Memoirs of Service Afloat, During the War Between the States

by Raphael Semmes

Why it is that the trade-winds do not extend all the way from the poles to the equator, but take their rise in about the thirtieth parallel of latitude, north and south, we do not know. The theory would seem to demand that they should spring up at the poles, and blow continuously to the equator; in which case we should have but two systems of winds covering the entire surface of the earth. This non-conformity of the winds of the extra-tropical regions to our theory, does not destroy it, however, but brings into the meteorological problem other and beautiful features. Having put the winds in motion, our next business is to follow them, and see what “circuits” they travel. The quantity of atmosphere carried to the equator by the north-east and south-east trade-winds, must find its way back whence it came, in some mode or other; otherwise, we should soon have all the atmosphere drawn away from the poles, and piled up at the equator. We can easily conceive this, if we liken the atmosphere to fleeces of wool, and suppose an invisible hand to be constantly drawing away the fleeces from the poles, and piling them up at the equator. But how to get it back is the difficulty. It cannot go back on the surface of the earth, within the tropics, for there is a constant surface current here toward the equator. There is but one other way, of course, in which it can go back, and that is, as an upper current, running counter to the surface current. We may assume, indeed, we must assume, that there are two upper currents of air, setting out from the equator, and travelling, one of them to the 30th degree of north latitude, and the other to the 30th degree of south latitude.

What becomes of these two upper currents, when they reach these parallels of latitude, is not quite so certain; but there is good reason for believing that they now descend, become surface currents, and continue their journey on to the poles. It is further supposed that, when they reach the poles, they “whirl about” them, ascend, become upper currents again, and start back to the 30th parallel; and that, when they have returned to this parallel, they descend, become a surface current again—in other words, the trade-wind—and proceed to the equator as before.

But there is another, and more beautiful problem still, connected with these winds. It is their crossing each other at the equator, of which the proofs are so abundant, that there can be but little doubt concerning it. And yet the proposition, looked at apart from the proofs, is a very startling one. One would think that when the two winds met at the equator, there would be a general intermingling, and confounding of particles, and that when they ascended to form the upper currents, of which I have spoken, the northern particle would be as likely to turn back to the north, as to cross the equator and go south. The preceding figure will illustrate the crossing. Let A represent the equator, the arrows near the surface of the circle the two trade-winds, and the two cross arrows, two particles of wind in the act of crossing. The difficulty is to conceive how these particles should cross, without mixing with each other, and losing their identity; or why they should not turn back, as well as continue their course. What law of nature is it, that makes the particles of atmosphere which have come from the north pole, so separate and distinct from those which have come from the south pole, as to prevent the two from fusing, and becoming one? Is it because the two particles, as they have gyrated around their respective poles, have received a repulsive polarity? Whatever may be the reason, there can be no doubt, as remarked, that they do actually cross. One strong proof of their crossing is, that we cannot conceive, otherwise, how the great atmospheric machine could perform its office of distributing rain over the earth in due proportions. The reader will recollect that there is from a fourth, to a third, more land than water, in the northern hemisphere, and that there is from a fourth to a third more water than land in the southern hemisphere. The consequence of this unequal distribution of land and water in the two hemispheres is, that the northern hemisphere requires more rain than the southern, in the proportion in which it has more land to be rained upon. Now it is these mysterious trade-winds, of which we have been speaking, that are the water-carriers of the two hemispheres. These winds, on their way to the equator, generally reach the 30th parallel as dry winds. These dry winds, sweeping over the tropical seas, take up, in the shape of vapor, the water with which, in due time, they are to fertilize the fields of the farmer, and make the rose blossom. The quantity which they take up is in proportion to the sea-surface, or evaporating surface, they have respectively passed over. Now, if we will examine the jars of these water-carriers, when they reach the equator, we shall find that the northern jars are not nearly so full as the southern jars; the reason being, that the northern winds have passed over less evaporating surface.

Now, if the two systems of winds, with their jars thus filled, were to turn back to their respective hemispheres, and pour down upon them their water, in the shape of rain, the consequence would be, as the reader sees, that we should have less rain in the northern hemisphere, than they would have in the southern hemisphere; whereas, we require more, having more land to be watered. The atmospheric machine would thus be at fault. But the all-wise and beneficent ruler of the universe, makes nothing faulty. We know from the evidence of that silent witness, the rain-gauge, that more water falls in the northern hemisphere, than in the southern; in other words, that the more heavily laden of those jars which we examined, a moment ago, at the equator, have come to us, instead of returning to the south; the less heavily laden jars going south. The crossing of the winds thus satisfies our theory, and nothing else can; which is, of course, the most conclusive of all proofs.

But we have other proofs. For a number of years past, as the East India ships would be returning home from their voyages, they would report a curious phenomenon to have befallen them, as they passed the parallel of the Cape de Verde. This was the falling, or rather silting down upon their decks and rigging, of a brick-dust or cinnamon-colored powder. This dust, which when rubbed between the thumb and forefinger would be impalpable, would sometimes nearly cover the entire deck and rigging. The ships would be hundreds of miles away from the land, and where could this dust come from? The fact puzzled the philosophers, but having been reported so often, it ceased to attract attention. Still it was a fact, and was laid away carefully in the archives of philosophy for future use. Years passed away, and the great traveller and philosopher, Humboldt, arose to instruct and delight mankind. He travelled extensively in South America; and, among other places, visited the lower valley of the Orinoco. He happened there in the dry season, and gives a graphic account of the wild and weird spectacle of desolation which met his eye in that season of universal drought.

All annual vegetation lay dead and desiccated on the immense pampas or plains. The earth was cracked open, gaping, as it were, for rain. The wild cattle were roaming about in herds, bellowing for their accustomed food and water; many of them perishing. Even the insect world, so numerous and vivacious in all southern climates, had perished. Their tiny little organisms lay in heaps, fast disintegrating, and being reduced to powder, by the scorching and baking rays of a perpendicular sun, between which and the parched earth, not so much as a speck of cloud appeared. The philosopher examined a number of these little organisms with his microscope. They were peculiar to the region in which he found them, and he was struck with the fact. There was another phenomenon which he observed. A number of little whirlwinds were playing their pranks about the arid waste, sporting, as it were, with dead nature. These little whirlwinds, as they travelled hither and thither, would draw up into their vortices, and toss high into the upper air, the impalpable dust that lay everywhere, and which was composed, in great measure, of the decomposed and decomposing organisms of which I have spoken. The atmosphere, at times, when filled with this dust, would assume a yellowish, or pale straw-colored hue.

The reader probably, by this time, sees my design of connecting the dusty remains, described by Humboldt, with the rain dust reported by the mariners to have fallen on the decks and rigging of their ships, in the neighborhood of the Cape de Verde islands. But the “rain-dust” was of brick-dust, or cinnamon color, when collected by the masters of the ships, as specimens, and the heavens, when filled with the dust thrown up by the whirlwinds, as described by Humboldt, appeared to him to be of a straw color. Here is a discrepancy to be reconciled, and we must call in the aid of another philosopher, Captain M. F. Maury, late Superintendent of the National Observatory, at Washington, before alluded to in these pages, and to whom I am indebted for many of the facts here quoted. Captain Maury was struck with this discrepancy, and in reconciling it with the theory here discussed, makes the following statement: “In the search for spider lines, for the diaphragms of my telescopes, I procured the finest, and best threads from a cocoon of a mud-red color; but the threads of this cocoon, as seen singly in the diaphragm, were of a golden color; there would seem, therefore, no difficulty in reconciling the difference between the colors of the rain-dust, when viewed in little piles by the microscopist, and when seen attenuated and floating in the wind by the regular traveller.”

There remains but another link in the chain of evidence, to render it complete. It remains to be shown how the whirlwind dust, of the valley of the lower Orinoco, can be identified with the rain-dust of the Cape de Verde. Ehrenberg, a German philosopher, has done this, in our day. Some specimens of the rain-dust having been sent him by ship-captains, he brought them under his microscope, as Humboldt had done the whirlwind-dust, and to his great astonishment, and delight, he found it to be the same. These facts correspond entirely with our theory of the crossing of the trade-winds at the equator. The reader has been with us near the mouth of the Orinoco. This great river disembogues near the island of Trinidad, which we visited in the Sumter, in about the latitude of 9° N. The vernal equinox is the dry season here, and at this season, the north-east trade-wind is quite fresh. Running counter to this wind, in the upper atmosphere, there is, according to our theory, a strong south-west wind blowing. Now, if the reader will inspect a map, he will find that a south-west wind, starting from the mouth of the Orinoco, will blow over the Cape de Verde islands. The rest is plain. The whirlwind-dust is tossed high enough into the upper atmosphere, to be taken in charge by the counter south-west wind, is carried to the Cape de Verde, and there silted down upon the decks and rigging of the passing ships, as gently as so many snow-flakes, becoming the rain-dust which so long puzzled the philosophers!

We have reasoned, hitherto, on the supposition, that the three calm-belts, one of which the Alabama is now passing, and the two systems of trade-winds which they enclose, are stationary within certain limits. But this is not so; the whole system of belts and winds is moved north and south, as the sun passes now into one hemisphere, and now into another. The calm-belt of Cancer is not always in the latitude of 30° N.; nor is the calm-belt of the equator always at the equator. The reader will recollect that we observed, on board the Alabama, on the 10th of November, in latitude 25° N., and that we were only just then entering the calm-belt of Cancer. The reason is, that the sun, on that day, was in the southern hemisphere, well advanced toward his extreme limit in that hemisphere, and that he had dragged, as it were, the whole system of belts and winds after him. The figures below will make this idea plain. Let the broad, dark lines in the circles represent the system of belts and winds, all in one; and in circle A let the sun be in the northern hemisphere, and in circle B let him be in the southern.