# Physical Geography of the Sea and its Meteorology/Chapter 7

CHAPTER VII.
341. Halley's theory not fully confirmed by observations.—Halley's theory of the trade-winds, especially so much of it as ascribes their easterly direction to the effect of the diurnal rotation of the earth, seems to have been generally received as entirely correct. But it is only now, since all the maritime nations of the world have united in a common system of research concerning the physics of the sea, and occupied it with observers, that we have been enabled to apply the experimentum crucis to this part of the famous theory. The abstract logs, as the observing-books are called, have placed within my reach no less than 632,460 observations—each one itself being the mean of many separate ones—upon the force and direction of the trade-winds. It appears from these that diurnal rotation being regarded as the sole cause, does not entirely account for the easting of these winds.
342. Observed course of the trade-winds.—From these observations the following table has been compiled. It shows the mean annual direction of the trade-winds in each of the six belts, north and south, between the parallels of 30° and the equator, together with the number of observations from which the mean for the belt is derived:—
| Between. | N.E. Trades. | N.E. Trades. | N.E. Trades. | N.E. Trades. | S.E. Trades. | S.E. Trades. | S.E. Trades. | S.E. Trades. |
| Between. | Course. | Course. | Course. | No. of Obs | Course. | Course. | Course. | No. of Obs |
| 30° and 25° | N | 51° | E. | 68,777 | S. | 46° | E. | 66,635 |
| 25° and 20° | | 51° 30' | | 44,527 | | 49° 20′ | | 66,395 |
| 20° and 15° | | 53° 30' | | 33,103 | | 52° | | 46,604 |
| 15° and 10° | | 52° 30' | | 30,339 | | 49° 40′ | | 43,817 |
| 10° and 5° | | 53° 30' | | 30,841 | | 51° 40′ | | 54,648 |
| 5° and 0° | | 54° 30' | | 67,829 | | 48° 40′ | | 72,945 |
| Mean | N. | 52°45' | E. | | S. | 40° 33′ | E. | |
​Between the equator and 5° north, the annual average duration of the trades is 67 days for the north-east, and 199 fur the south-east, with a mean direction for the latter—which are the prevailing winds between those parallels—of S. 47° 30' E. According to the Halleyan theory these should be south-west winds.
343. Velocities of the trade-winds.—In the Atlantic the average velocity of the south-east is greater than the average velocity of the north-east trades.I estimate one to be from 14 to 18, the other from about 25 to 30 miles an hour. Assuming their velocity to be 14 and 25 respectively the following departures show the miles of easting which the trade-winds average per hour through each of the above-named belts:
Hourly Rate of Departure of the Trade-winds across the Belts.
| Between | N.E. Trades. | S.E. Trades. |
| | Easting per hour. | Easting per Hour. |
| 30° and 25° | 10.9 miles | 18. miles |
| 25° and 20° | 11.,, | 19.,, |
| 20° and 15° | 11.2,, | 19.7,, |
| 15° and 10° | 11.,, | 19.1,, |
| 10° and 5° | 11.2,, | 19.6,, |
| 5° and 0° | 11.4,, | 18.8,, |
344. Difference between observation and theory.—That diurnal rotation does impart easting to these winds there is no doubt; but the path suggested by the table does not conform to that which, according to any reasonable hypothesis, the trade-winds would follow if left to obey the forces of diurnal rotation alone, as they would do were diurnal rotation the sole cause of their easting. As these winds approach the equator, the effect of diurnal rotation becomes more and more feeble. But the table shows no such diminution of effect. They have as much easting between 5° and 0° as they have between 30° and 25°. Nay, the south-east trades between the equator and 5° N.—where, by the Halleyan theory, they should have westing—have as much easting (§ 342) as they have between 30° and 25° south. We cannot tell how much the air is checked in its easterly tendency by resisting agents, by friction, etc., but we know that that tendency is about ten times stronger between 30° and 25° than it is between 5° and 0°, and jet actual observations show no difference in their course. This table reminds us that diurnal rotation should not, ​until more numerous and accurate observations shall better satisfy the theory than those half a million and more now do, be regarded as the sole cause of the easterly direction of the trade-winds. It suggests either that other agents are concerned in giving the trade-winds their easting, or that the effect of the upper and counter current, when drawn down and turned back (§ 232), is such as to counteract their unequal turning in obedience to the varying forces of diurnal rotation. No apology is needed for applying the tests of actual observation to this part of the Halleyan theory, notwithstanding the general concurrence of opinion as to its sufficiency. With equal favour that feature of it also was received which ascribes the rising up in the belt of equatorial calms to the direct influence of the solar ray. But the advancement which has been made in our knowledge of physical laws since Halley expounded his trade-wind theory suggested a review of that feature, and it was found that, though the direct heat of the sun is one of the agents which assists the air to rise there, it is not the sole agent; the latent heat which is set free by condensing vapour for the equatorial cloud-ring and its rains is now also (§ 252) recognized as an agent of no feeble power in this calm belt.
345. Faraday's discovery of magnetism in the air.—Where shall those who are disposed to search, look for this other agent that is supposed to be concerned with the trade-winds in their easting? I cannot say where it is to be found, but considering the recent discoveries in terrestrial magnetism—considering the close relations between many of its phenomena and those both of heat and electricity—the question may be asked whether some power capable of guiding "the wind in his circuits" may not lurk there? Oxygen comprises more than one-fifth part (two-ninths) of the atmosphere, and Faraday has discovered that oxygen is para-magnetic. If a bar of iron be suspended between the poles of a magnet, it will arrange itself axially, and point towards them; but if, instead of iron, a bar of bismuth be used, it will arrange itself equatorially, and point in a direction perpendicular to that in which the iron pointed. To distinguish these two kinds of forces. Dr. Faraday has said iron is para-magnetic, bismuth dia-magnetic. Oxygen and iron belong to the same class, and all substances in nature belong to one or the other of the two classes of which iron and bismuth are the types.
​346. Lines of magnetic force.—This eminent philosopher has also shown that if you place a magnetized bar of iron on a smooth surface, and sift fine iron filings down upon it, these filings will arrange themselves in curved lines as in Fig. 1; or, if the bar he broken, they will arrange themselves as in Fig. 2. The earth itself, or the atmospheric envelope by which it is surrounded, is a most powerful magnet, and the lines of force which proceed whether from its interior, its solid shell, or vaporous covering, are held to be just such lines as those are which surround artificial magnets; proceed whence they may, they are supposed to extend through the atmosphere, and to reach even to the planetary spaces.
Many eminent men and profound thinkers, Sir David Brewster among them, suspect that the atmosphere itself is the seat of terrestrial magnetism. All admit that many of those agents, both thermal and electrical, which play highly important parts in the meteorology of our planet, exercise a marked influence upon the magnetic condition of the atmosphere also.
347. The magnetic influences of the oxygen of the air and of the spots on the sun. —Now, when, referring to Dr. Faraday's discovery (§ 345), and the magnetic lines offers as shown by the iron filings (§ 346), we compare the particles of oxygen gas to ​these minute bits of ferruginous dust that arrange themselves in lines and curves about magnets; when we reflect that this great magnet, the earth, is surrounded by a para-magnetic gas, to the molecules of which the finest atom from the file is in comparison gross and ponderous matter;—that the entire mass of this air is equivalent to a sea of mercury covering the earth around and over to the depth of 30 inches, and that this very subtle mass is in a state of unstable equilibrium, and in perpetual commotion by reason of various and incessant disturbing causes;—when we reflect farther upon the recent discoveries of Schwabe and of Sabine concerning the spots on the sun and the magnetic elements of the earth, which show that if the sun or its spots be not the great fountain of magnetism, there is at least reason to suspect a close alliance between solar and terrestrial magnetism—that certain well-known meteorological phenomena, as the aurora, come also within the category of magnetic phenomena;—that the magnetic poles of the earth and the poles of maximum cold are at or near the same spot;—that the thermal equator is not parallel to or coincident with either the terrestrial or with that which the direct solar ray would indicate, but that it follows, and in its double curvatures conforms to the magnetic equator;—moreover, when we reflect upon Barlow's theory and Fox's observations, which go to show that the direction of metallic veins of the northern hemisphere, which generally lie north-east and south-westwardly, must have been influenced by the direction of the magnetic meridians of the earth or air;—finally, I say, when we reflect upon magnetism in all its aspects, we may well inquire whether such a mass of highly magnetic gas as that which surrounds our planet does not intervene, by reason of its magnetism, in influencing the circulation of the atmosphere and the course of the winds.
348. The needle in its diurnal variations, the barometer in its readings, and the atmosphere in its electrical tension, all have the same hours for their maxima and minima. — This magnetic sea, as the atmosphere may be called, is continually agitated; it is disturbed in its movements by various influences which prevent it from adjusting itself to any permanent magnetic or other dynamical status; and its para-magnetic properties are known to vary with every change of pressure or of temperature. The experiments of Faraday show that the magnetic force of the air changes with temperature; that it is least near the equator, and ​greatest at the poles of maximum cold; that it varies with the seasons, and changes night and day; nay, the atmosphere has regular variations in its electrical conditions expressed daily at stated hours of maximum and minimum tension. Coincident with this, and in all parts of the would, but especially in subtropical latitudes, the barometer also has its maxima and minima readings for the day. So also, and at the same hours, the needle attains the maxima and minima of its diurnal variations. Without other time-piece, the hour of the day may be told by these maxima and minima, each group of which occurs twice a-day and at six-hour intervals. These invisible ebbings and flowings—the diurnal change in the electrical tension—the diurnal variation of the needle,—and the diurnal rising and falling of the barometer,—follow each other as closely and as surely, if not quite as regularly, as night the day. Any cause which produces changes in atmospheric pressure invariably puts it in motion, giving rise to gentle airs or furious gales, according to degree; and here, at least, we have a relation between the movements in the air and the movements of the needle so close that it is difficult to say which is cause, which effect, or whether the two be not the effects of a common cause.
349. The question raised by modern researches.—Indeed, such is the nature of this imponderable called magnetism, and such the suggestions made by Faraday's discoveries, that the question has been raised in the minds of the most profound philosophers of the age whether the various forces of light, heat, and gravitation, of chemical affinity, electricity, and magnetism, may not yet be all traced to one common source. Surely, then, it cannot be considered as unphilosophical to inquire of magnetism for some of the anomalous movements that are observed in the atmosphere. These anomalies are many; they are not confined to the easting of the trade-winds; they are to be found in the counter-trades and the calm belts also. There is reason to believe, as has already been stated (§ 288), that there is a crossing of the winds at the calm belts (§ 212), and it was promised to go more into detail concerning the circumstances which seem to favour this belief. Our researches have enabled us, for instance, to trace from the belt of calms, near the tropic of Cancer, which extends entirely across the seas, an efflux of air both to the north and to the south. From the south side of this belt the air flows in a steady breeze, called the north-east trade-winds, towards the ​equator (Plate I.); on the north side of it, the prevailing winds come from it also, but they go towards the north-east. They are the well-known westerly winds which prevail along the route from this country to England in the ratio of two to one. But why should we suppose a crossing to take place here? We suppose so from these facts: because throughout Europe,—the land upon which these westerly winds blow,—precipitation is in excess of evaporation, and because at sea they are going from a warmer to a colder climate; and therefore it may be inferred that Nature exacts from them what we know she exacts from the air under similar circumstances, but on a smaller scale, before our eyes, viz., more precipitation than evaporation. In other words, they probably leave in the Atlantic as much vapour as they take up from the Atlantic. Then where, it may be asked, does the vapour which these winds carry along, for the replenishing of the whole extra-tropical regions of the north, come from? They did not get it as they came along in the upper regions, as a counter-current to the north-east trades, unless they evaporated the trade-wind clouds, and so robbed those winds of their vapour. They certainly did not get it from the surface of the sea in the calm belt of Cancer, for they did not tarry long enough there to become saturated with moisture. Thus circumstances again pointed to the south-east trade-wind regions as the place of supply. This question has been fully discussed in Chapter V., where it has been shown they did not get it from the Atlantic. Moreover, these researches afforded grounds for the supposition that the air of which the north-east trade-winds are composed, and which comes out of the same zone of calms as do these south-westerly winds, so far from being saturated with vapour at its exodus, is dry; for near their polar edge, the north-east trade-winds are, for the most part, dry winds.
350. Wet and dry air of the calm belts.—Facts seem to confirm this, and the calm belts of Cancer and Capricorn both throw a flood of light upon the subject. These are two bands of light airs, calms, and baffling winds, which extend entirely around the earth. The air flows out north and south from these belts. That which comes out on the equatorial side goes to feed the trades, and makes a dry wind; that which flows out on the polar side goes to feed the counter-trades (§ 349), and is a rain wind. How is it that we can have from the same trough or receiver, as these calm belts may be called, an efflux of dry air ​on one side and of moist on the other? Answer: upon the supposition that the air without rain comes from one quarter, that with rain from another—that, coming from opposite directions to this place of meeting, where there is a crossing, they pass each other in their circuits. They both meet here as upper currents, and how could there be a crossing, without an agent or influence to guide them? and why in the search should we not look to magnetism for this agent as well as to any other of the hidden influences which are concerned in giving to the winds their force and direction?
351. Principles according to which the physical machinery of our planet should be studied.—He that established the earth "created it not in vain; He formed it to be inhabited." And it is presumptuous, arrogant, and impious to attempt the study of its machinery upon any other theory: it was made to he inhabited. How could it be inhabitable but for the sending of the early and the latter rain? How can the rain be sent except by the winds? and how can the fickle winds do their errands unless they have a guide? Suppose a new piece of human mechanism were shown to one of us, and we were told the object of it was to measure time; now, if we should seek to examine it with the view to understand its construction, would we not set out upon the principle—the theory—that it was made to measure time? By proceeding on any other supposition or theory we should be infallibly led into error. And so it is with the physical machinery of the world. The theory upon which this work is conducted is that the earth was made for man; and I submit that no part of the machinery by which it is maintained in a condition fit for him is left to chance, any more than the bit of mechanism by which man measures time is left to go by chance.
352. Division into wind hands.—That I might study to better advantage the workings of the atmospherical machinery in certain aspects, I divided the sea into bands or belts 5° of latitude in breadth, and stretching east and west entirely around the earth, but skipping over the land. There are twelve of these bands on each side of the equator that are traversed more or less frequently by our fleet of observers; they extend to the parallel of 60° in each hemisphere. To determine the force and direction of the wind for each one of these bands, the abstract logs were examined until all the data afforded by 1,159,533 observations were obtained; and the mean direction of the wind ​for each of the four quarters in every band was ascertained. Considering difference of temperature between these various bands to be one of the chief causes of movement in the atmosphere,—that the extremes on one hand are near the equator, and on the other about the poles;—considering that the tendency of every wind (§ 234) is to blow along the arc of a great circle, and that consequently every wind that was observed in any one of these bands must have moved in a path crossing these bands more or less obliquely, and that therefore the general movements in the atmosphere might be classed accordingly, as winds either with northing or with southing in them;—we have so classed them; and we have so classed them that we might study to more advantage the general movements of the great atmospherical machinery. See Plate XV.
353. The medial hands.—Thus, when, after so classing them, we come to examine those movements in the band between 5° and 10° south, and to contrast them with the movements in the band between 55° and 60° south, for example, we find the general movements to be exactly in opposite directions. Observations show that during the year the winds in the former blow towards the equator 283, and from it 73 days;, and in the latter they blow toward the pole for 224, and from it 132 days. These facts show that there must be a place of rarefaction—of low barometer, an indraught towards the poles as well as the equator;—and that consequently, also, there must be a medial line or band somewhere between the parallels of 10° and 55° south, on one side of which the prevailing direction of the wind is towards the equator, on the other towards the pole. So, in the northern hemisphere, the same series of observations point this medial band out to us. They show that one is near the calm belt of Capricorn, the other near the calm belt of Cancer, and that they both probably lie between the parallels of 35° and 40°, where the winds north and south are equal, as per table, page 162.
Winds with Nothing and Winds with Southing in each Hemisphere, expressed by Average Number of Days for which they blow annually.
| Bands. | Northern Hemisphere. | Northern Hemisphere. | Northern Hemisphere. | Southern Hemisphere. | Southern Hemisphere. | Southern Hemisphere. |
| Bands. | Northing. | Southing. | No. of Obs. | Northing. | Southing. | No. of Obs. |
| Between | Days. | Days. | | Days. | Days. | |
| 0° and 5° | 78 | 268 | 67,829 | 84 | 269 | 72,945 |
| 5° and 10° | 158 | 182 | 36,841 | 73 | 283 | 54,648 |
| 10° and 15° | 278 | 73 | 27,339 | 82 | 275 | 43,817 |
| 15° and 20° | 272 | 81 | 33,103 | 91 | 266 | 46,604 |
| 20° and 25° | 246 | 101 | 44,527 | 128 | 227 | 66,395 |
| 25° and 30° | 185 | 162 | 68,777 | 146 | 208 | 66,635 |
| 30° and 35° | 155 | 195 | 62,514 | 150 | 204 | 76,254 |
| 35° and 40° | 173 | 178 | 41,233 | 178 | 177 | 107,231 |
| 40° and 45° | 163 | 186 | 33,252 | 202 | 155 | 63,669 |
| 45° and 50° | 164 | 188 | 29,461 | 209 | 148 | 29,132 |
| 50° and 55° | 147 | 204 | 41,570 | 208 | 151 | 14,286 |
| 55° and 60° | 141 | 213 | 17,874 | 224 | 132 | 13,617 |
| | | | 504,320 | | | 655,233 |
| | Total Observations 1,159,553 | Total Observations 1,159,553 | Total Observations 1,159,553 | Total Observations 1,159,553 | Total Observations 1,159,553 | Total Observations 1,159,553 |
crossing, between 35° and 40°. In the southern hemisphere, the conflict between the polar and equatorial indraught, as expressed by winds with southing and winds with northing, is more decided. There the two curves march, one up, the other down, and cross between the parallels of 35° and 40° S., thus confirming what from other data we had already learned, viz., that the condition of the atmosphere is more unstable in the northern than it is in the southern hemisphere.
354. The rainless regions and the calm belts.—Such, for the winds at sea, is their distribution between the two halves of the horizon in the several bands and in each hemisphere. Supposing a like distribution to obtain on shore, we shall find it suggestive to trace the calm belts of the tropics across the continents (Plate VIII.), and to examine, in connection with them, the rainless regions of the earth, and those districts of country which, though not rainless, are nevertheless considered as "dry countries," by reason of the small amount of precipitation upon them. So, tracing the calm belt of Cancer, which at sea lies between the parallels of 28° and 37° (Plate VIII.), but which, according to Sir John Herschel, reaches higher latitudes on shore, it will be perceived that the winds that flow out on the north side blow ​over countries abounding in rivers, which countries are therefore abundantly supplied with rains. Hence we infer (§ 350) that those winds are rain winds. On the other hand, the winds that flow out on the equatorial side blow either over deserts, rainless regions, or dry countries. Hence we infer that these winds are dry winds. These "dry" winds traverse a country abounding in springs and rivers in India, but it is the monsoons there which bring the water for them. The winds which come out of this calm belt on its equatorial side give out no moisture, except as dew, until they reach the sea, and are replenished with vapour thence in sufficient quantities to make rain of; whereas the winds which come out on the polar side leave moisture enough as they come for such rivers as the Obi, the Yenisei, the Lena, and the Amoor, in Asia; the Missouri, the Sascatchawan, the Red River of the North, and others, in America. Between this calm belt and the head waters of these rivers there are no seas or other evaporating surfaces, neither are they so situated with regard to the sea-coast that they may be, as the shores of Eastern China and the Atlantic slopes of the United States are, supplied with vapour by the winds from the sea-board. When we consider the table (§ 353), the situation of the rainless regions and dry countries with regard to the calm belt of Cancer, we are compelled to admit that, come whence it may and by what channels it may, there are flowing out of this calm belt two kinds of air, one well charged with moisture, the other dry and thirsty to a degree.
355. The theory of the crossings re-stated, and the facts reconciled by it.—The supposition that the dry air came from the north and the moist from the south, and both as an upper current, is the only hypothesis that is consistent with all the known facts of the case. The dry air gave up all its moisture when, as a surface wind, it played upon the frozen summits of the northern hills; the wet obtained its moisture when, as the south-east trade-winds, it swept across the bosom of intertropical seas of the southern hemisphere. Rising up at the equator, it did not leave all its moisture with the cloud-ring, but, retaining a part, conveyed it through the cloud region, above the north-east trades, to this calm belt, where there was a descent and a crossing. The fact that these dry places are all within or on the equatorial side of this calm belt, while countries abounding with rains and well watered with running streams are to be found all along its ​polar side, is clearly indicative of a crossing. Upon no other supposition can we account for the barrenness on one side, the fertility on the other. The following are also links in the chain of facts and circumstances which give strength to the supposition that the rains for the Lena and the Missouri are brought across the calm belt of Cancer by those currents of air which flow thence towards the pole as the prevailing counter-trades or south-westerly winds of the extra-tropical north. We have already seen (§ 353) that, on the north side of this calm zone of Cancer, the prevailing winds on the surface are from this zone towards the pole, and (Plate I., § 215) that these winds return as A B C through the upper regions from the pole; that, arriving at the calms of Cancer, this upper current, A B C, meets another upper current, S R, from the equator, where they neutralize each other, produce a calm, descend, and come out as surface winds, D E, or the trade-winds; and as T U, or the counter-trades. Now observations have shown that the winds represented by T U are rain winds; those represented by D E, dry winds; and it is evident that A B C could not bring any vapours to these calms to serve for T U to make rains of; for the winds represented by A B C have already performed the circuit of surface winds as far as the pole, during which journey they parted with all their moisture, and, returning through the upper regions of the air to the calm belt of Cancer, they arrived there as dry winds. The winds represented by D E are dry winds; therefore it was supposed that these are, for the most part, but a continuation of the winds A B C. On the other hand, if the winds A B C, after descending, do turn about and become the surface winds T U, they would first have to remain a long time in contact with the sea, in order to be supplied with vapour enough to feed the great rivers, and supply the rains for the whole earth between us and the north pole. In this case, we should have an evaporating region at sea and a rainless region ashore on the north as well as on the south side of this zone of Cancer; but investigation shows no such region. Hence it was inferred that B C and R S do come out on the surface as represented by Plate I. But what is the agent that should lead them out by such opposite paths? According to this mode of reasoning, the vapours which supply the rains for T U would be taken up in the south-east trade-wind region by O Q, and conveyed thence along the route Q R S to T. And if this mode of reasoning be admitted as plausible—if it be ​true that R S carry the vapour which, by condensation, is to water with showers the extra-tropical regions of the northern hemisphere, Nature, we may be sure, has provided a guide for conducting S T across this belt of calms, and for sending it on in the right way. Here it was, then, at this crossing of the winds, that I thought I first saw the footprints of an agent whose character I could not comprehend. Can it be the magnetism that resides in the oxygen of the air? Heat and cold, the early and the latter rain, clouds and sunshine, are not, we may rely upon it, distributed over the earth, by chance; they are distributed in obedience to laws that are as certain and as sure in their operations as the seasons in their rounds. If it depended upon chance whether the dry air should come out on this side or on that of this calm belt, or whether the moist air should return or not whence it came—if such were the case in nature, we perceive that, so far from any regularity as to seasons, we should have, or might have, years of drought the most excessive, and then again seasons of rains the most destructive; but, so far from this, we find for each place a mean annual proportion of both, and that so regulated withal, that year after year the quantity is preserved with remarkable regularity. Having thus shown that there is no reason for supposing that the upper currents of air, when they meet over the calms of Cancer and Capricorn, are turned back to the equator, but having shown that there is reason for supposing that the air of each current, after descending, continues on in the direction towards which it was travelling before it descended, we may go farther, and, by a similar train of circumstantial evidence, afforded by these researches and other sources of information, show that the air, kept in motion on the surface by the two systems of trade-winds, when it arrives at the belt of equatorial calms and ascends, continues on thence, each current towards the pole which it was approaching while on the surface. In a problem like this, demonstration in the positive way is difficult, if not impossible. We must rely for our proof upon philosophical deduction, guided by the lights of reason; and in all cases in which positive proof cannot be adduced, it is permitted to bring in circumstantial evidence; and the circumstantial evidence afforded by my investigations goes to show that the winds represented by O Q, § 215, do become those represented by R S T U V A, and A B C D E F respectively. In the first place, O Q represents the south-east trade-winds—i. e., all the ​winds of the southern hemisphere as they approach the equator; and is there any reason for supposing that the atmosphere does not pass freely from one hemisphere to another? On the contrary, many reasons present themselves for supposing that it does. If it did not, the proportion of land and water, and consequently of plants and warm-blooded animals, being so different in the two hemispheres, we might imagine that the constituents of the atmosphere in them would, in the course of ages, probably become different also, and that consequently, in such a case, man could not safely pass from one hemisphere to the other. Consider the manifold beauties in the whole system of terrestrial adaptations; remember what a perfect and wonderful machine (§ 268) is this atmosphere; how exquisitely balanced and beautifully compensated it is in all its parts. We know that it is perfect; that in the performance of its various offices it is never left to the guidance of chance—no, not for a moment. Wherefore I was led to ask myself why the air of the south-east la-ades, when arrived at the zone of equatorial calms, should not, after ascending, rather return to the south than go on to the north? Where and what is the agency by which its course is decided? Here I found circumstances which again induced me to suppose it probable that it neither turned back to the south nor mingled with the air which came from the regions of the north-east trades, ascended, and then flowed indiscriminately to the north or the south. But I saw reasons for supposing that what came to the equatorial calms as the south-east trade-winds continued to the north as an upper current, and that what had come to the same zone as north-east trade-winds ascended and continued over into the southern hemisphere as an upper current, bound for the calm zone of Capricorn. And these are the principal reasons and conjectures upon which these suppositions were based: At the seasons of the year when the area covered by the south-east trade-winds is large, and when they are evaporating most rapidly in the southern hemisphere, even up to the equator, the most rain is falling in the northern. Therefore it is fair to suppose that much of the vapour which is taken up on that side of the equator is precipitated on this. The evaporating surface in the southern hemisphere is greater, much greater, than it is in the northern; still, all the great rivers are in the northern hemisphere, the Amazon being regarded as common to both; and this fact, as far as it goes, tends to ​corroborate the suggestion as to the crossing of the trade-winds at the equatorial calms. Taking the laws and rates of evaporation into consideration, I could find (Chapter V.) no part of the ocean of the northern hemisphere from which the sources of the Mississippi, the St Lawrence, and the other great rivers of our hemisphere could be supplied. A regular series of meteorological observations has been carried on at the military posts of the United States since 1819. Rain maps of the whole country have been prepared from these observations by Mr. Lorin Blodget at the Surgeon-General's office, and under the direction of Dr. Cooledge, U.S.A. These maps, as far as they go, sustain these views in a remarkable manner, for they bring out facts in a most striking way to show that the dry season in California and Oregon is the wet season in the Mississippi Valley. The winds coming from the south-west, and striking upon the coast of California and Oregon in winter, precipitate there copiously. They then pass over the mountains robbed in part of their moisture. Of course, after watering the Pacific shores, they have not as much vapour to make rains of, specially for the upper Mississippi Valley, as they had in the summer-time, when they dispensed their moisture, in the shape of rains, most sparingly upon the Pacific coasts. According to these views, the dry season on the Pacific slopes should be the "wet, especially in the upper Mississippi Valley, and vice versá. Blodget's maps show that such is actually the case. Meteorological observations in the "Red River country" and other parts of British America would throw farther light and give farther confirmation, I doubt not, both to these views and to this interesting question. These army observations, as expressed in Blodget's maps, reveal other interesting features, also, touching the physical geography of the country. I allude to the two isothermal lines 45° and 65° (Plate VIII.), which include between them all places that have a mean annual temperature between 45° and 65°. I have drawn, for the sake of comparison, similar lines on the authority of Dove and Johnston (A. K., of Edinburgh), across Europe and Asia. The isotherm of 65° skirts the northern limits of the sugar-cane, and separates the intertropical from the extra-tropical plants and productions. I have drawn these two lines across America in order to give a practical exemplification of the nature of the advantages which the industrial pursuits and the political ​economy of the country would derive by the systematic extension of our meteorological observations from the sea to the land. These lines show how much we err when we reckon climates according to parallels of latitude. The space that these two isotherms of 45° and 65° comprehend between the Mississippi and the Rocky Mountains, owing to the singular effect of those mountains upon the climate, is larger than the space they comprehend between the Mississippi and the Atlantic. Hyetographically it is also different, being dryer, and possessing a purer atmosphere. In this grand range of climate between the meridians of 100° and 110° W., the amount of precipitation is just about one-half of what it is between those two isotherms east of the Mississippi. In this new country west of it, winter is the dry, and spring the rainy season. It includes the climates of the Caspian Sea, which Humboldt regards as the most salubrious in the world, and where he found the most delicious fruits that he saw during his travels. Such was the purity of the air there, that polished steel would not tarnish even by night exposure. These two isotherms, with the remarkable loop which they make to the north-west, beyond the Mississippi, embrace the most choice climates for the olive, the vine, and the poppy; for the melon, the peach, and almond. The finest of wool may be grown there; and the potato, with hemp, tobacco, maize, and all the cereals, may be cultivated there in great perfection. No climate of the temperate zone will be found to surpass in salubrity that of this Piedmont trans-Mississippi country. The calm zone of Capricorn is the duplicate of that of Cancer, and the winds flow from it as they do from that, both north and south, but with this difference: that on the polar side of the Capricorn belt they prevail from the northwest instead of the south-west, and on the equatorial side from the south-east instead of the north-east. Now if it be true that the vapour of the north-east trade-winds is condensed in the extra-tropical regions of the southern hemisphere, the following path, on account of the effect of diurnal rotation of the earth upon the course of the winds, would represent the mean circuit of a portion of the atmosphere moving according to the general system of its circulation over the Pacific Ocean, viz.: coming down from the north as an upper current, and appearing on the surface of the earth in about longitude 120° west, and near the tropic of Cancer, it would here commence to blow the north-east ​trade-winds of that region. To make this clear, see Plate VII., on which I have marked the course of such vapour-bearing winds; A being a breadth or swath of winds in the north-east trades; B, the same wind as the upper and counter-current to the south-east trades; and C, the same wind after it has descended in the calm belt of Capricorn, and come out on the polar tide thereof, as the rain winds and prevailing north-west winds of the extra-tropical regions of the southern hemisphere. This, as the north-east trades, is the evaporating wind. As the north-east trade-wind, it sweeps over a great waste of waters lying between the tropic of Cancer and the equator. Meeting no land in this long oblique track over the tepid waters of a tropical sea, it would, if such were its route, arrive somewhere about the meridian of 140° or 150° west, at the belt of equatorial calms, which always divides the north-east from the south-east trade-winds. Here, depositing a portion of its vapour as it ascends, it would, with the residuum, take, on account of diurnal rotation, a course in the upper region of the atmosphere to the south-east, as far as the calms of Capricorn. Here it descends and continues on towards the coast of South America, in the same direction, appearing now as the prevailing north-west wind of the extra-tropical regions of the southern hemisphere. Travelling on the surface from warmer to colder regions, it must, in this part of its circuit, precipitate more than it evaporates. Now it is a coincidence, at least, that this is the route by which, on account of the land in the northern hemisphere, the north-east trade-winds have the fairest sweep over that ocean. This is the route by which they are longest in contact with an evaporating-surface; the route by which all circumstances are most favourable to complete saturation; and this is the route by which they can pass over into the southern hemisphere most heavily laden with vapours for the extra-tropical regions of that half of the globe; and this is the supposed route which the north-east trade-winds of the Pacific take to reach the equator and to pass from it. Accordingly, if this process of reasoning be good, that portion of South America between the calms of Capricorn and Cape Horn, upon the mountain ranges of which this part of the atmosphere, whose circuit I am considering as type, first impinges, ought to be a region of copious precipitation. Now let us turn to the works on Physical Geography, and see what we can find upon this subject. In Berghaus and Johnston— ​department Hyetography—it is stated, on the authority of Captain King, R.N., that upwards of twelve feet (one hundred and fifty-three inches) of rain fell in forty-one days on that part of the coast of Patagonia which lies within the sweep of the winds just described. So much rain falls there, navigators say, that they sometimes find the water on the top of the sea fresh and sweet. After impinging upon the cold hill-tops of the Patagonian coast, and passing the snow-clad summits of the Andes, this same wind tumbles down upon the eastern slopes of the ranges as a dry wind; as such, it traverses the almost rainless and barren regions of cis-Andean Patagonia and South Buenos Ayres, Plate VIII. These conditions, the direction of the prevailing winds, and the amount of precipitation, may be regarded as evidence afforded by nature, if not in favour of, certainly not against, the conjecture that such may have been the voyage of this vapour through the air. At any rate, here is proof of the immense quantity of vapour which these winds of the extra-tropical regions carry along with them towards the poles; and I can imagine no other place than that suggested, whence these winds could get so much vapour.
356. The question. How can two currents of air cross? answered.—Notwithstanding the amount of circumstantial evidence that has already been brought to show that the air which the north-east and the south-east trade-winds discharge into the belts of equatorial calms, does, in ascending, cross—that from the southern passing over into the northern, and that from the northern passing over into the southern hemisphere (see O Q R S, and D E F G, § 215)—yet some have implied doubt by asking the question, "How are two such currents of air to pass each other?" And, for the want of light upon this point, the correctness of my reasoning, facts, inferences, and deductions has been questioned. In the first place, it may be said in reply, the belt of equatorial calms is often several hundred miles across, seldom less than sixty; whereas the depth of the volume of air that the trade-winds pour into it is only about three miles, for that is supposed to be about the height to which the trade-winds extend. Thus we have the air passing into these calms by an opening on the north side for the north-east trades, and another on the south for the south-east trades, having a cross section of three miles vertically to each opening. It then escapes by an opening upward, the cross section of which is sixty or one ​hundred, or even three hundred miles. A very slow motion upward there will carry off the air in that direction as fast as the two systems of trade-winds, with their motion of twenty miles an hour, can pour it in; and that curds or flakes of air can readily cross each other and pass in different directions without interfering the one with the other, or at least without interfering to that degree which prevents, we all know. The brown fields in summer afford evidence in a striking manner of the fact that, in nature, flakes, or streamlets, or curdles of air do really move among each other without obstruction. That tremulous motion which we so often observe above stubble-fields, barren wastes, or above any heated surface, is caused by the ascent and descent, at one and the same time, of flakes of air at different temperatures, the cool coming down, the warm going up. They do not readily commingle, for the astronomer long after nightfall, when he turns his telescope upon the heavens, perceives and laments the unsteadiness they produce in the sky. If the air brought to the calm belt by the north-east trade-winds differ in temperature (and why not?) from that brought by the south-east trades, we have the authority of nature for saying that the two currents would not readily commingle (§ 98). Proof is daily afforded that they would not, and there is reason to believe that the air of each current, in streaks, or patches, or flakes, does thread its way through the air of the other without difficulty. Therefore we may assume it as a postulate which nature concedes, that there is no physical difficulty as to the two currents of air, which come into those calm belts from different directions, crossing over, each in its proper direction, without mingling.
357. The rain winds in the Mississippi Valley.—The same process of reasoning which conducted us (§ 355) into the trade-wind region of the northern hemisphere for the sources of the Patagonian rains, now invites us into the trade-wind regions of the South Pacific Ocean to look for the vapour springs of the Mississippi. If the rain winds of the Mississippi Valley come from the east, then we should have reason to suppose that their vapours were taken up from the Atlantic Ocean and Gulf Stream; if the rain winds come from the south, then the vapour springs might, perhaps, be in the Gulf of Mexico; if the rain winds come from the north, then the great lakes might be supposed to feed the air with moisture for the fountains of that river; but if the rains come from the west, where, short of the ​great Pacific Ocean, should we look for the place of evaporation? Wondering where, I addressed a circular letter to farmers and planters of the Mississippi Valley, requesting to be informed as to the direction of their rain winds. I received replies from Virginia, Mississippi, Tennessee, Missouri, Indiana, and Ohio; and subsequently, from Colonel W. A. Bird, Buffalo, New York, who says, "The south-west winds are our fair-weather winds; we seldom have rain from the south-west." Buffalo may get much of its rain from the Gulf Stream with easterly winds. But I speak of the Mississippi Valley; all the respondents there, with the exception of one in Missouri, said, "The south-west winds bring us our rains." These winds certainly cannot get their vapours from the Rocky Mountains, nor from the Salt Lake, for they rain quite as much upon that basin as they evaporate from it again; if they did not, they would in the process of time have evaporated all the water there, and the lake would now be dry. These winds, that feed the sources of the Mississippi with rain, like those between the same parallels upon the ocean, are going from a higher to a lower temperature; and the winds in the Mississippi Valley, not being in contact with the ocean, or with any other evaporating surface to supply them with moisture, must bring with them from some sea or another that which they deposit. Therefore, though it may be urged, inasmuch as the winds which brought the rains to Patagonia (§ 355) came direct from the sea, that they therefore took up their vapours as they came along, yet it cannot be so urged in this case; and if these winds could pass with their vapours from the equatorial calms through the upper regions of the atmosphere to the calms of Cancer, and then as surface winds into the Mississippi Valley, it was not perceived why the Patagonian rain winds should not bring their moisture by a similar route. These last are from the north-west, from warmer to colder latitudes; therefore, being once charged with vapours, they must precipitate as they go, and take up less moisture than they deposit. The circumstance that the rainy season in the Mississippi Valley (§ 355) alternates with the dry season on the coast of California and Oregon, indicates that the two regions derive vapour for their rains from the same fountains.
359. Quetelet's observations.—The more we study the workings of the atmospherical machinery of our planet, the more are we impressed with the conviction that we as yet know very little concerning its secret springs, and the little "governors" here and there which regulate its movements. My excellent friend M. Quetelet, the astronomer royal at Brussels, has instituted a most excellent series of observations upon atmospherical electricity. He has shown that there is in the upper regions of the air a great reservoir of positive electricity, which increases as the temperature diminishes. So, too, with the magnetism of the oxygen in the upper regions.
​360. At sea in the southern hemisphere we have the rule, on land in the northern the exceptions, as to the general circulation of the atmosphere.—In the southern hemisphere, we may, by reason of its great aqueous area, suppose the general law of atmospherical moments to be better developed than it is in the northern hemisphere. We accordingly see by the table (§ 353) that the movements north and south between 45° and 50° correspond with the movements south and north between 25° and 30°; that as you go from the latter band towards the equator the winds with southing in them increase, while the winds with northing in them increase as you go from the former towards the pole.
361. The magnetic poles, the poles of the wind and of cold coincident.—This is the law in both hemispheres: thus indicating that there must be in the polar regions, as in the equatorial, a calm place, where these polar-bound winds cease to go forward, rise up, and commence their return (§ 214) as an upper current. So we have theoretically a calm disc, a polygon—not a belt—about each pole. The magnetic poles and the poles of maximum cold (§ 347) are coincident. Do not those calm discs, or "poles of the wind," and the magnetic poles, cover the same spot, the two standing in the relation of cause and effect? This question was first asked several years ago, and I was then moved to propound it by the inductions of theoretical reasoning. Observers, perhaps, may never reach those inhospitable regions with their instruments to shed more light upon this subject; but Parry and Barrow have found reasons to believe in the existence of a perpetual calm about the north pole, and later, Bellot has reported the existence of a calm region within the frigid zone. Professor J. H. Coffin, in an elaborate and valuable paper on the "Winds of the Northern Hemisphere," arrives by deduction at a like conclusion. In that paper he has discussed the records at no less than five hundred and seventy-nine meteorological stations, embracing a totality of observations for two thousand eight hundred and twenty-nine years. He places his "meteorological pole"—pole of the winds—near latitude 84° north, longitude 105° west. The pole of maximum cold, by another school of philosophers, Sir David Brewster among them, has been placed in latitude 80° north, longitude 100° west; and the magnetic pole, by still another school, in latitude 73° 35' north, longitude ​95° 39' west. Neither of these poles is a point susceptible of definite and exact position. The polar calms are no more a point than the equatorial calms are a line; and, considering that these poles are areas or discs, not points, it is a little curious that philosophers in different parts of the world, using different data, and following up investigation each through a separate and independent system of research, and each aiming at the solution of different problems, should nevertheless agree in assigning very nearly the same position to them all. Are these three poles grouped together by chance or by some physical cause? By the latter, undoubtedly. Here, then, we have another of those gossamer-like clews, that sometimes seem almost palpable enough for the mind, in its happiest mood, to lay hold of, and follow up to the very portals of knowledge, where we pause and linger, fondly hoping that the chambers of hidden things may be thrown open, and that we may be permitted to behold and contemplate the mysteries of the winds, the frost, and the trembling needle. In the polar calms there is (§ 215) an ascent of air; if an ascent, a diminution of pressure and an expansion; and if expansion, a decrease of temperature. Therefore we have palpably enough a connecting link here between the polar calms and the polar place of maximum cold. Thus we establish a relation between the pole of the winds and the pole of cold, with evident indications that there is also a physical connection between these and the magnetic pole. Here the out-croppings of a relation between magnetism and the circulation of the atmosphere again appear.
362. The barometer in the wind bands.—Thousands of observations, made by mariners and recorded in their abstract logs, have enabled us to determine approximately the mean height of the barometer for the various bands (§ 352) at sea. Between the parallels of 36° S. and 50° N., Lieut. Andrau, of the Dutch Navy, has collected from the abstract logs at the Meteorological Institute of Utrecht no less than 83,334 observations on the height of the barometer in the following bands. (See table, page 176.)
Number of Observations and Mean Height of the Barometer between the Parallels of 78° 37’ N. and 74° S.
| North | Baromoter. | No. | South. | Barometer. | No. |
| 0° and 5° | 29.915 | 5114 | 0° and 5° | 29.940 | 3692 |
| 5° and 10° | 29.922 | 5343 | 5° and 10° | 29.981 | 3924 |
| 10° and 15° | 29.964 | 4496 | 10° and 15° | .30.028 | 4166 |
| 15° and 20° | 30.018 | 3592 | 15° and 20° | 30.060 | 4248 |
| 20° and 25° | 30.081 | 3816 | 20° and 25° | 30.102 | 4536 |
| 25° and 30° | 30.149 | 4302 | 25° and 30° | 30.095 | 4780 |
| 30° and 35° | 30.210 | 4989 | 30° and 35° | 30.052 | 6970 |
| 35° and 40° | 30.124 | 5103 | 42° 53′ | 29.90 |
| 40° and 45° | 30.077 | 5898 | 45° 0′ | 29.66 |
| 45° and 50° | 30.060 | 8282 | 49° 08′ | 29.47 |
| 51° 29' | 29.99 | | 51° 33' | 29.50 |
| 59° 51′ | 29.88 | | 54° 26’ | 29.35 |
| 78° 37’ | 29.759 | | 55° 52' | 29.36 |
| | | | 60°0’ | 29.11 |
| | | | 66° 0′ | 29.08 |
| | | | 74°0′ | 28.93 | |
| ↑ From 50° N. to 36° S. the observations are the mean of 83,334 taken from " Maandelijksche Zeilaanwijzingen van Java naar Let Kanaal Koninklijk Nederlandsch Meteorologisch Instituut, 1859." ↑ Hobart Town; mean of 10 years' observations. ↑ Sir J. C. Ross ; " Erebus and Terror." ↑ Greenwich; mean of 4 years' observations. ↑ St. Petersburg; mean of 10 years' observations. ↑ Dr Kane; 12,000 observations (mean of 17 months' observation) ||||||
away from the austral regions, just as the vapour that is formed in the real steam-boiler expels the air from it. This difference of atmosphere over the two halves of the globe, as indicated by the barometer, is very suggestive.
364. A standard of comparison for the barometer at sea.—Admiral Fitzroy has also reduced from the abstract logs in the Meteorological Department of the Board of Trade in London a great number of barometrical observations. He has discovered the near the parallel of 5° N. in the Atlantic Ocean the pressure of the atmosphere is so uniform as to afford navigators a natural standard by which, out there at sea, they may, as they pass to and fro, compare their barometers. This pressure is said to be so uniform, that after allowing for the six-hourly fluctuations, the mariner may detect any error in his barometer amounting to the two or three thousandth part of an inch.
365. South-east trade-winds having no moisture traced over into ​rainless regions of the northern hemisphere.—According to the views presented in § 358 and Plate VII., the south-east trade-winds, which reach the shores of Brazil near the parallel of Rio, and which blow thence for the most part over the land, should be the winds which, in the general course of circulation, would be carried, after crossing the Andes and rising up in the belt of equatorial calms, towards Northern Africa, Spain, and the South of Europe. They might carry with them the infusoria of Ehrenberg (§ 358), but according to this theory, they would be wanting in moisture. Now, are not those portions of the Old World, for the most part dry countries, receiving but a small amount of precipitation? Hence the general rule: those countries to the north of the calms of Cancer, which have large bodies of land situated to the southward and westward of them, in the south-east trade-wind region of the earth, should have a scanty supply of rain, and vice versâ. Let us try this rule: The extra-tropical part of New Holland comprises a portion of land thus situated in the southern hemisphere. Tropical India is to the northward and westward of it; and tropical India is in the north-east trade-wind region, and should give extra-tropical New Holland a slender supply of rain. But what modifications the monsoons of the Indian Ocean may make to this rule, or what effect they may have upon the rains in New Holland, my investigations in that part of the ocean have not been carried far enough for final decision; though New Holland is a dry country.
366. Each hemisphere receives from the sun the same amount of heat.—The earth is nearer to the sun in the summer of the southern hemisphere than it is in the summer of the northern; consequently, it has been held that one hemisphere annually receives more heat than the other. But the northern summer is 7⋅7 days longer than the southern; and Sir John Herschel has shown, and any one who will take the trouble may demonstrate, that the total amount of direct solar heat received annually by each hemisphere is identically the same, and therefore the northern hemisphere in its longer summer makes up with heat for the greater intensity but shorter duration of the southern summer. But though the amount of heat annually impressed by the sun upon each hemisphere be identically the same, it by no means follows that the amount radiated off into space by each hemisphere again is also identically the same. There is no reason to believe that the earth is growing warmer or cooler, and ​therefore we infer that the total amount of heat received annually by the whole earth is again annually radiated from the whole earth. Nevertheless, the two hemispheres may radiate very unequally.
367. The northern radiates most.—Direct observations concerning the amount of radiation from different parts of the surface of our planet are meagre, and the results as to quantity by no means conclusive; but we have in the land and sea breezes a natural index to the actinometry of sea and land, which shows that the radiating forces of the two are very different. Notwithstanding the temperature of the land is raised so much above that of the waters during the day, its powers of radiation are so much greater than those of water that its temperature falls during the night below that of the sea, and so low as to produce the land breeze. From this fact it may be inferred that the hemisphere that has most land dispensed most heat by radiation.
368. Another proof of the crossings at the calm belts.—The question now may be well put: Since the two hemispheres receive annually the same amount of heat from the sun, and since the northern hemisphere, with its greater area of land, radiates most, whence does it derive the surplus? The theory of the crossing at the calm belts indicates both the way and the means, and suggests the answer; for it points to the latent heat of vapour that is taken up in the southern hemisphere, transported by the winds across the calm belts, and liberated, as the clouds drop down their fatness upon northern fields. It is not only the difference of radiating power between land and water that makes the northern continents the chimneys of the earth, but the difference of cloud in a continental and an oceanic sky must also greatly quicken the radiating powers of the northern hemisphere. Radiation goes on from the upper surface of the clouds and from the atmosphere itself, but we know that clouds in a great measure obstruct radiation from the surface of the earth; and as the surface of the earth receives more of the direct heat of the sun than the atmosphere, the point under discussion relates to the mode in which the surface of the earth gets rid of that heat. It gets rid of it chiefly in three ways: some is carried off by convection in the air; some by evaporation; and some by radiation; and such is the interference of clouds with this last-named process, that we are told that during the rainy season in intertropical countries, as on the coast of Africa, there ​is often not radiation enough to produce the phenomena of land and sea breezes. The absence of dew in cloudy nights is a familiar instance of the anti-radiating influence of clouds. The southern hemisphere, being so much more aqueous, is no doubt much more enveloped with clouds where its oceans lie, than is the northern where its continents repose, and therefore it is that one hemisphere radiates more than the other.
369. Facts and pearls.—Thus, by observing and discussing, by resorting to the force of reason and to the processes of induction, we have gathered for the theory that favours the air-crossings at the calm belts fact upon fact, which, like pearls for the necklace, seemed only to require a string to hang them together.
* ↑ "Average Force of the Trade-winds," p. 857, vol. ii., Maury's Sailing Directions, 1859.
* ↑ § 273, p. 614, vol. xvii. (Phys. Geog.), Encyclopaedia Britannica.
* ↑ See Army Meteorological Observations, published 1855.
* ↑ Maury's Sailing Directions.
* ↑ Smithsonian Contributions to Knowledge, vol. vi., 1854.
* ↑ Gauss
* ↑ Below the parallels of 50° N. and 36° S. the observations are reduced to the temp, of 32° Fahr.