Page:Physical Geography of the Sea and its Meteorology.djvu/398

372 condensation of the vapour for all this rain expands the air, causing it to boil over, flow off, and leave a low barometer—a diminished atmospheric pressure throughout all the region south of the Himalaya.

693. Dove and the monsoons.—As long ago as 1831, Dove maintained that the south-west monsoon was the south-east trade-wind rushing forward to fill the vacant places over the northern deserts. Dove admits the proofs of this to be indirect, and acknowledges the difficulty of finding out and demonstrating the problem.

694. The south-east trades passing into south-west monsoons.—But any navigator who, during the summer months, has occasion to traverse the Indian Ocean from north to south, may find that it is so. The outward-bound Indiaman, who, when on his way to Calcutta, crosses the equator in August, fur example, will find the south-east trades, as he approaches the line, to haul more and more to the south. As he advances still farther north they get to the west of south. Finally, he discovers that he has got the regular south-west monsoons, and that he has passed from the south-east trades into them without any intervening calm. This in summer is the rule; it has its exceptions, but they are rare. Examining the logs of a number of vessels taken at random for the passage in August, we find, by 421 observations therein recorded, they had the wind thus:

695. Lieutenant Jansen.—In like manner, and with like force, Jansen maintains that the north-west monsoon of Australia is the north-east trade-wind turned aside.

696. Monsoons in the Pacific.—The influence exerted upon rainless winds by the deserts of Africa and the overheated plains of Asia is felt at sea for a thousand miles or more. Thus, though the desert of Cobi and the sun-burned plains of Asia are, for the most part, north of latitude SO"", their influence in assisting to cause monsoons (§ 692) is felt south of the equator (Plate VIII.). So, too, with the great desert of Sahara and the African monsoons of the Atlantic; also with the Salt Lake country and the Mexican monsoons on one side, and those of Central America in the Pacific on the other. The influence (§ 298) of the deserts of