V 
MAGNETISM AND CIRCULATION OF THE ATMOSPHERE. US 
It rains more in the valley drained by that river than is evapo- 
rated from it again. The difference for a year is the volume of 
water annually discharged by that river into the sea (§ 117). 
At the time and place that the vapor which supplies this im- 
mense volume of water was lifted by the atmosphere up from the 
sea, the thermometer, we may infer, stood higher than it did at 
the time and place where this vapor was condensed and fell down 
as rain in the Mississippi Valley. 
210. I looked to the south for the springs in the Atlantic which 
supply the fountains of this river with rain. But I could not find 
spare evaporating surface enough for it, in the first place ; and if 
the vapor, I could not find the winds which would convey it to the 
right place. 
The prevailing winds in the Caribbean Sea and southern parts 
of the Gulf of Mexico are the northeast trade-winds. They have 
their offices to perform in the river basins of tropical America, and 
the rains which they may discharge into the Mississippi Valley 
now and then are exceptions, not the rule. 
211. The winds from the north can not bring vapors from the 
great lakes to make rains for the Mississippi, for two reasons : 
1st. The basin of the great lakes receives from the atmosphere 
more water in the shape of rain than they give back in the shape 
of vapor. The St. Lawrence River carries off the excess. 2d. 
The mean climate of the lake country is colder than that of the 
Mississippi Valley, and therefore, as a general rule, the tempera- 
ture of the Mississippi Valley is unfavorable for condensing vapor 
from that quarter. 
212. It can not come from the Atlantic, because the greater 
part of the Mississippi Valley is to the windward of the Atlantic. 
The winds that blow across this ocean go to Europe with their 
vapors ; and in the Pacific, from the parallels of California 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 expres- 
sion be too strong, with something like apparent plausibility, that 
the rain winds of the Mississippi Valley do not, as a general rule, 
get their vapors from the North Atlantic Ocean, nor from the Gulf 
of Mexico, nor from the great lakes, nor from that part of the Pa- 
cific Ocean over which the northeast trade-winds prevail. 
