94 THE PHYSICAL GEOGRAPHY OF THE SEA. 



strike the water on the coast of Africa. Traveling to the north- 

 west, they blow obliquely across the ocean until they reach the 

 coast of Brazil. By this time they are heavily laden with vapor, 

 which they continue to bear along across the continent, depositing 

 it as they go, and supplying with it the sources of the Bio de la 

 Plata and the southern tributaries of the Amazon. Finally they 

 reach the snow-capped Andes, and here is wrung from them the 

 last particle of moisture that that very low temperature can extract. 

 Beaching the summit of that range, they now tumble down as 

 cool and dry winds on the Pacific slopes beyond. Meeting with 

 no evaporating surface, and with no temperature colder than that 

 to which they were subjected on the mountain-tops, they reach 

 the ocean before they again become charged with fresh vapor, and 

 before, therefore, they have any which the Peruvian climate can 

 extract. The last they had to spare was deposited as snow on 

 the tops of the Cordilleras, to feed mountain streams under the 

 heat of the sun, and irrigate the valleys on the western slopes. 

 Thus we see how the top of the Andes becomes the reservoir from 

 which are supplied the rivers of Chili and Peru. 



196. The other rainless or almost rainless regions are the west- 

 ern coasts of Mexico, the deserts of Africa, Asia, North America, 

 and Australia. Now study the geographical features of the coun- 

 try surrounding those regions ; see how the mountain ranges run ; 

 then turn to Plate YIII. to see how the winds blow, and where 

 the sources are (§ 112) which supply them with vapors. This 

 plate shows the prevailing direction of the wind only at sea ; but, . 

 knowing it there, we may infer what it is on the land. Suppos- 

 ing it to prevail on the land as it generally does in corresponding 

 latitudes at sea, then the Plate will suggest readily enough how 

 the winds that blow over these deserts came to be robbed of their 

 moisture, or, rather, to have so much of it taken from them as to 

 reduce their dew-point below the Desert temperature ; for the air 

 can never deposit its moisture when its temperature is higher tha7i 

 its dew-point, 



197. We have a rainless region about the Bed Sea, because the 

 Bed Sea, for the most part, lies within the northeast trade-wind 

 region, and these winds, when they reach that region, are dry 



