§ 355. EASTING OF THE TRADE-WINDS, ETC. 1()1 



spheres, we might imagine that the constituents of the atmosphere 

 in them would, in the course of ages, probably become different 

 also, and that consequently, in such a case, man could not safely 

 pass from one hemisphere to the other. Consider the manifold 

 beauties in the whole system of terrestrial adaptations ; remember 

 what a perfect and wonderful machine (§ 261) 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. Wherefore I was led to ask myself why the 

 air of the southeast trades, when arrived at the zone of equatorial 

 calms, should not, after ascending, rather return to the south than 

 go on to the north ? Where and what is the agency by which its 

 course is decided ? Here I found circumstances which again in- 

 duced 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 northeast trades, ascended, and then flowed indiscriminately 

 to the north or the south. 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 con- 

 tinued over into the southern hemisphere as an upper current, 

 bound for the calm zone of Capricorn. And these are the prin- 

 cipal reasons and conjectures upon which these suppositions were 

 based : 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 tlie northern. Therefore it is fair to sup- 

 pose that much of the vapor which is taken up on that side of the 

 equator is precipitated on this. 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 cross- 

 ing of the trade-winds at the equatorial calms. Taking the laws 

 and rates of evaporation into consideration, I could find (Chapter 

 V.) no part of the ocean of the northern hemisphere from which 

 the sources of the Mississippi, the St. Lawrence, and the other 

 great rivers of our hemisphere could be supplied. A regular se- 



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