THE ATMOSPHERE. 89 



southeast trades, and the same causes which operate m California 

 to prevent rain there, operate in ChiU ; 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. 



138. The same phenomenon, from a like cause, is repeated in 

 inter-tropical 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 northeast trades prevail. They evap- 

 orate from the Bay of Bengal water enough to feed wuth 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 (§ 133) hold to the southeast trades ; it 

 first cools and then reheves them of their moisture, and they tum- 

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

 (§ 137), cool, rainless, and dry; wherefore that narrow strip of 

 country between the Ghauts and the Arabian Sea w^ould, like 

 that in Peru between the Andes and the Pacific, remain without 

 rain forever, wef e 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. 



139. After the northeast trades have blown out their season, 

 which in India ends in April (^ 138), the great arid plains of Cen- 

 tral Asia, of Tartary, Thibet, and Mongolia, become heated up, 

 react upon these northeast trades, turn them back, and convert 

 them, during the summer and early autumn, into southwest mon- 

 soons. These then come from the Indian Ocean and Sea of Ara- 

 bia loaded with moisture, and striking w4th it perpendicularly upon 

 the Ghauts, precipitate upon that narrow strip of land between 

 this range and the Arabian Sea an amount of water that is truly 

 astonishing. Here, then, are not only the conditions for causing 

 more rain, now on the w^est, now on the east side of this mount- 

 ain range, but the conditions also for the most copious precipita- 

 tion. Accordingly, when we come to consult rain gauges, and 

 to ask meteorological observers in India about the fall of rain, 

 they tell us that on the western slopes of the Ghauts it some- 



