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, i. 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 
