China Sea.
The countries and islands of South-Eastern Asia have thus a wet and a dry side. The S.W. monsoon drops much of its ram on the Western Ghats of the Peninsula of India, and moisture brought by the N.E. monsoon is deposited prin cipally on the eastern side of the Peninsula. The south side of an island in the S.W. monsoon has one continuous shower, but as the clouds spend their rain on the central mountains, the N. coast is quite dry. In the N.E. monsoon this is reversed.
In British India the S.W. monsoons commence at the north, and back down, or work their way towards the south. Thus they set in earlier at Calcutta than they do at and earlier in Ceylon than they do at the equator. The average rate of travel, or backing down to the south, as seamen express it, is from 15 to 20 miles a day. It takes the S.W. monsoons 6 or 8 weeks to back down from the tropic of Cancer to the equator. During this period there is a sort of barometric ridge in the air over this region, which may be called 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. monsoon, on the equatorial as the N.E. .
As the vernal equinox approaches, the heat of the sun begins to play upon the steppes and deserts of Asia, with power enough to rarefy the air, and cause an uprising sufficient to produce an indrought thitherward from the surrounding region. 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 become daily more and more heated, the sun more and more powerful, and the ascending columns more and more active ; the arc of inrush ing air, like a circle on the water, is winded, and thus the S.W. monsoons, backing down towards the equator, drive the N.E. monsoons from the
land, replace them, and gradually extend them selves out to sea.
The S.W. monsoon commences to change at Cal cutta, in lat. 22° 34' N., in February, and extends thence out to sea at the rate of 15 or 20 miles a day ; yet these winds do not gather vapour enough for the rainy season of Cherrapunji, in lat. 25° 16' N., to commence with until the middle or last of April, though this station, of all others in the Bengal Presidency, seems to be most favour ably situated for wringing the clouds. Selecting from Colonel Sykes' Report of the Rainfall 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 :— in each locality, so the irregularities, the excep tions to the rule, give a distinctive character to each season. For example, in 1868, a local depression in the north-west of the Bay of Bengal diverted the moisture-laden winds from Central and Northern India, and attracted an excessive rainfall towards Bengal, west of the delta, and the northern part of Orissa. In 1869 the frontiers of Bengal were surrounded by a belt of low atmospheric pressure which shut in the rainfall, threatening to afflict the north-west with a second drought, until an egress was found for the imprisoned clouds during September and October in a rise of pressure about Hazaribagh. Such local atmospheric irregularities throw out calcu lation. Probably the most striking evidence of this is their misleading effects' upon the deter mination of heights by the barometer. Thus the difference of Cuttack and Saugor Islands, which is known to be only 74 feet, appeared from the barometric readings of 1868 to be 205 feet, and from those of another year 166 feet. The track of cyclones also, though not their occurrence, ie influenced in a considerable degree by local depressions.