84 THE PHYSICAL OSTEOGRAPHY OF THE SEA. 



gions, where the water for all the rivers is evaporated ; and there 

 the saltest portions are found. 



127. Dr. Ruschenberger* of the Navy, on his late 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 

 heaviest water. Though so warm, the water there was heavier 

 than the cold w^ater 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. 



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. 



128. The Exijlanation. — The winds that are to blow as the 

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

 the moisture (§ 125) 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 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 ex- 

 posed to their action. 



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 parallels, 

 would make a belt equal to 120° of longitude by 22° of latitude. 



According to the hypothesis, illustrated by Plate I., p. 70, as to 

 the circulation of the atmosphere, it is these northeast trade-winds 

 that take up and carry over, after they rise up in the belt of equa- 

 torial calms, the vapors which make the rains that feed the rivers 

 in the extra-tropical regions of the southern hemisphere. 



Upon this supposition, then, two thirds only of the northeast 

 trade-winds are fully charged with moisture, and only two thirds 

 of the amount of rain that falls in the northern hemisphere should 



