# Physical Geography of the Sea and its Meteorology/Chapter 15

CHAPTER XV.
§ 621-680.—SEA ROUTES, CALM BELTS, AND VARIABLE WINDS.
621. Practical results of physical researches at sea.—Plate VIII., so far as the winds are concerned, is supplemental to Plate I. The former shows the monsoon regions, and indicates the prevailing direction of the winds in every part of the ocean; the latter indicates it generally for any latitude, without regard to any particular sea. Plate VIII. also exhibits the principal routes across the ocean. This plate indicates the great practical results of all the labour connected with this vast system of research; its aim is the improvement of navigation; its end, the shortening of voyages. Other interests and other objects, nay, the great cause of human knowledge, have been promoted by it; but the advancement that has been given to these do not, in this utilitarian age, and in the mind of people so eminently practical as mariners are, stand out in a relief half so grand and imposing as do those achievements by which the distant isles and marts of the sea have, for the convenience of commerce, been lifted up, as it were, and brought closer together by many days' sail.
622. Time-tables.—So to shape the course on voyages as to make the most of the winds and currents at sea is the perfection of the navigator's art. How the winds blow and the currents flow along this route or that, is no longer matter of opinion or subject of speculation, but it is a matter of certainty determined by actual observation. Their direction has been determined for months and for seasons, along many of the principal routes, with all the accuracy of which results depending on the doctrine of ​chances are capable; and farther, these results are so certain that there is no longer any room for the mariner to be in doubt as to the best route. When a navigator undertakes a voyage now, he does it with the lights of experience to guide him. The winds and the weather daily encountered by hundreds who have sailed on the same voyage before him, with "the distance made good" by each one from day to day, have been tabulated in a work called Sailing Directions, and they are so arranged that he may daily see how much he is ahead of time, or how far he is behind time; nay, his path has been literally blazed through the winds for him on the sea; mile-posts have been set up on the waves, and finger-boards planted, and time-tables furnished for the trackless waste, by which the ship-master, even on his first voyage to any port, may know as well as the most experienced trader whether he be in the right road or no.
623. Close running.—From New York to the usual crossing of the equator on the route to Rio, the distance, by an air line, is about 3400 miles; but the winds and currents are such as to force the Rio bound vessel out of this direct line. Nevertheless, they have been mapped down, studied, and discussed so thoroughly that we may compute with remarkable precision the detour that vessels attempting this route from New York, or any other port, would have to make. This computation shows that, instead of 3400 miles, the actual distance to be accomplished through the water by vessels under canvas on this part of the voyage is 4093 miles. More than a hundred sailing vessels have tried it by measuring and recording the distance actually sailed from day to day; their mean distance is 4099 miles, consequently their actual average differs only six miles from the computed average.
624. A desideratum on ship-hoard.—The best navigated steamships do not sail closer than this, and a better proof of the accuracy of our knowledge concerning the prevailing direction of the winds at sea could not be afforded. Unfortunately, anemometers are not used on shipboard. Had they been in common use there, and had we been furnished with data for determining the force of the wind as well as its direction, we could compute the time as well as the distance required for the accomplishment of any given voyage under canvas. Thus the average time required to sail from New York to the equator might be ​computed within an hour, for it has been computed within an hour's sail—six miles (§ 623).
625. How passages have been shortened.—By the knowledge thus elaborated from old and new log-books and placed before the nautical world, the average passage from Europe or the United States to all ports in the southern hemisphere has been shortened ten days, and to California a month and a half. Between England and her golden colony in the South Seas the time required for the round voyage has been lessened fifty days or more, and from Europe to India and China the outward passage has been reduced ten days. Such are some of the achievements that commend this beautiful system of research to the utilitarian spirit of the age.
626. Fast sailing.—The route that affords the bravest winds, the fairest sweep, and the fastest running for ships, is the route to and from Australia. But the route which most tries a ship's prowess is the outward-bound voyage to California. The voyage to Australia and back carries the clipper ship along a route which for more than 300° of longitude runs with the "brave west winds" of the southern hemisphere. With these winds alone, and with the bounding seas which follow them, the modern clipper, without auxiliary power, has accomplished a greater distance in a day than any sea steamer has ever been known to reach. Running before these fine winds and heaving seas those ships have performed their voyages of circumnavigation in 60 days.
627. The longest voyage.—The sea voyage to California, Columbia, and Oregon is the longest voyage in the world—longest both as to time and distance. Before these researches were extended ​to the tides and currents along that route, the average passage both from Europe and America to our north-west coast was not less than 180 days. It has been reduced so as to average only 135 days. This route is now so well established, and the winds of the various climates along it so well understood, that California bound vessels sailing about the same time from the various ports of Europe and America are, if they be at all of like prowess, almost sure to fall in with and speak each other by the way.
628. Obstructions to the navigator.—The calm belts at sea, like mountains on the land, stand mightily in the way of the voyager, but, like mountains, they have their passes and their gaps. In the legions of light airs, of baffling winds, and deceitful currents, the seaman finds also his marshes and his "mud-holes" on the water. But these, these researches have taught him how best to pass or entirely to avoid. Thus the forks to his road, its turnings, and the crossings by the way, have been so clearly marked by the winds that there is scarcely a chance for him who studies the lights before him, and pays attention to directions, to miss his way.
629. Plate VIII.—The arrows of Plate VIII. are supposed to fly with the wind; the half-bearded and half- feathered arrows denote monsoons or periodic winds; the dotted bands, the regions of calm and baffling winds. Monsoons, properly speaking, are winds which blow one half of the year from one direction, and the other half from an opposite, or nearly an opposite direction. The time of the changing of these winds, and their boundaries at the various seasons of the year, have been discussed in such numbers, and mapped down in such characters, that the navigator who wishes to take advantage of them or to avoid them altogether is no longer in any doubt as to when and where they may be found.
630. Deserts.—Let us commence the study of the calm belts as they are represented on Plates I. and VIII. The monsoons and trade-winds are also represented on the latter, they often occupy the same region. But, turning to the trade-winds for a moment, we see that the belt or zone of the south-east trade-winds is broader than the belt or zone of north-east trades. This phenomenon is explained by the fact that there is more land in the northern hemisphere, and that most of the deserts of the earth—as the great deserts of Asia and Africa—are situated in the rear, or behind the north-cast trades; so that, as these deserts become ​more or less heated, there is a call—a pulling back, if you please—upon these trades to turn about and restore the equilibrium which the deserts destroy. There being few or no such regions in the rear of the south-east trades, the south-east trade-wind force prevails, and carries them over to the northern hemisphere.
631. Diurnal rotation.—We see by the plate that the two opposing currents of wind called "the trades," are so unequally balanced that the one recedes before the other, and that the current from the southern hemisphere is larger in volume; i. e., it moves a greater zone or belt of air. The south-east trade-winds discharge themselves over the equator—i. e., across a great circle—into the region of equatorial calms, while the north-east trade-winds discharge themselves into the same region over a parallel of latitude, and consequently over a small circle. If, therefore, we take what obtains in the Atlantic as the type of what obtains entirely around the earth, as it regards the trade-winds, we shall see that the south-east trade-winds keep in motion more air than the north-east do, by a quantity at least proportioned to the difference between the circumference of the earth at the equator and at the parallel of latitude of 9° north. For if we suppose that those two perpetual currents of air extend the same distance upward from the surface of the earth, and move with the same velocity, a greater volume from the south should, as has already been shown (§ 343), flow across the equator in a given time than would flow from the north over the parallel of 9° in the same time ; the ratio between the two quantities would be as radius to the secant of 9°. Besides this, the quantity of land lying within and to the north of the region of the north-east trade-winds is much greater than the quantity within and to the south of the region of the south-east trade-winds. In consequence of this, the mean level of the earth's surface within the region of the north-east trade-winds is, it may reasonably be supposed, somewhat above the mean level of that part which is within the region of the south-east trade-winds. And as the north-east trade-winds blow under the influence of a greater extent of land surface than the south-east trades do, the former are more obstructed in their course than the latter by the forests, the mountain ranges, unequally heated surfaces, and other such like obstacles.
632. The land in the northern hemisphere.—That the land of the northern hemisphere does assist to turn these winds is rendered ​still more probable from this circumstance: All the great deserts are in the northern hemisphere, and the land surface is also much greater on our side of the equator. The action of the sun upon these unequally absorbing and radiating surfaces in and behind, or to the northward of the north-east trades, tends to check these winds, and to draw in large volumes of the atmosphere, that otherwise would be moved by them, to supply the partial vacuum made by the heat of the sun, as it pours down its rays upon the vast plains of burning sands and unequally heated land surfaces in our overheated hemisphere. The north-west winds of the southern are also, it may be inferred, stronger than the south-west winds of the northern hemisphere.
633. Why the south-east trades are the stronger.—That the south-east trade-winds should, as observations (§ 343) have shown, be stronger than the north-east trade-winds, is due in part also to the well-established fact that the southern (§ 446) is cooler than the northern hemisphere. The isothermal lines of Dove show that the air of the south-east is also cooler than the air of the north-east trade-winds. Being cooler, the air from the cool side would, for palpable causes, rush with greater velocity into the equatorial calm belt than should the lighter air from the warmer or northern side. The fact that the air in the lower latitudes of the southern hemisphere is the cooler will assist to explain many other contrasts presented by the meteorological conditions on opposite sides of the equator. Plate XIII. shows that we have more calms and more fogs, more rains and more gales, with more thunder, on our side than on the other, and that the atmosphere preserves its condition of unstable equilibrium with much more uniformity, being subject to changes less frequent and violent on the south side of the equator than on the north side.
634. Their uniformity of temperature.—The highest summer temperature in the world is to be found in the extra-tropical countries of the north. The greatest extremes of temperature are also to be found among the valleys of the extra-tropical north. In the extra-tropical south there is but little land, few valleys, and much water; consequently the temperature is more uniform, changes are less sudden, and the consequent commotions in the air less violent.
635. The mean place of the equatorial calm belt.—Following up these facts with their suggestions, we discover the key to many phenomena which before were locked up in "the chambers of ​the south." The belt of equatorial calms which separates the two systems of trade-winds is, as we know (§ 295), variable as to its position. It is also variable in breadth. Sometimes it covers a space of several degrees of latitude, sometimes not more than one. Its southern edge, in spring, sometimes goes down to 5° S.; its northern edge, in autumn, often mounts up to the parallel of 15° N. The key to these phenomena has been found; with it in hand, let us proceed to unlock, first remarking that the mean position of the equatorial calm belt in the Atlantic is between the equator and 9° N., and that as it is there, so I assume it to be in other oceans.
636. Never at rest.—This calm belt is produced by the meeting of the two trade-winds, and it occupies strictly a medial position between them. It is in the barometric valley, between the two barometric ridges (§ 667), from which the trade-winds flow. If one "trade" be stronger than the other, the stronger will prevail so far as to force their place of meeting over and crowd it back upon the weaker wind. It is evident that this place of meeting will recede before the stronger wind, until the momentum of the stronger wind is so diminished by resistance, and its strength so reduced as exactly to be counterbalanced by the weaker wind. Then this calm place will become stationary, and so remain, until, from some cause, one or the other of the meeting winds gains strength or loses force; then the stronger will press upon the weaker, and the calm belt will change place and adjust itself to the new forces. The changes that are continually going on in the strength of the winds keep the calm belt in a trembling state, moving now to the north, now to the south, and always shifting its breadth or its place under the restless conditions of our atmosphere.
637. The calm hells occupy medial positions.—The southern half of the torrid zone is cooler than the northern, and, parallel for parallel, the south-east trade-winds are consequently cooler than the north-east. They both blow into this calm belt, where the air, expanding, ascends, flows off above, produces a low barometer, and so makes room for the inflowing current below. Now if the trade-wind air which flows in on one side of this calm belt be heavier, whether from temperature or pressure, than the trade-wind air which flows in on the other, the wind from the heavy side will be the stronger. This is obvious, for it is evident that if the difference of temperature of the ascending column and the ​inflowing air were scarcely perceptible, the difference of specific gravity between the inflowing wind and the uprising air would be scarcely perceptible, and the movement of the inflowing wind would be very gentle; but if the difference of temperature were very great, the difference of specific gravity would be very great, and the violence of the inrushing wind proportionably great. Because the southern half of the torrid zone is the cooler, the diflerence in temperature between the air of the calm belt and the air of the trade-winds is greater, parallel for parallel, in the south-east than in the north-east trade-winds; consequently, the south-east trade-winds should be—as observations show them to be—stronger than the north-east;—and consequently, also, their meeting should take place, not upon the equator, but upon that side of it where the weaker winds prevail, and this is also in accordance (§ 343) with facts.
638. Strength of the trade-winds varies with the seasons.—It follows from these premises that the winter trade-winds should be stronger than the summer. In our summer, the air which the north-east trade-winds put in motion has its temperature raised and brought more nearly up to that of the air in the calm belt. At the same time, the temperature of the air which the south-east trade-winds put in motion is proportionably lowered. Thus they increase in strength, while the north-east diminish; the consequence is, they push their place of meeting with the north-east trades far over on this side of the equator, and for two or three months of the year maintain the polar edge of the calm belt as high up as the parallel of 15° N. But with the change of seasons these influences are all transposed and brought into play on opposite sides—only in the southern summer the strength of the south-east and the temperature of the north-east trade-winds are diminished so as to admit of the edge of the calm belt being pressed down only as far as 5° instead of 15° S. The causes which produce this alternation of trade-wind strength are cumulative; consequently, the north-east trade-winds should be weakest in August or September, strongest in February or March, after the period of maximum heat in one case and of minimum in the other.
639. Sailing through them in fall and winter.—In the other hemisphere, the period of strongest trades is coincident with that of the minimum in this. These deductions are also confirmed by observations; for such is the difference as to strength and regularity of ​the north-east trade-winds in September and February, that the average passage through them from New York to the line is 26.4 days in the winter against 38.8 in the fall month.
640. A thermal adjustment.—Thus it appears that the equatorial calm belt is made to shift its place with the seasons, not by reason of the greater intensity of the solar ray in the latitude where the calm belt may be at that season, but by reason of the annual variations in the energy of each system of trades; which variations (§ G38) depend upon the changes in the temperature and barometric weight of the air which each system puts in motion. This calm zone, therefore, may be considered as a thermal adjustment—the dynamical null-belt—between the trade-winds of the two hemispheres.
641. The barometer in the trade-winds and equatorial calms.—The observations on the barometer at sea (§ 858) shed light on this subject. According to the Dutch, that instrument stands higher by 0.055 inch in the south-east than it does in the north-east trade-winds. According to the observations of American navigators, it stands 0.050 inch higher. The former determination is derived from 80,873, the latter from 1899 observations; therefore 0.055 inch is entitled to most weight. The trade-winds are best developed between the parallels of 5° and 20°. The mean barometric pressure between these parallels is 29.968 inches for the north-east, and 30.023 inches for the south-east trade-winds; while for the calm belt it is 29.915 inches. The pressure, therefore, upon the air in each of the trade-winds is greater than it is in the calm belt; and it is this difference of pressure, from whatever cause arising, that gives the wind in each system of trades its velocity. The difference between the calm belt and trade-wind pressure is 0.108 for the south-east and 0.053 for the north-east. According to the barometer, then, the south-east should be stronger than the north-east trade-winds, and according to actual observations they are.
642. Experiments in the French Navy.—Now if we liken the equatorial calm belt with its diminished pressure to a furnace, the north-east and the south-east trade-winds may be not inaptly compared to a pair of double bellows that are blowing into it. In excess of barometric pressure, the former is a bellows with a weight of 3.8 lbs., the latter with a weight of 7.8 lbs. to the ​square foot. It is this pressure which, like the weight upon the real bellows in the smithy, keeps up the steady blast; and as the effective weight upon the one system of trades is about double that upon the other, the one under the greatest pressure should blow with nearly double the strength of the other, and this appears, both from actual observations and calculations, as well as from direct experiments ordered in the French brig of war "Zebra," by Admiral Chabannes, to be the case.
​643. Difference in tons of the barometric pressure upon the north-east and south-east trade winds.—With those barometric observations, and the assumed fact that the mean pressure of the atmosphere is 15 lbs, upon the square inch, we may readily determine in tons the total by which the superincumbent pressure upon the south-east trade-winds between the parallels of 5°and 20° exceeds that upon the north-east between corresponding parallels. For the whole girdle of the earth, the excess of pressure upon the south- ​east trade-winds is 1,235,250 millions of tons. This is the super-incumbent weight or pressure which is urging the south-east trade-winds forward faster than the north-east. It is inconceivably great; and to bring it within comprehensible terms, the mariner will be astonished to hear that the weight of atmosphere which is bearing down upon the deck of a first-class clipper ship is 15 or 20 tons greater when he is sailing in her through the south-east than it is when he is sailing in her through the north-east trade-winds.
644. Why the barometer should stand higher in the south-east than in the north-east trade-winds.—The question now suggests itself. Why-should the barometer stand higher in the south-east than it does in the north-east trade-winds? The theory of a crossing at the calm belts affords the answer. The air which the north-east trade-winds deliver into the calm belt is not as heavily laden with moisture as that of the south-east trades. It is not as heavily laden for two reasons; one is, the south-east trade-wind belt is broader than the north-east; consequently, in the former there is more air in contact with the evaporating surface. In the ​next place, the north-east trade-wind belt includes more land within it than the south-east; consequently, when the two winds arrive at the calm belt, they are, for this reason, also unequally charged with moisture. Now, when they rise up and precipitate this moisture, more heat is liberated from the south-east than
Experiments made on board the 'Alceste' (frigate), Commander Debernal, bearing the flag of Rear-Admiral Viscount de Chabannes, for determining the Velocity of the Frigate in all cases, from nearest the Wind to Wind aft.
REMARKS.
The different experiments, whose results are inscribed in this Table, were made during the passage from Bahia to France. They were made in the trade-winds, or in fixed and regular winds; nevertheless, even in the trades the regularity of the wind was not always such that some results did not appear to differ a little from the rule. On the other hand, the log itself gives rise to errors sometimes quite large.
In going eight knots we have thrown two logs, simultaneously, without touching the line; the two results differed a knot. The incomplete experiments were interrupted in consequence of the irregularity of the wind, and each result is a mean result. ​from the north-east trade-wind air; the latter, therefore, after rising up, is the cooler and the more compact; and as, by the theory of the crossings, it flows off to the south as an tipper current, it presses upon the barometer with more weight than the warmer and more moist air that feeds the current which is above and counter to the north-east trades. There is not in the whole range of marine meteorology a single well-established fact that is inconsistent with the theory of a crossing at the calm belts.
645. Cataclysms.—The geological record affords evidence that the climates of the earth were once very different from what they are now; that at one time intertropical climates extended far up towards the north; at another time polar climates reached, with their icebergs and their drift, far down towards the equator; that in remote ages most of what we now call dry land was covered with water, for we find on the mountains and far away in the interior of continents deposits many feet thick, consisting of sea-shells, marine animals, and organic productions of many sorts. These fossils, marks, and traces indicate that since their day, ages inconceivably great have elapsed. Not only so: the lines of drift, and boulders, and gashes with which the earth is scored and strewed, afford reason for the conjecture that there have been cataclysms, in which the waters have swept from north to south, and again from south to north, bearing with them icebergs, huge blocks of stone, rubble, drift, and sediment of various sorts. Lieutenant Julien, M. Le Hon, and M. Adhémar have, with much ingenuity, treated of this subject. They maintain that our earth has a " secular" as well as an annual summer and winter; that these "secular" seasons depend upon the precession of the equinoxes, and that the length of each is consequently 10,500 of our years; and that it is the melting of the polar ices in the " secular" season of one hemisphere, and their recongelation in the "secular" winter of the other, that causes a rush of the sea from one hemisphere into the other; and so cataclysms are produced at regular intervals of 10,500 years. In consequence of the inclination of the axis of the earth to the plane of its orbit, we have our change of seasons; and in consequence of the ellipticity of that orbit, the spring and summer of our hemisphere are at present longer than those of the southern. During the excess of time that the sun tarries on our side of the equator, the southern nights are prolonged, so that the night of ​the south pole—the antarctic winter—is annually a week longer (§ 366) than the arctic. Thus, during the period of 10,500;years, the antarctic regions will experience 142 years of night, or winter, in the aggregate, more than the arctic. Therefore it is manifest, say the cataclysmatists, that though the two hemispheres do receive annually the same amount of solar heat, yet the amount dispensed by radiation is very much greater on one side of the equator than the other. The total effect of the alternate cooling down on each side of the equator causes an accumulation of ice at the pole—when the nights are longest—sufficient, say they, to disturb the centre of gravity of the earth, causing it to take up its position on the icy side of the equator. As the ice accumulates, so is the water drawn over from the opposite hemisphere. Such, brie% stated, is the theory which has found very ingenious and able advocates in the persons of MM. Julien and Adhemar.
646. Are the climates of the earth changing?—This theory is alluded to here, not for the purpose of discussion, but for the purpose of directing attention to certain parts of this work in connection with it, as Chapters VII. and XXI., for example, and of remarking upon the stability of terrestrial climates. Though the temperate regions be cooler in the southern than in the northern hemisphere, it does not appear certain that the climates of the earth are now changing. Observations upon the subject, however, are lacking. The question is one of widespread and exceeding interest; and it may be asked if we have not in the strength of the trade-winds a gauge, or in their barometric weight an index, or in the equatorial calm belt a thermometer—each one of the most delicate construction and sensitive character—which would, within the compass of human life, afford unerring indications of a change of climates, if any such change were going on? If the temperature of the S.E. trade-winds, or the barometric pressure upon the N.E. (§ 641), were to be diminished, the S.E. trades would force this calm belt still farther to the north, and we might have a regular rainy season in what is now the great desert of Sahara; for where this ​calm belt is (§ 517) there is the cloud-ring, with its constant precipitation. Therefore, if there be any indications that the southern edge of the great desert is gradually approaching the equator, it would favour the supposition that the southern hemisphere is growing warmer; but if the indications be that the southern edge of the desert is receding from the equator, then the fact would favour the supposition that the southern hemisphere is growing still cooler. Nor are these the only latchets which a study of this calm belt and of the winds enables us to lift.
647. Temperature of the trade-winds and calm belts.—Theory suggests, and observation, as far as it goes, seems to confirm the suggestion, that the N.E. and S.E. trade-winds enter the equatorial calmbelt at the same temperature. I have followed 100 vessels with their thermometer across the equatorial calm belt of the Atlantic, and another 100 across it in the Pacific. Assuming its mean position to be as these observations indicate it to be—viz., between the parallels of 3° and 9° N.—the mean temperature is 81° at its northern, 81°.4 at its southern edge, and 82'^ in the middle of it. These 200 logs were taken at random, and for all months. The temperature of the air was noted also in each trade at the distance of 5° from its edge of the calm belt. Thus the temperature of the N.E. trades, 5° from the north edge of the calm belt, or in 14° N., is 78°.2; at a like distance in the S.E. trades from the equatorial edge, or in 2^ S., the mean temperature is 80.°2. From this it would seem that, in traversing this belt of 5°, the temperature of the N.E. is raised twice as much as the temperature of the S.E. trades ; which is another indication that the velocity of the S.E. is nearly or quite double the velocity of the N.E. trades (§ 642). For if it be supposed that it takes the N.E. trades twice as long to traverse 5° of latitude as it does the S.E., it is evident that the former would be exposed twice as long to the solar ray, and receive twice the amount of heat that is imparted to the S.E. trade-winds in traversing given differences of latitude. Thus the position of the calm belt, the barometer, the thermometer, and the rate of sailing, all indicate the S.E. trade- winds to be the stronger. It appears, moreover, that the temperature of the S.E. trade-wind is in 2° S. below the temperature of the N.E. in 9° N., the latter being 81°, the former 80°.2.
648. The thermal equator.—The foregoing observations show ​that after these winds enter the calm belt, the air they bring into It continues to rise, and this also is what might well be anticipated, for the sun continues to pour down upon it. But while the temperature of the surface is kept down by the rain-drops from above, the temperature of the air in the whole belt is raised both by the direct heat of the sun and the latent heat which is set free by the constant (§ 515) and oftentimes heavy precipitation there. This latent heat is much more effective than is the direct heat of the sun in rarefying the air; consequently we here unmask the influences which place the thermal equator in the northern hemisphere.
649. A natural actinometer in the trade-winds.—Nor is this the only chamber into which this calm belt key conducts us. Parallel for parallel (§ 446), the southern hemisphere is cooler than the northern; that is, the mean temperature for the parallel of 40° south, for example, is below the mean temperature for the parallel of 40° north, and so of all corresponding parallels between 40° and the equator. It appears, moreover, that the mean temperature of the north-east trade-winds as they cross the parallel of 9° north, and the mean temperature of the south-east trade-winds as they cross the equator, is about the same (§ 647), The difference of temperature, then, between the south-east trades as they cross the parallel of 9° south, and as they cross the equator, expresses the difference in the thermal forces which give difference of energy to the dynamical power of the trade-winds. Not only so: it expresses the difference of temperature between the two corresponding parallels of 9° north and 9° south, and discovers to us a natural actinometer on a grand scale, and of the most delicate and beautiful kind.
650. Heat daily received by the south-east trade-winds.—This actinometer measures for us the heat which the south-east trade-winds receive between the moment of crossing the parallel of 9° south and their arrival at the equator, for the heat thus received is just sufficient (§ 644) to bring so much of the south-east up to the temperature which the north-east trades have as they cross the parallel of 9° north. To complete this measurement of heat we should know how long the south-east trade-winds are on their march from the parallel of 9° south to the equator. According to the estimate, it takes them about a day to accomplish this distance; but, knowing the exact time, we should have in the band of winds an actinometer which would disclose to us the average ​quantity of heat daily impressed by the sun upon the tmosphere at sea between the equator and 9° south. I say it takes about a day, and so infer from these data, viz.: The mean annual direction of the south-east trade-winds between 10° south and the line is south 40° east. We suppose their average velocity to be (§ 313) about 25 miles an hour. At this rate it would take them 29h. 22m. 30s. to reach the equator. During this time they receive more heat than they radiate, and the excess is just sufficient to raise them from the normal temperature of the north-east trades as they enter the calm belt in 9° north. A series of observations on the temperature of the air in latitude 9° south at sea would, for the farther study of this subject, possess great value,
651. Equatorial calm belt never stationary.—If these views be correct, we should expect to find the equatorial calm belt changing Its position with night and day, and yielding to all those influences, whether secular, annual, diurnal, or accidental which are capable of producing changes in the thermal condition of the trade-winds. The great sun-swing of this calm belt from north to south is annual in its occurrence; it marks the seasons and divides the year (§ 296) into wet and dry for all those places that are within the arc of its majestic sweep. But there are other subordinate and minor influences which are continually taking place in the atmosphere, and which are also calculated to alter the place of this calm belt, and to produce changes in the thermal status of the air which the trade-winds move. These are unusually severe winters or hot summers, remarkable spells of weather, such as long continuous rains or droughts over areas of considerable extent, either within or near the trade-wind belts. It is tremblingly alive to all such influences, and they keep it in continual agitation; accordingly we find that such is its state that within certain boundaries it is continually chanoing-place and limits. This fact is abundantly proved by in speed of ships, for the log-books at the Observatory show that it is by no means a rare occurrence for one vessel, after she may have been dallying in the Doldrums for days in the vain effort to cross that calm belt to see another coming up to her, "hand over fist," with fair ​winds, and crossing the belt after a delay in it of only a few hours instead of days.
652. It varies with the strength of the trade-winds.—Hence we infer that the position of the equatorial calm belt is determined by the difference of strength between the north-east and south-east trade-winds, which difference, in turn, depends upon difference of barometric pressure (§ 642), and upon difference in temperature between them in corresponding latitudes north and south. In it the air which they bring ascends. Now if we liken this belt of calms to an immense atmospherical trough, extending, as it does, entirely around the earth, and if we liken the north-east and south-east trade-winds to two streams discharging themselves into it, we shall see that we have two currents perpetually running in at the bottom, and that, therefore, we must have as much air as these two currents bring in at the bottom to flow out at the top. What flows out at the top is carried back-north and south by these upper currents, which are thus proved to exist and to flow counter to the trade- winds.
653. Precipitation in it.—Captain Wilkes, of the Exploring Expedition, when he crossed this belt in 1838, found it to extend from 4° north to 12° north. He was ten days in crossing it, and during those ten days rain fell to the depth of 6.15 inches, or at the rate of eighteen feet and upwards during the year. In its motions from south to north and back, it carries with it the rainy seasons of the torrid zone, always arriving at certain parallels at stated periods of the year; consequently, by attentively considering Plate VIIL, one can tell what places within the range of this zone have, during the year, two rainy seasons, what one, and what are the rainy months for each locality.
654. The appearance of the calm belts from a distant planet.—Were the north-east and the south-east trades, with the belt of equatorial calms, of different colours, and visible to an astronomer in one of the planets, he might, by the motion of these belts or girdles alone, tell the seasons with us. He would see them at one season going north, then appearing stationary, and then commencing their return to the south. But, though he would observe (§ 295) that they follow the sun in his annual course, he would remark that they do not change their latitude as much as the sun does his declination; he would therefore discover that their extremes of declination are not so far asunder as the tropics of Cancer and Capricorn, though in certain seasons the ​changes from day to day are very great. He would observe that the zones of winds and calms have their tropics or stationary nodes, about which they linger near three months at a time; and that they pass from one of their tropics to the other in a little less than another three months. Thus he would observe the whole system of belts to go north from the latter part of May till some time in August. Then they would stop and remain nearly stationary till winter, in December; when again they would commence to move rapidly over the ocean, and down towards the south, until the last of February or the first of March; then again they would become stationary, and remain about this, their southern tropic, till May again. Having completed his physical examination of the equatorial calms and winds, if the supposed observer should now turn his telescope towards the poles of our earth, he would observe a zone of calms bordering the north-east trade-winds on the north (§ 210), and another bordering the south-east trade-winds on the south (§213). These calm zones also would be observed to vibrate up and down with the trade-wind zones, partaking (§ 296) of their motions, and following the declination of the sun. On the polar side of each of these two calm zones there would be a broad band extending up into the polar regions, the prevailing wands within which are the opposites of the trade-winds, viz., south-west in the northern and north-west in the southern hemisphere. The equatorial edge of these calm belts is near the tropics, and their average breadth is 10° or 12°. On one side of these belts (§ 210) the wind blows perpetually towards the equator; on the other, its prevailing direction is towards the poles. They are called (§ 210) the "horse latitudes" by seamen.
655. Rainy seasons of the tropical calm belts.—Along the polar borders of these two calm belts (§ 296) we have another region of precipitation, though generally the rains here are not so constant as they are in the equatorial calms. The precipitation near the tropical calms is nevertheless sufficient to mark the seasons; for whenever these calm zones, as they go from north to south with the sun, leave a given parallel, the rainy season of that parallel, if it be in winter, is said to commence. Hence we may explain the rainy season in Chili at the south, and in California at the north.
656. Their position.—We can now understand why the calm belts of Cancer and Capricorn occupy a medial position between ​the trades and the counter trades; why, on one side of it, the prevailing direction of the wind should be polarward, on tho other towards the equator; and we also discover the influences which determine their geographical position; for:—
657. A meteorological law.—An accumulation of atmosphere over one part of the earth's surface implies a depression over some other part, precisely as the piling up of water into a wave above the sea level involves a corresponding depression below; and in meteorology it may be regarded as a general law, that the tendency of all winds on the surface is to blow from the place where the barometer is higher to the place where the barometer is lower. This meteorological law is only a restatement of the dynamical truism about water seeking its own level.
658. The barometer in the calm belts.—The mean height of the barometer in the calm belts of the tropics is greater (Plate I.) than it is in any other latitude. The mean height of the barometer in the equatorial calm belt is less than it is on any other parallel between the tropical and equatorial calm belts. The difference for the calm belt of Cancer is 0.25 inch. This difference is permanent. It is sufficient to put both systems of trade-winds in motion, and to create an indraught of air flowing perpetually towards the equatorial calm belt from the distance of two thousand miles on each side of it.
659. Winds with northing and winds with southing in them.—In like manner, as we go from either tropical calm belt towards the nearest pole, the barometric pressure becomes less and less. The meteorological law just announced requires the prevailing wind on the polar side of these calm belts to be from them and in the direction of the poles; and observations (Plate I.) show that such is the case. Dividing the winds in each hemisphere into winds with northing and winds with southing in them as has been done in Chapter XXI. and Plate XV., actual observation shows (§ 852) that they balance each other in the southern hemisphere between the parallels of 35" and 40°, and in the northern between the parallels of 25° and 60°; that between these parallels the average annual prevalence of winds with northing and of winds with southing in them is the same, the difference (Plate XV.) being so small as to be apparently accidental; that, proceeding from the medial band towards the pole, polar-bound winds become more and more prevalent, and proceeding from it towards the equator, equatorial-bound winds become more and more ​lent. Now, in each case, the prevailing winds blow (§ 657) from the high to the low barometer (Plate I.).
660. The barometric ridges.—The fact of two barometric ridges encircling the earth, as the high barometer of the tropical calm belts do, and as they may be called (Plate I.), suggests a place of low barometer on the polar side as naturally as the ascent of a hill on one side suggests to the traveller a descent on the other; and, had not actual observations revealed the fact, theory should have taught us (§ 654) the existence of a low barometer towards the polar regions as well as towards the equatorial.
661. They make a depression in the atmosphere.—Let us contemplate for a moment this accumulation of air in the tropical belt about the earth in each hemisphere. Because it is an accumulation of atmospheric air about the calms;—because the barometer stands higher under the calm belt of Capricorn, for instance, than it does on any other parallel between that calm belt and the pole on one side, or the equator on the other, it is not to be inferred that therefore there is a piling—a ridging up—of the atmosphere there. On the contrary, were the upper surface of our atmosphere visible, and could we take a view of it from above, we should discover rather a valley than a ridge over this belt of greatest pressure; and over the belt of least pressure, as the equatorial calm belt, we should discover (§ 520), not a valley, but a ridge, and for these reasons: In the belts of low barometer, that is, in both the equatorial and polar calms, the air is expanded, made light, and caused to ascend chiefly by the latent heat that is liberated by the heavy precipitation which takes place there. This causes the air which ascends there to rise up and swell out far above the mean level of the great aerial ocean. This intumescence at the equatorial calm belt has been estimated to be several miles above the general level of the atmosphere. This calm belt air, therefore, as it boils up and flow^s off through the upper regions, north and south, to the tropical calm belts, does not so flow by reason of any difference of barometric pressure, like that which causes the surface winds to blow, but it so flows by reason of difference as to level.
662. The upper surface of the atmosphere.—The tropical calm belts (§ 278) are places where the mean amount of precipitation is small. The air there is comparatively dry air. So far from being expanded by heat, or swelled out by vapour, this air is contracted by cold, for the chief source of its supply is through ​the upper regions, from the equatorial side, where the cross section between any two given meridians is the larger; and this upper current, while on its way from the equator, is continually parting with the heat which it received at and near the surface, and which caused it to rise under the equatorial cloud-ring. In this process it is gradually contracted, thus causing the upper surface of the air to be a sort of double inclined plane, descending from the equator and from the poles to the place of the tropical calm belts.
663. Winds in the southern stronger than winds in the northern hemisphere.—Observations show that the mean weight of the barometer in high southern is much less (Plate I.) than it is in corresponding high northern latitudes; consequently, we should expect that the polar-bound winds would be much more marked on the polar side of 40° S., than they are on the polar side of 40° N. Accordingly, observations (Plate XV.) show such to be the case; and they moreover show that the polar-bound winds of the southern are much fresher than those of the northern hemisphere.
664. The waves and gales off the Cape of Good Hope.—To appreciate the force and volume of these polar-bound winds in the southern hemisphere, it is necessary that one should "run them down" in that waste of waters beyond the parallel of 40° S., where "the winds howl and the seas roar." The billows there lift themselves up in long ridges with deep hollows between them. They run high and fast, tossing their white caps aloft in the air, looking like the green hills of a rolling prairie capped with snow, and chasing each other in sport. Still, their march is stately and their roll majestic. The scenery among them is grand, and the Australian-bound trader, after doubling the Cape of Good Hope, finds herself followed for weeks at a time by these magnificent rolling swells, driven and lashed by the "brave west winds" most furiously. A sailor's bride, performing this voyage with her gallant husband, thus alludes in her "Abstract log" to these rolling seas: "We had some magnificent gales off the Cape, when the colouring of the waves, the transition from gray to clear brilliant green, with the milky-white foam, struck me as most exquisite. And then in rough weather the moral picture is so fine, the calmness and activity required is such an exhibition of the power of mind over the elements, that I admired the sailors fully as much as the sea, ​and, of course, the sailor in command most of all; indeed, a sea voyage more than fulfils my expectations."
665. Winds blow from a high to a low barometer.—It appears, therefore, that the low barometer about the poles and the low barometer of the equator cause an inrush of wind, and in each case the rushing wind comes from the high and blows towards the low barometer; that in one hemisphere the calm belt of Capricorn, and in the other the calm belt of Cancer, occupies the medial line between the equatorial and polar places of low barometer.
660. Polar rarefaction.—It appears, moreover, that the polar refraction is greater than the equatorial, for the mean height of the austral barometer is very much below that of the equatorial, and, consequently, its influence in creating an indraught is felt at a greater distance (Plate XV.)—even at the distance of 50° of latitude from the south pole, while the influence of the equatorial depression is felt only at the distance of 30° in the southern, and of 25° in the northern hemisphere. The difference as to degree of rarefaction is even greater than this statement implies, for the influx into the equatorial calm belt is assisted also by temperature in this, that the trade-winds blow from cooler to warmer latitudes. The reverse is the case with the counter-trades; therefore, while difference of thermal dilatation assists the equatorial, it opposes the polar influx.
667. The tropical calm belts caused by the polar and equatorial calms.—Thus we perceive that the tropical calm belts are simply an adjustment between the polar and equatorial calms; that the tropical calm belts assume their position and change their latitude in obedience to the energy with which the influence of the heated and the expanding columns of air, as they ascend in the polar and equatorial calms, is impressed upon them.
668. The meteorological power of latent heat.—This explanation of the calm places and of the movements of the low austral barometer shows, comparatively speaking, how much the latent heat of vapour, and how little the direct heat of the sun has to do in causing the air to rise up and flow off from these calm places, and consequently, how little the direct action of the solar ray has to do either with the trades or the counter trades. It regulates and controls them; it can scarcely be said to create them.
669. The low barometer off Cape Horn.—The fact of a low ​barometer off Cape Horn was pointed out as long as 1834. It was considered an anomaly peculiar to the regions of Cape Horn. It is now ascertained by the comparison of 6455 observations on the polar side of 40° south, and about 90,000 in all other latitudes, that the depression is not peculiar to the Cape Horn regions, but that it is general and alike in all parts of the austral seas, as the following table, compiled from the log-books of the Observatory by Lieutenants Warley and Young, shows:—
Barometric Table.
Mean Height of the Barometer as observed between
| the Parallels of | the Meridians of | the Meridians of | the Meridians of | the Meridians of | the Meridians of | the Meridians of | Mean of all. | Mean of all. |
| the Parallels of | 20° W. and 140° E. | 20° W. and 140° E. | 140°E.and 80°W. | 140°E.and 80°W. | Of Cape Horn. | Of Cape Horn. | | Mean of all. | Mean of all. |
| the Parallels of | No. of obs. | Barom. Inches | No. of obs. | Barom. Inches | No. of obs. | Barom. Inches | No. of obs. | Barom. Inches |
| 40° S. and 43° S. | 1115 | 29.90 | 210 | 29.84 | 378 | 29.86 | 1703 | 29.88 |
| 43,,45 | 788 | .80 | 155 | .73 | 237 | .75 | 1130 | .78 |
| 45,,48 | 611 | .58 | 226 | .71 | 337 | .68 | 1174 | .63 |
| 48,,50 | 174 | .53 | 247 | .56 | 250 | .61 | 672 | .62 |
| 50,,53 | 108 | .35 | 198 | .45 | 359 | .56 | 665 | .48 |
| 53,,55 | 6 | .14 | 92 | .35 | 377 | .37 | 475 | .36 |
| | 7 | .27 | 64 | .42 | 1055 | .28 | 1126 | .29 |
671. Barometer at the poles.—These are the observed heights; for the want of data, no corrections have been applied to them; and for the want of numbers sufficient to give correct means, they lack that uniformity which larger numbers would doubtless give. They show, however, most satisfactorily, that a low barometer is not peculiar to Cape Horn regions alone; they show that it is common to all high southern latitudes ; and other observations (§ 362) show that it is peculiar to these and not to northern latitudes. Projecting on a diagram A, with parallels of latitude and the barometric scale as ordinates and abscissae, a curve S, which will best represent the observations (§ 670), and continuing it to the south pole—also projecting another curve N, which will best represent the observations (§ 362) on the polar side of 40° N., and continuing it to the north pole—we discover that if the barometric pressure in polar latitudes continue to decrease for ​the unknown as it does for the known regions, the mean height of the barometer would be at the north pole about 29.6, at the south about 28 inches. These lines, N and S, represent what may be called the barometric descent of the counter-trades.
672. The "brave west winds"—their barometric descent.—The rarefaction of the air in the polar calms is, as we have seen
(§ 667), sufficient to create an indraught all around to the distance of fifty degrees of latitude from the south pole; also (§ 662) the rarefaction in the belt of equatorial calms, is sufficient ​to extend with its influence no farther than thirty degrees of latitude. The fact also favours the idea suggested by the diagram (§ 071), that the mean height of the barometer in the polar calms is very much less than it is in the equatorial. Moreover, the counter trades of the southern hemisphere are very much stronger (§ 026) than the counter trades of the other. They are also stronger than the trade-winds of either; these facts likewise favour the idea of a greater exhaustion of air in the antarctic than in the arctic calm place; and it is manifest that actual observations also, as far as they go, indicate such to be the case. In other words, "the brave west winds" of the southern hemisphere have the greatest "barometric descent," and should therefore be, as they are, the strongest of the four winds.
673. Study of the monsoons affords farther information touching the calm belts.—Farther information may be gained upon the subject of high and low barometers, of the "barometric declivity of winds," and of the meteorological influence of diminished atmospheric pressure by studying the calm belts in connection with the monsoons.
674. The south-east winds of the Atlantic.—Before, however, we proceed to these, let us take a hasty glance at the winds in certain other parts of the ocean. The winds which most prevail on the polar side of the calm belt of Cancer, and as far as 50° N. in the Atlantic, are the west wands. "Wind and weather in this part of the ocean," says Jansen, "are very unreliable and changeable; nevertheless, in the summer months, we find permanent north winds along the coast of Portugal. These north winds are worthy of attention, the more so from the fact that they occur simultaneously with the African monsoon, and because we then find northerly winds also in the Mediterranean, and in the Red Sea, and farther eastward to the north of the Indian monsoon. When, between the months of May and November, during which the African monsoon prevails, the Dutch ships, which have lingered in the calm belt of Cancer run with the north-east trade, and direct their course for the Cape Verde Islands, then it seems as if they were in another world. The sombre skies and changeable—alternately chilly and sultry—weather of our latitudes are replaced by a regular temperature and good settled weather. Each one rejoices in the glorious heavens, in which none save the little trade-clouds are to be seen—which clouds in the ​trade-winds region make the sunset so enchanting. The dark-blue water, in which many and strange kinds of echinas sport in the sunlight, and, when seen at a distance, make the sea appear like one vast field adorned with flowers; the regular swellings of the waves with their silvery foam, through which the flying-fishes flutter; the beautifully-coloured dolphins; the diving schools of tunnies—all these banish afar the monotony of the sea, awake the love of life in the youthful seaman, and attune his heart to goodness. Everything around him fixes his attention and increases his astonishment.
675. Sailing through the trade-wind.—"If all the breathings out of heartfelt emotion which the contemplation of nature forces from the sailor were recorded in the log-books, how much farther should we be advanced in the knowledge of the natural state of the sea! Once wandering over the ocean, he begins to be impressed by the grand natural tableau around him with feelings deep and abiding. The most splendid forecastle is lost in the viewless surface, and brings home to us the knowledge of our nothingness; the greatest ship is a plaything for the billows, and the slender keel seems to threaten our existence every moment. But when the eye of the mind is permitted to wander through space and into the depths of the ocean, and is able to form a conception of Infinity and of Omnipotence, then it knows no danger; it is elevated—it comprehends itself. The distances of the heavenly bodies are correctly estimated; and, enlightened by astronomy, with the aid of the art of navigation, of which Maury's Wind and Current Charts form an important part, the shipmaster marks out his way over the ocean just as securely as any one can over an extended heath. He directs his course towards the Cape Verde Islands, and is carried there by the lively trade-wind. Yet beyond the islands, sooner or later, according to the month, the clear skies begin to be clouded, the trade-wind abates and becomes unsteady, the clouds heap up, the thunder is heard, heavy rains fall; finally, the stillness is death-like, and we have entered the belt of calms. This belt moves towards the north from May to September. It is a remarkable phenomenon that the annual movements of the trades and calm belts from south to north, and back again, do not directly follow the sun in its ​declination, but appear to wait until the temperature of the sea water puts it in motion. If a ship which has come into the belt of calms between May and September could lie still in the place where it came into this belt—cast anchor, for example—then it would perceive a turning of the monsoon or of the trade-wind. It would see the belt of calms draw away to the north, and afterwards get the south-west monsoon, or, standing more westerly, perhaps the south-east trade. On the contrary, later than September, this ship lying at anchor will see the north-east gradually awake. The belt of calms then moves towards the south, and removes from the which remains there anchored on the north side."
676. The influence of the land upon the winds of the sea.—The investigations that have taken place at the Observatory show that the influence of the land upon the normal directions of the wind at sea is an immense influence. It is frequently traced for a thousand miles or more out upon the ocean. For instance, the action of the sun's rays upon the great deserts and arid plains of Africa, in the summer and autumnal months, is such as to be felt nearly across the Atlantic Ocean between the equator and the parallel of 13° north. Between this parallel and the equator, the north-east trade-winds, during these seasons, are arrested in their course by the rainy seasons and heated plains of Africa, as observation shows they are in India, and instead of "blowing home" to the equator, they stop and ascend over the desert sands of the continent. The south-east trade-winds, arriving at the equator during this period, and finding no north-east trades there to contest their crossing the line, continue their course, and blow home as a south-west monsoon, where they deposit their moisture and ascend. These southwardly monsoons bring the rains which divide the seasons in these parts of the African coast. The region of the ocean embraced by these monsoons is cuneiform in its shape, having its base resting upon Africa, and its apex stretching over till within 10° or 15° of the mouth of the Amazon. Indeed, when we come to study the effects of South America and Africa (as developed by the Wind and Current Charts) upon the winds at sea, we should be led to the conclusion—had the foot of civilized man never trod the interior of these two continents— ​that the climate of one is humid; that its valleys are, for the most part, covered with vegetation, which protects its surface from the sun's rays; while the plains of the other are arid and naked, and, for the most part, act like furnaces in drawing the winds from the sea to supply air for the ascending columns which rise from its over-heated plains. Pushing these facts and arguments still farther, these beautiful and interesting researches seem already sufficient almost to justify the assertion that, were it not for the great desert of Sahara and other arid plains of Africa, the western shores of that continent, within the trade-wind region, would be almost, if not altogether, as rainless and sterile as the desert itself.
677. A "Gulf Stream" in the air.—Lieutenant Jansen has called my attention to a vein of wind which forms a current in the air as remarkable as that of the Gulf Stream is in the sea. This atmospherical Gulf Stream is in the south-east trade-winds of the Atlantic. It extends from near the Cape of Good Hope, in a direct line to the equator, on the meridian of Cape St. Roque (Plate VIII.). The homeward route from the Cape of Good Hope lies in the middle of this vein; in it the winds are more steady than in any other part of the Atlantic. On the edges of this remarkable aerial current the wind is variable and often fitful; the homeward-bound Indiaman resorts to and uses this stream in the atmosphere as the European-bound American does the Gulf Stream. It is shaded on the plate.
678. Counterpoises.—These investigations, with their beautiful developments, eagerly captivate the mind; giving wings to the imagination, they teach us to regard the sandy deserts, and arid plains, the mountain ranges, and the inland basins of the earth, as compensations in the great system of atmospherical circulation. Like counterpoises to the telescope, which the ignorant regard as incumbrances to the instrument, these wastes serve as make-weights, to give certainty and smoothness of motion—facility and accuracy to the workings of the machine.
679. Normal state of the atmosphere.—When we travel out upon the ocean, and get beyond the influence of the land upon the winds, we find ourselves in a field particularly favourable for studying the general laws of atmospherical circulation. Here, beyond the reach of the great equatorial and polar currents of the sea, there are no unduly heated surfaces, no mountain ranges, ​or other obstructions to the circulation of the atmosphere—nothing to disturb it in its normal courses. The sea, therefore, is the field for observing the operations of the general laws which govern the movements of the great aerial ocean. Observations on the land will enable us to discover the exceptions, but from the sea we shall get the rule. Each valley, every mountain range and local district, may be said to have its own peculiar system of calms, winds, rains, and droughts. But not so the surface of the broad ocean; over it the agents which are at work are of a more uniform character.
680. Rain-winds.—Rain-winds are the winds which convey the vapour from the sea, where it is taken up, to other parts of the earth, where it is let down either as snow, hail, or rain. As a general rule, the trade-winds (§ 293) may be regarded as the evaporating winds; and when, in the course of their circuit, they are converted into monsoons, or the variables of either hemisphere, they then generally become also the rain-winds—especially the monsoons—for certain localities. Thus the south-west monsoons of the Indian Ocean are the rain-winds for the west coast of Hindostan (§ 298). In like manner, the African monsoons of the Atlantic are the winds which feed the springs of the Niger and the Senegal with rains. Upon every water-shed which is drained into the sea, the precipitation, for the whole extent of the shed so drained, may be considered as greater than the evaporation, by the amount of water which runs off through the rivers into the sea. In this view% all rivers may be regarded as immense rain-gauges, and the volume of water annually discharged by any one, may be taken as an expression of the quantity which is annually evaporated from the sea, carried back by the winds, and precipitated throughout the whole extent of the valley that is drained by it. Now, if we knew the rain winds from the dry for each locality and season generally throughout such a basin, we should be enabled to determine, with some degree of probability at least, as to the part of the ocean from which such rains were evaporated. And thus, notwithstanding all the eddies caused by mountain chains and other uneven surfaces, we might detect the general course of the atmospherical circulation over the land as well as the sea, and make the general courses of circulation in each valley as obvious to the mind of the philosopher as in the current of the Mississippi, ​or of any other great river, to his senses. The greatest move that can now be made for the advancement of meteorology is to extend this system of co-operation and research from the sea to the land, and bring the magnetic telegraph regularly into the service of meteorology.
* ↑ P. 146, vol. ii., Maury's Sailing Directions.
* ↑ "During the last year [1859] the 8th edition of Maury's Sailing Directions, in two quarto volumes, has been published at the Observatory in Washington. It affords abundant evidence of the activity, to which allusion has already been made, in this field of research, and with regard to which all geographers feel the most lively interest. >"Official tables have been received from San Francisco, showing the vessels that have arrived at that port during the year, with the length of passage. Of those arriving direct viâ Cape Horn, 124 were from the Atlantic ports of the United States, and 34 from Europe. Of these 124, 70 are known to have had the Wind and Current Charts on board; their average passage was 135 days, which is 11 days less than the average of those from the United States, and 24 days less than the average of those from Europe without the Charts. When these researches commenced, the general average was 180 days from the United States, and 183 from Europe to California."—Journal American Geographical Society.
* ↑ Maury's Sailing Directions: "Barometric Anomalies off Cape Horn."
* ↑ Nautical Monograph, No. 1.
* ↑  Letter to Admiral Chabannes, with extracts from his reply thereto:— "Observatory, Washington, 8th April, 1859. "My dear Admiral,—My last was dated 10th January ultimo. I hope the charts and Vol, i., 8th ed. Sailing Directions, and part of vol. ii. in the sheets, came safely to hand. Vol, ii, is just out, and I hasten, in homage of my respect, and as a token of good-will, to lay a copy before you, "Permit me, if you please, to call your attention to the chapter on the 'Average Force of the Trade-winds,' p, 857, and especially to the table of comparative speed (of sailing vessels) through the north-east and south-east trade-winds of the Atlantic, p. 865, The average speed, you observe, is nearly the same, notwithstanding that through the south-east trades the wind is aft, through the north-east just abaft the beam, "In order to treat this question thoroughly, it is very desirable to know the difference in the speed of vessels when sailing with the same wind aft, with it quartering, with it a point or two abaft the beam, and with it close hauled, With a good series of experiments upon this subject, we should be able to arrive at definite conclusions with regard to the average difference in force not only of the two systems of trade-winds, but of the winds generally in various parts of the ocean. "If we assume that a wind which, being dead aft, drives a vessel at the rate of six knots, will, when brought nearly abeam, drive her eight knots—as in this chapter I have supposed—and then if we apply the dynamical law of the resistance increasing as the squares of the velocity of the ship, we should be led to the remarkable conclusion that the average velocity of the north-east to south-east trades of the Atlantic is as 36 to 64. Therefore, in conducting these experiments, it would be very desirable to know the area of canvas that fairly feels the wind when it is aft, and the area upon which the wind blows when the ship is hauled up. Suffice it to say, that the facts which we already have, indicate that the south-east trades, both of the Atlantic and Indian Oceans are fresher than the north-east trades of the Atlantic, May we infer from this that the south-east trades of the Pacific are also fresher than the north-east trades of that ocean? If we may so infer, and be right, then there is another step which we may take with boldness, and pronounce the atmospherical circulation of the southern hemisphere to be much more active than that of the northern. And having reached this round in the ladder up which I am soliciting you to accompany me, we are prepared to pause and take a view of some of the new physical aspects which these facts and this reasoning spread out before us.  "That the atmospherical circulation is more active in the southern than in the northern hemisphere appears to be indicated also by the "brave west winds" of the extra-tropical south. If the air performs its circuit more rapidly through one system of trade-winds than the other, then it follows that it must perform its circuit more rapidly also along those regions through which it has to pass in order to reach such rapid trades. Consequently, there should be a great difference between the gales of the northern and those of the southern hemisphere. If we suppose the general circulation of the northern hemisphere to be sluggish, the air in its circuits there would have time to tarry by the way, as it were, and to blow gales of wind from all points of the compass. On the contrary, if the general circulation of the southern hemisphere be brisk and active, the air in its general circuits, like a fast train on the railway, would not have so much time to tarry by the way, because, like the cars, it must be up to time. Hence, admitting this view of the matter to be correct (and you perceive that for the want of the experiments alluded to we are groping in the darkness of conjecture), though we might expect gales of wind in the extra -tropical regions of the south, yet they would for the most part blow with the prevailing direction of the wind, and not against it. Thus the gales on the polar side of Capricorn should, particularly at sea, have westing in them always—almost. "In corroboration of this view, I may mention, on the authority of paper just received from Lieutenant Van Gogh, of the Dutch Navy, that the gales of wind which take place between the meridians of 14° and 32° E., and between the parallels of 33° and 37° S., have been discussed at the Meteorological Institute of Utrecht. For this purpose he tabulated the results for the whole year of 17,810 observations—an observation comprehending a period of eight hours. According to these observations, it is blowing a gale of wind off the Cape of Good Hope 7.16 per cent, of the whole year, and from the following quarters; namely, between N.N.W. and S.S.W. 6.43 per cent.: from all other quarters, 0.73 per cent. "Perhaps you may find it convenient to institute, with some of the vessels of your fleet, a regular series of experiments in the south-east trades upon speed, when sailing at various angles with the course of the wind. Besides answering our immediate purpose, the results might enable us to convert ships into very good anemometers for all winds except gales. "Pardon me for being so tedious upon this subject. If you have felt me so,  pray ascribe it to my desire to get by actual experiment an expression in the average speed of ships for the actual force and velocity of the winds. "Wishing you all success and good luck in the investigation which you have in hand, pray believe me, my dear admiral, yours very truly, "M. F. Maury. "Admiral C. de Chabaness, Commander-in-chief of the French Naval Division of Brazil and La Plata, Rio de Janeiro." Extract from a letter in reply to the foregoing:— " Montevideo, January 25, 1860. "My dear Sir,— * * * * As you have indicated to me in your letter of April, I have caused to be made, by a brig of my division, experiments upon the comparative velocities, wind abaft and wind abeam with a given force of wind, but I have not yet been able to deduce any positive rule, the experiments not having been sufficiently multiplied. I can, however, give as a result that the increase of headway given by wind abeam over the headway with wind aft has been a little less than two knots; when the velocity with wind aft was from 6 to 8 knots, the force of the wind aft might be expressed by 4, and of the wind abeam by 6. * * * * "C. de Chabannes." London, November 26, 1860. Just in time for insertion here, I receive from the gallant admiral the subjoined very valuable and interesting series of experiments on the speed of his ship:—first, before the wind, i.e. sixteen points, and so on for every two points to six, and close hauled. At eight points the wind is abeam and at right angles with the course of the ship; at ten it is two points abaft the beam. From these experiments we infer that an average sailer that goes six knots before the wind.  will, if hauled up and trimmed, go nine knots with the wind two points abaft the beam, and 8.8 wind abeam. ↑ See also Plat XII. and §632 and §633
* ↑ Courants et Révolutions de l'Atmosphère et de la Mer, comprenaut une Théorie nouvello sur les Déluges Périodiques. Par Felix Julien, Lieutenant de Vaisscau, etc. Paris, 1860.
* ↑ Révolutions de la Mer. Déluges Periodiques. Par J Adhémar. Paris, 1860.
* ↑ Maury's Nautical Monograph, No. 1.
* ↑ The mean temperature of sea water in the Atlantic is for 9° north, 80°.26 by 565 obs.; for equator, 79°.63, by 269 obs; and for 9° south, 78°.96, 223 obs—Maury's Thermal Charts
* ↑ American Journal of Science, vol. xxvi. p. 54 (1834).
* ↑ When we, as our forefathers did, preserve in the journals all that we observe at sea, then we shall have abundant material with which to keep ourselves pleasantly occupied.
* ↑ Natuurkündige Beschrijving der zeeën, door M. F. Maury, LL.D., Luitenant der Nord-Amerikaansche Marine, vertaald door M. H. Jansen, Luitenant der Zee. (Bijdrage.) Dordrecht, P. K. Braat. 1855.