120 I'lIYSICAL GEOGliArnY OF THE SEA, AND ITS METEOROLOGY. 



are found. There, too, the rains fall less frequently (Plate XIII.). 

 Dr. Ruschenbcrgcr, of the Navy, on his last voyage to India, was 

 kind enough to conduct a scries 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 

 Eodgers, 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 eva- 

 porating surface^ hoio, 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 atmo- 

 spherical circulation, it remains to be shown how it is, if tho 

 view bs correct, there should be ►^mailer 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 

 (jcean ; 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, com- 

 mencing 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 throT^^l into one body between these pa- 

 rallels, 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 



