EAINS AND MVEBS. 125 



298. The rainy side of mountains. — Why there is more rain on 

 one side of a mountain than on the other. — We may now, from what 

 has been said, see why the Andes and all other mountains which 

 lie athwart the course of the winds have a dry and a rainy side, 

 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 per- 

 petual winds and calms, that coast is left within the regions of 

 the north-west winds — the winds that are counter to the south- 

 east trades — which, cooled 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 

 south-east trades, and the same causes which operate in Cali- 

 fornia 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 mountains 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 alternately the wet 

 and the dry side by a change in the prevailing direction of the 

 wind. Plate VIII. shows India to be in one of the monsoon 

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

 April the north-east 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 south-east trades ; it first 

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

 down on the western slopes of the (rhauts, Peruvian-like, cool, 

 rainless, and dry ; wherefore that narrow strip of country be- 

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

 between the Andes and the Pacific, remain without rain for 

 ever, 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 north-east 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 north-east trades, and cause it to ascend. 

 This rarefaction and ascent, by their demand for an indraught, 



