84 
THE PHYSICAL GEOGRAPHY 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 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. 
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 Explanation. — 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 
