May 27, 1892.] 



SCIENCE. 



30- 



Ferrel, even by a reference; and if it had been proposed, before 

 the publication of his book in 1889, I have full confidence that he 

 would have known it, and either endorsed it or attempted to re- 

 fute it. But no lover of truth should blindly follow any leader, 

 however great; if his reasoning does not convince his understand- 

 ing, be should mark out a course for himself, if that be to him the 

 truth. 



Any theory of the physical universe, or of any of its parts, 

 which will account for the greatest number of known facts in- 

 volved, will be most satisfactory and acceptable; and such theory 

 will be held and entertained until another is discovered which will 

 account for more, or all, of the facts. 



Now any theory of the general circulation of the atmosphere 

 may be accepted tentatively, as a true theory, which will fully 

 explain and account for the following facts : — 



1. The trade- winds of the tropics. 



2. The belt of calms at the equator. 



3. The ascent of the trades to the upper strata at the equator. 



4. Their flow as upper strata toward the temperate latitudes. 



5. The belt of calms near the 30th parallel of north latitude. 



6. The high barometric pressure at this calm belt. 



7. The prevailing south-west winds in the north temperate 

 zone. 



8. The source of supply of the trades. 



9. The source of supply of the constant (or prevailing) south- 

 west winds in the north temperate latitudes. 



10. The limit of range of the return trades of the tropics and 

 the same limit of the upper strata in the north temperate lati- 

 tudes. 



11. The calm belt about the 60th parallel of north latitude, if 

 there be one, as alleged by some writers. 



12. The location of a calm at the north pole, if there be one, as 

 Professor Ferrel believes. 



Now it will require but slight reasoning to prove that a body of 

 air starting from the equator can never reach the pole. Take, 

 for example, the amount of air included between any two meridians 

 of longitude ; at the equator, the distance between these lines is 

 about or near 70 miles. This body of air, as a return trade or 

 upper current, starts moving toward the north. Concede for a 

 moment that the earth does not rotate upon its axis. This air in 

 its northern flight cannot veer to the right or left beyond these 

 lines, for there is always another body of air there on either side 

 to prevent it. As the meridians continually approach each other, 

 and at the pole unite, it will be readily seen that if the air should 

 ever reach the pole under these circumstances, it must be squeezed 

 into space where there is no space. The length of the equatorial 

 line is about 25,000 miles. The air starts all along this line to 

 move northward simultaneously; can it keep on converging, until 

 the entire 25,000 miles of air shall be forced into a space repre- 

 sented by a dot? It is physically impossible. A million cubic 

 miles of air cannot be compressed into a pint cup. Conversely, a 

 cubic foot of air can cover the north pole ; if this amount should 

 endeavor to reach the equator, it must, when it arrives there, ex- 

 pand to the extent of 25,000 miles. The theory of Professor Fer- 

 rel — the interchange between the poles and the equator — requires 

 that the air at the earth's surface in all northern latitudes should 

 move southerly with a western component of motion, similar to 

 the trades ; but the fact is, as everyone knows, and as he himself 

 admits, that in the north temperate zone the surface winds move 

 northward with an eastern component of motion; that is, from the 

 south-west. This motion is shown by the arrows in the figure on 

 page 155 of his book, referred to in the above quotation. Further- 

 more, the arrows in the same figure indicate that he would have 

 the motion of all surface winds except the trades at the tropics, 

 and all the upper strata, without exception, towards the east, 

 the surface winds moving northeast, and the upper strata moving 

 nearly east, but a little north of east ; and he endeavors to prove 

 this to be true, by elaborate reasoning and mathematical formulse, 

 extending over many pages. But this easterly motion of nearly 

 the entire atmosphere is directly opposed by another principle 

 which he recognizes on page 117, where he says: "This principle 

 was recognized by Hadley in his theory of the trade-winds, for he 

 states that all n>otions in any direction must have their counter- 



motions, else the eflfect upon the earth's surface would be to change 

 the earth's rotation upon its axis.'' If it be objected that this 

 reasoning is intended only to apply to surface winds, it may be 

 stated that Professor Ferrel, on pages 93 and 91, seems to recog- 

 nize the broad principle that all motions must have their counter- 

 motions, which he terms the "condition of continuity." It is 

 difScult to conceive how it is possible that all the upper strata, 

 from pole to pole, should move in one direction, and that direction 

 east, and so rapidly as to get ahead of the motion of the earth in 

 its rotation upon its axis. Where are the counter-currents to all 

 these easterly winds? Nowhere, except the surface trade- winds 

 within the tropics. What force, what energy is it that causes 

 all the winds to flow in one direction, and at so rapid a rate? 

 What is their source of supply ? and what is their limit of range 

 or destination ? Since their motion, as alleged, is not directly east, 

 but a little north and south, in the respective hemispheres, of an 

 east and west parallel, will they not ultimately fly off from the 

 earth at the poles? The western component of motion of the 

 trades, he says, is balanced by the corresponding eastern com- 

 ponent of the winds in the more northern latitudes, and there is 

 kept up and maintained his "condition of continuity." so far. 

 But as there are no counter-currents alleged to correspond to the 

 easterly motion of all the upper strata, it seems that his "condi- 

 tion of continuity " is thus destroyed. 



Now to follow the air in all its motions, let us begin at the be- 

 ginning of motion ; that is, at the point where the greatest amount 

 of energy is expended to produce motion, — at the equator, — and 

 follow it in its flight, if we can. Here, as everyone can readily 

 understand, the air is heated and rarified by the sun's rays, and 

 rises up to the higher altitudes. This necessitates the inflowing 

 of colder air from the north and south to prevent a vacuum. 

 When the sun's energy has raised the column of air to the limit 

 in height, it will then separate in two parts, one flowing north 

 and one south, because of the descent of the temperature gradients 

 in those directions. These motions and these causes are recog- 

 nized by all authorities (except those who attribute everything to 

 electricity), and we thus readUy account for the trade-winds, the 

 belt of calms at the equator, — the meeting of the two trades de- 

 stroying motion, ^the ascent of the trades to the upper strata, 

 and their flow towards the temperate latitudes. As the subsequent 

 motions of the atmosphere are similar in the two hemispheres, let 

 us confine our further pursuit to those of the northern. When 

 the upper stratum begins moving northward, it has acquired the 

 eastern motion of the earth near the equator, and soon coming 

 over portions of the earth with slower motion, it will get ahead of 

 the earth and veer to the north-east, — making the return trades, — 

 the cause being the reverse of that which produced the south-west 

 motion of the trade-winds. As the parallels of latitude grow 

 shorter and the meridians of longitude approach each other as we 

 go north from the equator, it is evident that this body of air will 

 soon become confined into narrower limits than it had at the 

 equator ; and it will sometime and somewhere happen that it will 

 become so heaped up and crowded that its northern motion will be 

 retarded, and finally cease entirely. When this happens, it will 

 find the line of least resistance down towards the surface, where 

 it will go to keep up the supply of the trades, and will then return 

 to the equator again. So it may be inferred that this body of air 

 will thus continue to make its eternal round in this grand cycle, 

 unless changed from i:s course by local causes, topography, un- 

 equal distribution of temperature, etc. Its most northern limit is 

 believed to be about the parallel of 30°, and its heaping up and 

 downpour will cause both a calm and a high barometric pressure 

 at this parallel. 



Turning our attention next to the atmosphere in the north tem- 

 perate latitudes, we discover that the air to the north of the above- 

 named calm belt has a north-east motion at the surface of the 

 earth. This eastern component of motion is doubtless produced 

 by the same causes that operated to give a north-east motion to 

 the upper strata of the tropics. When this body of air has 

 proceeded for some distance to the northward, it will also be con- 

 fined by the shortening of the parallels of latitude and the nearer 

 approach of the meridians of longitude; and, thus meeting with 

 resistance in front, it seeks the line of least resistance in the upper 



