§292,293. RAINS AND RIVERS. l^o 



ble in the northern hemisphere, with its excess of land, than in the 

 southern, with its excess of water. Eains, and fogs, and thunder, 

 and calms, and storms, all occur much more frequently, and are 

 more irregular also as to the time and place of their occurrence on 

 this side, than they are on the other side of the equator. Moist- 

 ure is never extracted from the air by subjecting it from a low to 

 a higher temperature, but the reverse. Thus all the air which 

 comes loaded with moisture from the other hemisphere, and is 

 borne into this with the southeast trade-winds, travels in the up- 

 per regions of the atmosphere (§ 213) until it reaches the calms 

 of Cancer ; here it becomes the surface wind that prevails from 

 the southward and westward. As it goes north it grows cooler, 

 and the process of condensation commences. We may now liken 

 it to the wet sponge, and the decrease of temperature to the hand 

 that squeezes that sponge. Finally reaching the cold latitudes, 

 all the moisture that a dew-point of zero, and even far below, can 

 extract, is wrung from it ; and this air then commences " to return 

 according to his circuits" as dry atmosphere. And here we can 

 quote Scripture again : " The north wind driveth away rain." This 

 is a meteorological fact of high authority, and one of great signif- 

 icance too. 



292. By reasoning in this manner and from such facts, we are 

 The trade-winds the ^^d to the couclusion that our rivers are supplied 

 evaporating winds. ^-^^-^ ^|^g-j, ^^^^^^ principally from the trade-wind 



regions — the extra-tropical northern rivers from the southern 

 trades, and the extra-tropical southern rivers from the northern 

 trade-winds, for the trade-winds are the evaporating winds. 



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

 The saitest part of wc Can catch from these facts, and supposing these 

 ^^^ '®^" views to be correct, then the saitest portion of the 

 sea should be in the trade-wind regions, where the water for all 

 the rivers is evaporated ; and thera the saitest portions are found. 

 There, too, the rains fall less frequently (Plate XIII.). Dr. Ru- 

 schenberger, of the Navy, on his last voyage to India, 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 wa- 

 ter to the south of the Cape of Good Hope. Lieutenant D. D. 



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