CHAPTER II

THE ATMOSPHERE*

Matthew Maury


Text converted in Fall 1997 by Maury Glass, Travis Phillips, Jonathon Moore, Hoan Le and Suran Wije.


6. There is no employment more worthy of the human mind than that which is afforded by tracing the evidences of design and purpose, which are visible in any 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 his barque is wafted or driven along. It is an envelope or covering for the dispersion 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 conveying the waters for their fountains on the ocean to their sources in the mountains.

Upon the proper working of this machine depends the well-being of every plant and animal that inhabits the earth; therefore, the management of it, its movement, 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 the machinery as obedient to order as are the planets in their orbits.

An 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 are laws which were put in force by the Creator when the foundations of the earth were laid; 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 winds and sea obey the voice of rebuke?

To one who looks abroad to contemplate the agents of nature, as he sees them at work upon our planet, no expression uttered nor act performed by them is without meaning. By such an one, the wind and rain, the vapor and the cloud, the tide, the current, the saltness, and depth, and warmth, and color 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 chooses to make known her own laws. To help us to understand that language, and to interpret aright those laws, is the object of the call which we have made upon sailors for observations at sea. No fact gathered in such a field, therefore, comes amiss to those who tread the walks of inductive philosophy; for, in the hand-book 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 and the philosopher on the mountain see spread out before them.

7. From the parallel of about 30o north and south, nearly to the equator, and extending entirely around the earth, are two zones of perpetual winds, viz: the zone of northeast trades on this side, and of southeast on that. They blow perpetually, and are as steady and as constant as the currents of the Mississippi river-always moving in the same direction, (Plate III.) As these two 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 or other to the place near 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.

This return or counter-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 always blowing on the surface. These direct and counter-currents are also made to move in a sort of spiral or loxodromic 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 to the poles. This turning is caused by the rotation of the earth on its axis.

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 toward the equator, we can easily see how this particle of air, coming from the very axis of the pole, where it did not partake of the diurnal motion of the earth, would, in consequence of its vis inertiae, find, as it travels south, the earth slipping from under it, as it were, and thus it would appear to be coming from the northeast and going towards the southwest; in other words, it would be a northeast 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 island; 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 Madeira or 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, in consequence of its vis inertiae, be going toward the east faster than the earth. It would, therefore, appear to be blowing from the southwest, and going towards the northeast, and exactly in the opposite direction to the other. Writing south for north, the same takes place between the south pole and the equator.

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, the equator being near one of the nodes, and there being 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 northeast trade-wind extending from the pole to the equator would satisfy it; and were this so, we should have, on the surface, no winds but the northeast trade-winds on this side, and none but southeast trade-winds on the other side of the equator.

8. Let us return now to our northern particle, (Plate III,) 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 to have been very satisfactorily explained by philosophers, instead of travelling (§ 7) on the surface all the way from the pole to the equator, travels in the upper regions of the atmosphere for a part of the way, and until it gets near the parallel of 30o. Here it meets, also in the clouds, the hypothetical particle that is coming from the south, and going north to take its place.

About this parallel of 30o 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 winds from the north and south.

From under this bank of calms, which seamen call the "horse latitudes," (I have called them the calms of Cancer,) two surface currents of wind are ejected: one towards the equator, as the northeast trades, the other toward the pole, as the southwest passage winds.

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.

The barometer, in this calm region, is said to stand higher than it does either to the north or to the south of it; and this is another proof as to the banking up here of the atmosphere, and pressure from its downward motion.

9. Following our imaginary particle of air from the north across this calm belt, we now feel it moving on the surface of the earth as the northeast trade-wind; and as such it continues till it arrives at the equator, where it meets a like particle, which, starting from the south pole at the same time the other started from the north pole, has blown as the southeast trade-wind.

Here, at this equatorial place of meeting, there is another conflict of winds, and another calm region, for a northeast and southeast wind cannot blow at the same time in the same place. The two particles have been put in motion by the same power; they meet with equal force; and, therefore, at their place of meeting, are stopped in their course. Hence this calm belt.

10. Warmed now by the heat of the sun, and pressed on each side by the whole force of the northeast and southeast 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 (§ 8) near the parallel of 30o.

11. This imaginary particle, then, having ascended to the upper regions of the atmosphere again, travels there counter to the southeast trades until it meets, near the calm belt of Capricorn, another particle from the south pole; here there is a descent as before, (§ 8;) it then (§ 7) flows on toward the south pole as a surface wind from the northwest.

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 continued gyrations; finally, reaching the vortex the calm place, it is carried upward to the regions of atmosphere above, whence it commences again its circuit to the north as an upper current an far as the calm belt of Capricorn; here it encounters (§ 1l) its fellow froth the north, (§ 7;) they stop, descend, and floor out as surface currents, (§ 8,) the one with which the imagination is travelling, to the equatorial calms as the southwest trade-wind; here (§ 9,) it ascends, travelling thence to the calm belt of Cancer as an upper current counter to the northeast trades. Here (§ 8) it ceases to be an upper current, but descending, (§ 8,) travels on with the southwest passage winds toward the pole.

Now, the course we have imagined an atom of air to take is this, (Plate III:) an ascent at P, the north pole; an efflux thence as an upper current (§ 8) until it meets G (also an upper current) over the calms of Cancer. Here (§ 8) there is supposed to be a descent, as shown by the arrows along the wavy lines which envelop the circle. This upper current from the pole (§ 7) now becomes the northeast trade-wind B (§ 9) on the surface until it meets the southeast trades in the equatorial calms, when it ascends and travels as C with the upper current to the calms of Capricorn, then as D with the prevailing northwest 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 E, F, G, and H.

The Bible frequently makes allusions 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 great 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 Bible called the earth "the round world;" yet for ages it was considered a heresy for Christian men to say the world is round; and, finally, sailors circumnavigated the globe, proved the Bible to be right, and confounded theologians so called.

"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 established 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, therefore, can ever "bind the sweet influences?"

And as for the general system of atmospherical circulation which I have been so long endeavoring to describe, the Bible tells it all in a single sentence: "The wind goeth toward tier, south, and turneth about unto the north; it whirleth about continually, and the wind returneth again according to his circuits."*

12. Of course, as the surface winds H and D (Plate III.) approach the poles, there must be a sloughing off, if I may be allowed the expression, of air from the surface winds, in consequence of their approaching the polls. For as they near the polls the parallels become smaller and smaller, and the surface current must either extend much higher up, and blow with greater rapidity as it approaches the polls, or else a part of it must be sloughed off above, and so turn back before reaching the polls. The latter is probably the case.

Investigations have shown that the southeast trade-wind region is much larger than the northeast. I speak now of its extent over the Atlantic ocean only; that the southeast trades are the fresher, and that they often push themselves up to 10o or 15o of north latitude; whereas the northeast trade-winds of the Atlantic seldom get south of the equator.

The peculiar clouds of the trade-winds are formed between the upper and lower currents of air. They are probably formed of vapor condensed from the upper current, and evaporated, as it descends, by the lower and dry current from the polls. It is the same phenomenon up there which is so often observed here below; when a cold and dry current of air meets a warm and wet one an evolution of vapor or fog ensues.

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 (§ 6) 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 they "sang together?"

13. There are at least two forces concerned in driving the wind through its circuits. We have seen (§ 7) whence that force is derived which gives casting to the winds as they approach the equator, and westing as they approach the polls; and allusion, without explanation, has been made (§ 10) to the source whence they derive their northing and their southing. The trade-winds are caused, it is said, by the intertropical heat of the sun, which, expanding the air, causes it to rise up near the equator; it then flows off in the upper currents north and south, and there is a rush of air at the surface, both from the north and the south, to restore the equilibrium-hence the trade-winds. But to the north side of the trade-wind belt in the northern, (§ 6,) and on the south side in the southern hemisphere, (§ 11,) the prevailing direction of the winds is not toward the source of heat about the equator, but exactly in the opposite direction. In the extra-tropical region of each hemisphere the prevailing winds blow from the equator toward the polls. It therefore at first appears paradoxical to say that heat makes the easterly winds of the torrid zone blow toward the equator, and the westerly winds of the temperate zones to blow toward the polls. Let us illustrate:

The primum mobile of the extra-tropical winds toward the equator is, as just intimated, generally ascribed to heat, and in this wise, viz: Suppose, for the moment, the earth to have no, diurnal rotation; that it is at rest; that the rays of the sun have been cut off from it; that the atmosphere has assumed a mean uniformity of temperature, the thermometer at the equator and the thermometer at the poles giving the same reading; that the winds are still, and that the whole aerial ocean is in equilibrium and at rest. Now imagine the screen which is supposed to have shut off the influence of the sun to be removed, and the whole atmosphere to assume the various temperatures in the various parts of the world that it actually has at this moment, what would take place, supposing the uniform temperature to be a mean between the actual temperature at the equator and that at the poles? Why, this would take place: a swelling up of the atmosphere about the equator by the expansive force of intertropical heat, and a contraction of it about the poles in consequence of the cold. These two forces, considering them under their most obvious effects, would disturb the supposed atmospherical equilibrium by altering the level of the great aerial ocean; the expansive force of heat elevating it about the equator, and the contracting powers of cold depressing it about the poles. And forthwith two systems of winds would commence to blow, viz: one in the upper regions from the equator towards the poles, and as this warm and expanded air should flow towards either pole, seeking its level, a wind would blow on the surface from either pole to restore the air to the equator which the upper current had carried off.

These two winds would blow due north and south; the effects of heat at the equator, and cold at the poles, would cause them so to do. Now suppose the earth to commence its diurnal rotation; then, instead of having these winds north and south winds, they will, for reasons already explained (§ 7,) approach the equator on both sides with easting in them, and each pole with westing.

The circumference of the earth, measured on the parallel of 60o , is only half what it is when measured on the equator. Therefore supposing velocity to be the same, only half the volume of atmosphere (§ 13) that sets off from the equator as an upper current toward the poles can cross the parallel of 60o north or south. The other moiety has been gradually drawn in and carried back (§ 12) by currents which are moving in the opposite direction.

Such, and such only, would be the extent of the power of the sun to create a polar and equatorial flow of air, were its power confined simply to a change of level. But the atmosphere has been invested with another property which increases its mobility, and gives the heat of the sun still more power to put it in motion, and it is this: as heat changes the atmospherical level, it changes also the specific gravity of the air acted upon. If, therefore, the level of the great aerial ocean were undisturbed by the sun's rays, and if the air were adapted to a change of specific gravity alone, without any change in volume, this quality would also be the source of at least two systems of currents in the air, viz: an upper and a lower. The two agents combined, viz: that which changes level or volume, and that which changes specific gravity, give us the general currents under consideration. Hence we say that the primum mobile of the air is derived from change of specific gravity induced by the freezing temperature of the polar regions, as well as from change of specific gravity due the expanding force of the sun's rays within the tropics.

Therefore, fairly to appreciate the extent of the influence due the direct heat of the sun in causing the winds, it should be recollected that we may with as much reason ascribe to the intertropical heat of the sun the northwest winds, which are the prevailing winds of the extra-tropical regions of the southern hemisphere, on the southwest winds, which are the prevailing winds of the extra-tropical regions of the northern hemisphere, as we may the trade-winds, which blow in the opposite directions. Paradoxical, therefore, as it seems for us to say that the heat of the sun causes the winds between the parallels of 25o or 30o north and south to blow toward the equator, and that it also causes the prevailing winds on the polar sides of the same parallels to blow toward the poles, yet the paradox ceases when we come to recollect that by the process of equatorial heating and polar cooling which is going on in the atmosphere, the specific gravity of the air is changed as well as its level. Nevertheless, as Halley said, in his paper reed before the Royal Society in London in 1686, "it is likewise very hard to conceive why the limits of the trade-wind should be fixed about the parallel of latitude 30o all around the globe, and that they should so seldom exceed or fall short of those bounds."

14. Operated upon by the equilibrating tendency of the atmosphere and by diurnal rotation, the wind approaches the north pole, for example, by a series of spirals from the southwest (§ 11.) 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 southwest, and, consequently, that a whirl ought to be created thereby, 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 northwest (§ 11,) 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 III.) These arrows are intended to represent the prevailing direction of the wind at the surface of the earth.

It is a singular coincidence between these two facts thus deduced, and other facts which have been observed, and which have been set forth by Redfield, Reid, Piddington, and others, viz: that all rotary storms in the northern hemisphere revolve as do the whirlwinds about the north pole, viz: from right to left, and that all circular gales in the southern hemisphere revolve in the opposite direction, as does the whirl about tile south pole.

How can there be any connexion between the rotary motion of the wind about the pole and the rotary motion of it in a gale caused here by local agents?

That there is probably such a connexion has been suggested by other facts and circumstances, for, although the theory of heat satisfies many conditions of the problem, and though heat, doubtless, if one of the chief agents in keeping up the circulation of the atmosphere, yet it can be made to appear that it is not the sole agent; magnetism, probably, has something to do with it.

15. So far, we see how the atmosphere moves; but the atmosphere, line every other department in the economy of nature, has its offices to perform, and they are many. I have already alluded to some of them; but I only propose, at this time, to consider some of the meteorological agencies at sea, which, in the grand design of creation, have probably been assigned to this wonderful machine.

To distribute moisture over the surface of the earth, and to temper the climate of different latitudes, it would seem, are two great offices assigned by their Creator to the ocean and the air.

When the northeast and southeast trades meet, and produce the equatorial calms (§ 9), the air, by this time, is heavily laden with moisture, for in each hemisphere it is travailed obliquely over a large space of the ocean. It has no room for escape but in the upward direction (§ 10.) It expands as it ascends, and becomes cooler; a portion of its vapor 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 surface of the sea.

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 taller 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-Amazon and the Mississippi, for example. We see them day after day and year after year discharging an immense volume of water into the ocean.

"All the rivers run into the sea, yet the sea is not full."-Ecc., i. 7. Where do the waters so discharged go, and whence do they come? They come from their sources you will say. But whence are their sources supplied? for, unless what the fountain sends forth be returned to it again, it will fail and be dry.

16. 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 are supplied from the rains of heaven, and these rains are formed of vapors 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."

17. 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, 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 constant.

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 to law as is the steam engine to the will of its builder.

18. It, too, is an engine. The South Seas themselves, in all their vast inter-tropical extent, are the boiler for it, and the northern hemisphere is its condenser.

19. Where does the vapor that makes the rain which feed the rivers of the northern hemisphere come from?

The proportion between the land and water in the northern hemisphere is very different from the proportion between them in the southern. In the northern hemisphere the land and water are nearly equally divided. In the southern there is several times more water than land. Most of the great rivers in the world are in the northern hemisphere, where there is less ocean to supply them. Whence, then, are their sources replenished? Those of the Amazon are supplied with rains from the equatorial calms and trade-winds of the Atlantic. That river runs east, its branches come from the north and south; it is always the rainy season on one side or the other of it; consequently, in its lower parts, it is without periodic stages of a very marked character. There it is always near its high-water mark: For one-half of the year its northern tributaries are flooded, and its southern for the other half. It discharges under the line, and as its tributaries come from both hemispheres it cannot be said to belong exclusively to either. It is supplied with water from the Atlantic Ocean. Taking the Amazon, therefore, out of the count, the Rio de la Plata is the only great river of the southern hemisphere.

There is no large river in New Holland. The South Sea Islands give rise to none nor is there one in South Africa that we know of.

The great rivers of North America and North Africa, and all the rivers of Europe and Asia, lie wholly within the northern hemisphere. How is it, then, considering that the evaporating surface lies mainly in the southern hemisphere-how is it, I say, that we should have the evaporation to take place in one hemisphere and the condensation in the other? The total amount of rain which falls in the northern hemisphere is much greater, meteorologists tell than that which falls in the southern. The annual amount of rain in the north temperate zone is half as much again as that of the south temperate.

20. How is it, then, that this vapor gets, as stated (§ 18), from the southern into the northern hemisphere, and comes with such regularity that our rivers never go dry, and our springs fail not? It is because of the beautiful operations and the exquisite compensation of this grand machine, the atmosphere. It is exquisitely and wonderfully counterpoised. Late in the autumn of the north, throughout its winter, and in early spring, the sun is pouring his rays with the greatest intensity down upon the seas of the southern hemisphere, and this powerful engine which we are contemplating is pumping up the water there (§ 18) for our rivers with the greatest activity. At these seasons the mean temperature of the entire southern hemisphere is said to be about 10o higher than the northern.

The heat which this heavy evaporation absorbs becomes latent, and, with the moisture, carried through the upper regions of the atmosphere until it reaches our climates. Here the vapor is formed into clouds, condensed, and precipitated. The heat which held this water in the state of vapor is set free, it becomes sensible heat, and it is that which contributes so much to temper our winter climate. It clouds up in winter, turns warm, and we say we are going to have falling weather. That is because the process of condensation has already commences though no rain or snow may have fallen: thus we feel this southern heat that has been collected from the rays of the sun by the sea, been bottled away by the winds in the clouds of a southern summer, and set free in the process of condensation in our northern winter.

21. If Plate III fairly represent the course of the winds, the southeast trade-winds would enter the northern hemisphere, and, as an upper current, bear into it all their moisture, except that which is precipitated in the region of equatorial calms.

The south seas, then, according to § 18, should supply mainly the water for this engine while the northern hemisphere condenses it; we should, therefore, have more rain in the northern hemisphere. The rivers tell us that we have-at least on the land: for the great water courses of the globe (§ 19), and half the fresh water in the world, are found on our side of the equator. This fact alone is strongly corroborative of this hypothesis.

The rain gauge tells us also the same story. The yearly average of rain in the north temperate zone is, according to Johnston, thirty-seven inches. He gives but twenty-six in the south temperate.

22. Moisture is never extracted from the air by subjecting it from a low to a higher temperature, but the reverse. Thus, all the air which comes loaded with moisture from the other hemisphere, and is borne into this with the southeast trade-winds, travels in the upper regions the atmosphere (§ 8) until it reaches the calms of Cancer; here it becomes the surface wind at prevails from the southward and westward. As it goes north it grows cooler, and the process of condensation commences.

We may now liken it to the wet sponge, and the decrease of temperature to the hand that squeezes that sponge. Finally reaching the cold latitudes, all the moisture that a dew-point of zero, and even far below, can extract is wrung from it; and this air then commences "to return according to his circuits" as dry atmosphere; and being dry, it licks up the clouds it meets on its way south, making clear weather as it goes. And here we can quote Scripture again: "The north wind driveth away rain." This is a meteorological fact of high authority and great importance in the study of the circulation of the atmosphere.

23. By reasoning in this manner we are led to the conclusion that our rivers are supplied with their waters principally from the trade-wind regions-the extra-tropical northern rivers from the southern trades, and the extra-tropical southern rivers from the northern trade-winds, for the trade-winds are the evaporating winds.

Taking for our guide such faint glimmerings of light as we can catch from these facts, and supposing these views to be correct, then the saltest portions of the sea should be in the trade-wind regions, where the water for all the rivers is evaporated; and there the saltest portions are found.

24. Dr. Ruschenberger, of the navy, on his late voyage to India, was kind enough to conduct a series of observations on the specific gravity of sea water. In about the parallel of 17o north and south-midway of the trade-wind regions-he found the heaviest water. Though so warm, the water there was heavier than the cold water to the south of the Cape of Good Hope. Lieutenant D. D. Porter, in the steamship Golden Age, found the heaviest water about the parallels of 20o north and 17o south.

In summing up the evidence in favor of this view of the general system of atmospherical circulation, it remains to be shown how it is, if the view be correct, there should be smaller rivers and less rain in the southern hemisphere.

25. The winds that are to blow as the northeast trade-winds returning as upper currents from the polar regions, where the moisture (§ 22) has been compressed out of them, remain, as we have seen, dry winds until they cross the calm zone of Cancer, and are felt on the surface as the northeast trades. About two-thirds of them only can then blow over the ocean; the rest blow over the land, over Asia, Africa, and North America, where there is but comparatively a small portion of evaporating surface exposed to their action.

The zone of the northeast trades extends, on an average, from about 29o north to 7o north. Now, if we examine the globe to see how much of this zone is land and how much water, we shall find, commencing with China and coming over Asia, the broad part of Africa, and so on, across the continent of America to the Pacific, land enough to fill up, as nearly as may be, just one-third of it. This land, if thrown into one body between these parallels, would make a belt equal to 120o of longitude by 22o of latitude.

According to the hypothesis, illustrated by Plate III, as to the circulation of the atmosphere, it is these northeast trade-winds that take up and carry over, after they rise up in the belt of equatorial calms, the vapors which make the rains that feed the rivers in the extra-tropical regions of the southern hemisphere.

Upon this supposition, then, two-thirds only of the northeast trade-winds are fully charged with moisture, and only two-thirds of the amount of rain that falls in the northern hemisphere should fall in the southern, and this is just about the proportion (§ 21) that observation gives.

26. In like manner, the southeast trade-winds take up the vapors which make our rivers, and as they prevail to a much greater extent at sea, and have exposed to their action about three tines as much ocean as the northeast trade-winds have, we might expect, according to this hypothesis, more rains in the northern-and, consequently, more and larger rivers-than in the southern hemisphere. A glance at Plate XV will show how very much larger that part of the ocean over which the southeast trades prevail is than that that where the northeast trade-winds blow.

27. This estimate as to the quantity of rain in the two hemispheres is one which is not capable of verification by any more than the rudest approximations; for the greater extent of southeast trades on one side, and of itch mountains on the other, must each of necessity, and independent of other agents, have their effects. Nevertheless, this estimate gives as close an approximation as we can make out from any other data.

28. The rainy seasons, how caused.-The calm and trade-wind regions or belts move up and down the earth, annually, in latitude nearly a thousand miles. In July and August the zone of equatorial calms is found between 7o north and 12o north; sometimes higher; in March ant April, between latitude 5o south and 2o north.

With this fact and these points of view before us, it is easy to perceive why it is that we have a rainy season in Oregon, a rainy and dry season in California, another at Panama, two at Bogota', none in Peru, and one in Chili.

In Oregon it rains every month, but more in the winter months.

The winter there is the summer of the southern hemisphere, when this steam engine is working with the greatest pressure. The vapor that is taken up by the southeast trades is borne along over the region of northeast trades to latitude 35o or 40o north (§ 21), where it descends and appears on the surface with the southwest winds of those latitudes. Driving upon the highlands of the continent, this vapor is condensed and precipitated, during this part of the year, almost in constant showers.

In the winter, the calm belt of Cancer approaches the equator. This whole system of zones, viz: of trades, calms, and westerly winds, follows the sun in declination; and they of our hemisphere are nearer the equator in the winter and spring months than at any other season.

The southwest winds commence at this season to prevail as far down as the lower part of California. In winter and spring the land in California is cooler than the sea air, and is quite cold enough to extract moisture from it. But in summer and autumn the land is the warmer, and can not condense the vapors of water held by the air. So the same cause which made it rain in Oregon, now makes it rain in California. As the sun returns to the north, he brings the calm belt of Cancer and the northeast trades along with him; and now, at places where, six months before, the southwest winds were the prevailing winds, the northeast trades are found to blow. This is the case in the latitude of California. The prevailing winds, then, instead of going from a former to a cooler climate, as before, are going the opposite way. Consequently, they cannot, if they have the moisture in them to make rains of, precipitate it under such circumstances.

Panama is in the region of equatorial calms. This belt of calms travels during the year back and forth, over about 17o of latitude, coming further north in the summer, where it tarries for several months, and then returns so as to reach its extreme southern latitude some time in March or April. Where these calms are, it is always raining, and the Chart shows that they hang over the latitude of Panama from June to November; consequently, from June to November is the rainy season at Panama. The rest of the year that place is in the region of the northeast trades, which, before they arrive there, have to cross the mountains of the isthmus, on the cool tops of which they deposit their moisture, and leave Panama rainless and pleasant until the sun returns north with the belt of equatorial calms after him. They then push the belt of northeast trades further to the north, occupy a part of the winter zone, and refresh that part of the earth with summer rains. This belt of calms moves over more than double of its breadth, and nearly the entire motion from south to north is accomplished generally in two months, May and June.

Take the parallel of 4o north as an illustration: during these two months, the entire belt of calms crosses this parallel, and then leaves it in the region of the southeast trades. During these two months, it was pouring down rain on that parallel. After the calm belt passes it, the rains cease, and the people in that latitude have no more wet weather till the fall, when the belt of calms recrosses this parallel on its way to the south. By examining the "Trade-wind Chart," it may be seen what the latitudes are that have two rainy seasons, and that Bogota? is within the bi-rainy latitudes.

29. The Rainless Regions.-The coast of Peru is within the region of perpetual southeast trade-winds. Though the Peruvian shores are on the verge of the great South Sea boiler, yet it never rains there. The reason is plain.

The southeast trade-winds in the Atlantic Ocean first strike the water on the coast of Africa. Travelling to the northwest, they blow obliquely across the ocean until they reach the coast of Brazil. By this time they are heavily laden with vapor, which they continue to bear along across the continent, depositing it as they go, and supplying with it the sources of the Rio de la Plata and the southern tributaries of the Amazon.

Finally, they reach the snow-capped Andes, and here is wrung from them the last particle of moisture that that very low temperature can extract.

Reaching the summit of that range, they now tumble down as cool and dry winds on the Pacific slopes beyond. Meeting with no evaporating surface, end with no temperature colder than that to which they were subjected on the mountain-tops, they reach the ocean before they become charged with fresh vapor, and before, therefore, they have any which the Peruvian climate can extract. Thus we see how the top of the Andes becomes the reservoir from which are supplied the rivers of Chili and Peru.

The other rainless or almost rainless regions are the western coasts of Mexico, the deserts of Africa, Asia, North America, and Australia. Now study the geographical features of the country surrounding those regions; see how the mountain ranges run; then turn to Plate XV to see how the winds blow, and where the sources are (§ 18) which supply them with vapors. This plate shows the prevailing direction of the wind only at sea; but knowing it there, we may infer what it is on the land. Supposing it to prevail on the land as it generally does in corresponding latitudes at sea, then the plate will suggest readily enough how the winds that blow over these deserts came to be robbed of their moisture, or, rather, to have so much of it taken from them as to reduce their dew-point below the desert temperature; for the air (§ 22) can never deposit its moisture when its temperature is higher than its dew-point.

We have a rainless region about the Red Sea, because the Red Sea, for the most part, lies within the northeast trade-wind region, and these winds, when they reach that region, are dry winds, for they have as yet, in their course, crossed no wide sheets of water from which them could take up a supply of vapor.

30. Most of New Holland lies within the southeast trade-wind region; so does most of inter-tropical South America. But inter-tropical South America is the land of showers. The largest rivers and most copiously watered country in the world are to be found there, whereas almost exactly the reverse is the case in Australia. Whence this difference? Examine the direction of the winds with regard to the shore-line of these two regions, and the explanation will at once be suggested. In Australia-east coast-the shore line is stretched out in the direction of the trades; in South America-east coast-it is perpendicular to their direction. In Australia, they fringe this shore only with their vapor, and so stint that thirsty land with showers that the trees cannot afford to spread their leaves out to the sun, for it evaporates all the moisture from them; their instincts, therefore, teach them to turn their edges to his rays. In America, they blow perpendicularly upon the shore, penetrating the very heart of the country 4 with their moisture. Here the leaves, as the plantain, &c., turn their broad sides up to the sun, and court his rays.

31. Why there is more rain on one side of a mountain than on the other.-We may now, from what has been said, see why the Andes, and all other mountains which run north and south, have a dry and a rainy side, and how the prevailing winds of the latitude determine which is the rainy and which the dry side.

Thus, let us take the southern coast of Chili for illustration. In our summer time, when the sun comes north, and drags after him his belts of perpetual winds and calms, that coast is left within the regions of the northwest winds-the winds that are counter to the southeast trades--which, cooled by the winter temperature of the high lands of Chili, deposit their moisture copiously. During the rest of the year, the most of Chili is in the region of the southeast trades, and the same causes which operate in California to prevent rain there operate in Chili; only the dry season in one place is the rainy season of the other.

Hence we see that the weather side of all such mountains as the Andes is the wet side and the lee side the dry.

32. The same phenomenon, from a like cause, is repeated in inter-tropical India, only it that country each side of the mountain is made alternately the wet and the dry side by a change in the prevailing direction of the wind. Plate XV shows India to be in one of the monsoon regions; it is the most famous of them all. From October to April the northeast trades prevail. They evaporate from the Bay of Bengal water enough to feed with rains during this season, the western shores of this bay and the Ghauts range of mountains. This range holds the relation to these winds that the Andes of Peru (§ 29) hold to the southeast trades; it first cools and then relieves them of their moisture, and they tumble down on the western slopes of the Ghauts, Peruvian-like, (§ 31,) cool, rainless, and dry; wherefore that narrow strip of country between the Ghauts and the Arabian Sea would, like that in Peru between the Andes and the Pacific, remain without rain forever, were it not for other agents which are at work about India and not about Peru. The work of the agents to which I allude is felt in the monsoons, and these prevail in India and not in Peru.

33. After the northeast trades have blown out their season, which in India ends in April, (§ 32,) the great arid plains of Central Asia, of Tartary, Thibet, and Mongolia, become heated up, react upon these northeast trades, turn them back, and convert them, during the summer and early autumn, into southwest monsoons. These then come from the Indian Ocean and Sea of Arabia loaded with moisture, and striking with it perpendicularly upon the Ghauts, precipitate upon that narrow strip of land between this range and the Arabian Sea an amount of water that is truly astonishing. Here, then, are not only the conditions for causing more rain, now on the west, now on the east side of this mountain range, but the conditions also for the most copious precipitation. Accordingly, when we come to consult rain gauges, and to ask meteorological observers in India about the fall of rain, they tell us that on the western slopes of the Ghauts it sometimes reaches the enormous depth of twelve or fifteen inches in one day.*

These winds then continue their course to the Himalaya range as dry winds. In crossing his range, they are subjected to a lower temperature than that to which they were exposed crossing the Ghauts. Here they drop more of their moisture in the shape of snow and rain, and then pass over into the thirsty lands beyond with scarcely enough vapor in them to make even a cloud. Thence they ascend into the upper air, there to become counter-currents in the general system of atmospherical circulation. By studying Plate XV, where the rainless

regions and inland basins, as well as the course of the prevailing winds, are shown, these facts will become obvious.

34. The Regions of Greatest Precipitation.-We shall now be enabled to determine, if the views which I have been endeavoring to present be correct, what parts of the earth are subject to the greatest fall of rain. They should be on the slopes of those mountains which the trade-winds first strike, after having blown across the greatest tract of ocean. The more abrupt the elevation, and the shorter the distance between the mountain top and the ocean, the greater the amount of precipitation.

If, therefore, we commence at the parallel of about 30o north in the Pacific, where the northeast trade-winds first strike that ocean, and trace them through their circuits till they first strike high mountains, we ought to find such a place of heavy rains.

Commencing at this parallel of 30o, therefore, in the north Pacific, and tracing thence the course of the northeast trade-winds, we shall find that they blow thence, and reach the region of equatorial calms near the Caroline Islands. Here they rise up; but, instead of pursuing the same course in the upper stratum of winds through the southern hemisphere, they, in consequence of the rotation of the earth, are made to take a southeast course. They keep in this upper stratum until they reach the calms of Capricorn, between the parallels of 30o and 40o; after which they become the prevailing northwest winds of the southern hemisphere, which correspond to the southwest of the northern. Continuing on to the southeast, they are now the surface winds; they are going from warmer to cooler latitudes; they become as the wet sponge, (§ 22) and are abruptly intercepted by the Andes of Patagonia, whose cold summit found the astonishing fall of water here of nearly thirteen feet (one hundred and fifty-one inches) in forty-one days; and Mr. Darwin reports that the sea water along this part of the South American coast is sometimes quite fresh from the vast quantity of rain that falls.

We ought to expect a corresponding rainy region to be found to the north of Oregon; but there the mountains are not so high, the obstruction to the southwest winds is not so abrupt the high lands are further from the coast, and the air which these winds carry in their circulation; to that part of the coast, though it be as heavily charged with moisture as at Patagonia, has a greater extent of country over which to deposit its rain, and consequently the fall to the square inch will not be as great.*

In like manner, we should be enabled to say in what part of the world the most equable climates are to be found. They are to be found in the equatorial calms, where the northeast and southeast trades meet fresh from the ocean, and keep the temperature uniform under a canopy of perpetual clouds.

35. Amount of Evaporation.-The mean annual fall of rain on the entire surface of the earth is estimated at about five feet.

To evaporate water enough annually from the ocean to cover the earth, on the average, five feet deep with rain; to transport it from one zone to another; and to precipitate it in the right places, at suitable times, and in the proportions due, is one of the offices of the grand atmospherical machine. This water is evaporated principally from the torrid zone. Supposing it all come thence, we shall have, encircling the earth, a belt of ocean three thousand miles in breadth, from which this atmosphere evaporates a layer of water annually sixteen feet in depth. And to hoist up as high as the clouds, and lower down again all the water in a lake sixteen feet deep, and three thousand miles broad, and twenty-four thousand long, is the yearly business of this invisible machinery. What a powerful engine is this atmosphere! and how nicely adjusted must be all the cogs, and wheels, and springs, and pinions of this exquisite piece of machinery, that it never wears out or break down, nor fails to do its work at the right time and in the right way.

36. In his annual report to the society, (Transactions of the Bombay Geographical Society, from May, 1849, to August, 1850, vol. ix,) Dr. Buist, the secretary, states, on the authority of Mr. Laidly, that the evaporation at Calcutta is "about fifteen feet annually; that between the Cape and Calcutta it averages, in October and November, nearly three-fourths of an inch daily; between 10o and 20o in the Bay of Bengal, it was found to exceed an inch daily. Supposing this to be double the average throughout the year, we should," continues the doctor, "have eighteen feet of evaporation annually."

If, in considering the direct observations upon the daily rate of evaporation in India, it be remembered that the seasons there are divided into wet and dry; that in the dry season, evaporation in the Indian Ocean, because of its high temperature, and also of the high temperature and dry state of the wind, probably goes on as rapidly as it does anywhere else in the world; if, moreover, we remember that the regular trade-wind regions proper are, for the most part, rainless regions at sea; that evaporation is going on from them all the year round, we shall have reason to consider the estimate of sixteen feet annually for the trade-wind surface of the ocean not too high.

37. We see the light beginning to break upon us, for we now begin to perceive why it is that the proportions between the land and water were made as we find them in nature. If there had been more water and less land, we should have had more rain, and vice versa; and then climates would have been different form what they now are, and the inhabitants, animal or vegetable, would not have been as they are. And as they are, that wise Being who, in his kind providence, so watches over and regards the thing of this world that he takes notice of the sparrow's fall, and numbers in the very hairs of our head, doubtless designed them to be.

The mind is delighted, and the imagination charmed, by contemplating the physical arrangements of the earth from such points of view as this is which we now have before us; from it the sea, and the air, and the land, appear each as a part of that grand machinery upon which the well-being of all the inhabitants of earth, sea, and air depends; and which, in the beautiful adaptations that we are pointing out, affords new and striking evidence that they all have their origin in ONE omniscient idea, just as the different parts of a watch may be considered to have been constructed and arranged according to one human design.

In some parts of the world the precipitation is greater than the evaporation; thus the amount of water borne down by every river that runs into the sea may be considered as the excess of the precipitation over the evaporation that takes place in the valley drained by that river.

This excess comes from the sea; the winds convey it to the interior; and the forces of gravity, dashing it along in mountain torrents or gentle streams, hurry it back to the sea again.

In other parts of the world the evaporation and precipitation are exactly equal, as in those inland basins such as that in which the City of Mexico, Lake Titicaca, the Caspian Sea, &c., &c., are situated, which basins have no ocean drainage.

If more rain fell in the valley of the Caspian Sea than is evaporated from it, that sea would finally get full and overflow the whole of that great basin. If less fell than is evaporated from it again, then that sea, in the course of time, would dry up, and plants and animals there would all perish for the want of water.

In the sheets of water which we find distributed over that and every other inhabitable inland basins we see reservoirs or evaporating surfaces just sufficient for the supply of that degree of moisture which is best adapted to the well-being of the plants and animals that people such basins.

In other parts of the earth still, we find places, as the Desert of Sahara, in which neither evaporation nor precipitation takes place, and in which we find neither plant nor animal.

38. ADAPTATIONS.-In contemplating the system of terrestrial adaptations, these researches teach one to regard the mountain ranges and the great deserts of the earth as the astronomer does the counterpoises to his telescope; though they may be mere dead weights, they are, nevertheless, necessary to make the balance complete-the adjustments of this machine perfect. These counterpoises give ease to the motions, stability to the performance, and accuracy to the workings of the instrument. They are compensations.

Whenever I turn to contemplate the works of nature, I am struck with the admirable system of compensation, with the beauty and nicety with which every department is poised by the others; things and principles are meted out in directions the most opposite, but in proportions so exactly balanced and nicely adjusted, that results the most harmonious are produced.

It is by the action of opposite and compensating forces that the earth is kept in its orbit, and the stars are held suspended in the azure vault of heaven; and these forces are so exquisitely adjusted, that, at the end of a thousand years, the earth, the sun, and moon, and every star in the firmament, is found to come to its proper place at the proper moment.

Nay, philosophy teaches us, when the little snow drop, which in our garden walks we see raising its beautiful head to remind us that spring is at hand, was created, that the whole mass of the earth from pole to pole, and from circumference to centre, must have been taken into account and weighed, in order that the proper degree of strength might be given to the fibres of even this little plant.

Botanists tell us that the constitution of this plant is such as to require that at a certain stage of its growth the stalk should bend, and the flower should bow its head, that an operation may take place, which is necessary in order that the herb should produce seed after its kind; and that after this its vegetable health requires that it should lift its head again and stand erect. Now if the mass of the earth had been greater or less, the force of gravity would have been different; in that case the strength of fibre in the snow drop, as it is, would have been too much or too little; the plant could not bow or raise its head at the right time, fecundation could not take place, and its family would have become extinct with the first individual that was planted, because its "seed" would not have been "in itself," and therefore it could not reproduce itself.

Now if we see such perfect adaptation, such exquisite adjustment, in the case of one of the smallest flowers of the field, how much more may we not expect "compensation" in the atmosphere and the ocean, upon the right adjustment and due performance of which depends not only the life of that plant, but the well-being of every individual that is found in the entire vegetable and animal kingdoms of the world?

When the east winds blow along the Atlantic coast for a little while, they bring us air saturated with moisture from the Gulf Stream, and we complain of the sultry, oppressive, heavy atmosphere; the invalid grows worse, and the well man feels ill, because when he takes this atmosphere into his lungs it is already so charged with moisture that it cannot take up and carry off that which encumbers his lungs, and which nature has caused his blood to bring and leave there, that respiration may take up and carry off. At other times the air is dry and hot; he feels that it is conveying off matter from the lungs too fast; he realizes the idea that it is consuming him, and he calls the sensation parching.

39. Therefore, in considering the general laws which govern the physical agents of the universe, and regulate them in the due performance of their offices, I have felt myself constrained to set out with the assumption that if the atmosphere had had a greater or less capacity for moisture; or if the proportion of land and water had been different-if the earth, air, and water, had not been in exact counterpoise-the whole arrangement of the animal and vegetable kingdoms would have varied from their present state. But God chose to make those kingdoms what they are; for this purpose it was necessary, in his judgment, to establish the proportions between the land and water, and the desert, just as they are, and to make the capacity of the air to circulate heat and moisture just what it is, and to have it to do all its work in obedience to law and in subservience to order. If it were not so why was power given to the winds to lift up and transport moisture, or the property given to the sea by which its waters may become first vapor and then fruitful showers or gentle dews? If the proportions and properties of land, sea, and air, were not adjusted according to the reciprocal capacities of all to perform the functions required by each, why should we be told that he "measured the waters in the hollow of his hand, and comprehended the dust in a measure, and weighed the mountains in scales, and the hills in a balance?" Why did he span the heavens, but that he might mete out the atmosphere in exact proportion to all the rest, and impart to it those properties and powers which it was necessary for it to have, in order that it might perform all those offices and duties for which he designed it?

Harmonious in their action, the air and sea are obedient to law and subject to order in all their movements; when we consult them in the performance of their offices they teach us lessons concerning the wonders of the deep, the mysteries of the sky, the greatness and the wisdom, and goodness of the Creator. The investigations into the broad-spreading circle of phenomena connected with the winds of heaven and the waves of the sea are second to none for the good which they do, and the lessons which they teach. The astronomer is said to see the hand of God in the sky; but does not the right-minded mariner, who looks aloft as he ponders over these things, hear his voice in every wave of the sea that "claps its hands," and feel his presence in every breeze that blows?

    ° Vide "Maury's Physical Geography of the Sea;" Harper and Brothers, New York.

    ° Eccl., i. 6.

    ° Keith Johnston.

    ° I have, through the kindness of A. Holbrook, Esq., United States Attorney for Oregon, received the Oregon Spectator of February 13, 1861, containing the Rev. G. H. Atkinson's Meteorological Journal, kept in Oregon City during the month of January, 1851. The quantity of rain and snow for that month is 13.63 inches, or about one third the average quantity that falls at Washington city during the year.