are found. There, too, the rains fall less frequently (Plate XIII.). Dr. Ruschenberger, of the Navy, on his last voyage to India, was kind enough to conduct a series of observations on the specific gravity of sea-water. In about the parallel of 17° north and south—midmay of the trade-wind regions—he found the heaviest water. Though so warm, the water there was heavier than the cold water to the south of the Cape of Good Hope. Lieutenant D. D. Porter, in the steam-ship Golden Age, found the heaviest water about the parallels of 20° north and 17° south. Captain Rodgers, in the United States ship Vincennes, found the heaviest water in 17° north, and between 20° and 25° south.
294. Seeing that the southern hemisphere affords the largest evaporating surface, how, unless there he a crossing, could we have most rain and the great rivers in the northern?—In summing up the evidence in favour of this view of the general system of atmospherical circulation, it remains to be shown how it is, if the view is correct, there should be smaller rivers and less rain in the southern hemisphere. The winds that are to blow as polar the north-east trade-winds, returning from the regions, where the moisture (§ 292) has been compressed out of them, remain, as we have seen, dry winds until they cross the calm zone of Cancer, and are felt on the surface as the north-east trades. About two-thirds of them only can then blow over the ocean; the rest blow over the land, over Asia, Africa, and North America, where there is comparatively but a small portion of evaporating surface exposed to their action. The zone of the north-east trades extends, on an average, from about 29° north to 7° north. Now, if we examine the globe, to see how much of this zone is land and how much water, we shall find, commencing with China and coming over Asia, the broad part of Africa, and so on, across the continent of America to the Pacific, land enough to fill up, as nearly as may be, just one-third of it. This land, if thrown into one body between these parallels, would make a belt equal to 120° of longitude by 22° of latitude, and comprise an area of about twelve and a half millions of square miles, thus leaving an evaporating surface of about twenty-five millions of square miles in the northern against about seventy-five millions in the southern hemisphere. According to the hypothesis, illustrated by Plate I., as to the circulation of the atmosphere, it is these north-east trade-winds that take up and carry over, after they rise up in the belt
