118 



when tlie belt of calms recrosses this 2:)arallel on its way to the 

 south. By examining the "Trade-wind Chart," it may be seen 

 what the latitudes are that have two rainy seasons, and that 

 Bogota is A^ithin the bi-rainy latitudes. 



297. The Bainless Piegions. — The coast of Peru is within the 

 The eainless ee- rogion of perpetual south-east trade-winds. Though 

 ^^°^^- the Peruvian shores are on the verge of the great 



South Sea boiler, yet it never rains there. The reason is plain. 

 The south-east trade-winds in the Atlantic Ocean first strike the 

 water on the coast of Africa. Travelling to the north-west, they 

 blow obliquely across the ocean till they reach the coast of Brazil. 

 By this time they are heavily laden with vapour, which they con- 

 tinue to bear along across the continent, depositing it as they go, 

 and supplying ^ith it the somxes of the Eio 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 

 moistui^e that that very low temperature can extract. Eeaching 

 the summit of that range, they now tumble do^n 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 vapour, and before, there- 

 fore, 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 supphe^ . 

 the rivers of Chili and Peru. The other rainless or almost rainless 

 regions are the western coast of Mexico, the deserts of Africa, 

 Asia, North America, and Austraha. Now study the geographical 

 features of the country surrounding those regions ; see how the 

 mountain ranges run ; then turn to Plate YIII. to see how the 

 winds blow, and where the soui'ces are (§ 276) which supply them 

 with vapours. , 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. Supposing it to prevail on the land as it generally 

 does in corresponding latitudes at sea, then the Plate mil 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 tempera- 

 true ; for the air can never deposit its moisture iclien its temjjera- 



