Magnetism and the Trade Winds. 281 



tropical America, and the rains which they may discharge 

 into the Mississippi valley now and then are exceptions, not 

 the rule. 



The winds from the north cannot bring vapours from the 

 great lakes to make rains for the Mississippi, for two reasons: 

 firsts the basin of the great lakes receives from the atmo- 

 sphere more water in the shape of rain than they give back 

 in the shape of vapour. The St Lawrence river carries off 

 the excess. 



Second^ the mean climate of the lake is colder than that of 

 the Mississippi valley ; and therefore, as a general rule, the 

 temperature of the Mississippi valley is unfavourable for 

 condensing vapour from that quarter. 



It cannot come from the Atlantic, because the greater part 

 of the Mississippi valley is to the windward of the Atlantic 

 ocean. The winds that blow across it go to Europe with 

 their vapours ; and in the Pacific, from the parallels of Cali- 

 fornia down to the equator, the direction of the wind at the 

 surface is from, not toward, the basin of the Mississippi. 

 Therefore it seemed to be established with some degree of 

 probability — or if that expression be too strong — with some- 

 thing like apparent plausibility, that the rain winds of the 

 Mississippi valley, as the general rule, do not get their 

 vapours from the North Atlantic ocean, nor from the Gulf 

 of Mexico, nor from the great lakes, nor from that part of 

 the Pacific ocean over which the NE. trade winds prevail. 



The same process of reasoning which induced me to look 

 into the trade-wind region of the Northern hemisphere for 

 the sources of the Patagonia rains, induced me to look in the 

 trade-wind regions of the South Pacific ocean, for the vapour 

 springs of the Mississippi. 



I therefore last summer addressed a circular letter to the 

 farmers and planters of the Mississippi valley, requesting to 

 be informed as to the direction of the rain winds of each 

 locality, and with the view of acquiring some idea as to the 

 general hygrometric condition of the atmosphere, I asked 

 also to be informed as to the kind and quality of fruits and 

 the like. 



To this I have received the following replies : — 



