MAGNETISM AND CIRCULATION OF THE ATMOSPHERE. 109 



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 success- 

 ively. 



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 atmos- 

 phere 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 conse- 

 quently of plants and warm-blooded animals, being so different in 

 the two hemispheres, we might imagine that the constituents of 

 the atmosphere in them w^ould, 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 ascend- 

 ing, 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 north- 



