90 THE PHYSICAL GEOGRAPHY OF THE SEA. 



ern^rivers from the northern trade-winds, for the trade-winds are 

 the evaporating winds. 



179. Taking for our guide such faint glimmerings of light as 

 we can catch from these facts, and supposing these views to be 

 correct, then the saltest portion of the sea should be in the trade- 

 wind regions, where the water for all the rivers is evaporated ; and 

 there the saltest portions are found. There, too, the rains fall less 

 frequently (Plate XIII.). 



180. Dr. Ruschenberger, of the Navy, on his last voyage to In- 

 dia, 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 — midway of the trade- wind regions — he found the heav- 

 iest water. Though so warm, the water there was heavier than 

 the cold water to the south of the Cape of Good Hope. Lieuten- 

 ant D. D. Porter, in the steam-ship Golden Age, found the heav- 

 iest water about the parallels of 20° north and 17° south. 



181. In summing up the evidence in favor of this view of the 

 general system of atmospherical circulation, it remains to be shown 

 how it is, if the view be correct, there should be smaller rivers and 

 less rain in the southern hemisphere. The winds that are to blow 

 as the northeast trade-winds, returning from the polar regions, 

 where the moisture (§ 176) has been compressed out of them, re- 

 main, as we have seen, dry winds until they cross the calm zone 

 of Cancer, and are felt on the surface as the northeast 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 but comparatively a small portion of evaporating 

 surface exposed to their action. 



182. The zone of the northeast 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 par- 

 allels, would make a belt equal to 120° of longitude by 22° of lat- 

 itude, and comprise an area of about twelve and a half millions 



