RAINS AND RIVERS. 123 



in the region of the north-cast trades, which befoi-e they arrive 

 there have to cross the mountains of the isthmus, on the cool 

 tops of which they deposit their moisture, and leave Panama 

 rainless and pleasant until the sun returns north with the belt of 

 equatorial calms after him. They then push the belt of north- 

 east trades farther to the north, occupy a part of the winter zone, 

 and refresh that part of the earth with summer rains. This belt 

 of calms moves over more 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 south- 

 east 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 w^et weather till the 

 fall, when the belt of calms recrosses 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 Bainless Beglons. — The coast of Peru is within the 

 region of perpetual south-east trade-winds. Though the Peru- 

 vian 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 continue to bear 

 along across the continent, depositing it as they go, and supply- 

 ing with it the sources of the Eio de la Plata and the southern 

 tributaiies 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. Peaching the sum- 

 mit of that range, they now tumble down as cool and dry winds 

 on the Pacific slopes beyond. Meeting with no evajDorating 

 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 be- 

 fore, 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 tho 

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



