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SCIENCE. 



[Vol. XIX. No. 486 



body of air moves to the north or south near the surface, it soon 

 acquires an eastern or western component of motion ; and if the 

 same body of air returns as the upper strata, the eastern or westera 

 component of motion is reversed. 



If the temperature of the entire body of atmosphere from pole 

 to pole and from top to bottom, were the same, it is believed 

 there would be no motion whatever of the air. Heat, therefore, 

 or difiference in the temperature, is the prime factor in the gen- 

 eration of air currents. It is maintained by some writers, how- 

 ever, that the rotation of the earth upon its axis, from the west to 

 the east, would propagate a current of air, in the opposite direc- 

 tion, at or near the equator. But this has not been demonstrated 

 to the satisfaction of all. So also it is believed that as aqueous 

 vapor gives buoyancy to the air, this might produce some gentle 

 breezes. As the sun is the source of all heat, we must look to 

 that luminary as the prodiicer of all winds and air currents. If 

 the earth's surface were all water, oral! land with a homogeneous 

 topography, the air currents of the earth would remain almost 

 constant and uniform. This continuity and uniformity are broken 

 up and interfered with by the various divisions of continents, 

 oceans, mountains, sandy plains, etc., and also by the variations 

 of the temperature at the same locality at different periods in the 

 year. 



As the earth revolves upon its axis, one-half of its surface is 

 always in the sunlight and the other half at the same time in 

 darkness. The temperature of the former therefore is always 

 higher than that of the latter ; and the atmosphere tends to flow 

 from the one to the other at the surface, and in the upper strata 

 to flow in the opposite direction. This is in obedience to the 

 known laws of dynamics: 1. That heated air will rise, and the 

 vacuum which this tends to produce will be filled up by cold air 

 flowing in at the bottom. 2. As gravity tends to keep the air in 

 equilibrium, when a current of air is observed to be moving in any 

 direction, either horizontally or perpendicularly, a like current 

 will somewhere be found flowing in the opposite direction, to re- 

 store the equilibrium. 



Let us liken the two hemispheres of air (one in the light and the 

 other in the dark) to two great thin metallic cups or bowls, each 

 enveloping one-half of the earth, their edges touching each other 

 and coinciding with a great circle or meridian of longitude. Let 

 them be pivoted at the poles, so that they may slide around the 

 earth, one following the other. The one in the sunlight we will 

 paint white, and the other black ; and they follow each other as 

 the earth revolves, the white being always turned towards the sun. 

 Again, as the sun always shines perpendicularly to the earth's 

 surface between the tropics, we find here a broad belt abnormally 

 heated ; and we will represent this heat on the white bowl by a 

 broad band of bright red, and on the black bowl by a similar band 

 of dark brown. Further, in examining the air along the edges of 

 the two bowls, we find but little difference in the temperature of 

 the two, the air growing colder as we recede from the edge towards 

 the centre of the black, and warmer in the direction of the centre 

 of the white bowl. We will therefore shade these edges, at first 

 both nearly alike, but gliding into a dark gray, and then black 

 towards one, and a light gray and then white towards the centre 

 of the other. Still further, as the sun is annually perpendicular 

 at each of the tropics, so the two poles will alternately be in light 

 and shade — heat and cold. We must therefore give some slight 

 shading to these parts of the bowls, to represent the different 

 amounts of sun energy employed to heat the earth at these 

 points. 



We have now before us, therefore, most of the factors that enter 

 into the production of air currents, or the circulation of the at- 

 mosphere. If the air could be seen with the naked eye, as we 

 have here colored and shaded these two enveloping bowls, and if 

 we could stand upon the moon or some inter-stellar planet and 

 look upon the earth, what a grand and magnificent kaleidoscopic 

 panorama would be presented to our view ! 



The theory of the general circulation of the atmosphere most 

 generally accepted is shown in the following extract, taken from 

 Buffon's "Natural History," which he quotes approvingly from 

 Maclaren: "The unequal distribution of heat over the surface of 

 the land and water necessarily disturbs the equilibrium of the 



atmosphere, and produces currents of air, or winds. These cur- 

 rents, however various, have been supposed to result from two 

 general movements, pervading the whole mass of the atmosphere. 

 The heavy and cold air of the temperate regions, having a ten- 

 dency to displace the warm and rarified air of the torrid zone, 

 generates a current in each atmosphere" (hemisphere?) "towards 

 the equator. To replace the air abstracted from the higher lati- 

 tudes, an upper and counter current flows back from the equator 

 to the pole; and thus the atmosphere, while it performs a constant 

 revolution, tempers the extremes of climate, by transporting the 

 cold of the frigid zone to the equator, and carrying back the heat 

 of the equator to the frigid zone." A writer on the article" Winds" 

 in "Chambers's EncyclopEedia" says: " When the part of the earth's 

 surface which is heated is a whole zone, as in the case of the 

 tropics, a surface wind will set in towards the heated tropical 

 zone from both sides, and uniting will ascend, and, there sepa- 

 rating, will flow as upper currents in opposite directions. Hence 

 a surface current will flow from the higher latitudes towards the 

 equator, and an upper current towards the poles." Professor 

 William Ferrel, author of a "Popular Treatise on the Winds," 

 published in 1889, practically adopts the above views. This is an 

 elaborate work, containing about 500 pages, and is considered one 

 of the best authorities on the subjects treated. In Chapter III., 

 from page 89 to 162, he gives a detailed and graphic account of 

 the "general circulation of the atmosphere." He frequently refers 

 to the upper strata flowing " from the equator to the poles," and 

 the surface currents flowing " from the poles to the equator," etc. 

 On page 154 he gives a "graphic summary," as follows : "In the 

 preceding part of this chapter it has been shown that if all parts 

 of the atmosphere had the same temperature there would be a 

 complete calm over all parts of the earth's surface. But that in 

 consequence of the difference of temperature between the equa- 

 torial and polar regions of the globe, and the consequent tempera- 

 ture gradient, there arise pressure gradients and forces, which 

 give rise to and maintain a vertical circulation of the atmosphere, 

 with a motion of the air of the upper strata of the atmosphere 

 from the equator towards the poles, and a counter current in the 

 lower part from the poles toward the equator, as represented by 

 the arrows in the following figure, and that this of course requires 

 a gradual settling down of the air from the higher to the lower 

 strata in the middle and higher latitudes, and the reverse in the 

 lower latitudes. It has also been shown that, in case the earth 

 had no rotation on its axis, this would be exclusively a vertical 

 circulation in the planes of the meridians, without any east or west 

 components of motion in any part; but that, in consequence of 

 the deflecting forces arising from the earth's rotation, the atmos- 

 phere at the earth's surface has also an east component of motion 

 in the middle and higher latitudes, and the reverse in the lower 

 latitudes; and that the velocities of the east components increase 

 with increase of elevation, so that, at great altitudes, they become 

 very much greater than those at the earth's surface ; while those 

 of the west components decrease with increase of altitude up to a 

 certain altitude, where they vanish and change signs and become 

 east velocities, now increasing with increase of altitude to the top 

 of the atmosphere." 



Now the foregoing theory seems to me not to be sustained by 

 the facts. It may seem presumptuous in a layman to question 

 the conclusions of such great and confessed authorities; but if I 

 am to follow what seems to me to be the truth, I must dissent. 

 I have a profound admiration for the untiring labors and great 

 researches of Professor Ferrel; he has placed the world under 

 many obligations for his valuable suggestions ; yet I fear he has 

 fallen into the same error which has characterized the reasoning 

 of all his predecessors. This is not strange or unexpected ; for we 

 all know that sometimes grave errors will for generations run 

 through the writings of the most astute and learned men, unde- 

 tected. Witness, for example. Lord Bacon's "Wisdom of the 

 Ancients," where his "explanations" need explaining, and are 

 more abstruse and muddled than the mythology of the Greeks 

 and Romans. I do not know that what I may here offer as the 

 true theory of the circulation of the atmosphere is new to the sci- 

 entific world, as I have not kept full pace with all the new dis- 

 coveries. I know, however, that it is not recognized by Professor 



