138 THE PHYSICAL GEOGRAPHY OF THE SEA. 



be an additional reason for their not mixing, and for their taking 

 the direction of opposite poles after ascending ? 



354. Therefore we may assume it as a postulate Avhich nature 

 concedes, that there is no difficulty as to the two currents of air, 

 which come into those calm belts from different directions, cross- 

 ing over, each in its proper du'ection, without mingling. 



355. Thus, having shown that there is nothing to jprevent the 

 crossing of the air in these calm belts, I return to the process of 

 reasoning by induction, and offer additional circumstantial evi- 

 dence to prove that such a crossing does take place. Let us there- 

 fore catechise, on this head, the waters which the Mississippi pours 

 into the sea, inquiring of them as to the channels among the clouds 

 through which they were brought from the ocean to the fountains 

 of that mighty river. 



356. It rains more in the valley drained by that river than is 

 evaporated from it again. The difference for a year is the vol- 

 ume of water annually discharged by that river into the sea 

 (§ 165). 



357. At the time and place that the vapor which supplies this 

 immense 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. 



358. 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 thence 

 to the right place. 



359. 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 inter-tropical 

 America, and the rains which they may discharge into the Missis- 

 sippi Valley now and then are exceptions, not the rule. 



360. The winds from the north can not bring vapors from the 

 great lakes to make rains for the ]\Iississippi, 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 



