§298. RAINS AND RIVERS. II9 



and how the prevailing winds of the latitude determine which is 

 the rainy and which the dry side. Thus, let us take the southern 

 coast of Chili for illustration. In our summer time, when the sun 

 comes north, and drags after him the belts of perpetual winds and 

 calms, that coast is left within the regions of the northwest winds 

 — the winds that are counter to the southeast trades — which, cool- 

 ed by the winter temperature of the highlands of Chili, deposit 

 their moisture copiously. During the rest of the year, the most 

 of Chili is in the region of the southeast trades, and the same 

 causes which operate in California to prevent rain there, operate 

 in Chili ; only the dry season in one place is the rainy season of 

 the other. Hence we see that the weather side of all such mount- 

 ains as the Andes is the wet side, and the lee side the dry. The 

 same phenomenon, from a like cause, is repeated in intertropical 

 India, only in that country each side of the mountain is made al- 

 ternately the wet and the dry side by a change in the prevailing 

 direction of the wind. Plate YIII. shows India to be in one of 

 the monsoon regions : it is the most famous of them all. From 

 October to April the northeast trades prevail. They evaporate 

 from the Bay of Bengal water enough to feed with rains, during 

 this season, the western shores of this bay and the Ghauts range 

 of mountains. This range holds the relation to these winds that 

 the Andes of Peru (§ 297) hold to the southeast trades; it first 

 cools and then relieves them of their moisture, and they tumble 

 down on the western slopes of the Ghauts, Peruvian-like, cool, 

 rainless, and dry ; wherefore that narrow strip of country between 

 the Ghauts and the Arabian Sea would, like that in Peru between 

 the Andes and the Pacific, remain without rain forever, were it 

 not for other agents which are at work about India and not about 

 Peru. The work of the agents to which I allude is felt in the 

 monsoons, and these prevail in India and not in Peru. After the 

 northeast trades have blown out their season, which in India ends 

 in April, the great arid plains of Central Asia, of Tartary, Thibet, 

 and Mongolia, become heated up; they rarefy the air of the 

 northeast trades, and cause it to ascend. This rarefaction and 

 ascent, by their demand for an indraught, are felt by the air 

 which the southeast trade-winds bring to the equatorial Doldrums 

 of the Indian Ocean : it rushes over into the northern hemisphere 

 to supply the upward draught from the heated plains as the south- 



