮力便会让冰体撕裂。这样一来，冰川的尾端冰体便会浮到水面，形成冰山。当冰山浮在水面时，差不多8/9的体积都是淹没在水面以下的。
+
+在它们彻底融化之前，冰山一般会浮游很远的距离。夏季的时候，北大西洋的轮船航线常常变来变去，就是因为怕碰上浮游冰山，曾经一些可怕的航海大灾难大多都是因为轮船撞上了冰山而导致的。随着冰山的融化，它里面所包含的岩石、砂土等等杂质便会沉入海中，这样的话，这些杂质便会被冰山从它们原来诞生的地方带到遥远的大海当中。具有南极洲地质状况的石头，就常常从海洋各处被打捞上来。
+
+北半球海洋的冰山已经很大了，而南极洲附件的冰山还要大得多。它们能在水面上形成200多英尺的高度，其宽大的顶部更是长达数英里，的确是名副其实的巨大浮岛。
+
+169．冰川的形成物——当一个地方有冰川形成，那里便会出现特殊的物质沉积现象，这在其他地方是不会发生的。由于前段冰川的慢慢融化，冰川的尾段因此有时会在一个地区保留很长的时间。在这样的情况下，冰川中的冰碛杂质便会在冰面上逐渐沉积，形成不规则的碎石砂土堆，并且中间随意散布着中空的凹槽。这些沉积物分布比较混乱，没有明显的分层，也几乎没有什么一致性。
+
+当冰川彻底消逝以后，它当初所在的洼地便会形成池塘与湖泊，一些溪流也会在这些低洼地带蜿蜒徘徊，池塘与湖泊的溢出水也会顺着小溪叮叮咚咚地流向远方。还有些池塘湖泊被包围得很严实，只能收纳很少的水流进入到里面，因此水也就流不到边缘外面来。因此这些溪流的分布也非常不对称，显得杂乱无序。整个表面几乎成了冰川废弃物的大杂烩，也就是一个终碛的集散地。
+
+在这个岩砾堆积场后面，还会发现其他种类的冰川沉积物。当一座冰川正不断沿着它的运动轨迹推进它下面的土石杂质时候，若它遇到障碍物，或者由于其自身融化以及流速降低而导致无法再继续承载之前的土石量的话，这些杂质也会因此而部分沉积下来。从它们身上碾压而过的冰体会将它们的棱角磨尽，像河流冲击出沙洲那样，也会让它们变成滩涂。
+
+但这些杂质也不会像水中沉积物那样分层分布。当冰川彻底融化，这些饱经磨砺的沉积物会形成高高矮矮的小山包，且由于这些沉积物在冰川运行的一路上都会形成，因此沿着它们来去的方向，这些沉积物会分布很长的距离，形成延绵的小山，也叫做冰丘。
+
+只要在冰川冰面以下存在溪流，这些溪流便会像河流超载那样，不断增加流量，将沟谷填得满满的。当冰川逐渐消退，沟谷沿岸形成的冰墙也会慢慢融化，水流冲积而下的土石砂砾也就会沿着河床一路杂乱堆积，最后往往形成不规则的小山脊，出现在流域尾端的冲积扇区或者三角洲上，这样的山脊也叫做蛇形丘。
+
+当冰川承载的杂质还很少的时候，比如在其源头附近，河床岩石便会裸露在表面，并且由于水流的冲刷显得非常光滑，当然也会有一些冰砾会对它们不断刮擦，轻轻地将其逐渐消磨。因此在较软的岩石区，谷底便会形成掏空的空腔，并最终形成湖泊；在坚硬的岩石区，则会最终形成圆滑的山脊。
+
+如此一来，冰川所行经过的河谷便会由于均匀的磨损而成U字形谷面，如果这时岸边的冰川和主冰川连成了一体，它们便不能像主冰川那样很快地冲蚀河谷，这样就会导致这些U形谷的出口的谷底高度，要高于其主流河段的谷底高度，进而形成悬谷。
+
+当冰体融化的时候，被它们所沿途携带的土石也会到处沉积下来，有的石头还很大，且被高高地安放在岸边高地上，周围也没什么其他力量可以移动它们，这些被冲出来又被高高在上安置在高岸上的不规则石头也叫做漂砾。
+
+170．冰川地区——在北美洲和欧洲的大面积人口稠密地区，人们发现了分布广泛的我们在刚才段落里提到的地质构造。那里的地表土壤与深层岩石的成分构造及年代都完全不一致，因此一定是从别处转运而来。经过仔细的研究与分析，地质学家们相信在人类历史早期，可能在数千年前吧，欧洲和北美洲的北部地区都覆盖着厚厚的冰层，经过漫长的地质变迁它们慢慢消失��，最终留下了我们今天的家园。
+
+尽管人们正在加紧对这一曾经冰雪覆盖地区进行研究，比如其冰盖的延伸边界，它在地表所发生的变化，但这一系列变化所发生的具体时间目前还没有定论。虽然归于这一现象的各种理论不断地被提出来，但还没有一种能对所有的事实进行圆满解释。
+
+冰盖在这些地方存在过是毫无疑问的，但是为什么会存在，存在了多久却还是个问题。让冰盖在陆地上广泛延伸的地质时期叫做冰河时期。在地球的不同历史阶段，可能已经出现过好几次这样的时期，但只有最后这一次给地球表面带来了如此巨大的改变，动植物纷纷出现，人类文明得以展开。
+
+171．冰川湖——处于寒带附近的国家会在冰盖消退的地方发现许多池塘与湖泊，这就是冰川地区常常能见到的另一种湖泊。其形成便是基于冰川，当水流经过这个地区的时候，高高低低的冰川会形成天然的障碍物，将水流隔断，进而形成由冰岸在末端围拢而成的湖泊，只要阻碍水流的冰体不消退，这些湖泊就会一直存在。
+
+瑞士著名的马杰伦湖就是这种湖泊的典型例子，瑞士最大的冰川——马莱奇冰川，阻碍了一个横向的河谷，进而形成了一面高达150英尺的冰墙。在它的背后，水流不断积累便形成了一个湖泊。冰川上的冰，不断掉入湖中，在湖面又形成了冰山。
+
+有时这面冰墙上会出现裂缝，这时湖水就会出现急速排泄，在冰岸的末端形成巨大洪流。这种破坏力量逐渐又会在河谷开口处形成新的水道，冰岸中的水便慢慢流失，不能再在里面大量积存了。湖水排掉之后，暴露的湖底渐渐就成了一个非常平整而干燥的平原；而让冰墙在此封冻之后，湖泊则又慢慢恢复了。
+
+在冰河时期快要结束的时候，一个很大的这类湖泊在美国北部地区形成了，但其湖水已经被现在北部的红河排掉了。那里的坡岸依然缓缓地滑向北方，随着冰川消退，这里便也成了水流的障碍栅栏，常常在里面关住一大塘水。随着冰雪融化，这个湖又慢慢被排泄掉，留下了如今红河流经的富饶平原。人们给这个远古冰川湖起了个名字叫做阿加西湖，以纪念这位为了解释冰川现象付出了巨大努力的伟大科学家。这类冰川湖所形成的平原并不多见，现在它们早已成了肥沃的农业经济区。
+
+172．冰川瀑布——随着冰川冰的到处漂流，在许多偏僻的河谷里都能看见它们支离破碎的身影，到处填塞着河道。当这些冰全部融化之后，河流也再也找不到它们之前的流通水道，只有再重新寻找。这也导致下列的情况时常发生，即在这类不断加深的河沟之中，河流常常遇到之前被土石掩埋的暗礁，并又将它们冲走，急流和瀑布也会因此而形成。在新英格兰和一些北部省份，人们便常常利用这类水利资源来为生活生产提供方便。
+
+梅里马克河就是这样由于冰川而产生巨大水利资源的例子，要不是它的冰体让它脱离原来的河道而进入现在的河道，并减缓了沿岸的坡度的话，其流域沿线上像洛厄尔、劳伦斯和黑弗里尔这样的工业城市也就不会存在了。尼亚加拉河则是另一个由于冰川冰对水量的影响而产生效益的著名范例，很可能在前冰河时期那里就存在一条河流，它的水流来源和现在的尼亚加拉河差不多，只是没有在如今的河道里流淌而已。
+
+173．冰河时期——在北美洲，人们已经找到一些关于远古冰川覆盖区域的证据，其覆盖面积很广，南至俄亥俄州，并且延伸到了如今气候温润的大部分地区。在这期间冰盖延伸最广的时候，从加拿大北部开始，横跨新英格兰直到大海，并穿越五大湖区、密西西比河上游以及密苏里峡谷，整个俄亥俄州、密苏里河及大西洋之间的区域，全被掩盖在冰川的冰盖之下。
+
+另一处冰川的巨大入侵则发生在斯堪的纳维亚高地。就像后来的北欧人一样，那时的冰川冰也还处在欧洲北部，但慢慢地将斯堪的纳维亚的巨石带到了波罗的海，也就是现在的北海平原。这几乎成了某种预兆，斯堪的纳维亚的武力战争在后来的历史中给这一地区带来了巨大的破坏。
+
+这一中部地区上所覆盖的冰川冰，达到了极深的厚度，差不多接近一英里，因此地表所承受的压力就是极其巨大的，而侵蚀磨损与挤压变形效应，自然也就在所难免了。之前的地表土壤，这时就被挤压到冰川边缘，只留下了光秃秃的岩石，依然被压在冰盖之下。
+
+史前人类可能就见到过这些巨大的冰盖，他们还可能在茫茫冰原上狩猎，因为我们曾发现过他们某些简陋工具就是由冰川岩石组成的。但就像这些冰体���样，他们也没有给我们留下任何历史记录，也没有关于冰河时期形成过程的相关信息。
+
+174．冰河时期对对动植物的影响——所有的动植物在冰河时期都面临选择，要么在冰川慢慢来袭之前撤离，要么留下来接受考验。单独的植物个体当然不能移动，但当冰盖走走停停，以极慢的速度向南移动的时候，寒带的植物便会感受到南边更舒适的生长环境，进而开始向南方生长，并朝着南方传播它们的种子。因此当巨大的冰川冰逐渐南下之时，曾经在寒带生长的植物也早已适应了南方的气候，远离故土去到了它们新的生长乐园。
+
+当冰川消退的时候，一些喜寒植物也便追随冰川而去，或者只有向高山攀爬以找到它们喜欢的气候。实际上很多植物都是二者兼而有之，这就是为什么在冰川覆盖过的地区，它们一些高山上的植物与很遥远的北方寒冷地区的植物很相似的原因。这样的情况对动物也是一样，因此冰河时期的一些地理气候状况，解释了很多植物学和动物学上的疑难问题。
+
+175．人类与冰河时期——尽管冰河时期处于数万年前，甚至早于人类在地球表面广泛生息繁衍，但它对我们人类的影响却是非常显著的。人类的早期制造业在很大程度上要依靠激流与瀑布的水力资源，而这就是冰川对河流系统影响的结果。一些中纬度地区的肥沃土壤，就是由冰川消融时植物尚未完全吸收养分的岩石碎末构成的。由于这些土壤大多来自人类不容易居住的北方地区，这样便无形中丰富了耕地的土壤成分，让土地更加容易耕种。
+
+另一些未被风化的岩土颗粒混合物广泛地分布在地表，虽然这让农业耕种变得困难了一些，但是却让土壤的肥沃持续性得到了增强。地表的湖泊也给人们带来了无数美景，它不但带来了天然的水源，而且限制了河流的过度泛滥。极其肥沃的冰川湖床也成了人们最佳的农耕场所，北方的耐寒植物也为中纬度的高山披上了生机勃勃的外衣。实际上，人们在这一地区的所有居住环境，都在一定程度上受惠于远古的冰川。
+
+176．风之流转——在影响地球的自然力量中，风与人类有着紧密的关系。当风在干燥地区吹过，尘土和沙粒也往往被它卷裹然后落到别的地方。我们家里的东西就常常被一层灰尘覆盖，它们就是被风从干燥的街面上带来的。即使在大海航行的轮船上，虽然远隔陆地数千英里，依然逃脱不了灰尘的侵袭。
+
+火山喷发时，大量的火山灰被抛向空中，然后会随着风飘落到很远的地方。在最高的山峰和最遥远的雪原上，我们依然能找到火山灰的身影。在喀拉喀托火山的一次大喷发后，火山灰几乎完全笼罩了地球一周，停落在空气中的火山灰让红色落日现象持续了好几个月才慢慢消失。
+
+砂土没有灰尘飘得远，但在强风作用下也能被吹到很远的地方。即使像房屋、树木、石头这样的大体积物体，有时也会被巨大的龙卷风吹到空中。被风卷起的砂砾土石也会对风的路径和流转产生阻碍影响，当风始终朝着一个方向吹刮尘土时，厚厚的沉积物就会慢慢形成。
+
+在堪萨斯州和内布拉斯加州，有许多火山灰形成的岩床，其厚度有的地方能达到二十多英尺，且在数百英里的范围内，还存在着数不清的活火山与死火山。在中国有一个类似灰尘土壤的沉积层，有的地方其深度达一千英尺，一般认为这就是一种风积土壤层。它形成了非常肥沃且层次分明的土地，让大量的人口可以在此繁衍生息，这一地带的许多居民都住在沿着坡谷挖出来的窑洞里，这都得缘于这种土质非常坚固及其层次分明的结构。这种风积土层也被叫做黄土层。
+
+177．风的侵蚀作用——风不仅能将沙土从一个地方吹到另一个地方，而且还能用这些沙土颗粒来侵蚀沿途的所有物体。现在我们常常能见到人工喷沙，就是一股沙土流以极高的速度被射向需要蚀刻的物体上。在大自然中，完成这样相同的任务的，就是风沙了。
+
+位于沙岸边的房屋的玻璃窗，常常就因为长久地被风沙侵蚀而变得模糊。裸露在风中的岩石也会被风沙侵蚀，且柔软的部分比坚硬的部分侵蚀得更快，这跟其他种类的侵蚀效应是一样的。在一些盛行风只吹往一个方向的地区，那里的岩石的迎风面往往被吹刮得很光滑，而其背风面却依然非常粗糙。
+
+178．风的掩藏与挖掘效应——在常刮强风的沙地上，阻碍风的物体会形成一堆沉积的沙土，就像阻碍水流的物体会导致水中杂质的沉积一样。并且就像河流能够冲积出沙洲，且由于水流的杂质量变化又会将沙洲冲走一样，沙地中的树林���房屋有时也会被沙土掩埋，但也会由于风沙中的沙土量的变化而重见天日。
+
+179．沙丘——被大风卷裹的沙土常常会形成土丘和山脊形状的沉积物堆积，叫做沙丘。当这样的堆积体一旦形成，它便会成为阻碍风流动的障碍物，进而又会更快地堆积变大。在一些只刮一个风向的风的大沙漠中，这些沙丘有时能长到数百英尺高，但一般而言，大多也就二三十英尺。
+
+它们一般都会在迎风面形成一个缓坡，而在背风面形成陡坡。且迎风面上的沙粒会不断地被吹过沙丘顶部，这样就导致沙丘会向风吹去的方向不断前移。
+
+沙丘会让人们徒步经过这里变得困难，不论向上爬还是向下走都很容易陷进绵软的沙层中去，在这些移动的沙堆上也几乎找不到任何植物，因此风会让这些沙土不断变得松散，这也导致了沙丘地区往往到处是流沙。由于沙丘会朝着风向不断移动，故而有时也会侵入到富饶的地区，因此很有必要在条件允许的情况下，找到一个阻止它们前进的办法。这个在有些地方已经办到了，就是在沙丘上种植一些能在沙土中存活的草皮，这样沙粒就不会被风四处吹散。
+
+在一些低洼的沙岸区都有不少的沙丘，它们的存在给通往海滩城镇的公路铁路建设增加了很多困难。
+
+
+
+
+
+总结——除了潮汐与河流以外，冰川和风也具有侵蚀作用。高山冰川是由大量的冰雪挤压而成，它们密集地分布在山谷中，长年累月都不会融化。冰川会自我横断形成破口，并且能够转运并积累冰碛石残片。冰山是冰川末端碎裂后的产物。
+
+美国北部在冰河时期曾被一个巨大冰川的冰盖所覆盖，这一冰川效应对该地区产生了非常大的影响。冰川能够让不规则的地表变得光滑，它们还能磨碎岩石、转运土壤和石头、围岸成湖、让河流改道，并通过这些方式形成瀑布。因此，冰河时期的冰川对地球的生命环境产生了显著影响。
+
+风不仅能吹散天上的流云，还能扬起沙土，并以此雕镂蚀刻岩石峭壁。它同时还能形成沙丘，并让它们在地表不断前移，有时因此而毁坏森林与农田。
+
+
+
+
+
+思考题
+
+
+冰川是如何形成的？一般能在哪里找到它们？它们会有哪些变化？
+
+格陵兰冰原有多大？冰层有多厚？
+
+冰山是如何形成的？为什么它们很危险？
+
+描述一下冰川的几种不同生成物和沉积物。
+
+像阿加西湖这样的冰川湖是如何形成的？为什么它们排干湖水以后，湖底土壤会非常肥沃？
+
+为什么冰川也能形成一些瀑布呢？
+
+冰河时期北美洲的冰原面积状况是怎样的？
+
+冰河时代对动植物及人类有哪些影响？
+
+风通过哪些途径改变了地球表面？
+
+沙丘是如何形成的？为什么它对植物有毁灭性的损害？
+
+
+
+
+
+译　注
+
+
+[1]高山冰川的英语词汇为Alpine Glaciers，直译过来便为“阿尔卑斯山的冰川”。
+
+[2]美国科学家在2009年给出的格陵兰岛冰川（或称冰盖）的相关最新数据为：总面积达180万平方公里，冰层平均厚度达到2300米，与南极大陆冰盖的平均厚度差不多。格陵兰岛所含有的冰雪总量为300万立方公里，占全球淡水总量的5.4%，如果格陵兰岛的冰雪全部消融，全球海平面将上升7.5米。
+
+[3]目前关于南极冰盖的最新数据为：面积约1398万平方千米，约占南极大陆面积的98%。平均厚度为2000至2500米，最大厚度达4000多米。冰盖的总体积约2450万立方千米，占世界陆地冰量的90%，淡水总量的70%。如果全部融化，全球洋面将升高60米，地球上的陆地面积将因此而缩小2000万平方千米。这将会给世界上人口相对稠密的低海拔地区造成巨大灾难。
+
+
+
+
+
+CHAPTER 12
+
+LOW AREAS OF THE EARTH
+
+地球上的低地
+
+
+180. Level Areas. —At different places on the earth's surface there are broad extents of nearly level land. Here the drainage is often poorly developed, and there are slight depressions often of considerable area. After a rainfall the shallow water stands in these depressions until it evaporates or sinks into the ground. In the parts where the drainage has been developed, the streams flow with slow currents in channels of little depth.
+
+
+
+A LEVEL, POORLY DRAINED AREA.
+
+Such an area is called young.
+
+
+
+When excavations are made, the rock beneath the soil is often found in horizontal or almost horizontal layers. Where the elevation of these areas is considerable, the streams may have deep gorges and the surface may be well dissected. Where these level areas are low, they are called plains, and where high, especially if surrounded by steeply descending sides, they are called plateaus. A good example of the low, level area is the plain of northern Russia and of the high area, the Arizona Plateau through which flows the famous Colorado River.
+
+
+
+
+
+181. Coastal Plains.
+
+
+
+
+
+Experiment 130. —Fill a tall glass jar nearly full of water. Pour into this very slowly a mixture of sand and finely pulverized clay. Note the effect upon the color of the water. Allow the water to stand for several days and then examine the deposition on the bottom of the jar. Are the sand and clay now mixed as they were when poured into the jar? What effect has the water had upon the mixture?
+
+
+
+
+
+We have already seen that the surface of the earth is not stable, but is subject to movements. If the land bordering a coast rises or the bottom of the ocean is depressed, it causes the water to withdraw from the land, and a strip of what was formerly sea bottom is changed into dry land.
+
+This new area is composed of clays, sands and gravels, often containing shells similar to those found on the neighboring shores. The surface is comparatively flat, but slightly irregular, and the drainage lines have not as yet been established. The water that falls here which neither evaporates nor sinks into the soil runs into the slight depressions and makes shallow lakes. When these become full, the water finds an outlet into a lower region until at last it works its way to the sea.
+
+These outlet streams gradually establish themselves and form a continuous line of streams and pools reaching to the sea, with broad, poorly drained areas lying between. The streams at once begin to cut down their beds and the pools to fill up with the silt washed into them, until at last all the pools are drained and a network of streams carries the run-off into the sea.
+
+Usually the dry land of the coastal plain has appeared very gradually, with long periods when there was no gain in its extent. Sometimes the waste brought to the ocean was of a different kind from what it was at other times. Thus the character and condition of the material composing the plain vary considerably, but all the strata are usually inclined slightly toward the sea. The boundaries of the different kinds of hard and soft material composing the plain are approximately parallel to the old shore line. The plain will thus become a belted plain.
+
+As streams wear back faster in the soft than in the hard material, the side streams become longer in the soft layers than in the hard, and in time streams of considerable length are found running in a direction nearly parallel to the old coast. These have their outlets through streams which run down from the old land across the plain, so that the general appearance of the drainage is something like a lengthwise cross section through the trunk and limbs of an oak.
+
+When mixtures of different materials are deposited in water, the coarsest sinks first and the finest last (Exp. 130). We should thus expect that of the material brought down by the river the coarser layers would lie back from the coast. This is often true, although there is frequently uncovered near the border of the old land back from the coast a belt of easily eroded material, and a lowland of erosion is formed in this by the streams. The Delaware River from Trenton to Wilmington and the Alabama River between Montgomery and Selma flow through such lowlands. These regions are called inner lowlands and possess a fertile, fine-textured soil, generally the best to be found in the coastal plain area.
+
+
+
+THE COAST NEAR ATLANTIC CITY.
+
+Showing marshes, lagoons and sand reefs.
+
+
+
+
+
+RICE SWAMP AT THE BORDER OF A NARROW COASTAL PLAIN.
+
+
+
+This inner lowland is bordered on the landward side by the old land, usually composed of firmly compacted rocks which often contain valuable minerals and building stones. On the seaward side it is bordered by the rather abruptly ascending edge of the coarse material of the plain which has not yet been removed. From the top of this ridge there is a gradual slope toward the sea. As the region back toward the old land is higher, and has been above the sea and exposed to erosion longer, it is much more dissected than the surface nearer the sea and is much more irregular and hilly.
+
+A coastal plain is a gradually emerged sea bottom, and so has shallow water extending out for a considerable distance from its edge. Along the shore are marshes and lagoons bordered on their seaward side by sand reefs, where the winds have piled up the sand brought in by waves. In some places these sand reefs are so situated that they are valuable for habitation, as at Atlantic City, New Jersey, where a large summer resort has grown up, or along the coast farther south, where a sparse population finds its home on the broader reef.
+
+A coastal plain increasing in width toward the south extends from New York to the Gulf. The western coast of Europe has a considerable plain of this kind. The Netherlands are situated on land which has been either reclaimed from the sea naturally in recent geological time or artificially by man in recent historical time. In the southern part this reclamation is largely due to the sediment brought down by the Rhine.
+
+Sometimes the materials of a coastal plain are found far inland in places which are now separated from the sea by mountain ranges, as near Lake Ontario. But the method of formation was the same, only thousands upon thousands of years have passed since these rocks were exposed, and vast geological changes have taken place in that time. Such areas as these are sometimes called ancient coastal plains.
+
+In the western part of the United States the coastal plain is not as well developed as on the Atlantic border. But the region about Los Angeles is a coastal plain, and almost all the characteristics of the broad eastern plain can be seen in traveling from the ocean to the coast mountains.
+
+
+
+CRUDE TURPENTINE STILL.
+
+In the pine belt of the North Carolina coastal plain.
+
+
+
+182. Industries on Coastal Plains. —The valuable minerals of the earth are usually found in the older rocks, so there is no mining on a coastal plain, and because the rivers are shallow and fall over no ledges as they flow across these plains, no great water power for manufacturing can be developed. The sluggish streams are often dammed and small water powers developed, but there is not the fall necessary for large factories, except sometimes in the hilly region back near the old land where the rivers have developed rather deep and narrow valleys, and mill ponds of considerable size may be made.
+
+As the different kinds of soil lie in belts, agriculture will vary with the belts. In warm climates rice can be raised along the shore where the land is marshy. On the sandy land most profitable truck farming is possible if the transportation facilities are good. In many places in the southern states these sandy areas support fine forests of pine which are most valuable for the production of turpentine, tar and lumber. Where the soil is not too sandy, cotton is raised in abundance. The materials for making glass, pottery and brick are widespread over coastal plains.
+
+
+
+COTTON.
+
+A most valuable product of the southern coastal plain.
+
+
+
+The cities on coastal plains are usually found either (1) near the coast, where the rivers have formed harbors and so have made ocean commerce possible, or (2) at the head of navigation in the rivers where water transportation begins, or (3) still farther up the river at the fall line, where manufacturing on a large scale is possible.
+
+The fall line is the point on a river where its bed passes from the harder rock of the old land to the softer material of the coastal plain. The softer material is worn away more easily than the hard material, and falls or rapids are produced suitable for water power. A glance at a map of the southeastern United States will show that the principal cities lie in lines nearly parallel to the coast. Of those near the coast are Norfolk, Wilmington, Charleston, Savannah, Jacksonville; at the fall line, Trenton, Philadelphia, Richmond, Columbia and Augusta.
+
+
+
+PINEAPPLES.
+
+A valuable crop of the southern coastal plain.
+
+
+
+Coastal plains furnish a most suitable place for the boring of artesian wells. As the strata are diversified in structure and all dip gently toward the sea, porous strata inclosed above and below by impervious strata are readily found. When the upper of these are tapped, water is forced by hydraulic pressure to a height nearly equal to the highest point reached by the upper stratum. Much of the drinking water on coastal plains is obtained in this way.
+
+183. Embayed Plains. —If a coastal plain is submerged after it has been somewhat eroded, the water backs up into the stream valleys and forms reëntrant bays. The little side streams which enter into the main streams near the coast no longer flow into these streams but into the bays. If the country is somewhat thoroughly dissected near the coast, there will be many small bays. The interstream areas will project out like long fingers with water between them.
+
+
+
+A SUBMERGED COASTAL PLAIN.
+
+
+
+The effect of a submerged and eroded coastal plain is seen in the Delaware and Chesapeake bay region. Here the old river courses have been submerged, and the land between the rivers extends into the ocean in narrow, rather flat strips with many little inlets along the sides. Easy water communication is here possible to a considerable distance inland and to almost every part of the land surface near the coast.
+
+When the country was first settled, these water courses were most advantageous to the settlers, as the produce of the farms could be transported to sea-going ships with comparatively little difficulty, much more easily than would have been the case if it had been necessary to carry it by land. There was little need of building roads, as each farmer had a protected water highway to his door. Thus a part of this region was known as “Tide-water Virginia.”
+
+184. Lake Plains. —Lakes which receive the drainage from the land gradually have their floors smoothed over by the sediment which the streams bring to them and the waves and currents spread out. The lake itself is thus filled, or in time the outlet wears back so as to drain the lake. Thus a plain is left, the elevation of which is determined by the elevation of the old lake bed.
+
+During the Glacial Period lakes were held in at some places by huge dams of ice and at other places by accumulations of sand or gravel brought down by the glaciers and deposited so as to obstruct the valleys. The ice has now disappeared and the gravelly material has often been easily eroded, so that lake plains are not uncommon in the northern United States. As the soil of these plains is fine and easily cultivated, they furnish excellent farm lands.
+
+As already stated, a plain of this kind, remarkable for its fertility and extent, is drained by the Red River of the North and comprises the eastern Dart of North Dakota and about half of the Province of Manitoba. A somewhat similar plain is found in northern New York, bordering Lake Ontario. This was formed at the time when the outlet of the lake was the Mohawk River, the present outlet then being obstructed by ice. The ice dam has since melted, the lake has been lowered, and a part of its old bed has been exposed.
+
+
+
+LAKE PLAIN.
+
+The ice dam in this lake has recently receded.
+
+
+
+A change in the amount of rainfall may cause the formation of a lake plain. If not as much water is furnished to the lake as evaporates, the lake dries up and exposes its bed as a flat plain with perhaps a small remnant of the former lake still existing at the lowest part. Such is the region around Great Salt Lake, Utah.
+
+185. River Plains. —Sometimes a river widens its valley enough so that it swings slowly from one side to the other, and, at high water, floods the valley for a considerable distance on either side of its course. A low, flat plain is thus developed, sometimes terminating near the mouth of the river in a delta.
+
+
+
+"BOTTOM LANDS."
+
+
+
+These plains are very fertile and are usually called "bottom lands" by the farmers. They are often unhealthy because of floods and poor drainage. Where the water in the river rises rapidly and to a considerable height, it is dangerous to inhabit these plains. Thus it is necessary to build strong levees along the river bank, as in the case of the lower Mississippi and some of its tributaries. But sometimes these plains are so fertile that they are densely populated, as the plain of the Ganges.
+
+186. Prairies of the United States. —North of the Ohio River and extending westward beyond the Mississippi is a region of rolling land with a deep, rich soil. Early in the last century it began to be rapidly populated on account of its great agricultural advantages. Owing partly to the fineness of the soil, but mostly to the frequent burning over of the region by the Indians, the area was destitute of trees except in some places along the river courses.
+
+Thus the emigrant did not need to go to the trouble and delay of clearing the forests before beginning to farm. Cultivation could begin in earnest with the first spring, and, as a rule, rich harvests could be obtained. The soil here is transported soil; it is deep and unlike that of the underlying rock. In some places it is rather stony and in others very fine and without stones. It is so deep that the underlying rock is only seen in deep cuts.
+
+This soil was probably deposited by the great continental glaciers which once covered the region and was spread out either by the action of the slowly moving ice or by the water from the melting ice. This water flowed over the surface in shallow débris-laden streams, bearing their silt into the still waters of transient ice-dammed lakes. Whatever the original surface of the region, at present it is an irregularly filled plain due to the ancient ice sheet. As the soil is composed of pulverized rock not previously exhausted by vegetable growth it is strong and enduring, so that this country has, since its settlement, been noted for its productivity.
+
+
+
+ALFALFA CUTTING ON THE FERTILE PRAIRIES.
+
+
+
+187. The Great Plains of the United States. —West of the Missis sippi River, and merging almost imperceptibly into the prairie region on the north and the coastal plain region on the south, there is a broad extent of territory usually called the Great Plains. This region consists of irregular intrenched valleys 50 to 100 feet deep. Sometimes there are hills and mountains, but viewed from an eminence the country appears flat.
+
+
+
+A HIGH, DRY PLAIN.
+
+
+
+The elevations are either flat topped hills, the strata of which are slightly inclined and correspond in position to those found in the plain beneath, or they are masses of igneous material which appear to have been thrust up through the rock surrounding them. In the former case the elevations are simply remnants of the layers of rocks which once extended over the country, but which have now been eroded away over the larger part of it; in the latter case they are the igneous masses which have withstood erosion. The Great Plains may thus be considered as an example of a plain of erosion.
+
+Here, as in the prairie region, trees are wanting, but their absence is due rather to the lack of the necessary rainfall than to the reasons assigned for the former region. Although formerly considered almost a desert on account of its small rainfall, this region now supports vast herds of cattle, and by the aid of irrigation will soon possess great agricultural wealth.
+
+188. Life on Plains. —The life conditions on plains are very dif ferent from those in places where the irregularities of the surface are great. The climate of plains is quite uniform and depends to a large ex tent upon their position on the earth's surface. Movement is as easy in one direction as in another, and the lines of travel tend to be straight. There is usually no reason for an accumulation of population in any one place, so the population tends to be uniformly distributed.
+
+
+
+A HERD OF CATTLE ON THE GREAT PLAINS.
+
+
+
+As movement from place to place is easy, it is not difficult for the inhabitants of a plain to mass themselves together at one point. In case of invasion by a superior enemy there is no place for hiding or safe retreat, and subjection or extermination are the alternatives, unless the plain is so large that the enemy is unable to spread over it. In the case of animals this has been shown in the practical extermination of the American bison and antelope. In the case of men it was shown on the plains of Russia in the thirteenth century when the Tartars conquered the region and threatened to overrun Europe.
+
+Another instance was that of the fatal invasion of Russia by Napoleon. The Russians, unable to find a strategic place to make a stand, retreated farther and farther into the plain. The depletion of Napoleon's army, due to the extent of territory which must be held in his rear, the distance from his base of supplies and the rigor of the Russian winter, forced him to begin that disastrous retreat, the fatal results of which probably led to his final overthrow.
+
+
+
+HERD OF BISON.
+
+
+
+The effect of plains on the distribution of population is shown in the early settlements on the coastal plain territory south of Philadelphia. Here there were almost no towns containing as many as twenty houses until the colonies had been settled for nearly two hundred years, and even now cities of considerable size are rare, but on the more rugged lands to the north the tendency to build towns began at the beginning of settlement.
+
+189. Plains in History. —Plains have always played an important part in history. Here armies can march and countermarch with comparative ease. Large bodies of men can easily be assembled. Military stores can be readily collected and all the operations of war carried on without natural obstructions. Thus it happens that certain plains have been the seats of almost innumerable wars. The great plain of the Tigris and Euphrates was the gathering ground and battlefield of vast ancient monarchies. The plains of the Po have been the arena in which embattled Europe has settled some of its deadliest strifes, while the level lands of Belgium have been dyed again and again with the blood of thousands and thousands of Europe's bravest sons.
+
+
+
+PART OF THE PLAIN OF WATERLOO, BELGIUM.
+
+
+
+Summary. —Level areas are called plains when low, plateaus when high. When a coast has been elevated and part of the continental shelf becomes exposed, this is called a coastal plain, as the east coast of the United States from New York to the Gulf of Mexico.
+
+Coastal plains have little mining and manufacturing; their agricultural products vary. Their large cities lie either at tide water or at the fall line of the rivers. The best drinking water on coastal plains usually comes from artesian wells.
+
+Besides coastal plains there are lake plains, like those of northern New York and eastern North Dakota, and river plains, of which the Mississippi is the best example. The prairies have a fine, rich, treeless, fertile soil, a result of ancient glaciation. The great plains of the United States have an irregular surface usually barren of trees.
+
+
+
+
+
+QUESTIONS
+
+
+How does the drainage of a coastal plain develop?
+
+What kind of a shore line will a coastal plain have?
+
+What are the usual industries of a coastal plain?
+
+Where are the largest cities on a coastal plain situated?
+
+Describe the kind of coast line that results from the depression of a dissected coastal plain.
+
+In what way are lake plains formed?
+
+How are river plains formed?
+
+What natural condlitions made its possible for the pioneer settlers to become quickly prosperous on the prairies?
+
+How have plains affected the welfare of their inhabitants?
+
+How have plains influenced history?
+
+
+
+
+
+【中文阅读】
+
+
+180．平坦地区——在地球表面的不同区域，存在着范围广阔的平坦陆地区。在这里，水流系统不是特别发达，而且还有面积不小的低洼地区。大雨过后，浅浅的积水便长时间存留在这些低洼处，直到最后全部蒸发，或者全部浸入土壤。而在那些有排水渠道的地方，浅浅的水流则会顺着沟渠缓缓流淌。
+
+如果对这些地区进行挖掘，会发���土壤下的岩石层会接近水平状，或者几乎完全就是水平状分布。当这类地区的海拔相对较高时，这些水流最终会导致形成深深的峡谷，将地表切割开来。而当这类地区海拔很低时，它们就叫做平原；在高一点的地方，尤其是周围全是向下的坡面的时候，就叫做高原。俄罗斯的北部大平原就是海拔较低的平坦区的典型例子，科罗拉多河畔的亚利桑那高原则是海拔较高的例子。
+
+181．海岸平原
+
+
+
+
+
+实验130：在一只玻璃广口瓶中加满水，然后慢慢向里面加入一些细沙和泥土粉末的混合物，注意观察水的颜色变化。然后将其静止放置几天，在仔细观察瓶底的沉淀物，现在细沙和泥土粉末还像它们被加入瓶中时那样混合在一起吗？水对混合物产生了什么作用？
+
+
+
+
+
+我们已经知道，地球的表面其实并不稳定，而是一直处于运动变化之中。当海岸的陆地抬升，或者海底发生沉降，海水便会从陆地上撤退一定距离，之前位于海底的条状地带现在就成了干燥的陆地。
+
+这一新生地带由粘土、沙石和碎砾石构成，而且常常有许多和邻近海滨上相似的贝壳。这一地区表面整体会非常平整，也会有一些轻微的不规则，且水流渠道还没有完全形成。雨水降落到这一地区以后，那些既没有蒸发掉，也没有浸入土壤的部分则会流到这些轻微洼地，形成浅水湖。水满之后，它们又会寻找出口流向更低的地方，直到最终流入海洋。
+
+这些顺着出口流走的水流不断壮大，在沿途广阔平坦的地域上形成了不断靠近海洋的水流干线和池塘。形成规模之后，这些水流便立即开始不断冲刷河床，池塘则不断被进入其中的淤泥慢慢填充起来，直到最后池塘的水全部流走，河流则将所有的地表径流水全部带进海洋。
+
+一般而言，海岸平原的干燥陆地总是显得很平整且很有规则，但在很长的时间里，其范围内的土地并没有多少利用价值。这样的荒废，有时给海洋带来的影响和平时显得不同。海岸平原地质构造成分的特性与状况，由此也千差万别，但所有底层一般都会轻微地向海洋倾斜。软硬地质材质之间的分界线也与原来的海岸线大致平行，这样的也就形成了分带平原。
+
+由于河流在软质河道中比在硬质河道中能更迅速地冲刷河床，因此柔软地层上的支流也一般比坚硬地层上的要长许多，同时较长的河流的流向也会与先前的海岸线几乎保持平行。这些情况都让这些河流从原先陆地上流出的出口，全都横向地陈列在平原上，这样一来，整个地区的河流网线便像一个纵向分布的栎树枝干图一样。
+
+当水流中的混合杂质不断沉淀的时候，最粗糙的沉淀得最快，最细微的沉淀得最慢。这样我们便可以想象，河流带来的沉淀物会出现明显的层次，粗糙的土石沉积物会在离海岸较远的地方沉淀下来。这的确是事实，人们常常在原先背向海岸的陆地边界处，发现一个极易侵蚀的带状地带，以及由于这些河流的侵蚀效应形成的低地。从特伦顿流向威明顿的特拉华河，以及蒙哥马利与塞尔玛之间的亚拉巴马河，就流淌在这一类型的低地上。这些地区也叫做内部低地，这里一般都有肥沃而纹理分明的土壤，是海岸平原上最好的土层区域。
+
+这一内部低地与原来的陆地接壤，并且位于分界线内侧，由挤压得很紧的岩石构成，这些岩石一般都含有非常有价值的矿物成分，也是很好的建筑石材。在分界线外则，即靠近海洋的一侧，平原中尚未被转运的粗糙土质层会突然升高，因此从分界线的顶部看去，便会出现一个平缓的斜坡，地势逐渐降低一直进入海洋。由于原来陆地的内侧地带较高，且高于海平面并经受了更长时间的侵蚀，因此地面上就比靠近海洋的地方显得更加纵横斑驳高低不平，到处的小山也让这一地带更加没有规律。
+
+海岸平原本来就是海底逐渐暴露形成的，因此也会有一些浅浅的水域会在海岸平原上延伸一段距离。沿着这样的海岸，海洋一侧的地带存在着不少沼泽地和环礁湖，再外侧一些便是许多沙质礁石，这是风将海浪带来的沙在这里不断堆积的结果。有些地方这类沙礁的规模很大，因此成了非常适合人居的地方，比如新泽西州的大西洋城就是如此，这里已经逐渐成为人们夏季的旅游胜地。如果再沿着海岸线往南走，还会发现少数居民在一些较宽的沙礁上生活。
+
+从纽约到整个海湾地区，宽度向南不断增加的这一地带也是一个海岸平原。整个欧洲的西部海岸地区也差不多是如此。荷兰的土地就既经历了晚近地质时代的海洋再造，也经历了近代以来的大规模人工开垦，其南部区域的开垦利用在很大程度上就得惠于莱茵河带来的土石沉积。
+
+有时海岸平原的地质成分也会在很远的内陆地区被发现，它们和海洋之间往往还隔着高高山脉，比如在安大略湖附近。其形成原因也是一样，皆是由于暴露的岩石在千万年的地质变化中演化而来，因此这类地区也叫做远古海岸平原。
+
+在美国西部，海岸平原便不像在大西洋沿岸那样成熟壮大，但洛杉矶地区就是一个典型的海岸平原区。辽阔的东部平原的所有地貌特征，在我们从海洋往海岸山脉行进的途中，都可以一览无余。
+
+182．海岸平原上的工业——地球上有价值的矿藏一般都位于年代久远的岩层中，因此在海岸平原上便没有矿业产生，同时由于河流在这里都很浅，在整个平原上流域中也没有大的落差，因此可用于制造业的水力资源在这里也很不发达。不过除了在靠近原来陆地区域的丘陵地带以外，这里也不是必须要有水力来支持工业发展，在丘陵地带的河流则可以冲刷出较深的河道以及狭窄的河谷，初具规模的作坊在那里也便能得以存在。
+
+不同类型的土壤在海岸平原上呈带状分布，因此这里的农业基本上也呈带状发展。在温暖的气候带，海滩沿岸的沼泽地上可以大面积种植稻谷。而在沙地上，只要物流条件不错的话，营利性的商品蔬菜栽培则可以大规模开展。在南部省份的许多地方都种植了松树林，其经济价值很大，可以制造松脂油、柏油，也是很好的木材资源。在土质沙化不太明显的地区，则可以大面积地栽种棉花。还有诸如制造玻璃、陶瓷、砖瓦等器物的材料，在海岸平原上也大量存在着，这为这类产业的发展提供了条件。
+
+海岸平原上的城市一般都具有下列特征之一：（1）靠近海岸，河流冲积出了良好的海港，让海洋贸易得以蓬勃发展；（2）处于河流航道的起始位置，水运交通发达；（3）位于河道前端的瀑布线上，制造业得以大规模发展。
+
+所谓瀑布线，就是在海岸平原上，河道从坚硬河床过渡到柔软材质的河床的地方。柔软的土质更容易被冲刷侵蚀，因而瀑布与激流便更容易形成，这也让水力资源得以丰富起来。看一眼地图我们就能发现，美国东南沿海的重要城市，都几乎分布在一条条与海岸线平行的线路上，它们中靠近海岸的有诺福克、威明顿、查尔斯顿、萨凡纳、杰克逊维尔；位于瀑布线上的有特伦顿、费城、里士满、哥伦比亚和奥古斯塔。
+
+海岸平原也是一个适合自流井勘探的地方。由于地层结构的多样化，且向海洋不断下降，夹在不易渗透底层之间的易渗透地层也常常在这里存在。当上层地层被挖掘开以后，地下水便会由于液压而上升到上层地层的最高位置。海岸平原上的大部分饮用水就是由此方法而获取的。
+
+183．港湾平原——如果海岸平原在被侵蚀之后沉入海中，海水便会倒灌进入河谷，形成折返湾。之前流入主干河流的小支流这时便不再流入河流，而是直接流入到这类海湾当中。如果这片靠海陆地上的河道很多的话，形成的小海湾也就不少。河流之间的地带就像一根根指头一样，排列在一条条河道之间。
+
+海岸平原的侵蚀下沉效应，可以在特拉华以及切萨皮克海湾地区一窥究竟。在那里，原有河道沉入海中，河流之间的陆地呈狭窄的条状长长地伸入海洋，地面依然比较平整，只是沿岸多了许多入水口。水流在这一地带能够充分地发生交流，并一直延伸到内陆很远的距离，因此整个陆地表面的每一部分都离海不远。
+
+当这类陆地形成之后，这些水道便对居民们特别有利，因为这为农作物的航海运输提供了便利，要是在内陆的话，物流问题可就麻烦多了。这里因此也不需要修路，因为每家每户都有一条直达家门口的水上高速路，故而这一地区也被人们称作“潮头上的弗吉尼亚”。
+
+184．湖岸平原——随着湖泊不断吸纳陆地水流，它的湖底会慢慢变得越来越平整光滑，因为河流以及潮水带来的沉积物会在湖底慢慢沉淀下来。因此湖泊会慢慢地被填充起来，或者湖泊的出水口总有一天会将湖水全部排走。这样就留下了一个平原地带，其海拔高度也就是以前湖床的海拔高度。
+
+在冰河时期，一些地方的湖泊被巨大的冰岸围堵起来，还有些地方的湖泊则被冰川带来的砂石沉积物所淤塞，让水道河谷被完全堵塞起来。而现在冰岸消失了，而砂质湖岸又很容易被侵蚀，因此湖岸平原在美国北部并不常见。由于这类平原的土壤很细，很容易被耕种，因而也是上佳的天然农场。
+
+我们之前提���过一个这样的平原，以它的肥沃和面积著称于世，其早前的湖水流进了红河的北段，湖岸范围包括北达科他州的东部地区，以及曼尼托巴省的半个普罗文斯地区。还有一个比较类似的平原在纽约北部地区，与安大略湖接壤。它形成的时候，湖泊的出水口就是今天的莫华克河，那时正被冰山堵得严严实实的。后来冰岸融化，湖水慢慢流出，一部分湖床也就暴露在光天化日之下了。
+
+降雨量的变化也能导致湖岸平原的形成。如果流进湖泊的水量少于蒸发的水量，湖泊就会渐渐干涸，暴露的湖床也就变成了一个平坦的平原，中间可能还残留着一汪水，依旧守护着最低洼的前湖区。犹他州的大盐湖就是这样的典型。
+
+185．河岸平原——河流有时会将河谷不断加宽，当其宽度足够大时，河水便会在两岸之间迂回荡漾，并且当水量很大时，洪水便会淹没沿岸很大的一片地区。这样，一个低矮、平坦的平原就慢慢形成了，并且一般都终止在河口的三角洲附近。
+
+这类平原都非常肥沃，农民们一般把它们叫做“河滩地”。不过这里的土壤有时对健康不利，因为常常发洪水，而排水又不太通畅。当河流的水位上涨到一定高度的时候，对居住在这一平原上的人们而言，就非常危险了。因此很有必要沿着河道修筑坚固的堤坝来抵御洪水，就像密西西比河下游以及它的许多小支流上那样。但尽管如此，由于土壤毕竟极其肥沃，这类平原有时依然会成为人口的密集区，比如恒河平原就是如此。
+
+186．美国的草原——从俄亥俄河北段向西，一直延展到密西西比河的这一片广袤地区，地表高低起伏，土壤肥沃，因此在19世纪早期便由于农业优势的发展而迅速成为了人口聚集区。由于土质松软，当然更主要是因为印第安人经常焚烧原林，这一地区除了沿河有少量树木以外，其他大部分区域都没有树木。
+
+因此这里的居民们，就省去了在土地耕种之前必须先清理树木的麻烦。春天一来，便可以从最东边开始播种，而且根据经验，收成往往十拿九稳。这里的土壤以运积土为主，因此土层很深，也与下层岩石的成分完全不同。有些地方的土壤中有很多石头，也有些地方土壤却极其细腻，没有一点石质成分。由于下层岩石所在的地层很深，因此只有在很深的挖掘通道口中才能一窥其面目。
+
+这些土壤多半是由很早以前覆盖这一地区的大陆冰川沉积而成，一方面是由于其冰盖的移动导致土石流转，另一方面由于冰体融化带来水流的沉淀。这些水流流淌在很浅的砂砾岩床河道中，将它们的泥沙带进了短期冰堰湖的澄静湖水中。不管过去这里的地表状态如何，如今呈现在我们眼前的，已经是一个由远古冰盖作用而生成的不规则填充平原。由于土壤中有不少粉碎石质成分，植物的生长也还没有耗尽其养分，因此土质依然很坚固，根据这一特点，人们现在已经注意到了这片土地的产业开发潜力。
+
+187．美国的大平原——在密西西比河西岸，北部与大草原轻微重叠、南部与海岸平原毗连的这片辽阔区域，就是我们常说的美国大平原。这一地区有不少起伏的沟谷，有的能达到50到100英尺深，也有一些丘陵和山脉，但若从高处俯瞰的话，整体还是比较平整的。
+
+这里的高地要么是一些平顶丘陵，但其地层与平原低矮处的地层依然完全一致。要么就是一些由杂乱的火山材质构成的区域，看起来似乎是由于火山喷发物冲击周围岩石形成的。在前一种情况下，这些高地都是曾经在这片土地上广泛延伸的岩石层的残余，但是现在它们大部分都已经被侵蚀而消失了；在后一种情况下，它们则是由一些能够经受侵蚀的火山岩石构成的。因此从这个意义上说，大平原其实也是自然侵蚀的产物。
+
+这里也跟大草原一样，树木稀少，但原因是必要的降雨量的缺乏，而不是由于被划定为农耕区而导致。由于降雨稀少，之前人们总认为这是一片荒芜的不毛之地，但近来由于畜牧业的发展，这一地区上养殖了不少家畜牲口。假以时日借助于灌溉的跟进，这里成为农业聚宝盆也是指日可待的事情。
+
+188．平原上的生命——平原上的生命状况与那些地表多样性很突出地方大为不同，平原上的气候比较单一，并在很大程度上取决于它所处的地理位置。向各个方向的运动与迁移都一样容易，没什么差别，而且行进的路线也几乎是一条直线。因此生物种群也没有理由在某一个地方大量聚集，故而在整个平原上，一般都是一致地分散分布。
+
+但也由于从一个地方到另一个地方很容易，居民在平原上某一个地方聚居也变得并不困难。不过当强敌入侵的时候，这里便也没有藏匿以及掩护撤退的地方，因此除非平原地区面积非常大，敌人无法横穿以外，臣服于敌人还是消灭他们，就成了生死之选。动物也会遭遇类似的情形，美洲野牛和羚羊就为我们提供给了活生生的例子；而这种情况之于人类，13世纪俄罗斯人的遭遇就是样本，那时蒙古大军征服了整个西伯利亚平原，并且已经危及到了整个欧洲。
+
+另一个类似的例子还有拿破仑入侵俄国。由于俄罗斯人找不到战略要塞加以抵抗，便向平原地区不断撤退。但由于平原地区非常辽阔，拿破仑的军队由于纵深太长以致消耗过大，后勤供给难以为继，加之西伯利亚地狱般的严寒冬天可不是说着玩儿的，最终他不得不开始灾难性的大撤退，这一致命结果在一定程度上也导致他的彻底瓦解。
+
+平原地貌对人口分布的影响，在费城南部海岸平原上的早期移民中可以清晰地看出来。在殖民地独立之前的近200年中，那里几乎没有一个房屋数量大于20的城镇，即使今天，规模较大的城市也不多，不过在其北部更加崎岖的陆地区域上，建立新城市的趋势已经渐渐拉开了帷幕。
+
+189．历史上的平原——平原在历史长河中一直扮演着重要角色。在这里，军队可以轻轻松松地前进后退，大量的人群也可以很容易地集合在一起。关于军事方面的故事在这里不断积累着，战争也常常在这里毫无障碍地上演着，因此有些平原几乎成了战争的渊薮。底格里斯河与幼发拉底河之间的大平原，就是古代君王们经常在此一决雌雄的战场。在波河平原战争中，几乎四面楚歌的欧洲在此进行了艰苦卓绝的抵抗。而比利时平原的土壤，则被千千万万勇敢的欧洲男儿的鲜血，染红了一遍又一遍。
+
+
+
+
+
+总结——陆地上的平坦区域，在海拔较低时叫做平原，海拔较高时叫做高原。当海岸抬升，部分大陆架暴露出来的时候，海岸平原就形成了。美国东海岸上，从纽约到墨西哥湾的这一地带就是一个典型的海岸平原。
+
+海岸平原上没有矿产业和制造业，但农产品很丰富。它们的大城市一般都位于潮汐海岸，或者位于河流的瀑布线上。海岸平原上优质的饮用水，一般都来自其自流井。
+
+除了海岸平原以外，还有湖岸平原，比如纽约北部地区以及北达科他州东部地区的平原就是此类；还有河岸平原，密西西比河沿岸便是典型例子。草原上的土壤一般都细腻肥沃，但树木稀少，这是远古冰川的作用结果。美国大平原的地貌很不规则，是一片荒芜的不毛之地。
+
+
+
+
+
+思考题
+
+
+海岸平原的河流渠道是如何发展壮大的？
+
+海岸平原一般会拥有什么样的海滩及海岸线？
+
+海岸平原上一般有哪些产业？
+
+海岸平原上的大城市一般位于哪些区域？
+
+描述一下地表沟壑较多的海岸平原在下沉之后，陆地海岸线会呈现出什么样的形态？
+
+湖岸平原是怎样形成的？
+
+河岸平原是怎样形成的？
+
+哪些自然条件能让大草原的首批居民们尽快地富起来？
+
+平原地理状况会对那里的居民的生活与发展产生哪些影响？
+
+平原对人类历史有哪些影响？
+
+
+
+
+
+CHAPTER 13
+
+THE HIGH AREAS OF THE EARTH
+
+地球上的高地
+
+
+190. Young Plateaus. —Sometimes large areas of horizontal rock are elevated high above the sea, forming lofty plains whose surfaces are often irregular, owing to previous erosion. Such areas are called plateaus. The descent from a plateau to the lower land is usually steep. Areas of this kind, where streams are present, suffer rapid and deep erosion, since the grades of the streams are steep because of the elevation.
+
+If there is not much rain there will be few streams, and these will have deepand steep-sided troughs. Such troughs render the area very difficult to cross. The valleys are too narrow for habitation or for building roads, and the deep troughs of the streams are too wide to bridge. Thus the uplands are isolated.
+
+If these high areas are in a warm latitude, they are desirable for habitation on account of their cool climate, due to the elevation; but if in temperate latitudes, their bleak surfaces are too cold.
+
+As the river troughs wear back, the harder rocks stand out like huge benches winding along the course of the rivers. From the different benches slopes formed from the crumbling of the softer strata slant backward. Thus the general outline of the stream sides will be something like that of a flight of stairs upon which a carpet has been loosely laid.
+
+An excellent example of a region of this kind which has been eroded by a strong river gaining its water from a distant region is that of the Colorado Cañon Plateau. Here is found the grandest example of erosion on the face of the earth. The rocks are of various colors, and the gorge is nearly a mile deep and in places some fifteen miles in width. Words are inadequate to express the grandeur of the panorama spread out before one who is permitted to see this gigantic exhibition of the results of erosion. Wonderful, grand, sublime, are mere sounds which lose themselves in the ears of one who looks out upon this overpowering display of Nature's handiwork.
+
+
+
+COLORADO PLATEAU.
+
+The river has cut a deep canon through the plateau.
+
+
+
+The region is very dry, and the river receives few and short branches for many miles of its course. The valley is widening much more slowly than it would if this were a land of considerable rainfall, and as yet the river fills the entire bottom of the gorge. The valley is in the early stages of its development and has just begun the vast work of wearing down the region. The side streams are small and the interstream spaces broad.
+
+191. Dissected Plateaus. —If a plateau has been elevated for considerable time in a region of abundant rainfall, the streams extend their courses in networks, thoroughly dissecting the area and leaving between their courses only narrow remnants of the upland. The valleys are still deep, but the intervening uplands are of small extent. Travel ing over the region in any direction except along the stream courses is a continual process of climbing out of and into valleys.
+
+There is very little level space that can be used for cultivation, and on account of the steepness of the slopes it is very hard to build roads. The river valleys are so narrow that unless the roads are perched high up on the sides, they are liable to be swept away at the time of flood. Farming in these regions is very discouraging because of the difficulty of transporting crops and of finding anything but a steep side hill on which to grow them.
+
+
+
+THE ENCHANTED MESA.
+
+With old Indian village in the foreground.
+
+
+
+Railroads can get through only by following the principal valleys, and here, on account of the narrowness, the engineering of the roads is difficult. Unless the region is rich in minerals, it can support only a small population, and that will of necessity be poor. As soon as the forests are cut off, the soil rapidly washes down the hillsides and leaves naught but bare surfaces. Regions of this kind are found in the Allegheny and Cumberland plateaus, extending from New York to Alabama.
+
+
+
+A BUTTE.
+
+
+
+192. Old Plateaus. —If a plateau remains elevated for a great length of time, the dissecting rivers are able to widen their valleys and wear away all the interstream spaces, except where these are very broad. Thus the rivers bring the surface down to a comparatively low level, with here and there a remnant which has not been worn away, but which shows in its steep sides the edges of the rock layers which formerly spread over the whole region. If these residual masses are large, they are called by the Spanish name mesas, meaning tables, and if small, buttes, from the French word which means land marks.
+
+
+
+AN INDIAN HOGAN.
+
+
+
+Some of these mesas are so high and so steep that it is impossible to climb them, and others are simply low, flattopped hills. A traveler in New Mexico and Arizona will see many of these mesas, which, like the lonely Indian huts or hogans, are but scattered remnants of what was formerly widespread.
+
+On old plateaus travel is easy. There are no deep valleys, and one can easily pass around the mesas, which only add charm to what would otherwise be a most monotonous landscape. When these mesas are high, they are sometimes occupied by a few Indian tribes who have fled to them for protection, as the medieval barons when hard pressed fled to their isolated castles.
+
+
+
+CLIFF DWELLINGS.
+
+A protected retreat in a mesa.
+
+
+
+193. Broken Plateaus. —The force which has uplifted the plateaus is not always uniform enough in its action to lift large areas without fracturing the rock layers. Thus plateaus are found which have the rock layers broken and displaced. The layers on one side of the break may stand thousands of feet higher than those on the other side.
+
+This is seen in the region of the Grand Cañon of the Colorado, where the Kaibab Plateau stands about 2000 feet above the Colorado Plateau and steep cliffs bound it on both its east and west sides. These fault cliffs, as they are called, are found at several other places in this region, showing that the whole area was much broken when it was uplifted. The Kaibab Plateau itself is so much higher than the plateaus on either side that it intercepts sufficient rainfall to support forests, whereas the plateaus about it are almost barren of trees.
+
+In the walls of the Colorado Cañon some of these great breakage lines can be traced and the same strata seen to be thousands of feet higher on one side of the line than on the other. In front of these breakage cliffs or fault cliffs, accumulations of débris extend along the entire distance, showing that since the uplift there has been time for much erosion even in this dry region. The Colorado River passes over these great faults regardless of their existence. The cañons in the region seem not to have been influenced by the faulting. Probably it took place too slowly.
+
+
+
+A FAULT.
+
+
+
+194. Hills and Mountains. —Irregular elevations of the earth's surface are called hills, or mountains when they are of considerable height. In the general use of these terms there is no exact line of separation. Elevations which in mountain regions would be called hills would in a flat region be called mountains. As a rule, elevations are not termed mountains unless they are at least 2000 feet high. But if the general elevation of the country is great, as in the lofty regions of the Rockies, an elevation to be termed a mountain must rise to a striking height above the generally elevated surface, which is itself nearly everywhere more than 4000 feet above the sea.
+
+195. Structure of Mountains. —Mountains are the results of de formations in the earth's crust, due to causes not fully understood and the study of which is a part of geology. The crust of the earth has been folded, pushed up, crumpled and in many ways distorted so that some portions have been elevated to a considerable height above sea level. Where these elevated portions have not remained long enough to be worn down, they form mountains.
+
+
+
+APPALACHIAN PLATEAU.
+
+A range of old mountains greatly reduced in height.
+
+
+
+All lofty mountains have been elevated in comparatively recent geological time, but this of course means millions of years ago. If mountains now lofty were geologically old, they would long ago have been worn down. The older mountains of the earth are all comparatively low, not because they were never elevated as high as the lofty mountains of to-day, but because their greater age has longer subjected them to erosion and thus reduced their height.
+
+It is difficult to classify the different kinds of mountains, for very few of them are simple in their structure, but certain kinds of mountain forms are easily distinguished.
+
+
+
+
+
+196. Block Mountains.
+
+
+
+
+
+Experiment 131. —Take three pieces of smooth, straight-edged boards, two of which are about 15 × 55 cm. on a side and the other 8× 55. Place these flat on a table with the smaller board in the middle and the longer edges close together. Sift corn meal, fine coal dust, powdered pumice, plaster of Paris and fine sawdust in even layers over the boards. Now lift carefully the inner edge of one of the wider boards and slip under it a narrow strip of wood 1 or 2 cm. thick. The layers of material spread over the boards will be broken and slant back from the line of breakage with their edges exposed along this line. Do the same with the wide board on the other side. The conditions shown will be similar to those exhibited in block mountains.
+
+
+
+
+
+In southern Oregon and extending southward are found long, narrow mountain ridges, having a steep cliff on one side and a gentle slope on the other. Between these ridges are flat, troughlike depressions in which small lakes are sometimes found. The ridges are formed of thick layers of rock inclined at the same angle as the long slope of the ridge. The short slope of the ridge exposes the edges of these layers which have been broken across.
+
+The débris slopes at the foot of the steep cliffs in some cases are slightly broken across in a direction parallel to the cliff. The steep cliffs sometimes face each other with a somewhat flat depression between, and sometimes the cliff on one ridge faces the long slope of the next. Some of the ridges are more gullied than others, showing longer exposure to erosion.
+
+These ridges are due to strains which have broken the rock layers and elevated those on one side of the fracture above those on the other side, so that a steep fracture cliff has been formed with the rock layers slanting backward from its elevated edge. (Fig. 118.) Mountains of this kind are called block mountains. As is seen from the fracturing of the débris slopes, the movement of elevation is not yet completed.
+
+
+
+Fig. 118.
+
+
+
+As some of the ridges are more gullied than others, it appears that the fracturing did not take place all at one time, but that the more gullied ridges were formed first. Earthquakes are not uncommon in this region. These are caused by a small slipping along the fault line.
+
+In Oregon these ridges are little eroded. They are simple in structure and young in age. The longer streams flow down the gentle slopes parallel to the surface of the rock layers and the shorter streams along the steeper slopes across the edges of the layers.
+
+
+
+
+
+197. Folded Mountains.
+
+
+
+
+
+Experiment 132. —To the long edge of a piece of board about 10 cm. wide and 20 cm. long tack securely one of the shorter edges of a piece of rather thick rubber dam about 20 × 25 cm. Tack the opposite edge to a strip of board about 2 cm. wide and 20 cm. long. Place the rubber dam thus arranged on a smooth table and secure the wide board firmly to the table by a clamp or nail. Taking hold of the strip, stretch the rubber dam as much as it will readily stand. Fasten the strip so as to hold the rubber dam in this stretched position.
+
+Sift fine sawdust, plaster of Paris, fine coal dust, ground pumice, corn meal, or any other distinctly colored substances in even layers over the stretched rubber dam. Slightly dampen the layers. Releasing the strip, allow the rubber dam to contract very slowly. When it has fully contracted, cut carefully through the layers of material with a thin knife and remove that which is on one side of the cut. The layers will have been folded into irregular undulating folds, thus simulating folded mountains.
+
+
+
+
+
+Where layers of rock are subjected to slow, uniform and tremendous lateral pressure, they may form undulating folds with little fracturing. (Fig. 119.) The contracting of the interior of the earth, due to cooling, has sometimes brought to bear such pressure, and in a few cases undulating folds have been produced.
+
+
+
+Fig. 119.
+
+
+
+The best example of this folding is that of the Jura Mountains, between Switzerland and France. Here the almost regular folding of the strata can be seen wherever the streams have cut across the mountains. The mountains are so young that there has been little carving by erosion and the downfolds still form the valleys and the upfolds the ridges.
+
+The rock layers composing these folds contain marine fossils, showing that they were once horizontal and must have been formed in the sea. The longer streams run down the troughs of the folds, but in some places, often where the folds are least high, streams cut across them and pass from one trough to another. Along these transverse stream courses are usually built the roads that cross the ridges.
+
+Sometimes the tops of the ridges have been sufficiently worn away or are broad enough to form considerable flat areas, where little villages are situated. But most of the population is found along the longitudinal valleys, especially where there are cross valleys. Some of the cross streams seem to have no connection with sags in the folds, but appear to have cut their valleys through the folds as fast as they rose, thus indicating that the rate of folding was slow.
+
+
+
+FOLDED STRATA.
+
+
+
+198. Massive Mountains. —The mountains already studied are all comparatively low. Probably none of them rises to height exceeding 6000 feet. They are simple in form and outline, and although pleasing features in the landscape, are a bit monotonous. Massive mountains, on the contrary, are varied in form, lofty in height and are among the grandest and most inspiring of Nature's marvels.
+
+In all ages mountains have been an inspiration to man's nobler thoughts and higher aspirations. With their heads piercing the azure vault of heaven and towering with gigantic mass above the lower world, they force man to look up, and in the contemplation of their nobility to forget his meaner self. Like everything else which holds enduring admiration, these are the result of strain and stress and never ceasing battle with the forces of destruction.
+
+
+
+MASSIVE MOUNTAINS.
+
+The high Sierras.
+
+
+
+The structure of massive mountains is complex in the extreme. Rock layers are often folded, twisted and contorted (Fig. 120) beyond all recognition of their initial condition. Their uplift has been no simple process, each age has added its peculiar impulse to their growth. As the forces of elevation have been lifting them up, those of degradation have been cutting them down. Their broad brows have been carved into peaks and pinnacles, and gorges and caverns have been cut into their flanks.
+
+
+
+Fig. 120.
+
+
+
+The different rock masses which enter into their structure have each assumed its own peculiar lineaments under the carving of the wind, rain, streams, avalanches and glaciers, and thus the variegated beauty of the whole mass has been produced. The central part of massive mountains is composed of igneous rocks, but on the sides overlying these, sedimentary rocks are found. The Rockies, the Alps and the Himalaya Mountains are of this kind.
+
+199. Mountains that no longer Exist. —The mountains which are now such prominent features of the earth's surface are neither all the same age nor are they the only representatives of this kind of land forms that have ever existed. All the kinds of mountains thus far considered are young in geological age, although some are older than oth ers.
+
+All parts of the earth's surface are being gradually worn down by the action of water, but the higher portions are worn more rapidly than those lower, as here the forces of denudation act more intensely. Thus if mountains stop growing, they decrease in height until finally they are too small to be called mountains. Their rocks will be crumpled and folded, and all the characteristics of mountain form will be present except the elevation.
+
+The slant of the rock layers may be such as to indicate a great elevation in former times, but now only the roots of the mountains are left and the region is of very moderate elevation. Regions of this kind are found in many parts of the earth.
+
+In the Appalachian highlands of Pennsylvania the rocks show that they were once folded into ridges and troughs something like those of the Jura. But now the arches have been worn away, and the existing ridges are due to the resistance which the harder layers offer to erosion. These ridges are as likely to occur where formerly the troughs of the folds were as in what were the crests.
+
+
+
+BEN NEVIS.
+
+A mountain much worn down but still high.
+
+
+
+The configuration of the country is not at all as it was when the rocks were folded. The elevation then was much greater than the highest ridges at present. If the beds should be reconstructed as they now lie, they would indicate a height much greater than the mountains ever had at any time. Research shows that these mountains have been lifted up and worn down more than once.
+
+
+
+THE MAT TERHORN.
+
+
+
+Another region in which the mountains have been reduced to inconspicuous heights is the Laurentian Plateau, the area around Hudson Bay. These mountains were very ancient and were worn down long, long ago. In some regions like southern New England, as the mountain structure has been worn down, it has left here and there a residual height like Mt. Monadnock which has not been fully reduced.
+
+Although the general features of such a country are those of hills and valleys and it has little of the appearance of a plain as one passes over it, yet it will be found that the uplands have a general uniformity of elevation. Such an area is called a peneplain. The residual mountains which rise above the general level of the country and of which Monadnock is a sample have been named monadnocks. This is simply a name for a mountain left above a region which has been cut down by erosion to an irregular plain.
+
+200. Mountain Peaks. —In mountain regions the features which are often most impressive are the serrated peaks which rise above the main mass of the mountains. The shapes of these peaks vary greatly in different mountain regions and tend to give individuality to the mountains. The peaks have been formed by erosion, and their peculiarities are due to the different kinds and positions of the rocks from which they have been carved.
+
+The younger mountains which have not been subjected to erosion for a long time do not show the peak and ridge structure. Their personal characteristics have not had time enough yet to assert themselves. All these peaks are the result, not only of original uplift, but of subsequent carving.
+
+
+
+THE TETON RANGE.
+
+
+
+201. Mountain Ranges. —As a rule mountains are found in ranges. The mountains in the range are by no means all the same elevation, nor is the range necessarily continuous, there being often gaps along its course. Neither were all ranges in a mountain region elevated at the same time. Those which make up the mountain region of the western United States differ much in the time of their elevation.
+
+
+
+FAULT LINE OF AN EARTHQUAKE.
+
+
+
+202. Earthquakes. —In mountain regions which are young or still growing, earthquakes are not uncommon. These are due to breaks or slips of a few inches or a few feet in the rock structure. From the place at which the break or slip takes place the motion is transmitted through the rock mass to the surface, where it causes sudden and often tremen dous shocks. These slippings may occur occasionally for ages along the same fault line. Sometimes they are intense enough to cause great damage; at other times only a slight tremor is felt.
+
+The rapidity of the transmission of the shock differs with the kind of material through which it is transmitted, varying from a few hundred feet to several thousand feet per second. The nearer a place is to the break or slip the greater is the intensity of the shock. Sometimes the crack or fault along which the movement occurs reaches to the surface and makes the displacement apparent.
+
+
+
+FENCE BROKEN BY SUPPING OF THE EARTH ALONG FAULT LINE.
+
+
+
+
+
+THE RESULT OF AN EARTHQUAKE.
+
+
+
+If an earthquake originates under the sea, a great wave may be developed which rushes inland from the coast, causing great destruction. One of the most fearful of these waves occurred at Lisbon, Portugal, in 1755, sweeping away thousands of people who had rushed into an open part of the city to get away from the falling buildings caused by the earthquake shock.
+
+
+
+PLACEE MINING IN THE SIERRAS.
+
+The sand is washed from the gold by huge streams of water.
+
+
+
+Sometimes earthquakes are followed by terrible fires which cannot be extinguished on account of the disarrangement of the water supply. This was the case in the San Francisco earthquake.
+
+203. Products of Mountain Regions. —When rocks are folded and crushed, in forming mountains, heat is generated, and heated water under pressure acts upon the components of the rocks and dissolves some of their minerals, which accumulate in cracks and crevices called veins. When the overly ing beds have been worn away, these mineral veins, formed deep below the surface, are exposed and can be mined. Mountains are therefore the great mining regions.
+
+
+
+DEEP DOWN IN THE CALUMET AND HECLA MINE.
+
+The world's greatest copper mine.
+
+
+
+In this country mining is a most important industry in the Sierra Nevada Mountains and in the Appalachian region. In one are found great quantities of copper, silver and gold, and in the other iron and coal. In the old Laurentian Mountain region, near the Great Lakes, much copper is found. The Alps and the Pyrenees are among those mountains that have few minerals.
+
+If mountains are not too high, they are also regions of forests and furnish great quantities of lumber. The surface is so rough that agriculture is not easily carried on, but they have great areas of pasture which often support large herds of cattle, sheep, and goats.
+
+204. Effect of Mountains on Climate. —All over the world where people have the money and the leisure they are accustomed to go ei ther to the mountains or the seashore in summer in order to get where it is cooler. They might for the same purpose travel northward in the northern hemisphere, but they would need to go many times as far to get the same fall of temperature.
+
+
+
+TOP OF PIKE'S PEAK IN SUMMER.
+
+Notice the snow and the rocks broken up by the freezing water.
+
+
+
+In summer one must ascend a mountain on an average about 300 feet vertically to get a mean fall of 1℉., whereas one must travel over 60 miles north to get the same change. In winter one must ascend farther on the mountain and travel not so far north, to get a change of a degree. As one ascends a mountain it grows colder and colder. In ascending a high mountain in the tropics one passes through all the changes in climate which one would pass in going from the equator toward the poles.
+
+As already stated, high mountains also affect the climate of the country near them. The windward side of mountains is moist, since the moisture in the air is condensed in rising over them. On the lee side the country is dry, as the air which moves over it has already been deprived of its moisture.
+
+The country on the lee side will also be subject to hot, dry winds like the chinook winds of the eastern Rockies and the foehn in Switzerland. As the moist winds pass over the mountains their moisture is condensed. This raises their temperature so that it is above what it would normally be at the altitude reached. As they come down on the lee side of the mountain, the air is compressed and thus heated so that on this side it is considerably warmer at the same altitude than on the windward side. Thus high mountains affect not only the rainfall, but the temperature changes of the region round about.
+
+
+
+POPOCATEPETL.
+
+A snow-covered mountain in the tropics.
+
+
+
+
+
+LANDSLIDE
+
+Covering one of the main roads of Norway.
+
+
+
+205. Avalanches. —In mountain regions where the inclination of the surface is steep, the loose material is liable to slide down the moun tain sides, especially when it becomes moist because of long rains, or of the thawing of the frozen ground. As the material slides, the quantity increases, and momentum or force of movement is gained until a vast mass sweeps with almost irresistible force down the side, wrenching away trees, bowlders and whatever lies in its path. On reaching the valley the débris is piled in irregular mounds.
+
+The scars of these avalanches are seen on the sides of almost all high mountains. In mountain regions which are inhabited, avalanches are frequently very destructive of life and property. In the Alps large forests are often maintained above villages to check the avalanches if possible and thus to protect the villages.
+
+206. Mountain Animals and Plants. —As the temperature of mountains varies greatly from bottom to top, so the animals and plants must vary. Near the foot of the mountain the plants will be similar to those of the surrounding country, but these will soon disappear as the slope is ascended, since the temperature will have decreased, and their place will be filled by those capable of withstanding greater cold. If the mountains are sufficiently high, the tops will be bare or covered with ice and snow.
+
+The animals of mountains vary somewhat as do the plants, but since animals have the power of movement, their distribution will not be so uniform. They may ascend the mountain during the summer and retreat down the slope when the weather becomes severe. Animals driven from the plains by other animals or by man often find a place of safety in mountain regions.
+
+
+
+LANDSLIDE AT AMALFI.
+
+This destroyed a part of the famous momastery.
+
+
+
+The buffalo of the western United States found their only place of safety, until protected by stringent laws, in the mountainous region of the Yellowstone. The last small herd of caribou made their final stand in central Maine on the heights of Mt. Katahdin; the deer which once roamed widely over New York State now are restricted to the Adirondack Mountains. In these mountainous retreats pursuit is difficult, and they can persist for a long time after being exterminated elsewhere.
+
+Some animals, such as the chamois of the Alps and the mountain goat of the Rockies, are particularly adapted to mountain life and find a congenial habitat nowhere else.
+
+
+
+ROCKY MOUNTAIN SHEEP.
+
+
+
+207. Mountain Peoples. —Mountains offer a retreat to persecuted people as well as to animals. Here are often found the races which once inhabited the surrounding plains, but which have been driven from them by conquerors. The people of Wales and the Scotch highlanders are probably descendants from more ancient inhabitants of the island than those in control to-day. The Pyrenees, the Caucasus and the Hi malaya Mountains each contain tribes which were driven from the lower plains, but have been able in these retreats to withstand invaders who were too powerful for them in their former homes.
+
+Flocks and herds frequently make up the greatest wealth of mountain peoples. Indeed in these regions it is common to reckon a man's wealth by the number of cows he can keep. In summer the cattle are driven to the higher slopes of the mountains, called alps in Switzerland and saeters in Norway. In winter they are brought down to the valleys, where the little villages are, and where every available foot of land has been utilized to produce hay for their feeding during the long winter months. Life is hard and meager, and industry, foresight and thrift are necessary.
+
+As mountain valleys are separated from each other by ridges which for a considerable part of the year are almost impassable, the inhabitants are divided into groups whose world consists largely of the small valley in which they live. Their customs and manners of dress become in time somewhat different from those of the valleys about them. In Norway many of the different valleys have developed various unique and beautiful costumes. Only in recent years, since travel has become more common, have these been laid aside for the humdrum, characterless costumes of the rest of Europe and of the United States. In some of the Scotch Highlands the natives still cling to their ancient dress.
+
+
+
+A MOUNTAIN SHEPHERD WITH HIS FLOCK.
+
+
+
+Old-fashioned customs still maintain their hold in mountain regions long after they have been discarded in the surrounding country where intercommunication is easier. In the southern Appalachian Mountains many of the customs of the early pioneers are still common. Homespun clothing is still manufactured, and hog and hominy are the principal diet.
+
+
+
+A NEAR VIEW OF THE JUNGFRAU.
+
+Such scenes as this produce the wealth of Switzerland.
+
+
+
+In mountain regions, such as the northern Appalachians and Alps, where travel has been made comparatively easy, caring for the summer tourist has become the most important business. Here the old-fashioned customs have been laid aside and the boarding-house and hotel industry has largely supplanted all others. Such mountain regions have become a playground for the rest of the world, and the bracing air and cool climate are as great revenue producers as are fine farming lands and water powers.
+
+
+
+CRIPPLE CREEK.
+
+One of the largest mining camps in the world.
+
+
+
+In mountain regions rich in ores, mining naturally becomes the chief industry, and here, if there were any secluded native inhabitants, these have been replaced by the energetic miners from distant places. The deep and remote valleys and mountain sides have become the homes of mining camps and cities. Railroads have been built to these, overcoming almost impassable obstructions, and ore crushing and smelting works supply the places of the mills and factories of the manufacturing cities. When the ore fails, the army of workers moves on, and the city, once thriving and booming, becomes suddenly simply an aggregation of empty dwellings.
+
+208. Effect of Mountains on History. —Not only have mountains been retreats for the vanquished, but they have been barriers against further conquest by the conquerors. It is very difficult for an army to traverse a mountain range. For a long time the Alps hemmed in the power of Rome. One of the greatest exploits of Hannibal and later of Napoleon was the passage of these same mountains.
+
+In our own country the Appalachian Mountains acted for a long time as an impassable barrier to the expansion of the Thirteen Colonies. The trails across them were so long and difficult that it was many years before the fertile plains on their western side became populated. The Mohawk valley opened a comparatively easy route at the north, but the Cumberland trail at the south was long, circuitous and full of places suitable for Indian ambushes.
+
+The little mountain country of Switzerland is a buffer state for the rest of Europe. Afghanistan, rough, mountainous and desert, is a buffer state for Asia.
+
+Mountains are often used as boundaries to countries, as in the case of the Pyrenees between France and Spain and the Carpathian Mountains between Austria-Hungary and Roumania. In early times it was thought sufficient to indicate the crest of the mountains as the boundary line, but soon it was found that what was to be called the crest was so open to controversy that definite lines, accurately determined from point to point, had to be substituted.
+
+Sometimes the determination of what shall be called the crest line has given rise to bitter international disputes, as was the case recently between Chile and Argentina. It may happen that mountain boundaries are so broad and complicated that a little country inserts itself along the boundary of two powerful nations and is able to protect itself from being absorbed by either. The little country of Andorra, containing only 150 square miles, situated in a lofty valley on the southern slope of the Pyrenees, with a population not exceeding 10,000, has remained independent for nearly a thousand years in spite of its powerful neighbors.
+
+
+
+
+
+Summary. —The high parts of the earth are plateaus and mountains. Some plateaus are dissected by the troughs of rivers that run through them and some are broken by faults. When plateaus are old and worn down they usually show remnants of their former surface in buttes and mesas.
+
+Mountains are elevations higher than hills. Block mountains are formed by breaks in the rock layers of the earth; folded mountains are due to folds caused by lateral pressure. Massive mountains are complex in structure and their causes are various. The peaks of mountains are formed by erosion. Many mountains are found in ranges.
+
+Mountains have a great effect upon climate. The windward side of mountains is usually wet and the leeward side dry. The wind, rain, and snow cause avalanches, which often do great harm to the plants and animals of the mountains and valleys.
+
+Mountains have also a great effect upon history. Not only do they form excellent boundaries between nations and states, but they offer protection to weak animals which are unable to withstand their stronger neighbors in the unprotected conditions of the plains.
+
+
+
+
+
+QUESTIONS
+
+
+Describe the characteristics of a young plateau.
+
+Why do not dissected plateaus attract a dense population?
+
+What are the characteristic features of an old plateau?
+
+Where in the United States are broken plateaus found?
+
+Why are there no lofty old mountains?
+
+How are block mountains formed? Where are mountains of this kind found? How are folded mountains formed? Where is a fine example of such mountains to be seen?
+
+What are the characteristics of massive mountains?
+
+What happens to mountains if they are exposed to erosion for a very long time? Where in the United States are such mountains found?
+
+What are the causes which produce mountain peaks?
+
+How are earthquakes caused?
+
+What are the principal industries in mountain regions?
+
+How do mountains affect climate?
+
+What influence have mountains had upon plants and animals?
+
+What influence do mountains have upon their human inhabitants?
+
+What has been the effect of mountains upon history?
+
+
+
+
+
+【中文阅读】
+
+
+190．早期高原——大面积的水平岩石层有时会抬升到高出海面很高的高度，并由于之前的侵蚀作用，形成地表极不规则的平原，这样的区域就叫做高原。从高原向低处下沉的地方都十分陡峭，这些区域一般还有河流，并由于河床落差很大，水流湍急，河谷被冲刷侵蚀得很深，这一切都根源于高原的地质抬升。
+
+如果这里降雨很少的话，河流也就很少，只有一些陡峭嶙峋的沟壑深谷。这就让行经穿越这一地点变得非常困难。这些沟谷由于过于狭窄，根本不适合居住，连修路都很困难，这里的河流又太宽，也不好架桥，因此这一类高地一般都荒无人烟。
+
+如果这些高地位于炎热地带，由于高海拔导致的凉爽气候则成为它们宜居的亮点，很受人们青睐。但若它们处于本来就很温和的气候带上的话，表面的温度就很冷了。
+
+随着河沟不断被侵蚀磨损，坚硬的岩石慢慢凸显出来，像弯曲的长条凳那样沿着沟岸蜿蜒盘曲。而且在不同的岩石台面上，剥落的软层岩石不断形成斜坡，一直向下倾斜，因此整体看上去，这些沟岸就像松软地毯上铺设的一段台阶一样。科罗拉多高原就是这样一个典型例子，由于被大河在很长的距离中不断侵蚀，地球上最显著的侵蚀地貌就存在于这里。这里的岩石呈现出不同颜色，有些峡谷深度达到1英里，宽度达到了15英里。站在大自然这样的侵蚀地貌面前，简直没有语言可以形容它的壮美雄奇。精彩绝伦、宏伟殊胜、庄严伟岸，这些词语在大自然如此梦幻般的杰作面前，都黯然失色。
+
+这一地带很干燥，河流的很长一段流域内都几乎没有支流。如果这里的降雨量很充沛的话，河道就会比现在要宽得多了，水流也会大不少，至少能充盈整个峡谷底部。这其实还是河谷的早期状态，它所肩负的对这一地区的侵蚀冲刷的繁重任务才刚刚开始，因此支流都还很小，支流之间的土地还依然辽阔。
+
+191．切��高原——如果一个在高原地区处于雨量充沛的地带，且发生抬升之后已经过了很长的时间的话，这里的河流就会纵横交错，把地面分割得支离破碎，只剩下河道之间一条条狭长的陆地依然暴露在高原上。沟谷很深，但是这些条状陆地的范围却非常小。在这里行进的话，只要不是沿着河道走，往其他任何方向，都是不断在山谷中爬进爬出。
+
+这里没有平坦的耕地用于耕种，由于坡谷陡峭险峻，也没有办法修路。山谷也非常狭窄，就算要修路也只有沿着河岸修，可是洪水一来的话，一切就泡汤了。因此这一地区的农业几乎没有，一是交通困难，二则由于根本没地方栽种。
+
+铁路也只能穿过这里一些比较大的山谷，当然了，由于地貌险峻狭窄，施工相当困难。除非这里矿产丰富，以此能聚集一些人口外，其余情况下这里都只是穷乡僻壤而已。若一旦这里的树木被砍伐，土壤就会很快从山坡上流失，最终只留下光秃秃的裸露岩石。这样的例子在阿勒尼格高原和坎伯兰群高原上就能发现，也就是从纽约向阿拉巴马州延伸的这一地带。
+
+192．晚期高原——如果高原抬升以后所经历的时间足够长，这些切割高原的河流便能将河谷冲刷出很宽的宽度，也会把各条河流之间的陆地慢慢冲刷殆尽，只有一些宽阔地域尚能幸存。这样，河流就让高原地表渐渐下降，变成一个低矮的平坦区域，上面还残留着尚未被冲刷而去的地块，但从陡峭坡岸的岩石层上也看得出来，它们曾经也在整个高原上广泛分布。如果剩下的地块较大的话，西班牙人便把它们叫做平顶山，本意是桌子；如果很小的话，就叫做孤峰，来源于法语中一个意义为地标的单词。
+
+有些平顶山由于太高太陡峭，几乎完全无法攀爬，不过其他大多数还是一些比较矮且顶部很平的山丘。若是在新墨西哥州和亚利桑那州旅行的话，这样的平顶山随处可见，它们就像印第安人的小草房和泥盖木屋那样看起来孤零零的，但其被分割的残留区域也依然广泛分布着。
+
+在晚期高原上的徒步旅行是很容易的事情，这里没有很深的沟谷，旅行者可以轻松地在平顶山上行进，在这茫茫天地之中，人也成了风景的一部分，因为这里的本来状态的确太单调了。一些很高的平顶山常常被某些印第安人部落所占领，他们一般是流落至此，以这些平顶山作为繁衍生息的屏障。就像中世纪的某些贵族那样，一旦遇到社会动荡便逃进偏远的城堡。
+
+193．破损高原——让高原发生抬升的力量有时也不一定非常均衡，尤其是当高原的面积很大时，这些力量的不均衡作用也会使岩石层发生破裂。因此我们常常发现一些岩层破损不堪且复杂交错的高原，而且那些破损岩层有时还比其他岩层高出一千多英尺。
+
+这样的地域在科罗拉多大峡谷地区便可以见到，在那里，凯巴布高原高出科罗拉多高原将近2000英尺，东西两面全是悬崖峭壁，这就是断层崖。这样的地貌在这一地区还有很多处，就像它们的名字一样，充分说明这一地区在曾经发生地质抬升的时候，岩层出现了严重的破裂。另外由于凯巴布高原比两边的地区都高出许多，因此也便阻挡拦截了降雨，让这些地区树木难以生长，成了不毛之地。
+
+在科罗拉多大峡谷的石壁上，岩石的裂口纹路清晰可见，相同的地层分布线有时会突然比旁边高出上千英尺。在这些裂口悬崖或者断层崖前面，沿线会堆积很多岩石砂砾的碎片，这说明地质抬升发生后，即使在干燥地区，侵蚀作用也依然非常显著。但科罗拉多河依然默默地流过这些断层，完全无视它们的沧桑故事。这一地区的峡谷似乎并没有收到断层作用的影响，也许这种影响会发生地很缓慢吧。
+
+194．丘陵与高山——地球表面不规则的高地区域叫做丘陵，而当它们海拔较高时，则被叫做高山。不过这二者之间并没有严格意义上的分界，在高山地区的高地常常被叫做丘陵，而在平坦地区的丘陵却又常常被叫做高山。就一般而言，海拔至少2000英尺以上才应该被叫做高山。但如果一个地区的总体高度已经很高了，这时的高山还必须明显高出该地区的总体高度才行，比如落基山脉的高地，海拔4000英尺的山体才有资格被叫做高山。
+
+195．高山的构架——高山的形成是地壳变形的结果，但这种变形的根本原因我们目前还没有完全了解，这也因此成为地质学的研究内容之一。地壳在漫长的地质年代中，会发生皱褶、挤压、弯曲等各类变形，并导致部分地区抬升到很高的海拔高度。那些抬升时间还不算特别长，还没有被侵蚀作用完全消磨的地区，则形成了高山。
+
+所有的高山都差不多形成于晚近地质时期，当然至少也是数百万年了。如果现在的高山都形成于远古地质时期的话，那可能在很早以前就已经被大自然的力量夷为平地了。相对而言，残留至今的远古高山都已经很低矮了，其原因并不是它们当年没有抬升到今天这些高山的高度，而是漫长岁月风霜雪雨的侵蚀作用已经消磨了它们年轻时的棱角，让其高度慢慢缩减了。
+
+给各类高山进行严格分类其实很困难，因为高山的结构都很复杂，但一些特别的高山形式还是可以略加区分的。
+
+196．断块山
+
+
+
+
+
+实验131：取三张光滑的直边木板，其中两张的尺寸为15×55cm，另一张为8×55cm。将它们平整地放在桌面上，小的那张夹在两张大的中间，长边对齐。然后在木板上分层均匀地撒上适量的玉米面、碳粉、石粉、熟石膏以及锯末，现在将宽板的内边缘小心抬起，再将一个厚约1到2cm的小木条从上面滑下去。可以看到，板面上的粉末物质被打乱了，都会向木条划过的纹路的一边挤压堆积。然后在宽板的另一边重复同样的实验，整个过程就与断块山的形成过程较为类似。
+
+
+
+
+
+在俄勒冈州南部并继续向南延伸的这一地带，我们能看到一些狭长的山脊，它们一边是陡峭的悬崖，另一边却是缓坡。这些山脊之间的区域是一些底部平坦的槽形洼地，有时还有一些小湖泊散布其中。这些山脊是由厚岩层像它们的斜坡那样，不断向一个方向倾斜而形成的，山脊边沿上的短坡则是由于破损效应而形成。
+
+在一些峭壁底部的残石坡上，有时能看见它们在与悬崖平行的方向上有轻微的破损。这些深深的峭壁，有的是隔着中间平坦的洼地面面相觑，有的则是面向其他山脊一侧的长坡望断云天。有些山脊上的裂口比其他地方更突出，这说明它们经受侵蚀的时间更长。
+
+这些山脊都是由于底层的压力作用而形成，这种力量让岩石层发生破损，并让断裂处的某些岩层发生抬升，让它们形成了高低不一的状态，这样如临深渊的悬崖峭壁就形成了，而之前的岩层这时则会发生倾斜，向上朝着抬升起来的地层。这类高山就叫做断块山。从它们断裂处的残损坡面开来，这种抬升运动还没有彻底结束呢。
+
+由于一些山脊的裂口比其他地方更突出，便可以推断地层断裂并不总是发生在同一时间，开口越剧烈的山脊形成得会更早一些。地震在这一地带也非常少，因为沿着断裂处的裂纹，地层的倾斜滑动能变得非常平缓。
+
+位于俄勒冈州的这类山脊，被侵蚀的程度都很小，结构也比较简单，年代也并不久远。这里较长的溪流从缓坡上沿着岩层分布的方向潺潺流下，较短的溪流则在深一点的坡谷中沿着岩层边缘默默流淌着。
+
+197．褶皱山
+
+
+
+
+
+实验132：取一张10cm宽20cm长的木板，在它的长边上牢固地钉上一张20×25cm的橡胶皮，让其一条短边与木板长边重合，另一个短边再钉上一个2cm宽20cm长的木条。然后将它们放在桌子上，将宽板用夹子或钉子在桌上固定牢实。拿着这根窄木条，让橡胶皮撑开到最大限度，然后再将其固定，并让橡胶皮始终保持其撑开的状态。
+
+然后在橡胶皮上均匀地分层撒上一些细锯末、熟石膏、细碳粉、沙石粉、玉米面或者其他容易区分层次颜色的细末物质。然后将这些细末层稍微打湿，解除窄木条的固定，让将胶皮缓慢收缩。当收缩彻底完毕以后，用锋利的小刀片小心地将这些细末层连同橡胶皮一起切开，观察桌面部分的切口，会发现这些细末层折叠成了不规则的波状褶皱，这就和褶皱山很类似了。
+
+
+
+
+
+当岩石层遭受到缓慢、统一而剧烈的横向压力的时候，它们就会形成波状褶皱，并伴随有部分的断裂。这里的收缩压力来源于地球内部，主要是由于温度冷却。于是，在某些特定条件下，岩层的波状褶皱就得以产生了。
+
+突出的例子就是位于瑞士和法国之间的侏罗山。在河流流经的河岸上，可以清晰看见地层的褶皱，这些山体的年代都比较短，因此侵蚀作用也不太显著。这样，向下的弯曲就形成了河谷，向上的弯曲就形成了山脊。
+
+这些褶皱的岩层中还含有海底化石，这表明它们曾经也是地平面，并且肯定形成于海洋当中。一些较长的河流会在皱褶的凹槽中流淌，但在一些褶皱高度不太高的地方，河流还可能将凹槽之间的凸起侵蚀掉，进而跨入别的凹槽。在这些横向的河道[1]之间，人们也修筑了不少横跨山脊的道路。
+
+有时某些山脊的顶部由于长时间的侵蚀，而显得非常宽阔平坦，因此往往坐落��一些村庄。但大量的人群聚集还是在狭长的山谷中，尤其是在一些横向的山谷。一些横向河流看起来与凹陷的褶皱并没有联系，但是又会随着它们的河道上升，在褶皱中不断侵蚀两边的谷岸，这从一个侧面也反映出褶皱的速度其实非常缓慢。
+
+198．巨型高山——我们研究过的高山海拔都不算高，几乎没有超过6000英尺的。而且它们的形式与外观都比较简单，尽管也有一些令人赏心悦目的风景，但总体上还是略显单调。巨型高山可就不同了，它们结构复杂，海拔极高，大自然的雄伟壮美在这里展现得淋漓尽致。
+
+在任何时代，高山都鼓舞着人类的高贵精神和雄心壮志。它们翘首苍宇，注目蓝天，高耸入云，俯瞰大地；它们迫使我们仰望，并启示我们关照内心、叩问自我。就像其他一切变动不居却又持之以恒的事物一样，高山也是地球张力与压力共同作用的结果，这种力量的矛盾与斗争从未停息。
+
+大型山脉的结构极其复杂，这里的岩石层不断褶皱扭曲，和它们最初的形态相比，都已经面目全非了。它们的抬升没有简单一致的过程，任何时期都会有特殊的动力推动它的成长。正因为这些力量的增强导致它们的不断抬升；同样的，当这些力量削弱时，它们也会慢慢降低。它们宽阔的山梁被削成了山峰，峡谷与洞穴也被压缩成了石壁。
+
+不同的岩层差别巨大，各自都有着自己的曲折经历，大风、雨水、溪流、雪崩、冰川等等自然力量都会对它们施加影响，进而形成各自独特的结构外形。斑驳陆离的美，自然就是应运而生了。大型山脉的中间部分都是由火成岩构成，两边则能看到沉积岩。落基山脉、阿尔卑斯山脉和喜马拉雅山脉，便都是这种类型。
+
+199．消逝的高山——目前地球表面上这些特征鲜明的高山，既不是诞生在同一地质年代，也不是陆地上存在过的地貌形态的唯一代表。从地质年代着眼，尽管它们各自的年龄也是千差万别的，但都还算很年轻。
+
+整个地球表面都被水流作用不断地侵蚀着，且海拔较高的地方比较低的地方侵蚀得更快，因为这里的剥蚀作用更加突出极端。因此，当高山一旦停止增加高度，便会慢慢变矮，直到最后完全不能称其为高山了。在此过程中，岩石层会褶皱弯曲，除了抬升以外的各类山体地质现象也会完全表现出来。
+
+倾斜的岩石层就可以表现出山体在很早之前的巨大抬升，剩下的山体根部区域的抬升在最后也变得极其缓慢，这类地区在地球上不胜枚举。
+
+在宾夕法尼亚的阿巴拉契亚高地，岩层的状态显现出它们曾经褶皱成为山脊和凹槽，跟侏罗山脉差不多。但现在这些拱形褶皱已经被侵蚀得无影无踪了，剩下的山脊也是由于坚硬岩层对侵蚀的抵抗才得以幸存。这类山脊的所在地，多半也是在之前褶皱凹槽与褶皱顶峰比较均衡类似的地方。
+
+高山地区的总体地貌，也和曾经褶皱发生时不尽相同。那里曾经抬升到的高度，比现存的最高峰要高得多。要是现在的岩床能再来一次重新构建的话，它们能比现在任何一座山峰还要高。研究还显示，高山的抬升与侵蚀也不仅仅只发生过一次。
+
+另一个高山退化的地区，便是哈德逊河湾的劳伦系高原。这里的高山的地质年代都非常久远，遭受侵蚀的时间自然也就极其漫长。在一些地区，比如新英格兰南部，由于山体结构被侵蚀得很严重，因此只剩下一些尚未被彻底侵蚀干净的残丘，比如莫纳蒂诺科山。
+
+尽管山地附近到处是丘陵与山谷，人们行经其中的时候也感觉不到任何平原的迹象，但由于这些高地的抬升有时也会呈现出一定的均衡性和一致性，因此总体上也能形成常说的准平原。这些剩余的山体部分，有些地方也会高于该地区的总体高度，它们被称作残丘，莫纳蒂诺科山就是例子。这是一个简单的命名，在由于侵蚀作用而被削为平地的区域残留下来的高山，都可被冠以此名。
+
+200．山峰——在高山地区，给人留下最深影响的，常常是那些高耸突兀又连绵不断的锯齿状山峰。不同高山的山峰形态各异，参差无穷，真可谓“横看成岭侧成峰，远近高低各不同”，每一座高山也都因为它们的存在而具有了自己独特的面目。山峰都是有大自然的侵蚀作用而形成，之所以出现不同的形态，是因为受侵蚀岩层的种类与位置有差异。
+
+年代较短的高山，由于遭受侵蚀的时间还不算太长，因此山峰和山脊构架并不十分明显突出，其自身特点还没有足够长的时间来加以表露。所有的山峰都不仅是山体抬升的结果，更是后来的侵蚀雕镂的杰作。
+
+201．���脉——就一般规律而言，高山总是以山脉的形式而存在。山脉中的高山不可能处于同样的抬升水平，而且山脉也不一定总是连续的，在它们的延伸路径上，也常常有一些断裂的缺口。当然，就算同一个山体的不同山脉，其抬升时间也是不一样的。美国西部的诸多山脉，其抬升时间就是千差万别的。
+
+202．地震——在地质年代很短或者依然处于上升期的山地，地震是常有的事情。这是因为在岩层之间，到处都有数英寸或者数英尺的裂口或间隙。通过这些地方，地层的运动从大量的岩石层中传播到地球表面，进而形成了突然的剧烈震动。这种岩层的相对移动，会间歇性地发生在不同时期的裂纹线上。有时强烈的地震能导致巨大的毁坏，但有时也仅仅是轻微地颤动一下而已。
+
+地震波的传播速度因地质构造的不同而差别巨大，从每秒数百英尺到数千英尺不等。靠地震断裂处越近的地方，遭受的地震影响越大。有时地震断裂带上的地层移位甚至破裂，能让地表变得面目全非。
+
+如果地震发生在海底，则会生成巨大的海浪，从海岸涌入内陆，造成巨大灾难。其中非常可怕的一次就发生在1755年的葡萄牙里斯本，成千上万的人涌入城市的空旷地带，以躲避地震造成的房屋倒塌。
+
+地震往往还会伴随着无法在短时间内扑灭的大火，因为一乱起来，水的供应也就成了问题。在旧金山大地震中就曾发生这样的事。
+
+203．高山地区的出产物——当岩石层发生褶皱弯曲形成高山以后，热量便得以积累，被加热的水流在压力之下又会作用于岩石本身，溶解它们的部分矿物质，这些矿物质一般都聚集在岩石的裂口与断层处，这些地方我们统称为矿脉。当覆盖岩层被侵蚀作用彻底剥离以后，矿脉便暴露出来，人们也就可以开采利用了。因此，山地往往也是矿产的出产地。
+
+在美国，矿产业已经成为内华达山脉与阿巴拉契亚地区最重要的工业。前者含有大量的铜矿、银矿和金矿，后者含有大量的铁矿和煤矿。在五大湖区附近，古老的劳伦山区也含有大量的铜矿。阿尔卑斯山脉和比利牛斯山脉则是没有什么矿藏的山脉[2]。
+
+如果高山地区的海拔不是特别高的话，那里也常常拥有大片森林，并能提供大量的木材。由于地表非常粗糙，农业耕种自然就很困难，但却是天然的牧场，“风吹草地见牛羊”则成为这里的常见一景。
+
+204．高山对气候的影响——全世界有钱有闲的人在夏季选择避暑地点时，一般都是要么去高山地区，要么去海滩地区，因为这些地方都非常凉爽。在北半球，为了同样的目的，他们也可能向北旅行，但是要的得到同样的温度落差，要走的路可就远多了。
+
+夏天，一个攀登高山的人需要攀爬大约300英尺的垂直距离，环境温度就会下降1°F，而要达到同样的结果，向北的旅行者则必须行进60英里。不过在冬天，为了温度有1°F的改变，登山的人要攀爬得更远，地面的旅行者则不需要走夏天那么远，因为山上的气温总是越来越冷的。在热带地区的高山，登山的人从山底到山顶所经历的气候变化，几乎跟地面旅行者从赤道走到极地差不多。
+
+如前所述，高山会对周围地区的气候产生影响。高山的迎风面会多雨而湿润，因为空气中的水分会在上升过程中不断凝聚。背风面则比较干燥，因为这里的空气已经失去了大部分的水分。
+
+背风面的地区常常还会遭受到干热风的吹刮，比如落基山东面的奇努克风，以及瑞士的焚风。由于湿润的风在爬升的过程中会将水分凝结，这就导致它们的温度上升，超过它们在相同海拔地区的平均温度。而当它们从背风面吹下来时，空气又因压缩而受热，因此这一面上的温度，便会高于迎风面上同海拔位置的温度。因此高山对气候的影响不仅仅是降雨量的增减，对周围地区的温度影响也是很显著的。
+
+205．塌方与雪崩——在高山上一些坡度很陡而且坡面很深的地方，往往地面土质都比较松软进而也很容易滑落，尤其当长期降雨或者冰雪融化之后，地面土石变得湿润，则向下滑落的可能性就更大了。随着土石不断滑落，其数量与运动量也逐渐增加，力量便也越来越大，到最后形成了不可阻挡的碎屑洪流冲下山坡，将树木连根拔起，留下满山的断石残垣。到达山谷以后，这些残碎土石又会堆积成小小的山丘，将山谷塞得满满的。
+
+这类塌方或雪崩的印记，几乎在所有高山的山坡上都能找到，在一些有人居住的高山地区，塌方与雪崩常常频繁地发生，给人们的生命财产带来巨大灾难。在阿尔卑斯山脉地区，人们在山坡上种植了大片的树林，以减少塌方与雪崩的发生，保护他们赖以生存的村庄与土地。
+
+206．高山上的动物与植物——由于从山底到山顶的温度变化很大，因此生在高山上的动植物也就千差万别。靠近山脚的植物跟周围地区的植物差不多，但随着山坡高度增加，它们马上就消失得无影无踪了，代替它们的则是更能抗寒的植物。如果山体足够高的话，山顶则一般是光秃秃的岩石，或者终年覆盖着积雪。
+
+高山上的动物也跟植物一样，不同海拔有不同的种类。但由于动物毕竟能自由移动，因此它们的分布并没有严格的统一性。夏天它们可能爬向高处避暑，冬天则有可能撤下山脚御寒。在平原地区被其他动物或人类追索的动物们，往往能在高山地区找到它们安全的乐园。
+
+美国西部的野牛，在受到法律的严格保护之前，它们的安乐窝就在黄石公园的高山上。一小群北美驯鹿，就是在缅因州中部的卡塔丁山上留下了它们最后的身影；曾经在纽约州悠然漫步的鹿群，现在也只在艾迪龙达克山上看到它们。在这类高山地区，自由地追逐与进退都很困难，而它们总能在新的地方接受生存的考验之后，默默地坚持下来。
+
+有一些动物，比如阿尔卑斯山脉的羚羊，以及落基山脉的山羊，便尤其能适应山地环境，并在那里大量繁衍生息，比在其他任何地方生存得优哉游哉。
+
+207．高山上的人们——除了对动物如此，高山往往对人类也提供了一个远离世俗纷扰的世外桃源。高山上常常生活着一些曾经居住在周围平原地区的人类族群，他们常常是为了躲避侵略者的战火而迁居于此的。威尔士和苏格兰高地的人们，很可能就是早期岛民的后裔，而并不是如今当地土著居民的后代。在比利牛斯山、高加索山、喜马拉雅山上，都有一些被从平原地区追赶至此的部落在这里繁衍生息。在这样的世外桃源，他们便能抵御侵略者的暴力。而在之前的家园，他们往往只有“三十六计走为上计”的份儿。
+
+牲口和家畜的数量常常是高山地区人们财富的象征。在这些地方，一个人财富的多寡，直接可以从他拥有的牲口数量来判断。夏天，牲口会被赶至山坡高处，这类地方在阿尔卑斯山叫做alps，在挪威则叫做saeters，都是高山牧场的意思。冬天，牲口则被赶至山谷，那里有一些小村庄，一些土地的干草也可以在漫长冬天作为牲口的口粮。这里的生活是很艰辛的，因此不停劳作、未雨绸缪、勤俭持家便成为这里的生活信条。
+
+高山的山谷常常也被山脊彻底分割开来，有时甚至一年到头无法通行，因此这里的人们也被分成不同的族群，每一部分都由他们聚居的小村庄组成。这样一来，他们的风俗、习惯、着装也与临近其他部分的族群不尽相同。在挪威许多不同山谷中，人们习惯各异，而且都有各自族群独特的漂亮服饰。只是在近些年，由于交通越来越方便，欧风美雨不断浸染，单调乏味、千篇一律的欧美服饰慢慢取代了这里本来色彩斑斓的世界。但是在苏格兰一些高地上，人们依旧坚守着他们古老的传统服饰。
+
+古老的传统习俗让他们在山区生活多年以后，依然能够顽强地保持他们的鲜明特色，并让他们在与周围的世界相比较时，显得与众不同。在阿巴拉契亚山脉南部，许多早期先民的风俗依然随处可见，手织服饰在这里依然是“慈母手中线，游子身上衣”，山猪肉和玉米粥也依然是这里的农家主食。
+
+在其他一些交通相对便利的地区，比如阿巴拉契亚山脉和阿尔卑斯山脉北部，招待夏季的旅游者，已经成为这里最重要的产业。这些地方，古老的风俗已被丢弃一边，木板房、小旅馆则成了最红火的替代对象。这些山区已经成了世界各地的人们的游乐场，凉爽的空气、舒适的气候也成了这里最著名的土特产，与这里的农田水利一样闻名遐迩。
+
+在矿石藏量丰富的山区，采矿业自然就成了这里的主要产业。如果那里早先就有土著居民居住的话，远道而来的雄纠纠气昂昂的矿工慢慢也会将他们取代。漫山遍野将搭建起采矿的工棚，当地也会渐渐成为矿业城镇。铁路慢慢地修筑到了这些地方，以前无法逾越的障碍如今全部被克服了，矿石的粉碎与熔炼使得这里的城镇到处是小作坊和加工厂。而当矿藏一旦开采完毕，矿工队伍便作鸟兽散，这里曾经繁荣兴旺的城镇也会很快变成死寂的空城，唯有一座座空无一人的房屋，在此茕茕孑立形影相吊。
+
+208．高山对人类历史的影响——高山不仅可以成为弱势族群的避难隐居之所，而且可以成为抵御外敌入���的天然屏障。因为军队毕竟很难轻松地横穿高山地带。在很长的一段时间理，阿尔卑斯山脉都处于罗马帝国的威严统治之下，伟大的汉尼拔大将[3]的生平壮举之一就是带领大军横穿这一大山，后来的拿破仑也是如此。
+
+在美国，阿巴拉契亚山脉在很长的时间里都成为一道无法逾越的障碍，阻挡了十三州殖民地的扩张。由于那里的山路又长又陡，穿越十分困难，因此过了很多年，它西面的肥沃平原上才慢慢有大量人口聚居。相对而言，莫霍克山谷则在其北部开出了一个较为容易进出的通道，而南边的坎伯兰山道则是弯弯曲曲的长长山路，这为印第安人的埋伏提供了很好的环境。
+
+瑞士的狭小山区是整个欧洲的战略缓冲地带，阿富汗广袤粗糙的山区的沙漠也是整个亚洲的缓冲地带。
+
+高山还常常被用来作为国界线，比如法国和西班牙之间的比利牛斯山，以及奥匈帝国和罗马尼亚之间的喀尔巴阡山就是如此。以前人们以为将山顶作为分界线就行了，但很快发现有的山顶太开阔，以致让国界线不好划分，因此后来就只能以固定点之间的连线，来精确划分国界。
+
+有时由山顶确定的国界会导致国与国之间的争吵，比如之前智利和阿根廷之间就上演过这类故事。有时还会出现这样的情况，就是高山的国界线区域很宽阔，而且很复杂，沿着山脊而存在的小国家就被夹在两个大国之间。这样往往很安全，两边讨好会对它们本身形成一种地缘政治保护。比如安道尔这个小国家，面积只有150平方英里，位于比利牛斯山南面的山坡上，人口还不到10000人，却在这里独立存在了上千年。而它的邻居，可都是膀大腰圆的大国。
+
+
+
+
+
+总结——地球表面海拔较高的地方叫做高原或者高山。有些高原会被河流的河槽分割，有的则因侵蚀而慢慢破损。当高原老化又被冲蚀殆尽之后，以前的表面上就会只剩下残丘和平顶山。
+
+高山的海拔要高于丘陵。断块山是由地球岩层运动而形成，褶皱山是由岩层受到侧向压力而形成。巨型高山的结构很复杂，形成原因也各不相同。山峰是由高山的侵蚀作用而形成。许多高山都是以山脉的形式存在着。
+
+高山会对气候产生重要影响。其迎风面会很湿润，而背风面则很干燥。风霜雨雪都能导致雪崩的发生，这对高山地区的动植物以及附近的村庄极为有害。
+
+高山也会影响到人类的历史进程。它们不仅可以成为国家之间的分界线，而且还能为弱小动物提供安全的生存屏障，让它们免于在毫无保护的平原地带被其他动物欺凌。
+
+
+
+
+
+思考题
+
+
+请描述一下早期平原的特征。
+
+为什么未被河流分割的高原往往聚居着大量的人口？
+
+晚期高原有哪些鲜明的特征？
+
+在美国的哪些地方能看到破损高原？
+
+为什么找不到海拔很高的晚期高山？
+
+断块山是如何形成的？能在什么地方找到这类高山？褶皱山是如何形成的？在哪里能看到它们的典型例子？
+
+巨型高山有哪些独特的特征？
+
+如果高山长时间暴露于侵蚀环境中，最终会发生什么情况？在美国的哪些地方能看到这类高山？
+
+山峰的形成原因都有哪些？
+
+地震是如何形成的？
+
+高山地区一般存在哪些主要产业？
+
+高山是如何影响气候的？
+
+高山对动植物有哪些影响？
+
+高山对生活在那里的人们有哪些影响？
+
+高山人类的历史产生了哪些影响？
+
+
+
+
+
+译　注
+
+
+[1]此处横向指的是山体岩层褶皱后形成的与自然延伸方向垂直的沟槽。想象一根围巾，长的方向为纵向，将围巾挤压折叠，与纵向垂直的沟槽就是这里“横向”的含义。
+
+[2]现在来看，这个结论并不正确。在过去的一个世纪中，这两个山脉都发现了大量的水晶或金属矿藏。
+
+[3]汉尼拔·巴卡（Hannibal Barca，公元前247年—前182年）北非古国迦太基名将，也是世界军事史上的卓越战略家，一生军功显赫，曾率领军队从西班牙翻越比利牛斯山和阿尔卑斯山。
+
+
+
+
+
+CHAPTER 14
+
+VOLCANOES
+
+火山
+
+
+209. Subsurface Earth. —Many excavations and borings have been made deep into the earth's surface, and it has been found that the temperature increases with the depth. The rate of increase is not the same in different places, nor is the increase always uniform in the same place. The average of a number of deep excavations in different parts of the earth gives a rise of 1° F. for each 70 or 80 feet of descent.
+
+
+
+MOUNT SHASTA.
+
+An extinct volcano.
+
+
+
+The greater the pressure to which rocks are subjected the more difficult it is to melt them. If it were not for this, the solid part of the earth could not be more than 40 or 50 miles thick, as the interior heat would melt rocks under ordinary pressure. But the earth is too rigid for its interior to be otherwise than solid. So great is the pressure to which it is subjected that probably none of the material deep down in the interior of the earth is in a molten condition.
+
+If the pressure near the surface should be decreased, or if the normal amount of heat at any place should be increased, the material might become fused, and under certain conditions might find its way to the surface. We know that heated material from below does rise toward the surface and intrude itself into the surface rocks and in some places pour forth over the surface.
+
+What causes the uprising and outpouring of this molten material from below the surface of the earth, and how and why it reaches the surface are questions which as yet are unanswerable. But as soon as this igneous material comes within the range of observation, its properties and actions can readily be studied. The following descriptions of some well-known typical volcanoes show some of the results of subsurface activity.
+
+210. Monte Nuovo. —In 1538，on the shore of the Bay of Naples near Baiæ, that once famous resort of the Roman nobles, after a period of severe earthquake shocks there suddenly occurred a tremendous eruption. From within the earth emerged a mass of molten material blown into fragments by the explosion of the included gases. Within a few days there was formed Monte Nuovo, a hill 440 feet high and half a mile in diameter, having in the top a cup-shaped depression or crater over 400 feet deep.
+
+So great was the explosive force of this eruption that none of the ejected material was poured out in the form of a liquid. The whole hill is made up of dust, small stones and porous blocks of rock which resemble the slag of a blast furnace. The small fragments in such eruptions are called ash or cinders. In a week the eruption was over, and nothing of the kind has since occurred in the region.
+
+When visited by the writer a few years ago, the bottom of the crater was a level field planted to corn. The whole process of formation of this volcanic cone was observed and recorded by residents of the region. Other similar eruptions have been observed, but perhaps this is the best known. We have here what may well be called a young volcano. The cone to-day is almost perfect in form.
+
+
+
+CINDER CONE NEAR MOUNT LASSEN.
+
+
+
+In northern California, near Mt. Lassen, which has itself recently become active, another almost perfect cone of this kind is found, which was probably formed much more recently than Monte Nuovo. From this cone both cinders and liquid material or lava were ejected.
+
+211. Vesuvius. —When the Roman nobles were building their magnificent villas and baths along the shore of the Bay of Naples, the scenic beauty of the region was greatly increased by a mountain in the shape of a truncated cone, which rose from the plain a few miles back from the shore. Its sides, nearly to the summit, were covered with beautiful fields.
+
+In the top of the mountain was a deep depression some three miles in diameter, partly filled with water and almost entirely surrounded by precipitous rock cliffs. There were no signs of internal disturbance. Around the mountain were scattered prosperous cities, the soil was fertile, the vegetation luxuriant. To this natural fortress Spartacus, the gladiator, retreated when he first began to defy the power of Rome.
+
+
+
+VESUVIUS AND NAPLES.
+
+
+
+In 63 A. D. the region about the mountain was shaken by a severe earthquake which did much damage. This was followed by other earthquakes during a period of six teen years. In August, 79，the whole region was frightfully shaken, and the previously quiet mountain began to belch forth volcanic dust, cinders and stones, so that for miles around the sun was obscured, and a pall of utter darkness shrouded the country, lighted at intervals by terrific flashes of lightning.
+
+A large part of the ancient crater, now known as Monte Somma, was blown away, and the villas and towns near the mountain were covered with the ash and cinders ejected. So deep were many of these buried that their sites were utterly forgotten. Pompeii and Herculaneum, after lying buried and almost forgotten for hundreds of years, have been recently partially uncovered.
+
+These fossil cities show the people of to-day how the ancient Romans lived and built. The topography of the country and the coast line were greatly changed by this eruption. Pompeii formerly was a sea coast city at the mouth of a river. It is now a mile or more from the sea and at a considerable distance from the river.
+
+
+
+MOUNT VESUVIUS.
+
+Showing the famous eruption of 1872.
+
+
+
+From the date of its first historic eruption until the present time Vesuvius has had active periods and periods when quiet or dormant. Sometimes the activity is mild, and at other times tremendously violent. At times the material ejected is fragmental and at other times streams of molten lava pour down its sides. Its ever changing cone, unlike that of Monte Nuovo, is composed partly of ash and partly of consolidated lavas. Even as late as 1907 a tremendous outpouring of ash took place which devastated a considerable area.
+
+
+
+MOUNT PELEE AND RUINS OF ST. PIERRE.
+
+
+
+212. Mount Pelee. —At the north end of the island of Martinique in the West Indies rose a conical-shaped mountain. In a hollow bowl-like depression at the top lay a beautiful little lake some 450 feet in circumference. The mountain and lake were pleasure resorts for the people of the city of St. Pierre. According to legend this mountain had been violently eruptive, but in historic time there had been no indication of this except one night in 1851 when the volcano had grumbled and a slight fall of volcanic ash was found in the morning over some of the surrounding region.
+
+On April 25, 1902, people began to see smoke rising from the vicinity of the mountain and from this time on till the final catastrophe smoke and steam came out in small quantities. By May 6 the volcano was in full eruption. On the morning of May 6 the cable operator at St. Pierre cabled, "Red-hot stones are falling here, don't know how long I can hold out." This was the last dispatch sent over the cable.
+
+About 8 o'clock on the morning of the 8th a great cloud of incandescent ash and steam erupted, swept rapidly down the mountain toward St. Pierre and in less than three minutes killed 30,000 people, set the city on fire and destroyed 17 ships at anchor in the harbor. Thus within two weeks from the time of the first warning a rich and densely populated region was made a desolate, lifeless, fireswept desert.
+
+213. The Azores. —About 800 miles west of Portugal rises from the depths of the Atlantic a group of nine islands, the Azores. They have an area of about 1000 square miles, and the soil is very fertile. The islands are mountainous, one of the mountains rising to between 7000 and 8000 feet above the sea. Like other lofty islands of the deep ocean these are volcanic. Although at present not actively eruptive they abound in hot springs and have frequent earthquakes.
+
+Volcanic cones are abundantly scattered over the islands, and comparatively fresh lava flows are not wanting. In recent times small islands have arisen in the group and eruptions have taken place. There are no other islands near them. Their formation is due entirely to volcanic forces. Islands of this kind and coral islands are the only projections rising to the surface from the deep ocean floor.
+
+
+
+SAN MIGUEL HARBOR IN THE AZORES.
+
+Notice the volcanic cones in the distance.
+
+
+
+214. Volcanoes of the United States. —In the Cordilleran region of the United States, west of the meridian of Denver, there are a score or more of lofty peaks which show conclusive evidence of volcanic origin. Until the summer of 1914 when Mt. Lassen suddenly began to erupt, none of these had been active since white men became familiar with the region. Some of the cones have been so recently formed that the forces of erosion have not had time to wear them away extensively. Thus they are almost perfect in shape like Mt. Shasta. Others, like Mt. Hood, have been deeply eroded, but not sufficiently to obliterate the conical outline.
+
+
+
+MOUNT LASSEN IN ERUPTION.
+
+This volcano, after being dormant for centuries, suddenly renewed its activity in 1914.
+
+
+
+
+
+MOUNT HOOD.
+
+A beautiful old volcanic cone.
+
+
+
+In the region around Mt. Taylor erosion has progressed so far that only the roots of the volcanoes still remain, the cones having been entirely worn away and only the central plug of lava left, forming what is called a volcanic neck. In the Aleutian Islands are numerous volcanoes which are still active, and in Hawaii are some of the greatest volcanoes on the earth.
+
+
+
+VOLCANIC NECKS NEAR MOUNT TAYLOR.
+
+
+
+
+
+CRATER LAKE.
+
+
+
+In Crater Lake we have a volcano whose normal development has been interrupted by an accident, its summit having fallen in, leaving a circular depression in the top of the mountain surrounded by steep walls and now nearly filled with water. Except for the water filling, this decapitated volcano or caldera quite closely resembles the probable condition of Vesuvius before the eruption of 79 A.D.
+
+
+
+LAVA FLOW IN THE HAWAIIAN ISLANDS.
+
+Liquid lava flowing over a cliff.
+
+
+
+
+
+A HAWAIIAN CRATER.
+
+
+
+215. Life History of a Volcano. —A volcano is simply a place in the earth's surface where molten rock or fragmental material from within the earth is extruded. If the extrusion of the lava is accompanied by gaseous explosions, it will be blown into fragments which will fall around the vent and build up a steep-sided cone, like that of Monte Nuovo. If the eruption is less violent, lava may flow from the crater or pour from openings formed in its sides.
+
+As the same volcano usually ejects both the fragmental and molten material, volcanic cones are generally complex in their composition. Sometimes, however, cones are found which are composed entirely of one sort of material. Those which are largely or entirely formed of lava have a much gentler slope than the others. Such are the great Hawaiian cones.
+
+Some volcanoes, like Stromboli, are in constant eruption; others, like Etna, vary their eruptions with irregular periods of rest, while still others remain quiet for very long periods and then suddenly break forth with terrific force, as did Vesuvius in 79. As a rule, but not always, eruptions are preceded and accompanied by earthquakes. Just why volcanoes erupt is unknown.
+
+
+
+CROSS SECTION OF A LAVA FLOW.
+
+
+
+After a volcanic cone has come into being it is subject to the action of the erosive forces, and unless its material is renewed by fresh outpourings it will in time be worn down. Unlike other kinds of mountains it is also liable to disruption by explosions from within.
+
+216. Distribution of Volcanoes. —The number of active volcanoes on the earth is about 300. Most of them are situated on the borders of the continents, on islands near the continents, or else they form islands in the deep sea. Soundings show that there are many peaks in the sea which have not reached the surface; these are probably volcanic. Few volcanoes are far from the sea although there is an active crater in Africa several hundred miles from the Indian Ocean.
+
+Extinct cones are sometimes found far in the interior of continents, as the Spanish Peaks of Colorado, which are more than 800 miles from the present coast. Many of the once active deep-sea cones have now become extinct, and their gently sloping shores have been cut back into cliffs which rise abruptly from the sea. One of these, St. Helena, rising from the depths of the Atlantic Ocean and bounded by precipitous cliffs, is noted as being the place of exile of the Emperor Napoleon I of France.
+
+
+
+THE CITY OF ST. HELENA.
+
+
+
+Summary. —Volcanoes are openings in the earth's crust through which portions of melted earth material pour forth. This material may be ash and cinders or it may be molten lava.
+
+Some of the most interesting or best-known volcanoes are in Italy. Monte Nuovo, the New Mountain near Naples, is so called because it came into being in a few days. Vesuvius, which dominates all views of Naples, is perhaps the world's most famous volcano. Mount Pelee in Martinique had perhaps the most disastrous and spectacular eruption in all history. The Azores islands are all volcanic in their formation, and Hawaii has some of the world's greatest volcanoes.
+
+The United States has a number of volcanic peaks, like Mt. Shasta and Mt. Hood, but until the recent eruption of Mt. Lassen, it had no active volcano.
+
+
+
+
+
+QUESTIONS
+
+
+What is the condition of the earth's interior?
+
+Describe the eruption and present condition of Monte Nuovo.
+
+What has been the history of Vesuvius?
+
+What is Mount Pelee's story?
+
+How were the Azores formed?
+
+What volcanoes are there in the United States?
+
+Give the life history of a volcano.
+
+Where are volcanoes found?
+
+
+
+
+
+【中文阅读】
+
+
+209．地下世界——人们对地下的挖掘与钻探已经达到了地表以下很深的地方，在这个过程中发现，随着深度增加，温度也不断增高。但温度增高的速度，在各个地方并不一样，就是在同一个地方，温度的增加也不尽相同。在地球上不同区域钻探时，温度的平均增加速度差不多是每70到80英尺深，则温度增加1°F。
+
+岩石受到的压力越大，则岩石越难以熔化。要不然的话，整个地球的固体部分的厚度就不会超过40或50英里，因为地球内部的热量足以在普通压力下将地下岩石层全部熔化。但事实情况是相反的，地球内部依然非常坚硬而不是柔软。因此，由于压力极其巨大，地球内部几乎没有物质处于完全的熔化状态。
+
+如果在靠近地表的地方压力减小，或者热量增加，则那里的物质就会被熔化，并在一定条件下通过一些渠道到达地球表面。我们知道，在地面以下被加热的物质会不断上升到地面，并且会浸入地表岩石，在有些地方还会向外流溢出来。
+
+是什么原因导致了熔化物质在地层中的上升和外流？以及它们为什么会到达地表？又是如何达到的？这些都还是没有确切答案的问题。但通过人们对这些火成物质的一系列观察，它们的特性与活动方式也可以逐渐被研究。接下来的关于一些我们所熟知的典型火山的描述，也在一定程度上显示了地下地质活动的最终结果。
+
+210．努奥伏奥火山——公元1538年，在罗马贵族著名的休闲疗养胜地——那不勒斯海滩附近，发生了几次很突然的地震，随后出现了令人恐怖的火山喷发。从地下涌出的大量熔融物质四散流淌，土石也被喷出的气体冲压成了碎片。几天之后，便形成了努奥伏奥火山，它440英尺高，直径差不多半英里，顶部有一个杯状的凹陷区，也叫火山口，差不多有400英尺深。
+
+由于喷发而形成的爆炸力量过于强大，使得喷发物几乎没有处于液态的。整个火山丘是由火山灰，以及像��炉中的矿渣一样的土石碎片构成，这些喷发出来的小碎片叫做火山灰。一周之内，这次喷发就结束了，同样的情形再也没有在这个地区出现过。
+
+我在数年前到访过这里，火山口的底部已经成了很平整的农田，并且种上了庄稼。当年的当地居民们，见证了整个火山丘锥体的形成过程。其他类似的喷发在其他地方也曾出现过，但这个是最著名的。尽管如此，我们把它还是定义为年轻火山。直到今天，它的火山丘的锥形外貌依然保持得非常完整。
+
+在加利福尼亚北部的拉森山区，也有一个这样外形完好的锥形山丘，最近似乎也出现了活跃的地质运动，不排除某一天也会形成一个比努奥伏奥更大的火山。因为这里曾经在喷发的时候，火山灰和液态的岩浆都大量出现过。
+
+211．维苏威火山——罗马帝国的贵族们当年沿着那不勒斯湾的海滩，修建了许多富丽堂皇的别墅与洗浴房。这里风景如画，离海滩只有几英里远的平原上，矗立着一座高山，其外形就像一个被削去顶部的圆锥。在它靠近顶部的山坡上，覆盖着郁郁葱葱色彩斑斓美丽的田野，远远望去，美不胜收。
+
+这座山的山顶上也有一个很深的凹陷区域，直径大约3英里，部分地方还有池塘，但整个洼地周围全是险峻的悬崖峭壁。这里与世隔绝，和谐宁静，高山周围分布着几个繁荣的城镇，整个地区土地肥沃，植物也生长得非常茂盛，俨然一个天府之国。当年伟大的斗士斯巴达克斯，在他开始挑战罗马帝国时，就曾盘踞在这里。
+
+公元63年，这个地方很不幸地发生了一场剧烈地震，毁坏严重。不仅如此，经过16年之后，又一系列大地震降临于此。在公元79年的八月份，整个地域在地震中剧烈摇晃，曾经平静祥和的大山，开始向外喷发火山灰与石土，让方圆数英里暗无天日，除了间断的闪电发出可怕的光芒，整个地区一片漆黑。
+
+现在这个古老的火山口的大部分，也即后来所称的索马山，已经被历史的风霜消磨殆尽了。周围的城镇和别墅，也已被盖上了一层厚厚的火山灰，有的地方甚至被完全深深地掩埋，因此也早被人们完全遗忘。庞培和赫库兰尼姆两座城镇就被彻底掩埋了，也被人们遗忘了千百年，最近才开始部分地被发掘出来。
+
+这些古老的城镇，让今天的人们见识了罗马先民们的生活情况和建筑风格。这里的地貌和海岸线分布，被这次喷发彻底改变了。庞培最早是一个海岸城镇，位于一条河流的出海口上，现在它已经离海岸有一英里多了，离那条河也有了很长的距离。
+
+从它第一次历史性的喷发到今天，维苏威火山已经经历了许多次的活跃期与休眠期。其活动有时极其轻微，有时却异常剧烈，而喷发时的喷发物，有时是土石碎片，有时则是熔化的岩浆，会顺着山坡流淌下来。因此它的锥丘山体不断地改变着形状，而且其构成也与努奥伏奥火山不同，它是由火山灰与凝固的岩浆构成。就在1907年，这里还发生了一次剧烈的喷发，并造成了大范围的灾难。
+
+212．培雷火山——在位于西印度群岛的马提尼克岛的北端，矗立着一座圆锥形的高山。其顶部有一个碗状的洼地，形成了一个周长约450英尺的美丽湖泊，如画般的湖光山色让这里成了圣皮埃尔市的度假村。据人们传说，这座山曾经发生过剧烈的火山喷发，但从历史记录来看，除了在1851年的一个晚上，这座山曾有一些轻微活动并在第二天早上发现了一些火山灰之外，这里并没有大规模喷发过的迹象。
+
+1902年4月25日，人们看到这座山开始冒烟，直到最终大难来临，烟尘和气流一直从山上不外涌出来。到了5月6日，大喷发终于来了，当天早上的电报播报员曾这样说道：“炽热红透的石头落在我面前，我不知道我还能坚持多久。”这就是从那里发出来的最后一通电报。
+
+到了8日早上8点，炽热的火山灰和热气流形成的云团迅速从山上飘下来，横扫了整个圣皮埃尔市区，30000多人因此丧生，整个城市变成地狱般的火海，连停靠在港口的17艘船只也被化为灰烬。从第一次出现喷发征兆到最后只用了两周时间，一个曾经繁荣富裕、人口密集的城市，变成了一个毫无生命的荒凉鬼城。
+
+213．亚速尔群岛——在葡萄牙以西，大约800英里的地方，从大西洋海底升起了一片高地，形成了一组由9个岛屿构成的群岛，这就是著名的亚速尔群岛。岛上总面积大约1000平方英里，土壤非常肥沃。岛上有许多高山，其中有一座的海拔高度达到了7000到8000英尺，跟其他深海岛屿一样，它们也全都是火山。现在它们已经没有非常活跃的喷发记录了，但是温泉依然是这里的名胜，地震也时有发生。
+
+岛上到处都分布着火山的锥丘，但是流淌着的新鲜岩浆并不少见。近年以来，新的小岛有时还会冒出来，小的喷发偶尔还是有的。在它们周围，再也没有其他岛屿。它们的形成，完全是由火山的喷发力量所导致。仅仅只有这类岛屿和珊瑚岛，能从大洋深处的海底不断升到海面上来。
+
+214．美国的火山——在美国西部山区，尤其是丹佛的西端，有十到二十座山峰可以找到确凿证据证明它们起源于火山。但自从白种人移居至此，直到1914年夏天拉森峰突然喷发之前，这里的每一座火山都没有处于活跃期。有一些火山锥丘，由于形成的时间太晚，大自然的侵蚀力量还没有足够的时间将它们充分消磨。因此它们中有的依然保有完美的外形，比如沙士达山；但也有像胡德山那样，已被严重侵蚀，只留下一个圆锥形的大概轮廓了。
+
+在泰勒山附近，大自然的侵蚀作用一直进行着，到今天便只剩下火山的山脚部分，其余部分则已经完全被冲蚀殆尽，只有中心部位的岩浆岩包块还留在那里，形成了我们所称的火山颈。而在阿留申群岛，那里数目众多的火山依然非常活跃；在夏威夷群岛上，还存在着地球上最大的几座火山。
+
+在克雷特莱克地区有一座火山，在其演化过程中被某些意外状况所干扰，造成其顶部向内凹陷，在山顶形成一个圆形洼地，四周都是高高的陡峭石壁，现在这个洼地里面已经存满了水。整体来看，除了里面有水这一点之外，其他方面这里和维苏威火山的状况非常相似。
+
+215．火山的生命历程——简单而言，火山就是在地球表面上，地球内部熔化的岩石和碎片被挤压出来的地方。如果喷出的岩浆还伴随着气体的爆炸，则最终的喷发残渣会在火山口周围堆积起来，形成一个陡面圆锥体，就像努奥伏奥火山那样。如果喷发不是特别猛烈，岩浆则会从火山口或山体的慢慢流淌下来，最后形成山坡。
+
+即使同一座火山，其喷发物也既有土石碎屑，也有熔岩物质，因此火山锥的构成就很复杂。不过有时也会发现个别火山锥体完全由某一种喷发物构成，比如主要由岩浆构成的火山锥，其山坡一般比其他种类火山更加平缓。夏威夷火山就是这样的例子。
+
+有些火山，比如像斯特隆博利火山，喷发得极其频繁；还有些火山，比如像埃特纳火山，喷发和休眠的时间段非常混乱，毫无规律可寻。但其他大部分都是长时间处于平静状态，只是一旦突然喷发起来，也挺够呛的，维苏威火山就是典型。通常而言，当然也不绝对，就是在火山喷发之前，往往都伴随着地震。但火山为什么要喷发，确切原因还不知道。
+
+一旦火山的锥体形成，便开始遭受大自然的侵蚀，如果没有新的喷发物不断覆盖上来的话，它最终将会被侵蚀力量完全消磨。不像其他种类的高山，火山的命运与地球内部物质的喷发与爆炸息息相关。
+
+216．火山的分布——地球上处于活跃期的火山大约有300座[1]，它们中的大多数都位于大陆板块边缘的岛屿上，或者干脆自己从海底直接升起，形成了火山岛。声纳探测显示，海底有许多没有接触到海面的山峰，而它们很可能都是火山。尽管在非洲大陆上，离印度洋数百英里远的地方也有火山存在，但绝大多数火山都是离海不远的。
+
+在大陆的内陆地区，有时也能发现休眠的火山，比如科罗拉多的西班牙峰，离海岸差不多有800英里。许多海底的活火山如今也已经休眠了，它们曾经平缓的山坡也被剥蚀成了陡立的峭壁，突兀地矗立在海中。圣海伦娜火山就是其中之一，险峻伟岸的悬崖从大西洋海底高高升起，当年的法国国王拿破仑一世就被流放于此，让这里因此被世人所熟知。
+
+
+
+
+
+总结——火山是地壳上面的一些开口处，地球内部的部分熔岩物质可以从这些开口处喷流出来。这些喷流物可能是火山灰、石土残渣，也可能是炽热的岩浆。
+
+一些最有趣也最著名的火山，位于意大利。比如努奥伏奥火山，也就是那不勒斯附近的“新山”；还有维苏威火山，那不勒斯的所有地理风貌几乎都拜它所赐，它也差不多算是全世界最著名的火山；马提尼克的培雷火山，地球上可能是有记录以来的最恐怖的一次喷发就发生于此。亚速尔群岛全部是由火山形成的，夏威夷也有不少在世界排名前列的大火山。
+
+美国有许多火山存在，比如沙士达山和胡德山，但直到近年拉森峰有过喷发以外，其余都未处于活跃期。
+
+
+
+
+
+思考题
+
+
+地球内部是怎样的一个环境？
+
+描述一下火山喷发的情形以及努奥伏奥火山的状况。
+
+维苏威火山有怎样的历史？
+
+培雷火山有哪些故事？
+
+亚速尔群岛是如何形成的？
+
+美国有哪些火山？
+
+描述一下火山的生命周期。
+
+火山一般位于什么地方？
+
+
+
+
+
+译　注
+
+
+[1]近年数据为：全世界的活火山有500多座，其中在海底的近70座，即海底活火山约占全世界活火山数量的1/8。
+
+
+
+
+
+APPENDIX
+
+附录
+
+
+217. Determination of Latitude. —In Fig. 121 consider the sun as vertically above the point where our meridian crosses the equator and the lines AB and ED as representing rays from the sun. The line FI tangent at the point A will represent a level surface at that point. Draw the line CH through the point A. It will be at right angles to the tangent line FI. The latitude of the point A is measured by the angle ECA, as this angle measures the number of degrees of latitude between the point E, which is on the equator, and the point A.
+
+
+
+Fig. 121.
+
+
+
+It can be proved by geometry that the angle HAB is equal to the angle HCE, since if the sun is vertical, the line CED is a straight line. The angle HAB is equal to a right angle, or 90° minus the angle BAI. As the angle BAI can be easily found by measuring the elevation of the sun above a horizontal plane, it is not a difficult thing to find the latitude of a place when the sun is vertical at the equator.
+
+As the sun is vertical at the equator but twice in a year, on March 21 and September 23, this method can be used without modification only on those days; but since the angle of the sun above or below the plane of the equator is given in the Nautical Almanac for every day in the year, by adding this angle to the angle BAI when the sun is above the equator, and subtracting it when the sun is below the equator, the latitude of a place can be found for any day.
+
+On board ship, every fair day, the officers will be seen just before noon coming on deck with their sextants to take the elevation of the sun. They find the elevation several times until they are sure that the sun has reached its highest point, and at this moment they call for the time to be taken on the chronometer; for when the sun reaches its highest point, it is noon for that place. Thus by making use of this one observation they are enabled, with the help of the chronometer, to find both their latitude and their longitude, or their exact position on the earth.
+
+218. Topographical Maps. —Maps which attempt to show the surface features of the earth are called topographical maps. There are several ways in which we may try to show on a map the irregularities of the surface. One of these is shading, that is, making the hills and ridges light, while the valleys are shaded dark. A somewhat similar way is to draw short broken lines in the direction of the slopes. This gives a more accurate representation of the steepness of the descents, since the lines are made short and heavy when the slope is steep and longer and lighter when it is gradual. Such maps are called hachure maps.
+
+The commonest way in this country is to draw lines connecting places of equal elevation. These lines wander in and out of the valleys and around the hills, but always pass through places which are of the same altitude. The distances apart of the lines vary continually, but the elevations never. From these maps the height of any place can be determined with great accuracy, for its height will be indicated by the line passing through it or near it. These maps are called contour maps.
+
+219. Contour Maps. —Although it is easy to find the elevations of places on a contour map, it is hard to get a clear idea of what a contour map really expresses. The best way to gain an appreciation of a contour map is to get a map of the region in which you live, take it into the field, and study map and region together. Another excellent way is to make a contour map of a model. When once you have made a map of this kind, you will readily understand all other similar maps.
+
+We must remember that a contour is the projection on a flat surface of a line which passes through places of equal elevation. It shows where the margin of water would come if the place in question were submerged to a given depth. No two contours can possibly cross each other, as no place can have two elevations. No contours can ever end except at the edge of the map, for a sheet of water must have a continuous boundary and only where the map terminates can the line representing the edge of the water appear to end.
+
+
+
+
+
+Experiment 133. —Provide each pupil with a contour map representing the home locality if possible; if not, use the contour map in section 160. Let the teacher or different pupils pick out places and ask some one to give their elevations. In this way you will get an idea of how elevations can be determined by use of a contour map. Notice the different topographical symbols used on the map.
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+220. Maps of Curved Surfaces. Projections. —The accurate mapping of small areas offers no great difficulty because these are practically flat, but when an attempt is made to represent a curved surface upon a flat surface, difficulties present themselves which are insurmountable. If the rind of an orange is taken off, it cannot be made to lie flat, and if crushed into this shape, it will break into pieces and only partly cover the surface over which it spreads. The same is true of any curved surface. Thus the continents of the earth, if they were flattened out, would of necessity be broken into fragments. If they could not themselves be made to occupy a flat surface, then no accurate map of them can be made on such a surface.
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+Although there are several ways of representing a curved surface upon a flat surface, yet no method has been found which is perfectly satisfactory. If the areas are in the right proportions, the outlines are not; and when the outlines are right, the areas are not. These different ways of mapping the surface of the earth are called projections. As a large part of our knowledge of the earth's surface is obtained from maps, it is very essential to have some idea of how these maps are made and wherein the essential error of each consists. Two very important kinds of projection are the cylindrical and the stereographic.
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+221. Cylindrical Projection. —In this projection it is considered that a cylinder is wrapped around the globe touching at the equator. The points on the globe are projected on to the cylinder by lines drawn from the center of the globe through each point to the surface of the cylinder. Thus the meridians become straight lines always the same distance apart; and the parallels of latitude are also straight lines, but their distances apart will increase with the latitude. The poles themselves, being in the diameter of the cylinder, lie at an infinite distance from the equator.
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+When such a cylinder is unrolled it forms a skeleton map on which can be plotted places whose latitude and longitude are known. The directions north and south will be up and down the map, and east and west to the right and left. This cylindrical projection causes a degree of latitude to vary from about 1/360 of the earth's circumference at the equator to infinity at the poles; and a degree of longitude, which near the poles has almost no length, is made to have a length everywhere equal to that of a degree on the equator. Thus passing from the equator toward the poles, the areas of surfaces on the earth are increased when represented on this projection, but the increase east and west and the increase north and south are not equal. This causes the shape of the portions of the earth farthest from the equator to be much distorted.
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+The Mercator projection is the most commonly used of all projections. It is a simple modification of the cylindrical, in which the exaggeration north and south is made equal to that east and west. In this projection the polar regions are greatly enlarged. This explains why Greenland, which on the globe is of comparatively small size, when seen on the ordinary map of the world is half the size of North America. The great advantage of this projection is that the meridians and parallels are both represented by straight lines. A navigator can thus at any time find his course by drawing a straight line joining the places between which he is sailing. This is why most nautical charts are constructed on this projection. But to geographers this projection is not of as great value as some others since the shapes of the land masses are so much distorted.
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+222. Stereographic Projection. —Of the hemispherical projections probably the best for study is the stereographic. This, or a slight modification of it, is the projection upon which are constructed the hemispherical maps usually seen. In it a plane is considered as held tangent at a certain point on the globe and from a point on the globe directly opposite the point of tangency, lines are drawn to the plane through the intersections of the parallels of latitude and longitude. Through these projected intersections the meridians and parallels of latitude are drawn.
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+In this projection, places near the point of tangency have their outlines correctly reproduced, but the farther away a place is from this point, the greater the distortion. This distortion, however, is never as great as that at the north and south in the cylindrical or Mercator projections. In the stereographic projection, however, the directions north and south and east and west must be traced on a curved line, thus making it much more difficult to tell at a glance the direction of one place from another. It is not possible on this projection to show more than one half the earth's surface on a single map.
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+【中文阅读】
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+217．纬度的测定——如图121所示，设想太阳正垂直照射在赤道与经线的交点上，直线AB和ED代表太阳的光线，切线FI则代表过A点的平直切面。过A点画直线CH，它会与切线FI形成直角。A点的纬度则是由∠ECA确定，其纬度的度数就是A点与位于赤道的E点之间的圆心角度数。
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+通过几何学可以证明，∠HAB等于∠HCE，因为太阳是垂直照射的话，CED的连线就是一条直线。∠HAB则要么是直角，要么等于90°减去∠BAI。由于∠BAI的度数可以很轻易地通过在水平面上测定太阳的仰角而得出，因此当太阳位于赤道正上方时，测量一个地方的纬度并不困难。
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+由于太阳位于赤道正上方的时间，一年只有两次，分别是每年3月21日和9月23日，因此用以上无需修正的办法测定纬度只能在这两个日子进行。不过又因为一年当中，太阳在赤道平面上下的仰角，每天都要在航海日历中标注出来，故而当太阳位于赤道平面以上时，用仰角加上∠BAI，当太阳位于赤道平面以下时，用∠BAI减去仰角，一个地方的纬度就可以在每天进行测定了。
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+在船的甲板上，每天中午之前都能看见船员拿着六分仪测量太阳的仰角，他们会反复测量好几次，直到确保太阳达到了它的最高点，然后在航海经线仪上记录下这个时间，因为当太阳到达其最高点的时间，就是这个地方的正午时间。因此，通过这样的观察，再借助于航海经线仪，他们便能确定当地的纬度和经度，进而确定其在地球表面的准确位置。
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+218．地形图——显示地表的地貌特征的地图就叫做地形图。在地图上表现地表的不规则状况，有几种不同的办法。其中一种就是描影法，具体方式是让山丘和山脊留白，而给山谷描上暗影。与此类似的办法还有用虚线表示山坡走向，这个办法能对地表山体的险峻程度和下降趋势给出更为精确的描述。当山坡很陡很长的时候，虚线就很短且颜色较深；当山坡平缓的时候，虚线则很长且颜色较浅。这样的地图叫做影线地图。
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+在美国最普通的办法，则是用一条线连接海拔相等的地方，以此来显示地貌。这些线条在山谷处弯进弯出，在山顶处则围成圆圈，但总是经过具有同样高度的地方。线条之间的距离变化很大，但是高度差始终不变。在这样的地图上，各个地方的海拔高度可以精确地读出，因为其高度由经过它的线条，或者靠近它的线条直接决定。这样的地图叫做等高线地图。
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+219．等高线地图——尽管在等高线地图上找到各个地方的海拔高度是轻而易举的事情，但它整体上表达的内容却不是特别清楚。查看等高线地图的最好办法是，找一张你所在地区的正常地图，将二者对照起来看。另一个好办法则是将它做成地图模型，一旦你将其变成实物模型之后，其他类似的地图就很容易理解查看了。
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+我们始终要记住，等高线其实是穿过同海拔地区的连线在平面上的投影。如果一个水源地有一定深度的沉降，它在等高线地图上便能清晰地反映出来。任意两条等高线绝不会相交，因为同一个地方不可能同时有两个海拔高度。另外，除非是在地图边缘，等高线也不可能终止，就像水流一定会有连续的边缘，只有在地图边缘截止的地方，代表水流的线条才会终止。
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+实验133：如果条件允许的话，给每位学生发一张他们家所在地区的等高线地图，如果没有这个条件，将本书第160节中的等高线地图插页将就用用也行。让老师或者不同学生相互在地图上指出一些地点，并进而找出它们的海拔高度。这样能让学生对如何用等高线地图确定不同地区的海拔高度有清晰的认识。并请注意地图上所使用的一些特殊地形符号。
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+220．曲面投影地图——给一些面积很小的地域制作地图并不困难，因为它们相对比较平整，但如果要将一个弯曲的地球表面表现在平面上，麻烦可就来了。将一个桔子的外皮取下来，怎么都摊不平，如果非要将其使劲压平的话，桔子皮就会成为碎片，只能覆盖那些碎片所在的地方。同样的道理，如果将地球的连续表面摊开变平的话，也一定会破碎不堪。但如果它们不能在平面上表现出来的话，精确的地图也就无从谈起了。
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+尽管已经有了好几种在平面上绘制曲面地图的办法，但还没一个是让人彻底满意的。如果让地图区域的比例精准，则轮廓会走样；如果让轮廓不走样，区域又会产生误差。这些不同的绘制地球表面地图的办法叫做投影。由于我们对地球表面的大部分认知都是通过地图来获取的，因此对地图是如何生成的，以及它们各自有哪些误差进行探究，就显得很有必要了。绘制地图时，两种非常重要的投影法就是圆柱投影和立体投影。
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+221．圆柱投影——这种方法是设想一个圆筒罩在地球周围，与赤道相切。地球上的每一点，通过球心与其连线，投影到圆筒的内表面上。这样，地球上的经线就变成了等长���直线，并且两两之间距离相等。同样纬线也成了等距的平行线。极点本身则成了圆筒的直径，似乎处于离赤道无限远的地方。
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+当这一圆筒展开后，它便形成了一张能确知各地经纬度的地图，方向为上北下南左东右西。圆柱投影法让纬度的一个度数的实际范围，从在赤道的1/360地球周长，到地球极点处的无穷大不断变化；而经度的一个度数的实际范围，则是在极点附近几乎没有长度，但在其他任何地方都和赤道上的范围相同。这样一来，图上所代表的从赤道到极点面积，无形之中就增加了，且东西向的增加与南北向的增加还并不相同。这就让远离赤道的地方的区域形态大大走样了。
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+墨卡托投影是应用最广泛的一种投影方式。它是对圆柱投影的一种简单修正，即让南北向的误差与东西向的误差相等。在这种投影法中，极地地区被严重放大了，这就是为什么在地球仪上看起来并不大的格陵兰岛，在这样的投影地球图上差不多有北美洲一半大。这种投影最大的好处是，让经线和平行线变成了直线，这样航海的船长就能用一条直线表示他航行的路线。这也便是许多航海地图之所以采用此法进行绘制的原因。但对于地理学家而言，这种投影方式可并没有那么有价值，因为许多地域的形状被改变得面目全非了。
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+222．立体投影——就学术研究而言，最理想的对半球的投影方式可能要数立体投影了。这种投影方式，或者它的类似升级版，就是我们常见的半球地图所用的投影方式。这种方法是，假设一个平面与地球相切于某个点，从与切点正对的一点出发，在切平面上画线，让其与经线和纬线的交叉点相连接，通过这样的投影交叉线，经线和平行的纬线就画出来了。
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+在这种投影法中，切点附近的地区轮廓非常精确，但是离切点越远的地方，变形越大，不过，这个变形远没有圆柱投影和墨卡托投影产生的南北向变形那么大。在立体投影中，南北向和东西向的轨迹全是曲线，因此这个给不同地域之间的相对方向的判别带来了困难。在这种投影法中，要将大于半个球面的区域绘制在同一张地图中也是不可能的。
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