352 PHYSICAL GEOGRAPHY OF THE SEA, AKD ITS METEOROLOGY. 



sort of barometric ridge in the air over this region, which we may 

 call the monsoon wave. In this time it passes from the northern 

 to the southern edge of the monsoon belt, and as it rolls along in 

 its invisible but stately march, the air beneath its pressure flows 

 out from under it both ways — on the polar side as the S.W. mon- 

 soon, on the equatorial as the N.E. 



685. As the vernal equinox approaches, the heat of the sun 

 How they begin, bogius to play upou the stoppes and deserts of Asia 



with power enough to rarefy the air, and cause an uprising suffi- 

 cient to produce an indraught thitherward from the surrounding 

 regions. The air that is now about to set off to the south as the 

 N.E. monsoon is thus arrested, turned back, and drawn into this 

 place of low barometer as the S.W. monsoon. These plains be- 

 come daily more and more heated, the sun more and more power- 

 ful, and the ascending columns more and more active ; the area 

 of inrushing air, like a circle on the water, is widened, and thus 

 the S.W. monsoons, "backing down" towards the equator, drive 

 the N.E. monsoons from the land, replace them, and gradually 

 extend themselves out to sea. 



686. Coming now from the water, they bring vapour, which, 

 The sun assisted by bein"" condonsed upon the hill-sides, liberates its 

 vapiu^* ^^'^^ ^^ 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 barometer still more. 

 It is not tillthe 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 pres- 

 sure 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 S.W. monsoon commences to change at Calcutta, in 

 Therain-faiiinindia. 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 



