Physical Geography of the Sea and its Meteorology/Chapter 4

by Matthew Fontaine Maury

200-268. THE ATMOSPHERE.

 

200. Likened to a machine.—There is no employment more ennobling to man and his intellect than to trace the evidences of design and purpose, which are visible in many parts of the creation. Hence, to the right-minded mariner, and to him who studies the physical relations of earth, sea, and air, the atmosphere is something more than a shoreless ocean, at the bottom of which he creeps along. It is an envelope or covering for the distribution of light and heat over the surface of the earth; it is a sewer into which, with every breath we draw, we cast vast quantities of dead animal matter; it is a laboratory for purification, in which that matter is recompounded, and wrought again into wholesome and healthful shapes; it is a machine for pumping up all the rivers from the sea, and for conveying the water (§ 191) from the ocean to their sources in the mountains; it is an inexhaustible magazine, marvellously stored. Upon the proper working of this machine depends the well-being of every plant and animal that inhabits the earth. How interesting, then, ought not the study of it to be! An examination of the uses which plants and animals make of the air is sufficient to satisfy any reasoning mind in the conviction that when they were created, the necessity of this adaptation was taken into account. The connection between any two parts of an artificial machine that work into each other, does not render design in its construction more patent than is the fact that the great atmospherical machine of our planet was constructed by an Architect who designed it for certain purposes; therefore the management of it, its movements, and the performance of its offices, cannot be left to chance. They are, we may rely upon it, guided by laws that make all parts, functions, and movements of this machinery as obedient to order and as harmonious as are the planets in their orbits.

201. The air and the ocean governed by stable laws.—Any examination into the economy of the universe will be sufficient to satisfy the well-balanced minds of observant men that the laws which govern the atmosphere and the laws which govern the ocean (§ 164) are laws which were put in force by the Creator when the foundations of the earth were laid, and that therefore they are laws of order; else, why should the Gulf Stream, for instance, be always where it is, and running from the Gulf of Mexico, and not somewhere else, and sometimes running into it? Why should there be a perpetual drought in one part of the world, and continual showers in another? Or why should the conscious winds ever heed the voice of rebuke, or the glad waves ever "clap their hands with joy?"

202. Importance of observing the works of nature.—To one who looks abroad to contemplate the agents of nature, as he sees them at work upon our planet, no expression uttered or act performed by them is without meaning. By such a one, the wind and rain, the vapour and the cloud, the tide, the current, the saltness, and depth, and warmth, and colour of the sea, the shade of the sky, the temperature of the air, the tint and shape of the clouds, the height of the tree on the shore, the size of its leaves, the brilliancy of its flowers—each and all may be regarded as the exponent of certain physical combinations, and therefore as the expression in which Nature chooses to announce her own doings, or, if we please, as the language in which she writes down or elects to make known her own laws. To understand that language and to interpret aright those laws is the object of the undertaking which we now have in hand. No fact gathered from such a volume as the one before us can therefore come amiss to those who tread the walks of inductive philosophy; for, in the handbook of nature, every such fact is a syllable; and it is by patiently collecting fact after fact, and by joining together syllable after syllable, that we may finally seek to read aright from the great volume which the mariner at sea as well as the philosopher on the mountain each sees spread out before him.

203. Materials for this chapter.—There have been examined at the Observatory more than a million of observations on the force and direction of the winds at sea.^[1] The discussion of such a mass of material has thrown much light upon the circulation of the atmosphere; for, as in the ocean (§ 201), so in the air, there is a regular system of circulation.

204. Different belts of winds.—Before we proceed to describe this system, let us point out the principal belts or bands of wind that actual observation has shown to exist at sea, and which, with more or less distinctness of outline, extend to the land also, and thus encircle the earth. If we imagine a ship to take her departure from Greenland for the South Shetland Islands, she will, between the parallels of 60° north and south, cross these several bands or belts of winds and calms nearly at right angles, and in the following order:—(1.) At setting out she will find herself in the region of south-west winds, or counter-trades of the north—called counter because they blow in the direction whence come the trade-winds of their hemisphere. (2.) After crossing 50°, and until reaching the parallel of 35° N., she finds herself in the belt of westerly winds, a region in which winds from the south-west and winds from the north-west contend for the mastery, and with nearly equal persistency. (3.) Between 35° and 30° she finds herself in a region of variable winds and calms; the winds blowing all around the compass, and averaging about three months from each quarter during the year. Our fancied ship is now in the "horse-latitudes." Hitherto winds with westing in them have been most prevalent; but, crossing the calm belt of Cancer, she reaches latitudes where winds with easting become most prevalent. (4.) Crossing into these, she enters the region of north-east trades, which now become the prevailing winds, until she reaches the parallel of 10° N., and enters the equatorial calm belt, which, like all the other wind-bands, holds fluctuating limits. (5.) Crossing the parallel of 5° N., she enters where the south-east trades are the prevailing winds, and so continue until the parallel of 30° S. is reached. (6.) Here is the calm belt of Capricorn, where, as in that of Cancer (3), she again finds herself in a region of shifting winds, light airs, and calms, and where the winds with westing in them become the prevailing winds. (7.) Between the parallels of 35° and 40° S., the north-west and south-west winds contend with equal power for the mastery. (8.) Crossing 40°, the counter-trades (1),—the north-west winds of the southern hemisphere,—become the prevailing winds, and so remain, as far as our observations at sea extend towards the south pole.

Such are the most striking movements of the winds at the surface of the sea. But, in order to treat of the general system of atmospherical circulation, we should consider where those agents reside which impart to that system its dynamical force. They evidently reside near the equator on one side, and about the poles on the other. Therefore, if, instead of confining our attention to the winds at the surface, and their relative prevalence from each one of the four quarters, we direct our attention to the upper and lower currents, and to the general movements back and forth between the equator and the poles, we shall be enabled the better to understand the general movements of this grand machine.

205. The trade-wind belts.—Thus treating the subject, observations show that from the parallel of about 30° or 35° north and south to the equator, we have, extending entirely around the earth, two zones of perpetual winds, viz., the zone of north-east trades on this side, and of south-east on that. With slight interruptions, these winds blow perpetually, and are as steady and as constant as the currents of the Mississippi River, always moving in the same direction (Plate I.) except when they are turned aside by a desert or a rainy region here and there to blow as monsoons, or as land and sea breezes. As these two main currents of air are constantly flowing from the poles toward the equator, we are safe in assuming that the air which they keep in motion must return by some channel to the place toward the poles whence it came in order to supply the trades. If this were not so, these winds would soon exhaust the polar regions of atmosphere, and pile it up about the equator, and then cease to blow for the want of air to make more wind of.

206. The return current.—This return current, therefore, must be in the upper regions of the atmosphere, at least until it passes over those parallels between which the trade-winds are usually blowing on the surface. The return current must also move in the direction opposite to that wind the place of which it is intended to supply. These direct and counter currents are also made to move in a sort of spiral or loxodronic curve, turning to the west as they go from the poles to the equator, and in the opposite direction as they move from the equator towards the poles. This turning is caused by the rotation of the earth on its axis.

207. Effect of diurnal rotation on the course of the trade-winds.—The earth, we know, moves from west to east. Now if we imagine a particle of atmosphere at the north pole, where it is at rest, to be put in motion in a straight line towards the equator, we can easily see how this particle of air, coming from the very axis of diurnal rotation, where it did not partake of the diurnal motion of the earth would, in consequence of its vis inertæ, find, as it travels south, the earth slipping from under it, as it were, and thus it would appear to be coming from the north-east and going towards the south-west; in other words, it would be a north-east wind. The better to explain, let us take a common terrestrial globe for the illustration. Bring the island of Madeira, or any other place about the same parallel, under the brazen meridian; put a finger of the left hand on the place; then moving the finger down along the meridian to the south, to represent the particle of air, turn the globe on its axis from west to east, to represent the diurnal rotation of the earth, and when the finger reaches the equator, stop. It will now be seen that the place on the globe under the finger is to the southward and westward of the place from which the finger started; in other words, the track of the finger over the surface of the globe, like the track of the particle of air upon the earth, has been from the northward and eastward. On the other hand, we can perceive how a like particle of atmosphere that starts from the equator, to take the place of the other at the pole, would, as it travels north, and in consequence of its vis inertiæ, be going towards the east faster than the earth. It would therefore appear to be blowing from the south-west, and going towards the north-east and exactly in the opposite direction to the other. Writing south for north, the same takes place between the south pole and the equator.

208. Two grand systems of currents.—Such is the process which is actually going on in nature; and if we take the motions of these two particles as the type of the motion of all, we shall have an illustration of the great currents in the air (§ 204), the equator being near one of the nodes, and there being at least two systems of currents, an upper and an under, between it and each pole. Halley, in his theory of the trade winds, pointed out the key to the explanation, so far, of the atmospherical circulation; but, were the explanation to rest here, a north-east trade-wind extending from the polo to the equator would satisfy it; and were this so, we should have, on the surface, no winds but the north-east trade-winds on this side, and none but south-east trade-winds on the other side, of the equator.

209. From the Pole to 30°-35°.—Let us return now to our northern particle (§ 207), and follow it in a round from the north pole across the equator to the south pole, and back again. Setting off from the polar regions, this particle of air, for some reason which does not appear, hitherto, to have been very satisfactorily explained by philosophers, instead of travelling (§ 208) on the surface all the way from the pole to the equator, travels in the upper regions of the atmosphere until it gets near the belt between 30°-35°. Here it meets, also in the clouds, the hypothetical particle that is coming from the south, and going north to take its place.

210. The "horse latitudes." About this belt of 30°-35° north, then, these two particles press against each other with the whole amount of their motive power, and produce a calm and an accumulation of atmosphere: this accumulation is sufficient to balance the pressure of the two currents from the north and south. From under this bank of calms, which seamen call the "horse latitudes," two surface currents of wind are ejected or drawn out; one towards the equator, as the north-east trades, the other towards the pole, as the south-west "passage-winds," or counter-trades. These winds come out at the lower surface of the calm region, and consequently the place of the air borne away in this manner must be supplied, we may infer, by downward currents from the superincumbent air of the calm region. Like the case of a vessel of water which has two streams from opposite directions running in at the top, and two of equal capacity discharging in opposite directions at the bottom, the motion of the water would be downward;—so is the motion of the air in this calm zone.

211. The barometer there.—The barometer, in this calm region, stands higher than it does either to the north or to the south of it; and this is another proof as to the accumulation of the atmosphere here, and pressure from its downward motion. And because the pressure under this calm belt is greater than it is on either side of it, the tendency of the air will be to flow out on either side; therefore, supposing we were untaught by observation as to direction of the wind, reason would teach us to look for the prevailing winds on each side of this calm belt to be from it.

212. The equatorial calm belt.—Following our imaginary particle of air, however, from the north across this calm belt of Cancer, we now perceive it moving on the surface of the earth as the north-east trade-wind; and as such it continues till it arrives near the equator, where it meets a like hypothetical particle, which, starting from the south at the same time the other started from the north pole, has blown as the south-east trade-wind. Here, at this equatorial place of meeting, there is another conflict of winds and another calm region, for a north-east and south-east wind cannot blow in the same place and at the same time. The two particles have been put in motion by the same power; they meet with equal force; and, therefore, at their place of meeting, they are arrested in their course. Here, therefore, there is a calm belt, as well as at Capricorn and Cancer. Warmed now by the heat of the sun, and of vapour in the process of condensation, and pressed on each side by the whole force of the north-east and south-east trades, these two hypothetical particles, taken as the type of the whole, cease to move onward and ascend. This operation is the reverse of that which took place at the meeting (§ 210) near the belt between the parallels of 30°-35°.

213. The calm belt of Capricorn.—This imaginary particle then, having ascended to the upper regions of the atmosphere again, travels there counter to the south-east trades, until it meets, near the calm belt of Capricorn, another particle from the south pole; here there is a descent as before. (§ 210); it then (§ 211) flows on towards the south pole as a surface wind from the north-west.

214. The polar calms and the return current.—Entering the polar regions obliquely, it is pressed upon by similar particles flowing in oblique currents across every meridian; and here again is a calm place or node; for, as our imaginary particle approaches the parallels near the polar calms more and more obliquely, it, with all the rest, is whirled about the pole in a continued circular gale; finally, reaching the vortex of the calm place, it is carried upward to the regions above, whence it commences again its flow to the north as an upper current, as far as the calm belt of Capricorn; here it encounters (§ 213) its fellow from the north (§ 207); they stop, descend, and flow out as surface currents (§ 210), the one with which the imagination is travelling, to the equatorial calm as the south-east trade-wind; here (§ 212) it ascends, travelling thence to the calm belt of Cancer as an upper current counter to the north-east trades. Here (§ 210 and 209) it ceases to be an upper current, but, descending (§ 210), travels on with the south-west passage-winds towards the pole.

215. Diagram of the winds.—Now the course we have imagined an atom of air to take, as illustrated by the "diagram of the winds" (Plate I.), is this: an ascent in a place of calms about the north pole, as at V P; an efflux thence as an upper current, A B C, until it meets R S (also an upper current) over the calms of Cancer. Here there is supposed to be a descent, as shown by the arrows, C D, S T. This current, A B C D, from the pole, now becomes the north-east trade-wind, D E, on the surface, until it meets the south-east trades, O Q, in the equatorial calms, where it ascends as E F, and travels as F G with the upper current to the calms of Capricorn, thence as H J K, with the prevailing north-west surface current to the south pole, thence up with the arrow P′, and around with the hands of a watch, and back, as indicated by the arrows along L M N O Q R S T U V.

216. As our knowledge of the laws of nature has increased, so have our readings of the Bible improved.—The Bible frequently makes illusion to the laws of nature, their operation and effects. But such allusions are often so wrapped in the folds of the peculiar and graceful drapery with which its language is occasionally clothed, that the meaning, though peeping out from its thin covering all the while, yet lies in some sense concealed, until the lights and revelations of science are thrown upon it; then it bursts out and strikes us with exquisite force and beauty. As our knowledge of Nature and her laws has increased, so has our understanding of many passages in the Bible been improved. The Psalmist called the earth "the round world;" yet for ages it was the most damnable heresy for Christian men to say the world is round; and, finally, sailors circumnavigated the globe, proved the Bible to be right, and saved Christian men of science from the stake. "Canst thou bind the sweet influences of Pleiades?" Astronomers of the present day, if they have not answered this question, have thrown so much light upon it as to show that, if ever it be answered by man, he must consult the science of astronomy. It has been recently all but proved, that the earth and sun, with their splendid retinue of comets, satellites, and planets, are all in motion around some point or centre of attraction inconceivably remote, and that that point is in the direction of the star Alcyon, one of the Pleiades! Who but the astronomer, then, could tell their "sweet influences?" And as for the general system of atmospherical circulation which I have been so long endeavouring to describe, the Bible tells it all in a single sentence: "The wind goeth towards the south, and turneth about unto the north; it whirleth about continually, and the wind returneth again according to his circuits."—Eccl. i. 6.

217. Sloughing off from the counter trades.—Of course, as the surface winds, H J K, and T U V, approach the poles, there must be a sloughing off,—if I may be allowed the expression,—of air from them, in consequence of their approaching the poles. For as they near the poles, the parallels become smaller and smaller, and the surface current must either extend much higher up, and blow with greater rapidity, or else a part of it must be sloughed off above, and so turn back before reaching the calms about the poles. The latter is probably the case. Such was the conjecture. Subsequent investigations^[2] have established its correctness, and in this way: they show that the south-east trade-winds, as in the Atlantic, blow, on the average, during the year, 124 days between the parallels of 25° and 30° S., and that as you approach the equator their average annual duration increases until you reach 6° S. Here between 5° and 10° S. they blow on the average for 329 out of the 365 days.

218. The air which supplies the south-east trade-wind in the band 5° does not cross the band 25°.—Now the question may be asked, Where do the supplies which furnish air for these winds for 329 days come from? The "trades" could not convey this fresh supply of air across the parallel of 25° S. during the time annually allotted for them to blow in that latitude. They cannot for these reasons: (1.) Because the trade-winds in lat. 5° are stronger than they are in lat. 25°, and therefore, in equal times, they waft larger volumes of air across 5° than they do across 25°. (2.) Because the girdle of the earth near the equator is larger than it is farther off, as at 25°; therefore, admitting equal heights and velocities for the wind at the two parallels, it would, in equal times, bear most air across the one of larger circumference. Much less, therefore, can the air which crosses the parallel of 25° S. annually in the 124 trade-wind days of that latitude be sufficient to supply the trade-winds with air for their 329 days in lat. 5°. Whence comes the extra supply for them in 5°? (3.) Of all parts of the ocean the trade-winds obtain their best development between 5° and 10° S. in the Atlantic Ocean, for it is there only that they attain the unequalled annual average duration of 329 days. But referring now to the average annual duration of the south-east trade-wind in all seas, we may, for the sake of illustration, liken this belt of winds which encircles the earth, say between the parallels of 5° and 25° S., to the frustum of a hollow cone, with its base towards the equator.

219. Winds with northing and winds with southing in them contrasted.—Now, dividing the winds into only two classes, as winds with northing and winds with southing in them, actual observations show, taking the world around, that winds having southing in them blow into the southern or smaller end of this cone for 209 days annually, and out of the northern and larger end for 286 days.^[3] They appear (§ 221) to come out of the larger end with greater velocity than they enter the smaller end. But we assume the velocity at going in and at coming out to be the same, merely for illustration. During the rest of the year, either winds with northing in them are blowing in at the big end, or out at the little end of the imaginary cone, or no wind is blowing at all-: that is, it is calm. Now, if we suppose, merely for the sake of assisting farther in the illustration, that these winds with northing and these winds with southing move equal volumes of air in equal times, we may subtract the days of the one from the days of the other, and thus ascertain how much more air comes out at one end than goes in at the other of our frustum. Winds with northing in them blow in at the big end for 72 days, and out at the little end for 146 days annually. Now, if we subtract the whole number of winds (146) with northing in them that blow out at the south or small end, from the whole number (209) with southing in them that blow in, we shall have for the quantity that is to pass through, or go from the parallel of 25° to 5°, the volume expressed by the transporting power of the south-east trade-winds at latitude 25° for 63 days (209-1466=3). In like manner we obtain, in similar terms, an expression for the volume which these winds bring out at the large or equatorial end, and find it to be as much air as the south-east trade-winds can transport across the parallel of 5° S. in 214 days (28—672=214). Again:

220. South-east trade-winds stronger near the equatorial limits.—The south-east trade-winds, as they cross the parallel of 5° and come out of this belt, appear to be stronger ^[4] than they are when they enter it. But assuming the velocity at each parallel to be the same, we have (§ 219) just three times as much air with southing in it coming out of this belt on the equatorial side as with southing in it we find entering (§ 218) on the polar side. From this it is made plain that if all the air, whether from the southward and eastward, or from the southward and westward, which enters the south-east trade-wind belt near its polar borders, were to come out at its equatorial edge as south-east trade-winds, there would not be enough air to feed the south-east trade-winds between these two parallels of 5° and 10° S: the annual deficiency of air here would be the volume required to supply the trades for 151 days (214-63=151).

221. Speed of vessels through the trade-winds.—The average speed which vessels make in sailing through the trade-winds in different parts of the world has been laboriously investigated at the National Observatory.^[5] By this it appears that their average speed through the south-east trade-winds of the Atlantic is, between the parallels of 5° and 10°, 6.l knots an hour, and 5.7 between 25° and 30°.

222. The question, Whence are the south-east trade-winds supplied with air? answered.—All these facts being weighed, they indicate that the volume of air which investigations show that the south-east trade-winds of the world annually waft across the parallels of 10°-5° S. in 285^[6] days—for that is their average duration for all oceans taken together—is at least twice as great as the volume which they annually sweep across the parallel of 25° in 139 days, which is their like average here. Hence in answer to the question (§ 218), "Whence comes the excess?" the reply is, it can only come from above, and in this way, viz.: the south-east trade-winds, as they rush from 25° S. towards the equator, act upon the upper air like an under-tow. Crossing, as they approach the equator, parallels of larger and larger circumference, these winds draw down and turn back from the counter current above air enough to supply pabulum to larger and larger, and to stronger and stronger currents of surface-wind.

223. Whither it goes.—The air which the trade-winds pour into the equatorial calm belt (§ 213) rises up, and has to flow off as an upper current, to make room for that which the trade-winds are continually pouring in below. They bring it from towards the poles—back, therefore, towards the poles the upper currents must carry it. On their journey they cross parallel after parallel, each smaller than the other in circumference. There is, therefore, a constant tendency with the air that these upper currents carry polarward to be crowded out, so to speak—to slough off and turn back. Thus the upper current is ever ready to supply the trade-winds, as they approach the equator, with air exactly at the right place, and in quantities just sufficient to satisfy the demand.

224. How is it drawn down from above?—This upper air, having supplied the equatorial cloud-ring (§ 514) with vapour for its clouds, and with moisture for its rains, flows off polarward as comparatively dry air. The dryest air is the heaviest. This dry and heavy air is therefore the air most likely to be turned back with the trade-winds, imparting to them that elasticity, freshness, and vigour for which they are so famous, and which help to make them so grateful to man and beast in tropical climates. The curved arrows, f g and f' g', r s and r' s', are intended to represent, in the "diagram of the winds" (Plate I.), this sloughing off and turning back of air from the upper currents to the trade-winds below.

225. Velocity of south-east shown to be greater than north-east trade-winds.—According to investigations which are stated at length in Maury's Sailing Directions, on his Wind and Current Charts, and in the Monographs of the Washington Observatory, the average strength and annual duration of the south-east trade-winds of the Atlantic may be thus stated for every band or belt of 5° of latitude in breadth, from 30° to the equator. For the band between the parallels of—

	Ann. duration.	Force.		No. of obs.
30° and 25° S	124	days.	5.6	miles.[7]	19,817
25° and 20°	157	,,	5.7	,,	20,762
20° and 15°	244	,,	5.9	,,	17,844
15° and 10°	295	,,	6.3	,,	14,422
10° and 5°	329	,,	6.1	,,	13,714
5° and 0°	314	,,	6.0	,,	15,463

It thus appears that the south-east trade-winds of the Atlantic blow with most regularity between 10° and 5°, and with most force between 10° and 15°.

226. The air sloughed off from the counter trades, moist air.—On the polar side of 35°-40°, and in the counter trades (§ 204 [7]), a different process of sloughing off and turning back is going on. Here the winds are blowing towards the poles; they are going from parallels of large to parallels of smaller circumference, while the upper return current is doing the reverse; it is widening out with the increasing circumference of parallels, and creating room for more air, while the narrowing current below is crowding out and sloughing off air for its winds.

227. The air sloughed off from the upper trade current dry.—In the other case (§ 224), it was the heavy dry air that was sloughed off to join the winds below. In this case it is the moist and lightest air that is crowded out to join the current above.

228. The meteorological influences of ascending columns of moist air.—This is particularly the case in the southern hemisphere, where, entirely around the globe between the parallels of 40° and 60° or 65°, all, or nearly all, is water. In this great austral band the winds are in contact with an evaporating surface all the time. Aqueous vapour is very much lighter than atmospheric air: as this vapour rises, it becomes entangled with the particles of air, some of which it carries up with it, thus producing, through the horizontal flow of air with the winds, numerous little ascending columns. As these columns of air and vapour go up, the superincumbent pressure decreases, the air expands and cools, causing precipitation or condensation of the vapour. The heat that is set free during this process expands the air still farther, thus causing here and there in those regions, and wherever it may chance to be raining, intumescences, so to speak, from the wind stratum below; the upper current, sweeping over these protuberances, bears them off in its course towards the equator, and thus we have another turning back, and a constant mingling. Tho curved arrows, h j k and h' f' k', are intended, on the "diagram of the winds" (Plate I.), to represent this rising up from the counter trades and turning back with the upper current.

229. Supposing the air visible, the spectacle that would he presented between the upper and lower currents.—Let us imagine the air to be visible, that we could see these different strata of winds, and the air as it is sloughed off from one stratum to join the other. We can only liken the spectacle that would be presented between the upper and the lower stratum of these winds to the combing of a succession of long waves as they come rolling in from the sea, and breaking one after another, upon the beach. They curl over and are caught up, leaving foam from their white caps behind, but nevertheless stirring up the sea and mixing up its waters so as to keep them all alike.

230. The importance of atmospherical circulation.—If the ordinances of nature require a constant circulation and continual mixing up of the water in the sea, that it become not stagnant, and that it may be kept in a wholesome state for its inhabitants, and subserve properly the various offices required of it in the terrestrial economy, how much more imperative must they not be with the air? It is more liable to corruption than water; stagnation is ruinous to it. It is both the sewer and the laboratory for the whole animal and vegetable kingdoms. Ceaseless motion has been given to it; perpetual circulation and intermingling of its ingredients are required of it. Personal experience teaches us this, as is manifest in the recognized necessity of ventilation in our buildings—the wholesome influences of fresh air, and the noxious qualities of "an atmosphere that has in circulation." Hence, continual mixing up of particles in the atmosphere being required of the winds in their circuits, is it possible for the human mind to conceive of the appointment of "circuits" for them (§ 216) which are so admirably designed and exquisitely adapted to the purpose as are those which this view suggests?

231. Its vertical movements—how produced.—As a physical necessity, the vertical circulation of the air seems to be no less important than its horizontal movements, which we call wind. One begets the other. The wind, when it blows across parallels of latitude—as it always must, except at the equator, for it blows in arcs of great circles, and not in small ones^[8]—creates a vertical circulation either by dragging down air from the upper regions (§ 224), or by sloughing it off and forcing it up from the lower (§ 228), according as the wind is approaching the pole or equator.

232. Vertical and horizontal movements in the air consequents of, and dependent upon each other.—Indeed the point may be well made whether the horizontal circulation of the air be not dependent upon and a consequent of its vertical circulation;—so nearly allied are the two motions in their relations as cause and effect. Upward and downward movements in fluids are consequent upon each other, and they involve lateral movements. The sea, with its vapour, is the great engine which gives upward motion in the air. As soon as aqueous vapour is formed it rises; the air resists its ascent; but it is lighter than the air, therefore (§ 177) it forces the resisting particles of air up along with it, and so produces ascending columns in the atmosphere. The juxta air comes in to occupy the space which that carried up by the vapour leaves behind it, and so there is a wind produced. The wind arising from this source alone is so slight generally, as scarcely to be perceived. But when the ascending vapour is condensed, and its latent heat liberated and set free in the upper air, we often have the most terrific storms.

233. Cold belts.—Now suppose the surface from which this vapour rises, or on which it is condensed, be sufficiently large to produce a rush of wind from afar; suppose it, moreover, to be an oblong lying east and west somewhere, for example, in the temperate zone of the northern hemisphere. The wind that comes rushing in from the south side will be in the category of the counter trades of the southern hemisphere (§ 228), viz.: going from larger to smaller parallels, and giving rise to ascending columns; while that from the northern side, moving in the opposite direction, is, like the trade-winds (§ 223), bringing down air from above.

234. The upper currents—their numbers and offices.—By the motion of the clouds upper currents of wind are discerned in the sky. They are arranged in layers or strata one above the other. The clouds of each stratum are carried by its winds in a direction and with a velocity peculiar to their stratum. How many of these superimposed currents of wind there may be between the top and bottom of the atmosphere we know not. As high up as the cloud-region several are often seen at the same time. They are pinions and ratchets in the atmospherical machinery. We have seen (§ 230) some of their uses: let us examine them more in detail. Now, as the tendency of air in motion is (§ 120) to move in arcs of great circles, and as all great circles that can be drawn about the earth must cross each other in two points, it is evident that the particles of the atmosphere which are borne along as wind must have their paths all in diverging or converging lines, and that consequently each wind must either be, like the trade-winds (§ 222), drawing down and sucking in air from above, or, like the counter trades (§ 226), crowding out and forcing it off into the upper currents.

235. Tendency of air when put in motion to move in the plane of a great circle.—This tendency to move in great circles is checked by the forces of diurnal rotation, or by the pressure of the wind when it blows towards a common centre, as in a cyclone. In no case is it entirely overcome in its tendency, but in all it is diverted from the great circle path and forced to take up its line of march either in spirals about a point on the surface of the earth, or in loxodromics about its axis. In either case the pushing up or pulling down of the combing, curdling air from layer to layer is going on.

236. The results upon its circulation of this tendency.—Thus the laws of motion, the force of gravity, and the figure of the earth all unite in requiring every wind that blows either to force air up from the surface into the regions above, or to draw it down to the earth from the crystal vaults of the upper sky. Add to these the storm-king:—traversing the air, he thrusts in the whirlwind or sends forth the cyclone, the tornado, and the hurricane to stir up and agitate, to mix and mingle the whole in one homogeneous mass. By this perpetual stirring up, this continual agitation, motion, mixing, and circulation, the airy covering of the globe is kept in that state which the well-being of the organic world requires. Every breath we draw, every fire we kindle, every blade of grass that grows or decays, every blaze that shines and burns adds something that is noxious, or takes something that is healthful away from the surrounding air. Diligent, therefore, in their offices must the agents be which have been appointed to maintain the chemical status of the atmosphere, to preserve its proportions, to adjust its ingredients, and to keep them in that state of admixture best calculated to fit it for its purposes.

237. Experiments by the French Academy.—Several years ago the French Academy sent out bottles and caused specimens of air from various parts of the world to be collected and brought home to be analyzed. The nicest tests which the most skilful chemists could apply were incapable of detecting any, the slightest, difference as to ingredients in the specimens from either side of the equator; so thorough in the performance of their office are these agents. Nevertheless, there are a great many more demands on the atmosphere by the organic world for pabulum in one hemisphere than in the other; and consequently a great many more inequalities for these agents to restore in one than in the other. Of the two, the land of our hemisphere most teems with life, and here the atmosphere is most taxed. Here the hearthstone of the human family has been laid. Here, with our fires in winter and our crops in summer, with our work-shops, steam-engines, and fiery furnaces going night and day—with the ceaseless and almost limitless demands which the animal and vegetable kingdoms are making upon the air overhead, we cannot detect the slightest difference between atmospherical ingredients in different hemispheres; and yet, notwithstanding the compensations and adjustments between the two kingdoms of the organic world, there are almost in every neighbourhood causes at work which would produce a difference were it not for these ascending and descending columns of air;—were it not for the obedient winds,—for this benign system of circulation,—these little cogs and ratchets which have been provided for its perfect working. The study of its mechanism is good and wholesome in its influences, and the contemplation of it well calculated to excite in the bosom of right-minded philosophers the deepest and best of emotions.

238. How supplies of fresh air are brought down from the upper sky.—Upon the proper adjustments of the dynamical forces which keep up these ceaseless movements the life of organic nature depends. If the air that is breathed were not taken away and renewed, warm-blooded life would cease; if carbon, and oxygen, and hydrogen, and water were not in due quantities dispensed by the restless air to the flora of the earth, all vegetation would perish for lack of food. That our planet may be liable to no such calamity, power has been given to the wayward wind, as it "bloweth where it listeth," to bring down from the pure blue sky fresh supplies of life-giving air wherever it is wanted, and to catch up from the earth wherever it may be found, that which has become stale—to force it up, there to be deflagrated among the clouds, purified and renovated by processes known only to Him whose ministers they are. The slightest change in the purity of the atmosphere, though it may be too slight for recognition by chemical analysis in the laboratory, is sure to be detected by its effects upon the nicer chemistry of the human system, for it is known to be productive of disease and death. No chemical tests are sensitive enough to tell us what those changes are, but experience has taught us the necessity of ventilation in our buildings, of circulation through our groves. The cry in cities for fresh air from the mountains or the sea, reminds us continually of the life-giving virtues of circulation. Experience teaches that all air when pent up and deprived of circulation becomes impure and poisonous.

239. Beautiful and benign arrangements.—How minute, then, pervading, and general, benignant, sure, and perfect must be that system of circulation which invests the atmosphere and makes the whole world kin?" In the system of vertical circulation which I have been endeavouring to describe, we see, as in a figure, the lither sky filled with crystal vessels full of life-giving air continually ascending and descending between the bottom and the top of the atmospherical ocean; these buckets are let down by invisible hands from above, and, as they are taken up again, they carry off from the surface, to be purified in the laboratory of the skies, phials of mephitic vapours and noxious gases, with the dank and deadly air of marshes, ponds, and rivers.

240. Their influences upon the mind.—Whenever, by study and research, we succeed in gaining an insight, though never so dim, into any one of the offices for which any particular part of the physical machinery of our planet was designed by the Great Architect, the mind is enriched with the conviction that it has comprehended a thought that was entertained at the creation. For this reason the beautiful compensations which philosophers have discovered in terrestrial arrangements are sources of never-failing wonder and delight. How often have we been called on to admire the benign provision by which fresh water is so constituted that it expands from a certain temperature down to freezing! We recognize in the formation of ice on the top instead of at the bottom of freezing water, an arrangement which subserves, in manifold ways, wise and beneficent purposes. So, too, when we discern in the upper sky (§ 234) currents of wind arranged in strata one above the other, and running hither and thither in different directions, may we not say that we can here recognize also at least one of the fore-ordained offices of these upper winds? That by sending down fresh air and taking up foul, they assist in maintaining the world in that state in which it was made and for which it is designed—"a habitation fit for man?"

241. The effect of downward currents in producing cold.—The phenomena of cold and warm "spells" are often observed in the United States, and I suppose in other parts of the world also; and here in these downward currents we have the explanation and the cause of sudden and severe local changes in the weather. These belts often lie east and west rather than north and south, and we frequently have much colder or hotter weather in them than we have even several degrees to the north or to the south of them. The conditions required for one of these cold "snaps" in America appear to be a north or north-west wind of considerable breadth from west to east. As it goes to the south, its tendency is, if it reach high enough, to bring down cold air from above in the manner of the trade-winds (§ 238); and when the air thus brought down chances to be, as it often is, dry and cold, we have the phenomenon of a cold belt, with warmer weather both to the north and the south of it. While I write the thermometer is—4° in Mississippi, lat. 32°, and they are having colder weather there than we have either in Washington or Cincinnati, 7° farther to the north.

242. The winter northers of Texas.—The winter "northers" of Texas sometimes bring down the cold air there with terrific effect. These bitter cold winds are very severe at Nueces, in the coast country, or the south-west corner of Texas bordering the Gulf of Mexico, lat. 27°.5. They are often felt to the west in Mexico, but rarely in eastern or northern Texas. The fact that they are not known in northern Texas goes to show that the cold they bring is not translated by the surface winds from the north.

243. Their severe cold.—A correspondent in Nueces, lat. 27° 36' N., long. 97° 27' W., has described these winds there during the winter of 1859-60: They prevail from November to March, and commence with the thermometer at about 80° or 85°. A calm ensues on the coast; black clouds roll up from the north; the wind is heard several minutes before it is felt; the thermometer begins to fall; the cold norther bursts upon the people, bringing the temperature down to 28°, and sometimes even to 25°, before the inhabitants have time to change clothing and make fires. So severe is the cold, so dry the air, that men and cattle have been known to perish in them.^[9] These are the winds which, entering the Gulf and sucking up heat and moisture therefrom, still retain enough of strength to make themselves terrible to mariners—they are the far-famed northers of Vera Cruz.

244. "Cold Snaps."—The temperature of the atmosphere at the height of three or four miles is variable — observations and balloonists tell us so. Air may be brought below the normal temperature due the height at which it may be, by radiation and other processes. It may also be raised above that normal temperature by the setting free there of the latent heat of vapour or by the action of the solar ray upon the cloud stratum. When this upper air is brought to the surface in this abnormal condition, the people of the district upon which it descends find themselves in a "cold snap" or "hot term," as the case may be.

245. Anemometers to determine the inclination of the wind wanted.—That our climates, especially the continental, are affected by, and that many of the changes in the weather are due to, the vertical circulation of the atmosphere, seems clear.^[10] We have other evidence besides that of induction (§ 224) as to upward and downward movements amongst the particles of air. In violent winds especially are these upward and downward currents made obvious by the feathers, leaves, thistledown, dust, and trash that are blown about. It would be well if our wind gauges and vanes therefore were so constructed as to show the inclination as well as the azimuth of the wind. With such an improvement we might ascertain whether certain sudden changes in the weather be not owing quite as much to the inclination as to the direction of the wind.

246. The hot winds of the Andes.—We may seek in the vertical circulation of the atmosphere for an explanation in part, not only of hot and cold terms, but in a measure also of seasons of excessive drought, as well as of other phenomena with which all are familiar. Travellers in crossing the Andes tell of hot winds encountered there even on the mountain tops. Streaks of hot air are also frequently encountered in various parts of America, and I have no doubt in other countries also.

247. Certain "Hot Spells" explained.—To explain one of these sudden and severe "hot spells," let us suppose the neighbouring atmosphere to be well loaded with moisture at the temperature of 80° for example, and with the barometer at 30 in.; that from some cause this rain-laden air commences to ascend, and its vapour to be condensed. In this process the heat which was latent in the vapour becomes sensible in the all. Now the height to which this air rises may be such, were it dry air, as to reduce its temperature 80°, and bring it down to zero; but it is moist air, and the liberated heat may be sufficient to raise it to 20°, and so prevent the temperature from going below that reading. Thus this air is at least 20° above^[11] the normal temperature of the height to which it may have risen. Suppose that now, in the process of vertical circulation, it be brought down to the surface again, and submitted to the same barometric pressure as before: its temperature now will not be 80°, as before, but it will be 80° + 20°, or 100°. Thus by going up, precipitating its moisture; and coming down, it is made hot.

248. Reservoirs in the sky.—Whenever and wherever air in this condition descends to the surface, there will be a longer or shorter period of excessively warm weather.^[12] Thus we infer the existence in the upper air of reservoirs for the heat as well as of chambers for the cold.

249. The warm winds of the Andes caused by the trade-winds.—The streaks of warm air on the Andes (§ 246) derive their warmth in all probability from the liberated heat of the trade-wind vapours as they are condensed into snow-storms.

250. Dormant powers of the telegraph in meteorology.—Spells of wet and dry, as well as "terms" of hot and cold, weather sometimes pass over portions of the country like great waves. They occupy hours, or days, or weeks in their march. The magnetic telegraph would, were the system of combined research out of which this work has grown so enlarged as to permit us to use it as a meteorological implement,^[13] enable us to give warning of all such changes in the weather in time for farmers and others, as well as mariners to profit by the foreknowledge. We could foretell the coming of storms also.

251. The wind in his circuits.—We now see the general course of the "wind in his circuits," as we see the general course of the water in a river. There are many abrading surfaces, irregularities, &c., which produce a thousand eddies in the main stream; yet, nevertheless, the general direction of the whole is not disturbed nor affected by those counter-currents; so with the atmosphere and the variable winds which we find here in this latitude. Have I not, therefore, very good grounds for the opinion (§ 200) that the "wind in his circuits," though apparently to us never so wayward, is as obedient to law and as subservient to order as were the morning stars when first they "sang together?"

252. Forces which propel the wind.—There are at least two forces concerned in driving the wind through its circuits. We have seen (§ 207) whence that force is derived which gives easting to the winds as they approach the equator, and westing as they approach the poles; and allusion, without explanation, has been made (§ 212) to the source whence they derive their northing and their southing. Philosophers formerly held that the trade-winds are drawn towards the equator by the influence of the direct rays of the sun upon the atmosphere there. They heated it, expanded it, and produced rarefaction, thereby causing a rush of the wind both from the north and south; and as the solar rays played with greatest effect at the equator, there the ascent of the air and the meeting of the two winds would naturally be. So it was held, and such was the doctrine.

253. Effect of the direct heat of the sun upon the trade-winds.—But the direct rays of the sun, instead of being most powerful upon the air at the equator, are most powerful where the sun is vertical; and if the trade-winds were produced by direct heat alone from the sun, the place of meeting would follow the sun in declination much more regularly than it does. But, instead of so following the sun, the usual place of meeting between the trade-winds is neither at the equator nor where the sun is vertical. It is at a mean between the parallels of 5° and 10° or 12° N. It is in the northern hemisphere, notwithstanding the fact that in the southern summer, when the sun is on the other side of the line, the earth is in perihelion, and the amount of heat received from the vertical ray in a day there is very much greater (1/15) than it is when she is in aphelion, as in our mid-summer. For this reason the southern summer is really hotter than the northern; yet, notwithstanding this, the south-east trade-winds actually blow the air away from under this hot southern sun, and bring it over into the northern hemisphere. They cross over into the northern hemisphere annually, and blow between 0°and 5° N. for 193 days,^[14] whereas the north-east trades have rarely the force to reach the south side of the equator at all. 254. The two systems of trade-winds unequal both in force, duration, and stability.—By examining the log-books of vessels while sailing through the north-east and south-east trade-wind belts, and comparing their rate of sailing, it has been ascertained that ships sail faster with the south-east than they do with the north-east trade-winds, and that the south-east blow more days during the year than do the north-east trades.^[15] The logs of vessels that spent no less than 166,000 days in sailing through these two belts of wind show that the average sailing speed through the south-east trade-wind belt, which lies between the equator and 30° S., is about eight miles an hour, and the average number of uninterrupted south-east trade-wind days in the year is 227. For the north-east it is 183 days, with strength enough to give ships an average speed of only 5.6 miles an hour. Hence it appears that the two systems of trade-winds are very unequal both as to force and stability, the south-east surpassing in each case.

255. Effects of heat and vapour.—Moreover, the hottest place within the trade-wind regions is not at the equator: it is where these two winds meet (§ 253). Lieutenant Warley has collated from the abstract logs the observations on the temperature of the air made by 100 vessels, indiscriminately taken, during their passage across the trade-wind and equatorial calm belts of the Atlantic. The observations were noted at each edge of the calm belt, in the middle of it, and 5° from each edge in the trade-winds, with the following averages: In the north-east trades, 5° north of the north edge of the equatorial calm belt, say in latitude 14° N, air 78°.69. North edge calm belt, say 9° N., air 80°.90. Middle of calm belt, say 4½° N., air 82°. South edge, say 0°, air 82°.30; and 5° S. (in south-east trades), air 81°.14. These thermometers had not all been compared with standards, but their differences are probably correct, notwithstanding the means themselves may not be so. Hence we infer the south edge of the calm belt is 1°.4 warmer than the north. The extreme difference between the annual isotherms that lie between the parallels of 30° N. and 30° S.—between which the trade-wind belts are included—does not probably exceed 12°. According to the experiments of Gay-Lussac and Dalton, the dilatation of atmospheric air due to a change of 12° in temperature is 2½ per cent.; that is, a column of atmosphere 100 feet high will, after its temperature has been raised 12°, be 102½ feet high. However, only about one-third of the direct heat of the sun is absorbed in its passage down through the atmosphere. The other two-thirds are employed in lifting vapour up from the sea, or in warming the crust of the earth, thence to be radiated off again, or to raise the temperature of sea and air by conduction. The air at the surface of the earth receives most heat directly from the sun; as you ascend, it receives less and less, and the consequent temperature becomes more and more uniform; so that the height within the tropics to which the direct rays of the sun ascend is not, as reason suggests, and as the snow-lines of Chimborazo and other mountains show, very great or very variable.

256. Hurricanes not due to direct heat of the sun.—Moreover, daily observations show most conclusively that the strong winds and the great winds, the hurricanes and tornadoes, do not arise from the direct heat of the sun, for they do not come in the hottest weather or in the clearest skies. On the contrary, winter is the stormy period in the extra-tropical regions of the north;^[16] and in the south, rains and gales—not gales and sunshine^[17]—accompany each other. The land and sea breezes express more than double the amount of wind force which the direct heat of the sun is capable of exerting upon the tradewinds. I say more than double, because in the land and sea breezes the wind-producing power acts alternately on the land and on the sea—in opposite scales of the balance; whereas in the trade-winds it acts all the time in one scale—in the sea scale; and the thermal impression which the solar ray makes through the land upon the air is much greater than that which it makes by playing upon the water.

257. The influence of other agents required.—From these facts it is made obvious that other agents besides the direct and reflected heat of the sun are concerned in producing the trade-winds. Let us inquire into the nature of these agents.

258. Where found.—They are to be found in the unequal distribution of land and sea, and rains, as between the two hemispheres. They derive their power from heat, it is true, but it is chiefly from the latent heat of vapour which is set free during the processes of precipitation. The vapour itself, as it rises from the sea, is (§ 232) no feeble agent^[18] in the production of wind, nor is it inconsiderable in its influence upon the trade-winds.

259. Vapour as one of the causes of the trade-winds.—Let us consider this influence. A cubic foot of water, being converted into vapour, occupies the space of 1800 cubic feet.^[19] This vapour is also lighter than the 1800 cubic feet of air which it displaces. Thus, if the displaced air weigh 1000 ounces, the vapour will weigh 623; consequently, when air is surcharged with vapour, the atmosphere is bulged out above, and the barometric pressure is diminished in proportion to the volume which flows off above in consequence of this bulging out. Thus, if we imagine the air over the Atlantic Ocean to be all in a state of rest, and that suddenly during this calm, columns of vapour were to commence rising from the middle of this ocean, we can understand how the wind would commence to flow into this central space from all around. Now, if we imagine no other disturbing cause to arise, but suppose the evaporation from this central area to go on with ceaseless activity, we can see that there would be a system of winds in the Atlantic as steady, but perhaps not so strong as the trades, yet owing their existence, nevertheless, merely to the formation of aqueous vapour. But this is not all.

260. Black's law.—"During the conversion of solids into liquids, or of liquids into vapours, heat is absorbed, which is again given out on their recondensation." ^[20] In the process of converting one measure of water into vapour, heat enough is absorbed—i. e., rendered latent, without raising the temperature of the vapour in the least—to raise the temperature of 1000 *such measures of water 1°; when this vapour is condensed again into water, wherever the place of recondensation may be, this heat is set free again. If it be still further condensed, as into hail or snow, the latent heat rendered sensible during the process of congelation would be sufficient to raise the temperature of 140 additional measures of water 1°.

261. The latent heat transported in vapour.—In this heat rendered latent by the processes of evaporation, and transported hither and thither by the winds, resides the chief source of the dynamical power which gives them motion. In some aspects vapour is to the winds what fuel is to the steam-engine: they carry it to the equatorial calm belt; there it rises, entangling the air, and carrying it up along with it as it goes. As it ascends it expands, as it expands it grows cool; and as it does this its vapour is condensed, the latent heat of which is thus liberated; this raises the temperature of the upper air, causing it to be rarefied and to ascend still higher. This increased rarefaction calls for increased velocity on the part of the inpouring trade-winds below.

262. The effect of the deserts upon the trade-winds.—Thus the vapours uniting with the direct solar ray would, were there no counteracting influences, cause the north-east and south-east trade-winds to rush in with equal force. But there is on the polar side of the north-east trade-winds an immense area of arid plains for the heat of the solar ray to beat down upon, also an area of immense precipitation. These two sources of heat hold back the north-east trade-winds, as it were, and, when the two are united, as they are in India, they are sufficient not only to hold back the north-east trade-wind, but to reverse it, causing the south-west monsoon to blow for half the year instead of the north-east trade.

263. Indications of a crossing at the calm belts.—We have, in this difference as to strength and stability (§ 254) between the north-east and south-east trade-winds, another link in the chain of facts tending to show that there is a crossing of the winds at the calm belts. The greatest amount of evaporation takes place in the southern hemisphere, which is known by the simple circumstance that there is so much more sea-surface there. The greatest quantity of rain falls in the northern hemisphere, as both the rain-gauge and the rivers show. So likewise does the thermometer; for the vapour which affords this excess of precipitation brings the heat—the dynamical power—from the southern hemisphere; this vapour transports the heat in the upper regions from the equatorial cloud-ring to the calms of Cancer, on the polar side of which it is liberated as the vapour is precipitated, thus assisting to make the northern warmer than the southern hemisphere. In those northern latitudes where the precipitation of vapour and liberation of heat take place, aerial rarefaction is produced, and the air in the calm belt of Cancer, which is about to blow north-east trade, is turned back and called in to supply the indraught towards the north. Thus the north-east, trade-winds being checked, the south-east are called on to supply the largest portion of the air that is required to feed the ascending columns in the equatorial calm belt.

264. The counter trades-they approach the pole in spirals.—On the north side of the trade-wind belt in the northern, and on the south side in the southern hemisphere, the prevailing direction of the winds is not towards the equator, but exactly in the opposite direction. In the extra-tropical region of each hemisphere the prevailing winds blow from the equator towards the poles. These are the counter-trades (§ 204). The precipitation and congelation that go on about the poles produce in the amount of heat set free, according to Black's law (§ 260), a rarefaction in the upper regions, and an ascent of air about the poles similar to that about the equator, with this difference however: the place of ascent over the equator is a line, or band, or belt; about the poles it is a disc. The air rushing in from all sides gives rise to a wind, which, being operated upon by the forces of diurnal rotation as it flows north, for example, will approach the north pole by a series of spirals from the south-west.

265. They turn with the hands of a watch about the south pole, against them about the north.—If we draw a circle about this pole on a common terrestrial globe, and intersect it by spirals to represent the direction of the wind, we shall see that the wind enters all parts of this circle from the south-west, and that, consequently, there should be about each pole a disc or circular space of calms, in which the air ceases to move forward as wind, and ascends as in a calm; about the Arctic disc, therefore, there should be a whirl, in which the ascending column of air revolves from right to left, or against the hands of a watch. At the south pole the winds come from the north-west (§ 213), and consequently there they revolve about it with the hands of a watch. That this should be so will be obvious to any one who will look at the arrows on the polar sides of the calms of Cancer and Capricorn (Plate I., § 215). These arrows are intended to represent the prevailing direction of the wind at the surface of the earth on the polar side of these calms.

266. The arrows in the diagram of the winds.—The arrows that are drawn about the axis of this diagram are intended to represent, by their flight, the mean direction of the wind, and by their length and their feathers the mean annual duration from each quadrant. Only the arrows nearest to the axis in each belt of 5° of latitude are drawn with such nicety. The largest arrow indicates that the wind in that belt blows annually, on the average, for ten months as the arrow flies. The arrow from the next most prevalent quarter is half-feathered, provided the average annual duration of the wind represented is not less than four months. The unfeathered arrows represent winds having an average duration of less than three months. The arrows are on the decimal scale; the longest arrow—which is that representing the south-east trade-winds between 5° and 10° S., where their average duration is ten months—being half an inch. Winds that blow five months are represented by an arrow half this length, and so on. The half-bearded arrows are on a scale of two for one. It appears, at first, as a singular coincidence that the wind should whirl in these discs about the poles as it does in cyclones, viz., against the hands of a watch in the northern, with them in the southern hemisphere.

267. The offices of sea and air in the physical economy.—To act and react upon each other, to distribute moisture over the surface of the earth, and to temper the climate of different latitudes, it would seem, are two of the many offices assigned by their Creator to the ocean and the air. When the north-east and south-east trades meet and produce the equatorial calms (§ 212), the air, by the time it reaches this calm belt, is heavily laden with moisture, for in each hemisphere it has travelled obliquely over a large space of the ocean. It has no room for escape but in the upward direction (§ 223). It expands as it ascends, and becomes cooler; a portion of its vapour is thus condensed, and comes down in the shape of rain. Therefore it is that, under these calms, we have a region of constant precipitation. Old sailors tell us of such dead calms of long continuance here, of such heavy and constant rains, that they have scooped up fresh water from the sea to drink. The conditions to which this air is exposed here under the equator are probably not such as to cause it to precipitate all the moisture that it has taken up in its long sweep across the waters. Let us see what becomes of the rest; for Nature, in her economy, permits nothing to be taken away from the earth which is not to be restored to it again in some form, and at some time or other. Consider the great rivers—the Amazon and the Mississippi, for example. We see them day after day, and year after year, discharging immense volumes of water into the ocean. All the rivers run into the sea, yet the sea is not full."—Eccl. i. 7. Where do the waters so discharged go, and where do they come from? They come from their sources, is the ready answer. But whence are their sources supplied? for, unless what the fountain sends forth be returned to it again, it will fail and be dry. We see simply, in the waters that are discharged by these rivers, the amount by which the precipitation exceeds the evaporation throughout the whole extent of valley drained by them; and by precipitation I mean the total amount of water that falls from, or is deposited by the atmosphere, whether as dew, rain, hail, or snow. The springs of these rivers (§ 191) are supplied from the rains of heaven, and these rains are formed of vapours which are taken up from the sea, that "it be not full," and carried up to the mountains through the air. "Note the place whence the rivers come, thither they return again." Behold now the waters of the Amazon, of the Mississippi, the St. Lawrence, and all the great rivers of America, Europe, and Asia, lifted up by the atmosphere, and flowing in invisible streams back through the air to their sources among the hills (§ 191), and that through channels so regular, certain, and well-defined, that the quantity thus conveyed one year with the other is nearly the same: for that is the quantity which we see running down to the ocean through these rivers; and the quantity discharged annually by each river is, as far as we can judge, nearly a constant.

268. Powerful machinery.—We now begin to conceive what a powerful machine the atmosphere must be; and, though it is apparently so capricious and wayward in its movements, here is evidence of order and arrangement which we must admit, and proof which we cannot deny, that it performs this mighty office with regularity and certainty, and is therefore as obedient a law as is the steam-engine to the will of its builder. It, too, is an engine. The South Seas themselves, in all their vast intertropical extent, are the boiler for it, and the northern hemisphere is its condenser (§ 24). The mechanical power exerted by the air and the sun in lifting water from the earth, in transporting it from one place to another, and in letting it down again, is inconceivably great. The utilitarian who compares the waters power that the Falls of Niagara would afford if applied to machinery, is astonished at the number of figures which are required to express its equivalent in horse-power. Yet what is the horse-power of the Niagara, falling a few steps, in comparison with the horse-power that is required to lift up as high as the clouds and let down again all the water that is discharged into the sea, not only by this river, but by all the other rivers and all the rain in the world? The calculation has been made by engineers, and, according to it, the force for making and lifting vapour from each area of one acre that is included on the surface of the earth is equal to the power of 30 horses.

1. Nautical Monograph, No. 1, 1859.
2. Nautical Monographs, No. 1, Observatory, Washington, October, 1859.
3. Nautical Monographs, No. 1, "The Winds of the Sea," Observatory, Washington, 1859.
4. The force of the trade-winds, as determined by the average speed of 2235 vessels sailing through them, is greater between 5V and 10° S. than it is between 25° and 30°.—Maury's Sailing Directions, 1859.
5. See "Average Force of the Trade-winds, "p. 857, vol. ii., 8th ed., Maury's Sailing Directions, 1859.
6. Nautical Monographs, Plate I., No. 1, "The Winds at Sea."
7. Distance per hour that vessels average while sailing through it.
8. The tendency of all bodies, when put in motion on the surface of the earth, is, "whether fluid, solid, or gaseous, to go from the point of departure to the point of destination by the shortest line possible; and this, when the motion is horizontal, is an arc of a great circle. If we imagine a partial vacuum to be formed at the north pole, we can readily enough perceive that the wind for 5°, 10° 20° of polar distance, all around, would tend to rush north, and strive to get there along the meridians—arcs of great circles. This would be the case whether the earth be supposed to be with or without diurnal rotation, or motion of any sort. Now suppose the place of refraction to be anywhere away from the poles, then draw great circles to a point in the middle of it, and the air all around would, in rushing into the vacuum, seek to reach it by these great circles. Force may turn it aside, but such is the tendency (§ 120).
9. "Two men," says Mr. M. A. Taylor, in a letter dated January 11th, 1860, at Nueces, Texas, "were actually frozen to death within a few miles of this place this winter in a norther. Animals seem to tell by instinct when the norther is coming, and make their way from the open prairies to timber and other shelter, starting often on a run when the heat is not oppressive. This is when the change is to be sudden and violent. Many cattle, horses, and sheep are frozen to death at such times."
10. Vide Chapter XXI.
11. Balloonists often in their voyages pass through layers of warm and cool air, made so doubtless by unequal radiation on one hand, or the liberation of the latent heat of vapour on the other.
12. "Heated Wind Storm.—A heated wind storm passed over a portion of Kansas on the 7th instant (July 1860), which proved nearly as destructive to animal life as the recent tornadoes that visited with such terrible effect portions of Iowa. The wind arose about half-past ten o'clock a.m., and continued until three o'clock in the afternoon. At one o'clock the mercury rose to 119½°, and continued so for about an hour, and then began gradually to decrease. The effect can scarcely be imagined. The wind blew a brisk gale, carrying with it a salty, sulphurous smell. Two men, in attempting to cross the country from Iola to Humboldt (distance eight miles), were overtaken and perished. There were three others at Humboldt who were caught out with teams, which perished, the men alone surviving, and are now in a fair way to recover. There was scarcely a chicken left in the country. Hogs and cattle fell in their tracks and suffocated. Various reasons and conjectures as to its cause are given, but all unsatisfactory."—Newspaper.
13. Arrangements for so using it have already been made in Holland, France, and England, and we hope to see them extended ere long to all other countries, and wherever lines of telegraph may go. Though the plan only went into operation in England in Sept. 1860, Admiral Fitzroy informs me, it is already rich with the promise of practical results the most valuable and important.—London, Nov. 14, 1860.
14. "The Winds of the Sea," Maury's Nautical Monographs, No. 1.
15. See Maury's Wind and Current Charts, vol. ii., 8th edition. Sailing Directions.
16. Gales of the Atlantic, Observatory, Washington, 1856.
17. Storm and Rain Charts.
18. I am sustained in this view by a recent paper on "the forces that produce the great currents of the air, and of the ocean," recently read before the Royal Society by Thomas Hopkins.
19. Black and Watt's Experiments on Heat.
20. Black's law. It is an important one, and should be remembered.