Physical Geography of the Sea and its Meteorology/Chapter 5

by Matthew Fontaine Maury

CHAPTER V.

§ 270-303.—RAINS AND RIVERS.

270. Rivers considered as rain-gauges—the ten largest.—Rivers are the rain-gauges of nature. The volume of water annually discharged by any river into the sea expresses the total amount by which the precipitation upon the valley drained by such river exceeds the evaporation from the same valley during the year. There are but ten rivers that we shall treat as rain-gauges; and there are only ten in the world whose valleys include an area of more than 500,000 square miles. They are:

	Square miles.
The Amazon, including the Tocantines and Orinoco	2,048, 000
Mississippi	982,000
La Plata	886,000
Yenisei	785,000
Ohi	725,000
Lena	594,000
Amoor	583,000
Yang-tse-kiang	548,000
Hoang-ho	537,000
Nile	520,000

These areas are stated in round numbers, and according to the best authorities. The basin of the Amazon is usually computed at 1,512,000 square miles; but such computation excludes the Tocantines, 204,000 square miles, which joins the Amazon near its mouth, and the Orinoco, with a hydrographic area of 252,000 square miles, which, by means of the Casiquiare, is connected also with the Amazon. We think that these three rivers should all be regarded as belonging to one hydrographic basin, for a canoe may pass inland from any one to either of the others with out portage. Of these hydrographic basins, three, including an area of 3,916,000 square miles, are American; six, which contain an area of 3,772,000 square miles, belong to Asia, one to Africa, and none to Europe. The three largest rivers of Asia, the Yenisei, Obi, and Lena (2,104,000 square miles), discharge their waters into the Arctic Ocean; their outlets are beyond the reach of the commercial world; consequently they do not possess the interest which, in the minds of men generally, is attached to the rest. The three others of Asia drain 1,668,000 square miles, and run into the Pacific; while the whole American system feed with their waters and their commerce the Atlantic Ocean. These rivers, with their springs, give drink to man and beast, and nourish with their waters plants and reptiles, with fish and fowl not a few. The capacity of their basins for production and wealth is without limits. These streams are the great arteries of inland commerce. Were they to dry up, political communities would be torn asunder, the harmonies of the earth would be destroyed, and that beautiful adaptation of physical forces to terrestrial machinery, by which climates are regulated, would lose its adjustment, and the seasons would run wild, like a watch without a balance.

271. Heat required to lift vapour for these rivers.—We see these majestic streams pouring their waters into the sea, and from the sea we know those waters must come again, else the sea would be full. We know, also, that the sunbeam and the sea-breeze suck them up again; and it is curious to fancy such volumes of water as this mighty company of ten great rivers is continually marching down to the sea, taken up by the winds and the sun, and borne away again through the invisible channels of the air to the springs among the hills. This operation is perpetually going on, yet we perceive it not. It is the work of that invisible, imponderable, omnipresent, and wonderful agent called heat. This is the agent which controls both sea and air in their movements and in many of their offices. The average amount of heat daily dispensed to our planet from the source of light in the heavens is enough to melt a coating of ice completely encasing the earth with a film 1½ in. thickness. ^[1] Heat is the agent that distils for us fresh water from the sea. It pumps up out of the ocean all the water for our lakes and rivers, and gives power to the winds to transport it as vapour thence to the mountains. And though this is but a part of the work which in the terrestrial economy has been assigned to this mighty agent, we may acquire much profitable knowledge by examining its operations here in various aspects. To assist in this undertaking I have appealed to the ten greatest rivers for terms and measures in which some definite idea may be conveyed as to the magnitude of the work and the immense physico-mechanical power of this imponderable and invisible agent called heat. Calculations have been made which show that the great American lakes contain 11,000 cubic miles of water. This, according to the best computation, is twice as much as is contained in all the other fresh-water lakes, and rivers, and cisterns of the world. The Mississippi River does not, during a hundred years, discharge into the sea so large a volume of water as is at this moment contained in the great northern lakes of this continent; and yet this agent, whose works we are about to study, operating through the winds, has power annually to lift up from the sea and pour down upon the earth in grateful showers fresh water enough to fill the great American lakes at least twenty times over.

272. Rain-fall in the Mississippi Valley.—That we may be enabled the better to appreciate the power and the majesty of the thermal forces of the sun, and comprehend in detail the magnitude and grandeur of their operations, let us inquire how much rain falls annually upon the water-sheds of one of these streams, as of the Mississippi; how much is carried off by the river; how much is taken up by evaporation; and how much heat is evolved in hoisting up and letting down all this water. In another chapter we shall inquire for the springs in the sea that feed the clouds with rain for these rivers. If we had a pool of water one mile square and six inches deep to be evaporated by artificial heat, and if we wished to find out how much would be required for the purpose, we should learn from Mr. Joule's experiments that it would require about as much as is evolved in the combustion of 30,000 tons of coal. Thus we obtain (§ 271) our unit of measure to help us in the calculation; for if the number of square miles contained in the Mississippi Valley, and the number of inches of rain that fall upon it annually be given, then it will be easy to tell how many of such huge measures of heat are set free during the annual operation of condensing the rain for our hydrographic basin. And then, if we could tell how many inches of this rain-water are again taken up by evaporation, we should have the data for determining the number of these monstrous measures of heat that are employed for that operation also.

273 Its area, and the latent heat liberated during the processes of condensation there.—The area of the Mississippi Valley is said by physical geographers to embrace 982,000 square miles; and upon every square mile there is an annual average rain-fall of 40 inches. Now if we multiply 982,000 by the number of times 6 will go into 40, we shall have the number of our units of heat that are annually set free among the clouds that give rain to the Mississippi Valley. Thus the imagination is startled, and the mind overwhelmed with the announcement that the quantity of heat evolved from the vapours as they are condensed to supply the Mississippi Valley with water is as much as would be set free by the combustion of 30,000 tons of coal multiplied 6,540,000 times. Mr Joule, of Manchester, is our authority for the heating power of one pound of coal; the Army Meteorological Register, compiled by Lorin Blodget, and published by the Surgeon General's Office at Washington in 1855, is the authority on which we base our estimate as to the average annual fall of rain; and the annals of the National Observatory show, according to the observations made by Lieutenant Marr at Memphis in Tennessee the annual fall of rain there to be 49 inches, the annual evaporation 43, and the quantity of water that annually passes by in the Mississippi to be 93 cubic miles. The water required to cover to the depth of 40 inches an area of 982,000 square miles would, if collected together in one place, make a sea one mile deep, with a superficial area of 620 square miles.

274. Annual discharge of the Mississippi River.—It is estimated that the tributaries which the Mississippi River receives below Memphis increase the volume of its waters about one-eighth, so that its annual average discharge into the sea may be estimated to be about 107 cubic miles, or about one-sixth of all the rain that falls upon its water-shed. This would leave 513 cubic miles of water to be evaporated from this river-basin annually. All the coal that the present mining force of the country could raise from its coal measures in a thousand years would not, during its combustion, give out as much heat as is rendered latent annually in evaporating this water. Utterly insignificant are the sources of man's mechanical powers when compared with those employed by nature in moving machinery which brings the seasons round and preserves the harmonies of creation!

275. Physical adaptations.—The amount of heat required to reconvert these 513 cubic miles of rain-water into vapour and bear it away, had accumulated in the Mississippi Valley faster than the earth could throw it off by radiation. Its continuance there would have been inconsistent with the terrestrial economy. From this stand-point we see how the rain-drop is made to preserve the harmonies of nature, and how water from the sea is made to carry off by re-evaporation from the plains and valleys of the earth their surplusage of heat, which could not otherwise be got rid of without first disturbing the terrestrial arrangements, and producing on the land desolation and a desert. Behold now the offices of clouds and vapour—the adaptations of heat. Clouds and vapour do something more than brew storms, fetch rain, and send down thunder-bolts. The benignant vapours cool our climates in summer by rendering latent the excessive heat of the noonday sun; and they temper them in winter by rendering sensible and restoring again to the air, that self-same heat.

276. Whence come the rains for the Mississippi.—Whence came, and by what channels did they come, these cubic miles of water which the Mississippi River pours annually into the sea? The wisest of men has told us they come from the sea. Let us explore the sea for their place and the air for their channel. The Gulf of Mexico cannot furnish rain for all the Mississippi Valley. The Gulf lies within the region of the north-east trades, and these winds carry its vapours off to the westward, and deliver them in rain to the hills, and the valleys, and the rivers of Mexico and Central America. The winds that bring the rains for the upper Mississippi Valley come not from the south; they come from the direction of the Rocky Mountains, the Sierra Nevada, and the great chain that skirts the Pacific coast. It is, therefore, needless to search in the Gulf, for the rain that comes from it upon that valley is by no means sufficient to feed one half of its springs. Let us next examine the Atlantic Ocean, and include its slopes also in the investigation.

277. The north-east trades of the Atlantic supply rains only for the rivers of Central and South America.—The north-east trade-wind region of this ocean extends (§ 210) from the parallel of 3° to the equator. These winds carry their vapour before them, and, meeting the south-east trade-wind, the two form clouds which give rain not only to Central America, but they drop down, also, water in abundance for the Atrato, the Magdalena, the Orinoco, the Amazon, and all the great rivers of intertropical America; also for the Senegal, the Niger, and the Congo of Africa. So completely is the rain wrung out of these winds for these American rivers by the Andes, that they become dry and rainless after passing this barrier, and as such reach the western shores of the continent, producing there, as in Peru, a rainless region. The place in the sea whence our rivers come, and whence Europe is supplied with rains, is clearly not to be found in this part of the ocean.

278. The calm belt of Cancer furnishes little or no rain.—Between the parallels of 30° and 35° N. lies the calm belt of Cancer, a region where there is no prevailing wind (see Diagram of the winds, Plate I.). It is a belt of light airs and calms—of airs so baffling that they are often insufficient to carry off the "loom," or that stratum of air, which, being charged with vapour, covers calm seas as with a film, as if to prevent farther evaporation. This belt of the ocean can scarcely be said to furnish any vapour to the land, for a rainless country, both in Africa, and Asia, and America, lies within it.

279. The North Atlantic insufficient to supply rain for so large a portion of the earth as one-sixth of all the land.—All Europe is on the north side of this calm belt. Let us extend our search, then, to that part of the Atlantic which lies between the parallels of 35° and 60° N., to see if we have water surface enough there to supply rains for the 8½ millions of square miles that are embraced by the water-sheds under consideration. The area of this part of the Atlantic is not quite 5 millions of square miles, and it does not include more than one-thirtieth of the entire sea surface of our planet, while the water-sheds under consideration contain one-sixth part of its entire land surface. The natural proportion of land and water surface is nearly as 1 to 3. According to this ratio, the extent of sea surface required to give rain for these 8½ millions of square miles would be a little over 25, instead of a little less than 5 millions of square miles.

280. Daily rate of evaporation at sea less than on land—observations wanted.—The state of our knowledge concerning the actual amount of evaporation that is daily going on at sea has, notwithstanding the activity in the fields of physical research, been but little improved. Records as to the amount of water daily evaporated from a plate or dish on shore afford us no means of judging as to what is going on even in the same latitude at sea. Sea-water is salt, and does not throw off its vapour as freely as fresh water. Moreover, the wind that blows over the evaporating dish on shore is often dry and fresh. It comes from the mountains, or over the plains where it found little or no water to drink up; therefore it reaches the observer's dish as thirsty wind, and drinks up vapour from it greedily. Now had the same dish been placed on the sea, the air would come to it over the water, drinking as it comes, and arriving already quite or nearly saturated with moisture; consequently, the observations of the amount of evaporation on shore give no idea of it at sea.

281. Rivers are gauges for the amount of effective evaporation.—There is no physical question of the day which is more worthy of attention than the amount of effective evaporation that is daily going on in the sea. By effective I mean the amount of water that, in the shape of vapour, is daily transferred from the sea to the land. The volume discharged by the rivers into the sea expresses (§ 270) that quantity; and it may be ascertained with considerable accuracy by gauging the other great rivers as I procured the Mississippi to be ganged at Memphis in 1849.

282. Importance of rain and river gauges.—The monsoons supply rains to feed the rivers of India, as the north-east and south-east trade-winds of the Atlantic supply rains to feed the rivers of Central and South America. Now rain-gauges which will give us the mean annual rain-fall on these water-sheds, and river-gauges which would give us the mean annual discharge of the principal water-courses, would afford data for an excellent determination as to the amount of evaporation from some parts of the ocean at least, especially for the trade-wind belts of the Atlantic and the monsoon region of the Indian Ocean. All the rain which the monsoons of India deliver to the land the rivers of India return to the sea. And if, in measuring this for the whole of India, our gauges should lead us into a probable error, amounting in volume to half the discharge of the Mississippi River, it would not make a difference in the computed rate of the effective daily evaporation from the North Indian Ocean exceeding the one two-thousandth part of an inch (0.002 in.).

283. Hypsometry in the North Atlantic peculiar.—That part of the extra-tropical North Atlantic under consideration is peculiar as to its hypsometry. It is traversed by large icebergs, which are more favourable to the recondensation of its vapours than so many islets would be. Warm waters are in the middle of it, and both the east and the west winds, which waft its vapours to the land, have, before reaching the shores, to cross currents of cool water, as the in-shore current counter to the Gulf Stream on the western side, and the cool drift from the north on the east side. In illustration of this view, and of the influence of the icebergs and cold currents of the Atlantic upon the hypsometry of that ocean, it is only necessary to refer to the North Pacific, where there are no icebergs nor marked contrasts between the temperature of its currents. Ireland and the Aleutian Islands are situated between the same parallels. On the Pacific islands there is an uninterrupted rain-fall during the entire winter. At other seasons of the year sailors describe the weather, in their log-books, there as "raining pretty much all the time." This is far from being the case even on the western coasts of Ireland, where there is a rain-fall of only 47 ^[2] inches—probably not more than a third of what Oonalaska receives. And simply for this reason: the winds reach Ireland after they have been robbed (partially) of the vapours by the cool temperatures of the ice-bergs and cold currents which lie in their way; whereas, such being absent from the North Pacific, they arrive at the islands there literally reeking with moisture. Oregon in America, and France on the Bay of Biscay, are between the same parallels of latitude; their situation with regard both to wind and sea is the same, for each has an ocean to windward. Yet their annual rain-fall is, for Oregon,^[3] 65 inches, for France, 30. None of the islands which curtain the shores of Europe are visited as abundantly by rains as are those in the same latitudes which curtain our north-west coast. The American water-shed receives about twice as much rain as the European. How shall we account for this difference, except upon the supposition that the winds from the Pacific carry (§ 171) more rain than the winds from the Atlantic? Why should they do this, except for the icebergs and cool streaks already alluded to?^[4]

284. Limited capacity of winds to take up and transport, for the rivers of Europe and America, vapour from the North Atlantic.—It may well be doubted whether the south-westerly winds—which are the prevailing winds in this part of the Atlantic—carry into the interior of Europe much more moisture than they bring with them into the Atlantic. They enter it with a mean annual temperature not far from 60°, and with an average dew-point of about 55°. They leave it at a mean temperature varying from 60° to 40°, according to the latitude in which they reach the shore, and consequently with an average dew-point not higher than the mean temperature. Classifying the winds of this part of the ocean according to the halves of the horizon as east and west, the mean of 44,999 observations in the log-books of the National Observatory shows that, on the average, the west winds blow annually 230 and the east winds 122 days.

285. The vapour-strings for all these rivers not in the Atlantic Ocean.—Taking all these facts and circumstances into consideration, and without pretending to determine how much of the water which the rivers of America and Europe carry into this part of the ocean comes from it again, we may with confidence assume that the winds do not get vapour enough from this part of the ocean to give rain to Europe, to the Mississippi Valley, to our Atlantic slopes, and the western half of Asiatic Russia. We have authority for this conclusion, just as we have authority to say that the evaporation from the Mediterranean is greater in amount than the volume of water discharged into it again by the rivers and the rains; only in this case the reverse takes place, for the rivers empty more water into the Atlantic than the winds carry from it. This fact also is confirmed by the hydrometer, for it shows that the water of the North Atlantic is, parallel for parallel, lighter than water in the Southern Ocean.

286. The places in the sea whence come the rivers of the north, discovered—proves the crossing at the calm belts.—The inference, then, from all this is, that the place in the sea (§ 276) whence come the waters of the Mississippi and other great rivers of the northern hemisphere is to be found in these southern oceans, and the channels by which they come are to be searched out aloft, in the upper currents of the air. Thus we bring evidence and facts which seem to call for a crossing of air at the calm belts, as represented by the diagram of the winds, Plate I. It remains for those who deny that there is any such crossing—who also deny that extra-tropical rivers of the northern are fed by rains condensed from vapours taken up in the southern hemisphere—to show whence come the hundreds of cubic miles of water which these rivers annually pour into the Atlantic and the Arctic Oceans. In finding the place" of all this water, it is incumbent upon them to show us the winds which bring it also, and to point out its channels.

287. Spirit in which the search for truth should be conducted.—"In the greater number of physical investigations some hypothesis is requisite, in the first instance, to aid the imperfection of our senses; and when the phenomena of nature accord with the assumption, we are justified in believing it to be a general law." ^[5]

288. The number of known facts that are reconciled by the theory of a crossing at the calm belts.—In this spirit this hypothesis has been made. Without any evidence bearing upon the subject, it would be as philosophical to maintain that there is no crossing at the calm belts as it would be to hold that there is; but nature suggests in several instances that there must be a crossing. (1.) In the homogeneousness of the atmosphere (§ 237). The vegetable kingdom takes from it the impurities with which respiration and combustion are continually loading it; and in the winter, when the vegetable energies of the northern hemisphere are asleep, they are in full play in the southern hemisphere. And is it consistent with the spirit of true philosophy to deny the existence, because we may not comprehend the nature, of a contrivance in the machinery of the universe which guides the impure air that proceeds from our chimneys and the nostrils of all air-breathing creatures in our winter over into the other hemisphere for re-elaboration, and which conducts across the calm places and over into this that which has been replenished from the plains and sylvas of the south? (2.) Most rain, notwithstanding there is most water in the southern hemisphere, falls in this. How can vapour thence come to us except the winds bring it, and how can the winds fetch it except by crossing the calm places? (3.) The "sea-dust" of the southern hemisphere, as Ehrenberg calls the red fogs of the Atlantic, has its locus on the other side of the equator, but it is found on the wings of the winds in the North Atlantic Ocean. If this be so, it must cross one or more of the calm belts.^[6] (4.) Parallel for parallel, the southern hemisphere from the equator to 40° or 45° S., is the cooler. This fact is consistent with the supposition that the heat that is rendered latent and abstracted from that hemisphere by its vapours is set free by their condensation in this. Upon no other hypothesis than by these supposed crossings can this fact he reconciled, for the amount of heat annually received from the sun by the two hemispheres is, as astronomers have shown, precisely the same.^[7]

(5.) Well-conducted observations made with the hydrometer^[8] (§ 285) for every parallel of latitude in the Atlantic Ocean from 40° S. to 40° N., show that, parallel for parallel, and notwithstanding the difference of temperature, the specific gravity of sea-water is greater in the southern than it is in the northern hemisphere. This difference as to the average condition of the sea on different sides of the line is reconciled by the hypothesis which requires a crossing at the calm belts. The vapour which conveys fresh water and caloric from the southern hemisphere to the northern will in part account for this difference both of specific gravity and temperature, and no other hypothesis will. This hydrometric difference indicates the amount of fresh water which, as vapour in the air, as streams on the land, and as currents in the sea,^[9] is constantly in transitu between the two hemispheres. All these facts are inconsistent with the supposition that there is no crossing at the calm belts, and consistent with the hypothesis that there is. It is no argument against the hypothesis that assumes a crossing, to urge our ignorance of any agent with power to conduct the air across the calm belts. It would be as reasonable to deny the red to the rose or the blush to the peach, because we do not comprehend the processes by which the colouring matter is collected and given to the fruit or flower, instead of the wood or leaves of the plant. To assume that the direction of the air is, after it enters the calm belts, left to chance, would be inconsistent with our notions of the attributes of the great Architect. The planets have their orbits, tho stars their course, and the wind "his circuits." And in the construction of our hypotheses, it is pleasant to build them up on the premiss that He can and has contrived all the machinery necessary for guiding every atom of air in the atmosphere through its channels and according to its circuits, as truly and as surely as He has contrived it for holding comets to their courses and binding the stars in their places. These circumstances and others favouring this hypothesis as to these air-crossings, are presented in further detail in Chaps. VII., IX., XII., and XII., also §349.

289. The atmosphere to he studied like any other machinery, by its operations.—In observing the workings and studying the offices of the various parts of the physical machinery which keeps the world in order, we should ever remember that it is all made for its purposes, that it was planned according to design, and arranged so as to make the world as we behold it:—a place for the habitation of man. Upon no other hypothesis can the student expect to gain profitable knowledge concerning the physics of sea, earth, or air. Regarding these elements of the old philosophers as parts only of the same piece of machinery, we are struck with the fact, and disposed to inquire why is it that the proportion of land and water in the northern hemisphere is very different from the proportion that obtains 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. Is there no connection between the machinery of the two hemispheres? Are they not adapted to each other? Or, in studying the physical geography of our planet, shall we regard the two hemispheres as separated from each other by an impassable barrier? Rather let us regard them as made for each other, as adapted to each other, the one as an essential to the other, and both as parts of the same machine. So regarding them, we observe that all the great rivers in the world are in the northern hemisphere, where there is less ocean to supply them. Whence, then, are their resources replenished? Those of the Amazon are, as we have seen (§ 277), supplied with rain 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, it is a river without periodic stages of a very marked character. 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 made of vapour that is taken up 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 entitled to be called great that we know of.

290. Arguments furnished by the rivers.—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 us, 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. How is it, then, that this vapour gets, as stated, 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 these air-crossings—these 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 wonderful engine which we are contemplating is pumping up the water there (§ 208) with the greatest activity, and sending it over here for our rivers. The heat which this heavy evaporation absorbs becomes latent, and, with the moisture, is carried through the upper regions of the atmosphere until it reaches our climates. Here the vapour is formed into clouds, condensed, and precipitated. The heat which held this water in the state of vapour is set free, it becomes sensible heat, and it is that [(4), § 288] which contributes so much to temper our winter climate. It clouds up in winter, turns warm, and we say we are going to have fallen weather. That is because the process of condensation has already commenced, 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. If Plate I. fairly represent the course of the winds, the south-east 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, and in the crossing of high mountain ranges, such as the Cordilleras of South America.

291. More rain in the northern than in the southern hemisphere.—The South Seas, then (§ 290), 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—the rain-gauge also. 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. The observations of mariners are also corroborative of the same. Log-books, containing altogether the records for upwards of 260,000 days in the Atlantic Ocean north and south (Plate XIII.), have been carefully examined for the purpose of ascertaining, for comparison, the number of calms, rains, and gales therein recorded for each hemisphere. Proportionally the number of each as given is decidedly greater for the north than it is for the south. The result of this examination is very instructive, for it shows the status of the atmosphere to be much more unstable in the northern hemisphere, with its excess of land, than in the southern, with its excess of land. Rains, and fogs, and thunder, and calms, and storms, all occur much more frequently, and are more irregular also as to the time and place of their occurrence on the north side, than they are on the other side of the equator. 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 south-east trade-winds, travels in the upper regions of the atmosphere (§ 213) until it reaches the calms of Cancer; here it becomes the surface wind that 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 here we can quote Scripture again: "The north wind driveth away rain." This is a meteorological fact of high authority, and one of great significance too.

292. The trade-winds the evaporating winds.—By, reasoning in this manner and from such facts, we are forced 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.

293. The saltest part of the sea.—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 portion 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. There, too, the rains fall less frequently (Plate XIII.). Dr. Ruschenberger, of the Navy, on his last voyage to India, was kind enough to conduct a series of observations on the specific gravity of sea-water. In about the parallel of 17° north and south—midmay 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 steam-ship Golden Age, found the heaviest water about the parallels of 20° north and 17° south. Captain Rodgers, in the United States ship Vincennes, found the heaviest water in 17° north, and between 20° and 25° south.

294. Seeing that the southern hemisphere affords the largest evaporating surface, how, unless there he a crossing, could we have most rain and the great rivers in the northern?—In summing up the evidence in favour of this view of the general system of atmospherical circulation, it remains to be shown how it is, if the view is correct, there should be smaller rivers and less rain in the southern hemisphere. The winds that are to blow as polar the north-east trade-winds, returning from the regions, where the moisture (§ 292) 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 north-east 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 comparatively but a small portion of evaporating surface exposed to their action. The zone of the north-east trades extends, on an average, from about 29° north to 7° 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 120° of longitude by 22° of latitude, and comprise an area of about twelve and a half millions of square miles, thus leaving an evaporating surface of about twenty-five millions of square miles in the northern against about seventy-five millions in the southern hemisphere. According to the hypothesis, illustrated by Plate I., as to the circulation of the atmosphere, it is these north-east trade-winds that take up and carry over, after they rise up in the belt of equatorial calms, the yapours 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 northern 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 (§ 292) that observation gives. In like manner, the south-east trade-winds take up the vapours which make our rivers, and as they prevail to a much greater extent at sea, and have exposed to their action about twice as much ocean as the north-east 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 XIII. will show how very much larger that part of the ocean over which the south-east trades prevail is than that where the north-east trade-winds blow. 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 south-east trades on one side, and of high 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 our data.

295. 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 7° north and 12° north; sometimes higher; in March and April, between latitude 5° south and 2° north.^[10] 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 about five times more in the winter than in the summer months. The winter there is the summer of the southern hemisphere, when this steam-engine (§ 24) is working with the greatest pressure. The vapour that is taken up by the south-east trades is borne along over the region of north-east trades to latitude 35° or 40° north, where it descends and appears on the surface with the south-west winds of those latitudes. Driving upon the highlands of the continent, this vapour is condensed and precipitated, during this part of the year, almost in constant showers, and to the depth of about thirty inches in three months.

296. The rainy seasons of California and Panama.—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; and they of our hemisphere are nearer the equator in the winter and spring months than at any other season. The south-west 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 cannot condense the vapours 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 north-east trades along with him; and now, at places where, six months before, the south-west winds were the prevailing winds, the north-east trades are found to blow. This is the case in the latitude of California. The prevailing winds, then, instead of going from a warmer to a cooler climate, as before, are going the opposite way. Consequently, if, under these circumstances, they have the moisture in them to make rains of, they cannot precipitate it. Proof, if proof were wanting that the prevailing winds in the latitude of California are from the westward, is obvious to all who cross the Rocky Mountains or ascend the Sierra Madre. In the pass south of the Great Salt Lake basin those west winds have worn away the hills and polished the rock by their ceaseless abrasion and the scouring effects of the driving sand. Those who have crossed this pass are astonished at the force of the wind and the marks there exhibited of its geological agencies. Panama is in the region of equatorial calms. This belt of calms travels during the year, back and forth, over about 17° of latitude, coming farther north in the summer, where it tarries for several months, and then returning 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^[11] 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 north-cast 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 north-east trades farther 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 4° 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 south-east 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.

297. The Rainless Regions.—The coast of Peru is within the region of perpetual south-east 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 south-east trade-winds in the Atlantic Ocean first strike the water on the coast of Africa. Travelling to the north-west, they blow obliquely across the ocean till they reach the coast of Brazil. By this time they are heavily laden with vapour, 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, and with no temperature colder than that to which they were subjected on the mountain-tops, they reach the ocean before they again become charged with fresh vapour, and before, therefore, they have any which the Peruvian climate can extract. The last they had to spare was deposited as snow on the tops of the Cordilleras, to feed mountain streams under the heat of the sun, and irrigate the valleys on the western slopes. 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 coast 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 XIII. to see how the winds blow, and where the sources are (§ 276) which supply them with vapours. 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 Plato 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 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 north-east tradewind 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 they could take up a supply of vapour. Most of New Holland lies within the south-east trade-wind region; so does most of intertropical South America. But intertropical 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 vapour; thus that thirsty land is so stinted with showers that the trees cannot afford to spread their leaves out to the sun, for it evaporates all the moisture from them; their vegetable instincts teach them to turn their edges to his rays. In inter-tropical South America the trade-winds blow perpendicularly upon the shore, penetrating the very heart of the country with their moisture. Here the leaves, measuring many feet square—as the plantain, &c.—turn their broad sides up to the sun, and court his rays.

298. The rainy side of mountains.—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 lie athwart the course of the winds 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 the belts of perpetual winds and calms, that coast is left within the regions of the north-west winds—the winds that are counter to the south-east trades—which, cooled by the winter temperature of the highlands of Chili, deposit their moisture copiously. During the rest of the year, the most of Chili is in the region of the south-east 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. The same phenomenon, from a like cause, is repeated in intertropical India, only in 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 VIII. shows India to be in one of the monsoon regions: it is the most famous of them all. From October to April the north-east 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 (§ 297) hold to the south-east trades; it first cools and then relieves them of their moisture, and they tumble down on the western slopes of the (Ghauts, Peruvian-like, 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 for ever, 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. After the north-east trades have blown out their season, which in India ends in April, the great arid plains of Central Asia, of Tartary, Tibet, and Mongolia become heated up; they rarefy the air of the north-east trades, and cause it to ascend. This rarefaction and ascent, by their demand for an indraught, are felt by the air which the south-cast trade-winds bring to the equatorial Doldrums of the Indian Ocean: it rushes over into the northern hemisphere to supply the upward draught from the heated plains as the south-west monsoons. The forces of diurnal rotation assist (§113) to give these winds their westing. Thus the south-east trades, in certain parts of the Indian Ocean, are converted, during the summer and early autumn, into south-west 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.^[12] Were the Andes stretched along the eastern instead of the western coast of America, we should have an amount of precipitation on their eastern slopes that would be truly astonishing; for the water which the Amazon and the other majestic streams of South America return to the ocean would still be precipitated between the sea-shore and the crest of these mountains. These winds of India then continue their course to the Himalaya range as high winds. In crossing this range, they are subjected to a lower temperature than that to which they were exposed in 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 vapour 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 XIII., where the rainless regions and inland basins, as well as the course of the prevailing winds, are shown, these facts will become obvious.

299. The regions of greatest precipitation—Cherraponjie and Patagonia.—We shall now be enabled to determine, if the views which I have been endeavouring 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 or monsoons first strike after having "blown across an extensive tract of ocean. The more abrupt the elevation, and the shorter the distance between the mountain top and the ocean (§ 298), the greater the amount of precipitation. If, therefore, we commence at the parallel of about 30° north in the Pacific, where the north-east trade-winds first strike that ocean, and trace them through their circuits till they first meet high land, we ought to find such a place of heavy rains. Commencing at this parallel of 30°, therefore, in the North Pacific, and tracing thence the course of the north-east 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 (§ 207), are made to take a south-east course. They keep in this upper stratum until they reach the calms of Capricorn, between the parallels of 30° and 40°, after which they become the prevailing north-west winds of the southern hemisphere, which correspond to the south-west of the northern. Continuing on to the south-east, they are now the surface winds; they are going from warmer to cooler latitudes; they become as the wet sponge (§ 292), and are abruptly intercepted by the Andes of Patagonia, whose cold summit compresses them, and with its low dew-point squeezes the water out of them. Captain King 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 surface water of the sea along this part of the South American coast is sometimes quite fresh, from the vast quantity of rain that falls. A similar rain-fall occurs on the sides of Cherraponjie, a mountain in India. Colonel Sykes reports a fall there during the south-west monsoons of 605¼ inches. This is at the rate of 86 feet during the year; but King's Patagonia rain-fall is at the rate of 114 feet during the same period. Cherraponjie is not so near the coast as the Patagonia range, and the monsoons lose moisture before they reach it. 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 south-west winds is not so abrupt, the highlands are farther 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 north-east and south-east trades meet fresh from the ocean, and keep the temperature uniform under a canopy of perpetual clouds.

300. Amount of evaporation greatest from the Indian Ocean.—The mean annual fall of lain 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. All this evaporation, however, does not take place from the sea, for the water that falls on the land is re-evaporated from the land again and again. But in the first instance it is evaporated principally from the torrid zone. Supposing it all to be evaporated thence, we shall have, encircling the earth, a belt of ocean three thousand miles in breadth, from which this atmosphere hoists up 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 the atmosphere and how nicely adjusted must be all the cogs, and wheels, and springs, and compensations of this exquisite piece of machinery, that it never wears out nor breaks down, nor fails to do its work at the right time and in the right way. The abstract logs at the Observatory in Washington show that the water of the Indian Ocean is warmer than that of any other sea; therefore it may be inferred that the evaporation from it is also greater. The North Indian Ocean contains about 4,500,000 square miles, while its Asiatic water-shed contains an area of 2,500,000. Supposing all the rivers of this water-shed to discharge annually into the sea four times as much water as the Mississippi (§ 274) discharges into the Gulf, we shall have annually on the average an effective evaporation (§ 282) from the North Indian Ocean of 6.O inches, or 0.0165 per day.

301. The rivers of India, and the measure of the effective evaporation from that ocean.—The rivers of India are fed by the monsoons, which have to do their work of distributing their moisture in about three months. Thus we obtain 0.065 inch as the average daily rate of effective (§ 282) evaporation from the warm waters of this ocean. If it were all rained down upon India, it would give it a drainage which would require rivers having sixteen times the capacity of the Mississippi to discharge. Nevertheless, the evaporation from the North Indian Ocean required for such a flood is only one-sixteenth of an inch daily throughout the year.^[13] Availing myself of the best lights—dim at best—as to the total amount of evaporation that annually takes place in the trade-wind region generally at sea, I estimate that it does not exceed four feet.

302. Physical adjustments.—We see the light breaking in 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 from what they are now, and the inhabitants, neither animal nor vegetable, would not have been as they are. And as they are, that wise Being who, kind providence, so watches over and regards the things of this world that he takes note of the sparrow's fall, and numbers 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 endeavouring; to point 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 earth the precipitation is greater than the evaporation: thus the amount of water borne down by every river that runs into the sea (§ 270) may be considered as the excess of the precipitation over the evaporation that takes place in the valley drained by that river. In other parts of the earth 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, etc., etc., 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 basin, 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 to fit the land for man's use.

303. Adaptations—their beauties and sublimity.—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 be mere dead weights, they are, nevertheless, necessary to make the balance complete, the adjustment of his 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 adjusted, adapted, and regulated according to the others; things and principles are meted out in directions apparently the most opposite, but in proportions so exactly balanced 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 and twinkle in its proper place at the proper moment. Nay, philosophy teaches us that when the little snowdrop—which in our garden walks we see raising its head at "the singing of birds," to remind us that "the winter is passed and gone"—was created, 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 its tiny fibres. 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 fecundation, 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 snowdrop, 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 there fore could not have reproduced itself, and its creation would have been a failure. Now, if we see such a 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 burning. Therefore, in considering the general laws which govern the physical agents of the universe, and which 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, for reasons which man may never know, 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, and to feed the plants with nourishment? or why was the property given to the sea by which its waters may become first vapour, 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 of 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 manifold and marvellous 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, which make us wiser and better men. The investigations into the broad-spreading 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 for 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?

1. Deduced from the experiments of Pouillet.
2. Keith Johnson
3. Army Meteorological Register, 1855.
4. Keith Johnson, "Physical Atlas."
5. Mrs. Somerville.
6. After this had been written, I received from my colleague, Lieut. Andrau, an account of the following little tell-tale upon this subject:— "I found a confirmation of your theory in a piece of vegetable substance caught in a small sack (hoisted up above the tops) between 22°-25° lat. N., and 38°-39°long. W. This piece is of the following dimensions:—14 millim. long. 1 to 1½ mm. large, $\scriptstyle {\frac {1}{5}}$ mm. thick, and weighing 1½ milligrams. Our famous microscopist and naturahst, Professor P. Harting, at Utrecht, told me, after an exact inquiry, 'that this vegetable fragment issued from a leaf of the family Monocotyledon, probably not from a palm-tree, but from a Padanacege or Scitaminese—consequently, from trees belonging to the tropical regions. Now I am sure it comes from the tropics. I am greatly surprised to perceive that a piece of leaf of this dimension could run off a distance of more than 1200 geographical miles in the upper regions of the atmosphere; for the nearest coast-lines of the two continents, America and Africa, lay at the said distance from the place where this vegetable fragment was caught, by the carefulness of Capt. S. Stapert, one of the most zealous co-operators. There can be no doubt that it comes from South America, because the direction of the trade-winds on the west coast of Africa is too northerly to bring this fragment to the finding-place in 20° N. and 38° W."—Letter from Lieut. Andrau, of the Dutch Navy, dated Utrecht, Jan. 2, 1860.
7. The amount of solar heat annually impressed upon the two hemispheres is identically the same; yet within certain latitudes the southern hemisphere is, paralled for parallel, the cooler. How does it become so? If it be the cooler by radiation, then it must be made so by radiating more heat than it receives; such a process would be cumulative in its effects, and were it so, the southern hemisphere would be gradually growing cooler. There is no evidence that it is so growing, and the inference that it is seems inadmissible. In fact, the southern hemisphere radiates less heat than the northern, though it receives as much from the sun. And it radiates more, for this reason: there is more land in the northern—land is a better radiator than water—therefore the northern radiates more heat than the southern hemisphere; the southern has more water and more clouds—clouds prevent radiation—therefore the southern hemisphere radiates less heat than the northern; still it is the cooler. How is this paradox to be reconciled but upon the supposition that the southern surplusage is stowed away in vapours, transported thence across the calm belts by the winds, and liberated by precipitation on our side of the equator?
8. Rodgers, in the Vincennes. Maury's Sailing Directions, 8th ed., vol. i., p. 235.
9. The water which the rivers empty into the North Atlantic has to find its way south with the currents of the sea.
10. See the Trade-wind Chart.
11. Vide Trade-wind Chart (Maury's Wind and Current).
12. Keith Johnston.
13. In his annual report of the Society {Transactions of the Bombay Geographical Society from May, 1849, to August, 1850, vol. ix.), the late Dr. Buist, the secretary, stated, on the authority of Mr. Laidly, the evaporation at Calcutta to be "about fifteen feet annually; that between the Cape and Calcutta it averages, in October and November, nearly three-fourths of an inch daily; between 10° and 20° 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." All the heat received by the intertropical seas from the sun annually would not be sufficient to convert into vapour a layer of water from them sixteen feet deep. It is these observations as to the rate of evaporation on shore that have led (§ 280) to such extravagant estimates as to the rate at sea.