130 THE PHYSICAL GEOGRAPHY OF THE SEA. 



rent, after descending, continues on in the direction toward which 

 it was traveling before it descended, we may go farther, and, hy 

 a similar train of circumstantial evidence, afforded by these re- 

 searches 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 tlience, each current toward the pole which it was approaching 

 while on the surface. 



317. 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 cases in which positive proof can not be adduced, it is permit- 

 ted to bring in circumstantial evidence. 



318. I am endeavoring, let it be borne in mind, to show cause 

 for the conjecture that the magnetism of the oxygen of the atmos- 

 phere is concerned in conducting the air which has blown as the 

 southeast trade-winds — and after it has arrived at the belt of equa- 

 torial calms and risen up — over into the northern hemisphere, and 

 so on through its channels of circulation, as traced on Plate I. 



319. But, in order to show reasonable grounds for this conjec- 

 ture, I want to establish, by circumstantial evidence and such in- 

 direct 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, C, D, 

 and E successively. 



320. 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. 



321. If it did not, the proportion of land and water, and con- 

 sequently 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. 



