Page:Physical Geography of the Sea and its Meteorology.djvu/400

374 of pressure. In the sea breeze, the land barometer would be low and the sea high, and vice versa in the land breeze; and when the barometer was highest and when it was lowest it would be calm at the barometric stations.

699. The changing of the monsoons.—It is these calm bands or "medial belts," as the crest and trough of the barometric wave may be called, which, with their canopy of clouds, follow the departing and herald the coming monsoon. They move to and fro, up and down the earth, like the sun in declination. As they have a breadth of 200 or 300 miles, they occupy several days in passing any given parallel, and while they overshadow it, then the monsoons are dethroned. During the interregnum, which lasts a week or two, the fiends of the storm hold their terrific sway in these bands. The changing of the monsoons is marked by storm and tempest. Becalmed in them, meanings are said by seamen to be heard in the air—a sign of the coming storm—a warning of impending danger to ship and crew. Then the props and stays are taken away from the air, and the wind seems ready to rush violently hither and thither, and whenever there is from any cause a momentary disturbance of the equilibrium. In such an atmosphere, the latent heat that is liberated by every heavy rain-shower has power to brew a storm. Throughout the monsoon region, the people know beforehand, almost to a day, the coming of this interregnum, which they call the changing of the monsoons, for the annual changing at the same place is very regular.

700. How the calm belt of Cancer is pushed to the north.—Theory, therefore, points to a place in Northern India, which is near the northern limits of the south-west monsoon, where the mean height of the barometer during the rainy season (§ 691) is about 29.5 inches, the mean height at the equator being 29.92 inches. Into this monsoon place of low barometer over the land the wind rushes from the north-east as well as the south-west. The place of high pressure towards the north from which it rushes is under the calm belt of Cancer. Hence this belt is also pushed north, and made to occupy, in summer at least, the position over land somewhat like that assigned to it on Plate VIII. In the south-west monsoon the Malabar coast has its rainy season, so that the air over the peninsula is permanently kept more or less in a rarefied state, by the liberation of latent heat from vapour as actual observations abundantly show.