EAINS AND RIYERS. 115 



parallels of 20° nortli and 17° south. Captain Eodgers, in the 

 United States ship Yincennes, found the heaviest water in 17° N., 

 and between 20' and 25° S. _ 



291. In summing up the evidence in favour of this view of the 

 seeingthat the south- general sjstem of atmospherical cii^culation, it re- 

 fo7dsX largest''^' maius to be shown how it is, if the view be correct, 

 how^uniSf there'^be ^^^^'^ should be Smaller rivers and less rain in the 

 a crossing.^ could we southem hemisphere. The winds that are to blow as 

 the^^at r[ve?sTi pokr tho uorth-cast trade- winds, returning from the 

 the northern? rcgious, whcrc the moistm^c (§ 292) has been com- 

 pressed out of them, remain, as we have seen, dry winds until they 

 cross thecalm 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, Ahica, and North 

 America, where there is comparatively but a small portion of 

 evaj)orating 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, illus- 

 trated 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 of equatorial cahns, the vapours which make 

 the rains that feed the rivers in the extra-tropical regions of the 

 southem hemisphere. Upon this supposition, then, two thirds 

 only of the northern trade-winds are fully charged with moisture, 

 and only two thu'ds of the amount of rain that falls in the northern 

 hemisphere should faU in the southem ; and this is just about the 

 proportion (§ 292) that observation gives. In like manner, the 

 south-east trade-winds take up the vapom^s which make our rivers, 

 and as they prevail to a much greater extent at sea, and have 

 exposed to their action about twice as much ocean as the north-east 

 trade-v^dnds have, we might expect, according to this hvpothesis, 



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