222 THE PHYSICAL GEOGRAPHY OF THE SEA. 



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

 already been detailed (^ 226), 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. 



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

 northeast trade-winds which come from a lower and go to a higher 

 temperature are the evaporating winds, i. e., they evaporate more 

 than they precipitate ; while those winds which come from a high- 

 er and go to a lower temperature are the rain-winds, ^. e., they 

 precipitate more than they evaporate. That such is the case, not 

 only do researches indicate, but reason teaches, and philosophy 

 tells. 



These views, therefore, suggest the inquiry as to the sufficiency 

 of the Atlantic, after supplying the sources of the Amazon and its 

 tributaries with their waters, to supply also the sources of the Mis- 

 sissippi and the St. Lawrence, and of all the rivers, great and small, 

 of North America and Europe. 



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" which supply the vapors for the rains that are car- 

 ried 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." 



474. Monsoons (^ 462) are, for the most part, formed of trade- 

 winds. When a trade-wind is turned back or diverted by over- 

 heated districts from its regular course at stated seasons of the 

 year, it is regarded as a monsoon. Thus the African monsoons 

 of the Atlantic (Plate VIII.), the monsoons of the Gulf of Mexico, 

 and the Central American monsoons of the Pacific, are, for the 

 most part, formed of the northeast trade-winds, which are turned 

 back to restore the equilibrium which the overheated plains of 



