110 THE PHYSICAL GEOGRAPHY OF THE SEA. 



east trades, ascended, and then flowed indiscriminately to the 

 north or the south. 



But I saw reasons for supposing that what came to the equato- 

 rial 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 south- 

 ern 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 : 



195. At the seasons of the year when the sun is evaporating 

 most rapidly in the southern hemisphere, 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. 



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 corrobo- 

 rate the suggestion as to the crossing of the trade-winds at the 

 equatorial calms. 



196. Independently of other sources of information, my investi- 

 gations also taught me to believe that the mean temperature of the 

 tropical regions was higher in the northern than in the southern 

 hemisphere, for they show that the difference is such as to draw 

 the equatorial edge of the southeast trades far over on this side of 

 the equator, and to give them force enough to keep the northeast 

 trade-winds out of the southern hemisphere almost entirely. 



197. Consequently, as before stated, the southeast trade-winds 

 being in contact with a more extended evaporating surface, and 

 continuing in contact with it for a longer time or through a 

 greater distance, they would probably arrive at the trade-wind 

 place of meeting more heavily laden with moisture than the others. 



198. Taking the laws and rates of evaporation into considera- 

 tion, I could find 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. 



Hence, by this process of reasoning, I was induced to regard 

 the extra-tropical regions of the northern hemisphere as standing 



