274 THE PHYSICAL GEOGRAPHY OF THE SEA. 



land as well as the sea, and make the general courses of circulation 

 in each valley as obvious to the mind of the philosopher as is the 

 current of the Mississippi, or of any other great river, to his senses. 



781. These investigations as to the rain-winds at sea indicate 

 that the vapors which supply the sources of the Amazon with rain, 

 are taken up from the Atlantic Ocean by the northeast and south- 

 east trade-winds ; and many circumstances, some of which have 

 already been detailed (§ 389), tend to show that the winds which 

 feed the Mississippi with rains get their vapor in the southeast 

 trade- wind region of the other hemisphere. For instance, we know 

 from observation that the trade-wind regions of the ocean, beyond 

 the immediate vicinity of the land, are, for the most part, rainless 

 regions, and that the trade-wind zones may be described, in a hy- 

 etographic sense, as the evaporating regions (§ 32). They also 

 show, or rather indicate, as a general rule, that, leaving the polar 

 limits of the two trade-wind systems, and approaching the nearest 

 pole, the precipitation is greater than the evaporation until the 

 point of maximum cold is reached. 



782. And we know also that, as a general rule, the southeast 

 and northeast trade-winds, wliich come from a lower and go to a 

 higher temperature, are the evaporating winds, i. <3., they evaporate 

 more than they precipitate ; while those winds which come from 

 a higher and go to a lower temperature are the rain-winds, i. e., 

 they precipitate more tlian they evaporate. That such is the 

 case, not only do researches indicate, but reason teaches, and phi- 

 losophy intimates. 



783. These views, therefore, suggest the inquiry as to the suf- 

 ficiency of the Atlantic, after supplying the sources of the Amazon 

 and its tributaries with their waters, to supply also the sources of 

 the j\Iississippi and the St. Lawrence, and of all the rivers, great 

 and small, of North America and Europe. 



786. A careful study of the rain-winds (§ 32), in connection 

 with the Wind and Current Charts, will probably indicate to us 

 the " springs in the ocean" wliich supply the vapors for the rains 

 that are carried off by those great rivers. "All the rivers run into 

 the sea ; yet the sea is not full ; unto the place from whence the 

 rivers come, thither they return again." 



