§-97. RAINS AND KIVERS. 117 



than double of its breadth, and nearly the entire motion from 

 south to north is accomplished generally in two months, May and 

 June. Take the parallel of 4° north as an illustration : during 

 these two months the entire belt of calms crosses this parallel, and 

 then leaves it in the region of the southeast trades. During these 

 two months it was pouring down rain on that parallel. After the 

 calm belt passes it the rains cease, and the people in that latitude 

 have no more wet weather till the fall, when the belt of calms re- 

 crosses this parallel 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 within the bi-rainy 

 latitudes. 



297. The Rainless Eegions. — The coast of Peru is within the re- 

 the Rainless Re- gi^^ ^^ pcrpctual southcast tradc-wiuds. Though 

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



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

 The southeast trade-winds in the Atlantic Ocean first strike the 

 water on the coast of Africa. Traveling to the northwest, they 

 blow obliquely across the ocean till they reach the coast of Bra- 

 zil. 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 Rio 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 parti- 

 cle of moisture that that very low temperature can extract. Reach- 

 ing the summit of that range, they now tumble down as cool and 

 dry winds on the Pacific slopes beyond. Meeting with no evap- 

 orating surface, and with no temperature coldei^ 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. The other rainless or al- 

 most rainless regions are the western coasts of Mexico, the deserts 

 of Africa, Asia, North America, and Australia. Now study the 

 geographical features of the country surrounding those regions ; 



