120 PHYSICAL GEOaEAPHY OF THE SEA, AND ITS METEOEOLOGY. 



tains as the Andes is the Tvet side, and the lee side the dry. The.- 

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

 India, only in that comitry 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 YIII. 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 Ghauts, Peru^dan-like, cool, 

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

 the Ghauts and the Arabian Sea would, lil^e that in Peru between 

 the Andes and the Pacific, remain "VNdthout 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 blo^^Ti out their season, w^hich in India ends 

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

 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, are felt by the air which 

 the south-east 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- 

 west monsoons. The forces of diurnal rotation assist (§ 113) to 

 give these winds their westing. Thus the south-east trades, in 

 certain parts of the Indian Ocean, are converted, during the sum- 

 mer and early autumn, mto south-west monsoons. These then 

 come fi'om the Indian Ocean and Sea of Arabia loaded vdth. 

 moisture, and, striking with it perpendicularly upon the Ghauts, 

 precipitate upon that narrow strip of land between this range and 

 the Arabian Sea an amoimt of water that is truly astonishing. 

 Here, then, are not only the conditions for causing more rain, 

 now on the west, now on the east side of this mountain range, but 

 the conditions also for the most copious precipitation. Accord- 

 ingly, when we come to consult rain gauges, and to ask meteoro- 

 logical observers in India about the fall of rain, they tell us that 

 on the western slopes of the Ghauts it sometimes reaches the enor- 



