MAGNETISM AND CIRCULATION OF THE ATMOSPHERE. 131 



322. Consider the manifold beauties in the whole system of 

 terrestrial adaptations ; remember what a perfect and wonderful 

 machine (§ 169) 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 ? 



323. 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- 

 east trades, ascended, and then flowed indiscriminately to the 

 north or the south. 



324. 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 : 



325. At the seasons of the year when the area covered by the 

 southeast trade-winds is large, and when they are evaporating 

 most rapidly in the southern hemisphere, even up to the equator, 

 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. 



326. 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. 



327. Independently of other sources of information, my inves- 

 tigations also taught me to believe that the mean temperature of 



