Page:Physical Geography of the Sea and its Meteorology.djvu/393

Rh drive the N.E. monsoons from the land, replace them, and gradually extend themselves out to sea.

686. The 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 barometer 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, 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 statement 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: