# Physical Geography Of The Sea 1855/5

CHAPTER V. — ON THE PROBABLE RELATION BETWEEN MAGNETISM AND THE CIRCULATION OF THE ATMOSPHERE.
Reasons for supposing that the Air of the Northeast and of the Southeast Trades cross at the calm Belts, § 174. — What Observations have shown, 184. — Physical Agencies not left to Chance, 188. — Conjectures, 192. — Reasons for supposing that there is a crossing of Trade-wind Air at the Equator, 194. — Why the extra-tropical Regions of the Northern Hemisphere are likened to the Condenser of a Steam boiler in the South, 199. — Illustration, 200. — A Coincidence, 202. — Proof, 203. Nature affords nothing in contradiction to the supposed System of Circulation, 204. Objections answered, 205. — Why the Air brought to the Equator by the Northeast Trades will not readily mix with that brought by the Southeast, 207. — Additional Evidence, 209. — Rains for the Mississippi River are not supplied from the Atlantic, 210. — Traced to the South Pacific, 213. — Anticipation of Light from the Polar Regions, 216. — Received from the Microscope of Ehrenberg, 217, and the Experiments of Faraday, 219. — More Light, 221. — Why there should be a calm Place near each Pole, 222. — Why the Whirlwinds of the North should revolve against the Sun, 223. — Why certain Countries should have scanty Rains, 228. — Magnetism the Agent that causes the Atmospherical Crossings at the calm Places, 231.
172. OXYGEN, philosophers say, comprises one fifth part of the atmosphere, and Faraday has discovered that it is magnetic. This discovery presents itself to the mind as a great physical fact, which is perhaps to serve as the keystone for some of the grand and beautiful structures which philosophy is building up for monuments to the genius of the age.
173. Certain facts and deductions elicited in the course of these investigations had directed my mind to the workings in the atmosphere of some agent, as to whose character and nature I was ignorant. Heat, and the diurnal rotation of the earth on its axis, were not, it appeared to me, sufficient to account for all the currents of both sea and air which investigation was bringing to light.
174. For instance, there was reason to suppose that there is a crossing of winds at the three calm belts; that is, that the southeast trade-winds, when they arrive at the belt of equatorial calms and ascend, cross over and continue their course as an upper current to the calms of Cancer, while the air that the northeast trade.
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winds discharge into the equatorial calm belt continues to go south, as an upper current bound for the calms of Capricorn. But what should cause this wind to cross over? Why should there not be a general mingling in this calm belt of the air brought by the two trade-winds, and why should not that which the southeast winds convey there be left, after its ascent, to flow off either to the north or to the south, as chance directs?
175. In the first place, it was at variance with my belief in the grand design; for I could not bring myself to believe that the operations of such an important machine as the atmosphere should be left to chance, even for a moment. Yet I knew of no agent which should guide the wind across these calm belts, and lead it out always on the side opposite to that on which it entered; nevertheless, certain circumstances seemed to indicate that such a crossing does take place.
176. Evidence in favor of it seemed to be afforded by this circumstance, viz., our researches enabled us to trace from the belt of calms, near the tropic of Cancer, which extends entirely across the seas, an efflux of air both to the north and to the south; from the south side of this belt the air flows in a never-ceasing breeze, called the northeast trade-winds, toward the equator. (PLATE I.) On the north side of it, the prevailing winds come from it also, but they go toward the northeast. They are the well-known southwesterly winds which prevail along the route from this country to England, in the ratio of two to one. But why should we suppose a crossing to take place here?
177. We suppose so, because these last-named winds are going from a warmer to a colder climate; and therefore it may be inferred that nature exacts from them what we know she exacts from the air under similar circumstances, but on a smaller scale, before our eyes, viz., more precipitation than evaporation.
178. But where, it may be asked, does the vapor which these winds carry along, for the replenishing of the whole extra-tropical regions of the north, come from? They did not get it as they came along in the upper regions, a counter-current to the northeast trades. They did not get it from the surface of the sea in the calm belt of Cancer, for they did not tarry long enough there to become saturated with moisture. Thus circumstances again pointed to the southeast trade-wind regions as the place of supply.
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179. Moreover, these researches afforded grounds for the supposition that the air of which the northeast trade-winds are composed, and which comes out of the same zone of calms as do these southwesterly winds, so far from being saturated with vapor at its exodus, is dry; for near their polar edge, the northeast trade winds are, for the most part, dry winds. Reason suggests, and philosophy teaches, that, going from a lower to a higher temperature, the evaporating powers of these winds are increased; that they have to travel, in their oblique course toward the equator, a distance of nearly three thousand miles; that, as a general rule, they evaporate all the time, and all the way, and precipitate little or none on their route; investigations have proved that they are not saturated with moisture until they have arrived fully up to the regions of equatorial calms, a zone of constant precipitation. This calm zone of Cancer borders also, it was perceived, upon a rainy region.
180. Where does the vapor which here, on the northern edge of this zone of Cancer, is condensed into rains, come from?and where, also — was the oft-repeated question — does the vapor which is condensed into rains for the extra-tropical regions of the north generally come from? By what agency is it conveyed across this calm belt from its birth-place between the tropics?
181. I know of no law of nature or rule of philosophy which would forbid the supposition that the air which has been brought along as the northeast trade-winds to the equatorial calms does, after ascending there, return by the counter and upper currents to the calm zone of Cancer, here descend and reappear on the surface as the northeast trade-winds again. I know of no agent in nature which would prevent it from taking this circuit, nor do I know of any which would compel it to take this circuit; but while I know of no agent in nature that would prevent it from taking this circuit, I know, on the other hand, of circumstances which rendered it probable that such, in general, is not the course of atmospherical circulation — that it does not take this circuit. I speak of the rule, not of the exceptions; these are infinite, and, for the most part, are caused by the land.
182. And I moreover knew of facts which go to strengthen the supposition that the winds which have come in the upper
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regions of the atmosphere from the equator, do not, after arriving at the calms of Cancer, and descending, return to the equator on the surface, but that they continue on the surface toward the pole. But why should they? What agent in nature is there that can compel these, rather than any other winds, to take such a circuit?
183. The following are some of the facts and circumstances which give strength to the supposition that these winds do continue from the calm belt of Cancer toward the pole as the prevailing southwesterly winds of the extra-tropical north: We have seen (PLATE I.) that, on the north side of this calm zone of Cancer, the prevailing winds on the surface are from this zone toward the pole, and that these winds return as A through the upper regions from the pole; that, arriving at the calms of Cancer, this upper current A meets another upper current G from the equator, where they neutralize each other, produce a calm, descend, and come out as surface winds, viz., A as B, or the trade winds; and G as H, or the variable winds.
184. Now observations have shown that the winds represented by H are rain winds; those represented by B, dry winds; and it is evident that A could not bring any vapors to these calms to serve for H to make rains of; for the winds represented by A have already performed the circuit of surface winds as far as the pole, during which journey they parted with all their moisture, and, returning through the upper regions of the air to the calm belt of Cancer, they arrived there as dry winds. The winds represented by B are dry winds; therefore it was supposed that these are but a continuation of the winds A.
185. On the other hand, if the winds A, after descending, do turn about and become the surface winds H, they would first have to remain a long time in contact with the sea, in order to be supplied with vapor enough to feed the great rivers, and supply the rains for the whole earth between us and the north pole. In this case, we should have an evaporating region on the north as well as on the south side of this zone of Cancer; but investigation shows no such region; I speak exclusively of the ocean.
186. Hence it was inferred that A and G do come out on the surface as represented by PLATE I. But what is the agent that should lead them out by such opposite paths?
187. According to this mode of reasoning, the vapors which
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supply the rains for H would be taken up in the southeast trade wind region by F, and conveyed thence by G, and delivered to H. And if this mode of reasoning be admitted as plausible — if it be true that G have the vapor which, by condensation, is to water with showers the extra-tropical regions of the northern hemisphere, Nature, we may be sure, has provided a guide for conducting G across this belt of calms, and for sending it on in the right way. Here it was, then, at this crossing of the winds, that I thought I first saw the foot-prints of an agent whose character I could not comprehend. Could it be the magnetism that resides in the oxygen of the air?
188. Heat and cold, the early and the latter rain, clouds and sunshine, are not, we may rely upon it, distributed over the earth by chance; they are distributed in obedience to laws that are as certain and as sure in their operations as the seasons in their rounds. If it depended upon chance whether the dry air should come out on this side or on that of this calm belt, or whether the moist air should return or not whence it came — if such were the case in nature, we perceive that, so far from any regularity as to seasons, we should have, or might have, years of droughts the most excessive, and then again seasons of rains the most destructive; but, so far from this, we find for each place a mean annual proportion of both, and that so regulated withal, that year after year the quantity is preserved with remarkable regularity.
189. Having thus shown that there is no reason for supposing that the upper currents of air, when they meet over the calms of Cancer and Capricorn, are turned back to the equator, but having shown that there is reason for supposing that the air of each current, after descending, continues on in the direction toward which it was traveling before it descended, we may go farther, and, by a similar train of circumstantial evidence, afforded by these researches and other sources of information, show that the air, kept in motion on the surface by the two systems of trade-winds, when it arrives at the belt of equatorial calms, and ascends, continues on thence, each current toward the pole which it was approaching while on the surface.
190. In a problem like this, demonstration in the positive way is difficult, if not impossible. We must rely for our proof upon philosophical deduction, guided by the lights of reason; and in all
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cases in which positive proof can not be adduced, it is permitted to bring in circumstantial evidence. I am endeavoring, let it be borne in mind, to show cause for the conjecture that the magnetism of the oxygen of the atmosphere is concerned in conducting the air which has blown as the southeast trade-winds, and after it has arrived at the belt of equatorial calms and risen up, over into the northern hemisphere, and so on through its channels of circulation, as traced on PLATE I. But, in order to show reasonable grounds for this conjecture, I want to establish, by circumstantial evidence and such indirect proof as my investigations afford, that such is the course of the “wind in his circuits,” and that the winds represented by F, PLATE I., do become those represented by G, H, A, B, and C successively.
191. In the first place, F represents the southeast trade-winds i.e., all the winds of the southern hemisphere as they approach the equator; and is there any reason for supposing that the atmosphere does not pass freely from one hemisphere to another? On the contrary, many reasons present themselves for supposing that it does.
192. If it did not, the proportion of land and water, and consequently of plants and warm-blooded animals, being so different in the two hemispheres, we might imagine that the constituents of the atmosphere in them would, in the course of ages, probably become different, and that consequently, in such a case, man could not safely pass from one hemisphere to the other.
193. Consider the manifold beauties in the whole system of terrestrial adaptations; remember what a perfect and wonderful machine (§ 118) is this atmosphere; how exquisitely balanced and beautifully compensated it is in all its parts. We know that it is perfect; that in the performance of its various offices it is never left to the guidance of chance — no, not for a moment. Therefore I was led to ask myself why the air of the southeast trades, when arrived at the zone of equatorial calms, should not, after ascending, rather return to the south than go on to the north. Where and what is the agency by which its course is decided?
194. Here I found circumstances which again induced me to suppose it probable that it neither turned back to the south nor mingled with the air which came from the regions of the northeast
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trades, ascended, and then flowed indiscriminately to the north or the south. But I saw reasons for supposing that what came to the equatorial calms as the southeast trade-winds continued to the north as an upper current, and that what had come to the same zone as northeast trade-winds ascended and continued over into the southern hemisphere as an upper current, bound for the calm zone of Capricorn. And these are the principal reasons and conjectures upon which these suppositions were based
195. At the seasons of the year when the sun is evaporating most rapidly in the southern hemisphere, the most rain is falling in the northern. Therefore it is fair to suppose that much of the vapor which is taken up on that side of the equator is precipitated on this. The evaporating surface in the southern hemisphere is greater, much greater, than it is in the northern; still, all the great rivers are in the northern hemisphere, the Amazon being regarded as common to both; and this fact, as far as it goes, tends to corroborate the suggestion as to the crossing of the trade-winds at the equatorial calms.
196. Independently of other sources of information, my investigations also taught me to believe that the mean temperature of the tropical regions was higher in the northern than in the southern hemisphere, for they show that the difference is such as to draw the equatorial edge of the southeast trades far over on this side of the equator, and to give them force enough to keep the northeast trade-winds out of the southern hemisphere almost entirely.
197. Consequently, as before stated, the southeast trade-winds being in contact with a more extended evaporating surface, and continuing in contact with it for a longer time or through a greater distance, theywould probably arrive at the trade-wind place of meeting more heavily laden with moisture than the others.
198. Taking the laws and rates of evaporation into consideration, I could find no part of the ocean of the northern hemisphere from which the sources of the Mississippi, the St. Lawrence, and the other great rivers of our hemisphere could be supplied. Hence, by this process of reasoning, I was induced to regard the extra-tropical regions of the northern hemisphere as standing
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in the relation of a condenser to a grand steam machine (§ 120), the boiler of which is in the region of the southeast trade-winds, and to consider the trade-winds of this hemisphere as performing the like office for the regions beyond Capricorn.
199. The calm zone of Capricorn is the duplicate of that of Cancer, and the winds flow from it as they do from that, both north and south; but with this difference: that on the polar side of the Capricorn belt they prevail from the northwest instead of the southwest, and on the equatorial side from the southeast in stead of the northeast. Now if it be true that the vapor of the northeast trade-winds is condensed in the extra-tropical regions of the southern hemisphere, the following path, on account of the effect of diurnal rotation of the earth upon the course of the winds, would represent the mean circuit of a portion of the atmosphere moving according to the general system of its circulation over the Pacific Ocean, viz., coming down from the north as an upper current, and appearing on the surface of the earth in about longitude 120º west, and near the tropic of Cancer, it would here commence to blow the northeast trade-winds of that region.
200. To make this clear, see PLATE VII., on which I have marked the course of such vapor-bearing winds; A being a breadth or swath of winds in the northeast trades; B, the same wind as the upper and counter-current to the southeast trades; and C, the same wind after it has descended in the calm belt of Capricorn, and come out on the polar side thereof, as the rain winds and prevailing northwest winds of the extra-tropical regions of the southern hemisphere. This, as the northeast trades, is the evaporating wind. As the northeast trade-wind, it sweeps over a great waste of waters lying between the tropic of Cancer and the equator.
201. Meeting no land in this long oblique track over the tepid waters of a tropical sea, it would, if such were its route, arrive somewhere about the meridian of 140º or 150º west, at the belt of equatorial calms, which always divides the northeast from the southeast trade-winds. Here, depositing a portion of its vapor as it ascends, it would, with the residuum, take, on account of diurnal rotation, a course in the upper region of the atmosphere to the southeast, as far as the calms of Capricorn. Here it descends
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and continues on toward the coast of South America, in the same direction, appearing now as the prevailing northwest wind of the extra-tropical regions of the southern hemisphere. Traveling on the surface from warmer to colder regions, it must, in this part of its circuit, precipitate more than it evaporates.
202. Now it is a coincidence, at least, that this is the route by which, on account of the land in the northern hemisphere, the northeast trade-winds have the fairest sweep over that ocean. This is the route by which they are longest in contact with an evaporating surface; the route by which all circumstances are most favorable to complete saturation; and this is the route by which they can pass over into the southern hemisphere most heavily laden with vapors for the extra-tropical regions of that half of the globe; and this is the supposed route which the northeast trade-winds of the Pacific take to reach the equator and to pass from it.
203. Accordingly, if this process of reasoning be good, that portion of South America between the calms of Capricorn and Cape Horn, upon the mountain ranges of which this part of the atmosphere, whose circuit I am considering as a type, first impinges, ought to be a region of copious precipitation. Now let us turn to the works on Physical Geography, and see what we can find upon this subject. In Berghaus and Johnston — department Hydrography — it is stated, on the authority of Captain King, Royal Navy, that upward of twelve feet (one hundred and fifty-three inches) of rain fell in forty-one days on that part of the coast of Patagonia which lies within the sweep of the winds just described. So much rain falls there, navigators say, that they sometimes find the water on the top of the sea fresh and sweet. After impinging upon the cold hill-tops of the Patagonian coast, and passing the snow-clad summits of the Andes, this same wind tumbles down upon the eastern slopes of the range as a dry wind; as such, it traverses the almost rainless and barren regions of Andean Patagonia and South Buenos Ayres.
204. These conditions, the direction of the prevailing winds, and the amount of precipitation, may be regarded as evidence afforded by nature, if not in favor of, certainly not against, the conjecture that such may have been the voyage of this vapor through
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the air. At any rate, here is proof of the immense quantity of vapor which these winds of the extra-tropical regions carry along with them toward the poles; and I can imagine no other place than that suggested, whence these winds could get so much vapor. I am not unaware of the theory, or of the weight attached to it, which requires precipitation to take place in the upper regions of the atmosphere on account of the cold there, irrespective of proximity to mountain tops and snow-clad hills. But the facts and conditions developed by this system of research upon the high seas are in many respects irreconcilable with that theory. With a new system of facts before me, I have, independent of all preconceived notions and opinions, set about to seek among them for explanations and reconciliations. These may not in all cases be satisfactory to every one; indeed, notwithstanding the amount of circumstantial evidence that has already been brought to show that the air which the northeast and the southeast trade-winds discharge into the belts of equatorial calms, does, in ascending, cross — that from the southern passing over into the northern, and that from the northern passing over into the southern hemisphere (see F, G, B and C, PLATE I.) — yet some have implied doubt by asking the question, “How are two such currents of air to pass each other?” And, for the want of light upon this point, the correctness of reasoning, facts, inferences, and deductions have been questioned.
205. In the first place, it may be said in reply, the belt of equatorial calms is often several hundred miles across, seldom less than sixty; whereas the depth of the volume of air that the trade-winds pour into it is only about three miles, for that is supposed to be about the height to which the trade-winds extend. Thus we have the air passing into these calms by an opening on the north side for the northeast trades, and another on the south for the southeast trades, having a cross section of three miles vertically to each opening. It then escapes by an opening upward, the cross section of which is sixty or one hundred, or even three hundred miles. A very slow motion upward there will carry off the air in that direction as fast as the two systems of trade-winds, with their motion of twenty miles an hour, can pour it in; and that curds or columns of air can readily cross each other and pass in different directions without interfering the one with the other, H or at least to that degree which obstructs or prevents, we all know.
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206. For example, open the window of a warm room in winter, and immediately there are two currents of air ready at once to set through it; viz., a current of warm air flowing out at the top, and one of cold coming in below. But the brown fields in summer afford evidence on a larger scale, and in a still more striking manner, of the fact that, in nature, columns, or streamlets, or curdles of air do readily move among each other without obstruction. That tremulous motion which we so often observe above stubble-fields, barren wastes, or above any heated surface, is caused by the ascent and descent, at one and the same time, of columns of air at different temperatures, the cool coming down, the warm going up. They do not readily commingle, for the astronomer, long after nightfall, when he turns his telescope upon the heavens, perceives and laments the unsteadiness they produce in the sky.
207. If the air brought down by the northeast trade-winds differ in temperature (and why not?) from that brought by the southeast trades, we have the authority of nature for saying that the two currents would not readily commingle. Proof is daily afforded that they would not, and there is reason to believe that the air of each current, in streaks, or patches, or curdles, does thread its way through the air of the other without difficulty. Now, if the air of these two currents differs as to magnetism, might not that be an additional reason for their not mixing, and for their taking the direction of opposite poles after ascending?
208. Therefore we may assume it as a postulate which nature concedes, that there is no difficulty as to the two currents of air, which come into those calm belts from different directions, crossing over, each in its proper direction, without mingling.
209. Thus, having shown that there is nothing to prevent the crossing of the air in these calm belts, I return to the process of reasoning by induction, and offer additional circumstantial evidence to prove that such a crossing does take place. Let us therefore catechise, on this head, the waters which the Mississippi pours into the sea, inquiring of them as to the channels among the clouds through which they were brought from the ocean to the fountains of that mighty river.
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It rains more in the valley drained by that river than is evaporated from it again. The difference for a year is the volume of water annually discharged by that river into the sea (§ 117). At the time and place that the vapor which supplies this immense volume of water was lifted by the atmosphere up from the sea, the thermometer, we may infer, stood higher than it did at the time and place where this vapor was condensed and fell down as rain in the Mississippi Valley.
210. I looked to the south for the springs in the Atlantic which supply the fountains of this river with rain. But I could not find spare evaporating surface enough for it, in the first place; and if the vapor, I could not find the winds which would convey it to the right place. The prevailing winds in the Caribbean Sea and southern parts of the Gulf of Mexico are the northeast trade-winds. They have their offices to perform in the river basins of tropical America, and the rains which they may discharge into the Mississippi Valley now and then are exceptions, not — the rule.
211. The winds from the north can not bring vapors from the great lakes to make rains for the Mississippi, for two reasons: 1st. The basin of the great lakes receives from the atmosphere more water in the shape of rain than they give back in the shape of vapor. The St. Lawrence River carries off the excess. 2d. The mean climate of the lake country is colder than that of the Mississippi Valley, and therefore, as a general rule, the temperature of the Mississippi Valley is unfavorable for condensing vapor from that quarter.
212. It can not come from the Atlantic, because the greater part of the Mississippi Valley is to the windward of the Atlantic. The winds that blow across this ocean go to Europe with their vapors; and in the Pacific, from the parallels of California down to the equator, the direction of the wind at the surface is from, not toward the basin of the Mississippi. Therefore it seemed to be established with some degree of probability, or, if that expression be too strong, with something like apparent plausibility, that the rain winds of the Mississippi Valley do not, as a general rule, get their vapors from the North Atlantic Ocean, nor from the Gulf of Mexico, nor from the great lakes, nor from that part of the Pacific Ocean over which the northeast trade-winds prevail.
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The same process of reasoning which conducted us (§ 203) into the trade-wind region of the northern hemisphere for the sources of the Patagonian rains, now invites us into the trade-wind regions of the South Pacific Ocean to look for the vapor springs of the Mississippi.
213. If the rain winds of the Mississippi Valley come from the east, then we should have reason to suppose that their vapors were taken up from the Atlantic Ocean and Gulf Stream; if the rain winds come from the south, then the vapor springs might, perhaps, be in the Gulf of Mexico; if the rain winds come from the north, then the great lakes might be supposed to feed the air with moisture for the fountains of that river; but if the rains come from the west, where, short of the great Pacific Ocean, should we look for the place of evaporation? Wondering where, I addressed a circular letter to farmers and planters of the Mississippi Valley, requesting to be informed as to the direction of their rain winds.
214. I received replies from Virginia, Mississippi, Tennessee, Missouri, Indiana, and Ohio; and they all, with the exception of one person in Missouri, said, “The southwest winds bring us our rains.”
215. These winds certainly can not get their vapors from the Rocky Mountains, nor from the Salt Lake, for they rain quite as much upon that basin as they evaporate from it again; if they did not, they would, in the process of time, have evaporated all the water there, and the lake would now be dry. These winds, that feed the sources of the Mississippi with rain, like those between the same parallels upon the ocean, are going from a higher.to a lower temperature; and these winds in the Mississippi Valley, not being in contact with the ocean, or with any other evaporating surface to supply them with moisture, must bring with them from some sea or another that which they deposit. Therefore, though it may be urged, inasmuch as the winds which brought the rains to Patagonia came direct from the sea, that they therefore took up their vapors as they came along, yet it can not be so urged in this case; and if these winds could pass with their vapors from the equatorial calms through the upper regions of the atmosphere to the calms of Cancer, and then as surface winds into the Mississippi Valley, it was not perceived
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why the Patagonian rain winds should not bring their moisture by a similar route. These last are from the northwest, from warmer to colder latitudes; therefore, being once charged with vapors, they must precipitate as they go, and take up less moisture than they deposit.
216. This was circumstantial evidence. No fact had yet been elicited to prove that the course of atmospherical circulation suggested by my investigations is the actual course in nature. It is a case in which I could yet hope for nothing more direct than such conclusions as might legitimately flow from circumstances. My friend Lieutenant De Haven was about to sail in command of the American Arctic Expedition in search of Sir John Franklin. Infusoria are sometimes found in sea-dust, rain-drops, hailstones, or snow-flakes; and if by any chance it should so turn out that the locus of any of the microscopic infusoria which might be found descending with the precipitation of the Arctic regions should be identified as belonging to the regions of the southeast trade winds, we should thus add somewhat to the strength of the many clews by which we have been seeking to enter into the chambers of the wind, and to “tell whence it cometh and whither it goeth.” It is not for man to follow the “wind in his circuits;” and all that could be hoped was, after a close examination of all the facts and circumstances which these researches upon the sea have placed within my reach, to point out that course which seemed to be most in accordance with them; and then, having established a probability, or even a possibility, as to the true course of the atmospheric circulation, to make it known, and leave it for future investigations to confirm or set aside.
217. It was at this stage of the matter that my friend Baron von Gerolt, the Prussian minister, had the kindness to place in my hand Ehrenberg’s work, “Passat-Staub und Blut-Regen.” Here I found the clue which I hoped, almost against hope, De Haven would place in my hands (§ 216). That celebrated microscopist reports that he found South American infusoria in the blood-rains and sea-dust of the Cape Verde Islands — Lyons, Genoa, and other places (§ 158). Thus confirming, as far as such evidence can, the indications of our observations, and increasing the probability that the general course of atmospherical circulation is in conformity with the suggestions
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of the facts gathered from the sea as I had interpreted them, viz., that the trade-winds of the southern hemisphere, after arriving at the belt of equatorial calms, ascend and continue in their course toward the calms of Cancer as an upper current from the southwest, and that, after passing this zone of calms, they are felt on the surface as the prevailing southwest winds of the extratropical parts of our hemisphere; and that, for the most part, they bring their moisture with them from the trade-wind regions of the opposite hemisphere.
218. I have marked on PLATE VII. the supposed track of the “Passat-Staub,” showing where it was taken up in South America, as at P, P, and where it was found, as at S, S; the part of the line in dots denoting where it was in the upper current, and the unbroken line where it was wafted by a surface current; also on the same plate is designated the part of the South Pacific in which the vapor-springs for the Mississippi rains are supposed to be. The hands ( ) point out the direction of the wind. Where the shading is light, the vapor is supposed to be carried by an upper current. Such is the character of the circumstantial evidence which induced me to suspect that some agent, whose office in the grand system of atmospherical circulation is neither understood nor recognized, was at work in these calm belts.
219. Dr. Faraday has shown that, as the temperature of oxygen is raised, its paramagnetic force diminishes, being resumed as the temperature falls again. “These properties it carries into the atmosphere, so that the latter is, in reality, a magnetic medium, ever varying, from the influence of natural circumstances, in its magnetic power. If a mass of air be cooled, it becomes more paramagnetic; if heated, it becomes less paramagnetic (or diamagnetic), as compared with the air in a mean or normal condition.”*
220. Now, is it not more than probable that here we have, in the magnetism of the atmosphere, that agent which guides the air from the south (§ 217) through the calms of Capricorn, of the equator, and of Cancer, and conducts it into the north; that agent which causes the atmosphere, with its vapors and infusoria, to above the clouds from one hemisphere into the other, and whose footprints had become so palpable?
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221. Taking up the theory of Ampere with regard to the magnetic polarity induced by an electrical current, according as it passes through wire coiled with or coiled against the sun, and expanding it in conformity with the discoveries of Faraday and the experiments of a Prussian philosopher,* we perceive a series of facts and principles which, being applied to the circulation of the atmosphere, make the conclusions to which I have been led touching these crossings in the air, and the continual “whirl” of the wind in the Arctic regions against, and in the Antarctic with the hands of a watch, very significant. In this view of the subject, we see light springing up from various sources, by which the shadows of approaching confirmation are clearly perceived. One such source of light comes from the observations of my excellent friend Quetelet, at Brussels, which show that the great electrical reservoir of the atmosphere is in the upper regions of the air. It is filled with positive electricity, which increases as the temperature diminishes.
222. May we not look, therefore, to find about the north and south magnetic poles these atmospherical nodes or calm regions which I have theoretically pointed out there? In other words, are not the magnetic poles of the earth in those atmospherical nodes, the two standing in the relation of cause and effect, the one to the other? This question was first asked several years ago† and I was then moved to propound it by the inductions of theoretical reasoning.
Observers, perhaps, will never reach those inhospitable regions with their instruments to shed light upon this subject; but Parry and Barrow have found reasons to believe in the existence of a perpetual calm about the north pole. Professor J. H. Coffin, in an elaborate and valuable paper‡ on the “WINDS OF THE NORTHERN HEMISPHERE,” arrives at a like conclusion. In that paper he has discussed the records at no less than five hundred and seventy-nine meteorological stations, embracing a totality of observations
† Lt. Maury’s Sailing Directions.
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for two thousand eight hundred and twenty-nine years. He places his “meteorological pole”— pole of the winds — near latitude 84° north, longitude 105° west. The pole of maximum cold, by another school of philosophers, Sir David Brewster among them, has been placed in latitude 800 north, longitude 1000 west; and the magnetic pole, by still another school,* in latitude 73° 35’ north, longitude 95° 39’ west.
223. Neither of these poles is a point susceptible of definite and exact position. The polar calms are no more a point than the equatorial calms are a line; and, considering that these poles are areas, not points, is it not a little curious that philosophers in different parts of the world, using different data, and following up investigation each through a separate and independent system of research, and each aiming at the solution of different problems, should nevertheless agree in assigning very nearly the same position to them all? Are these three poles grouped together by chance, or by some physical cause? By the latter, undoubtedly. Here, then, we have another of those gossamer-like clews, that sometimes seem almost palpable enough for the mind, in its happiest mood, to lay hold of, and follow up to the very portals of knowledge, where pausing to knock, we may boldly demand that the chambers of hidden things be thrown wide open, that we may see and understand the mysteries of the winds, the frost, and the trembling needle.
224. In the polar calms there is (§ 113) an ascent of air; if an ascent, a diminution of pressure and an expansion; and if expansion, a decrease of temperature. Therefore we have palpably enough a connecting link here between the polar calms and the polar place of maximum cold. Thus we establish a relation between the pole of the winds and the pole of cold, with evident indications that there is also a physical connection between these and the magnetic pole. Here the outcroppings of the relation between magnetism and the circulation of the atmosphere again appear. May we not find in such evidence as this, threads, attenuated and almost air drawn though they be when taken singly and alone, yet nevertheless proving, when brought together, to have a consistency sufficient, with the lights of reason, to guide us as we seek to trace the wind in his circuits? The winds (§ 106) approach
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these polar calms by a circular or spiral motion, traveling in the northern hemisphere against, and in the southern with the hands of a watch. The circular gales of the northern hemisphere are said also to revolve in like manner against the hands of a watch, while those in the southern hemisphere travel the other way. Now, should not this discovery of these three poles, this coincidence of revolving winds, with the other circumstances that have been brought to light, encourage us to look to the magnetism of the air for the key to these mysterious but striking coincidences? Indeed, so wide for speculation is the field presented by these discoveries, that we may in some respects regard this great globe itself, with its “cups” and spiral wires of air, earth, and water, as an immense “pile” and helix, which, being excited by the natural batteries in the sea and atmosphere of the tropics, excites in turn its oxygen, and imparts to atmospherical matter the properties of magnetism.
225. With the lights which these discoveries cast, we see (PLATE I.) why air, which has completed its circuit to the whirl * about the Antarctic regions, should then, according to the laws of magnetism, be repelled from the south, and attracted by the opposite pole toward the north. And when the southeast and the northeast trade-winds meet in the equatorial calms of the Pacific, would not these magnetic forces be sufficient to determine the course of each current, bringing the former, with its vapors of the southern hemisphere, over into this, by the courses already suggested?
226. This force and the heat of the sun would propel it to the north. The diurnal rotation of the earth propels it to the east; consequently, its course, first through the upper regions of the atmosphere, and then on the surface of the earth, after being conducted by this newly-discovered agent across the calms of Cancer, would be from the southward and westward to the northward and eastward. These are the winds (§ 122) which, on their way to the north from the South Pacific, would pass over the Mississippi Valley, and they appear (§ 214) to be the rain winds there. Whence, then, if not from the trade-wind regions of the South Pacific, can the vapors for those rains come?
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227. According to this view, and not taking into account any of the exceptions produced by the land and other circumstances upon the general circulation of the atmosphere over the ocean, the southeast trade-winds, which reach the shores of Brazil near the parallel of Rio, and which blow thence for the most part over the land, should be the winds which, in the general course of circulation, would be carried, after crossing the Andes and rising up in the belt of equatorial calms, toward Northern Africa, Spain, and the South of Europe.
They might carry with them the infusoria of Ehrenberg (§ 158), but, according to this theory, they would be wanting in moisture. Now, are not those portions of the Old World, for the most part dry countries, receiving but a small amount of precipitation?
228. Hence the general rule: those countries to the north of the calms of Cancer, which have large bodies of land situated to the southward and westward of them, in the southeast trade-wind region of the earth, should have a scanty supply of rain, and vice versa.
229. Let us try this rule: The extra-tropical part of New Holland comprises a portion of land thus situated in the southern hemisphere. Tropical India is to the northward and westward of it; and tropical India is in the northeast trade-wind region, and should give extra-tropical New Holland a slender supply of rain. But what modifications the monsoons of the Indian Ocean may make to this rule, or what effect they may have upon the rains in New Holland, my investigations in that part of the ocean have not been carried far enough for final decision; though New Holland is a dry country. Referring back to p. 79 for what has been already said concerning the “METEOROLOGICAL AGENCIES” (§ 115) of the atmosphere, it will be observed that cases are there brought forward which afford trials for this rule, every one of which holds good.
230. Thus, though it be not proved as a mathematical truth that magnetism is the power which guides the storm from right to left and from left to right, which conducts the moist and the dry air each in its appointed paths, and which regulates the “wind in his circuits,” yet that it is such a power is rendered very probable; for, under the supposition that there is such a crossing of the air at the five calm places, as PLATE, p.70, represents (§ 106), we
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can reconcile a greater number of known facts and phenomena than we can under the supposition that there is no such crossing. The rules of scientific investigation always require us, when we enter the domains of conjecture, to adopt that hypothesis by which the greatest number of known facts and phenomena may be reconciled; and therefore we are entitled to assume that this crossing does take place, and to hold fast to the theory so maintaining until it is shown not to be sound.
231. That the magnetism of the atmosphere is the agent which guides the air across the calm belts, and prevents that which enters them from escaping on the side upon which it entered, we can not, of our own knowledge, positively affirmn. Suffice it to say, that we recognize in this property of the oxygen of air an agent that, for aught we as yet know to the contrary, may serve as such a guide; and we do not know of the existence of any other agent in the atmosphere that can perform the offices which the hypothesis requires. Hence the suspicion that magnetism and electricity are among the forces concerned in the circulation of the atmosphere.