MONSOONS. 



367 



drive tbe N.E. monsoons from the land, replace them, and 

 gradually extend themselves out to sea. 



686. Tlie sun assisted by the latent heat of vapour. — Coming now 

 from the water, they bring vapour, which, being condensed upon 

 the hill-sides, liberates its latent caloric, and so, adding fuel to 

 the flame, assists the sun (§ 648) to rarefy the air, to cause it to 

 rise up and flow off more rapidly, and so to depress the baro- 

 meter still more. It is not till the S.W. monsoons have been 

 extended far out to sea that they commence to blow strongly, or 

 that the rainy season begins in India. By this time the mean 

 daily barometric pressure in this place of ascending air, which is 

 also a calm place, has become less than it is in the equatorial 

 calm belt ; and the air which the S.E. trade-winds then bring to 

 the equator, instead of rising up there in the calm belt, pass over 

 without stopping, and flows onward to the calms of Central Asia 

 as the S.W. monsoon. It is drawn over to supply the place of 

 rarefaction over the interior of India. 



687. The rain-fall in India. — The S.W. monsoon commences to 

 change at Calcutta, in 22° 34' N., in February, and extends 

 thence out to sea at the rate of fifteen or twenty miles a day ; yet 

 these winds do not gather vapour enough for the rainy season of 

 Cherraponjie, in lat. 25^ 16', to commence with until the middle 

 or last of April, though this station, of all others in the Bengal 

 Presidency, seems to be -most favourably situated for wringing 

 the clouds. Selecting from Colonel Sykes's report of the rain-fall 

 of India, those places which happen to be nearest the same 

 meridian, and about 2° of latitude apart, the following state- 

 ment is made, with the view of showing, as far as such data can 

 show, the time at which the rainy season commences in the 

 interior : 



