114 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



examination is very instrnctiye, for it shows tlie status of tlie atmo- 

 sphere to be much more unstable in the northern hemisphere, ^yith its 

 excess of land, than in the southern, with its excess of water. Eains, 

 and fogs, and thunder, and calms, and storms, all occm' much more fre- 

 quently, and are more irregular also as to the time and place of their 

 occmTence on the north side, than they are on the other side of the 

 equator. Moistm-e is never extracted from the air by subjecting it 

 from a low to a higher temperatm^e, but the reverse. Thus all the 

 au" Avhich comes loaded with moistm'e from the other hemisphere, 

 and is borne into this with the south-east trade- winds, travels in the 

 upper regions of the atmosphere (§ 213) until it reaches the cahns 

 of Cancer ; here it becomes the sm'&ce "wind that prevails from the 

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

 the process of condensation commences. We may now hken it to 

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

 squeezes that sponge. Finally reaching the cold latitudes, all the 

 moistm'e that a dew-point of zero, and even far below, can extract, 

 is wrung from it ; and this an' then commences " to retm'n accord- 

 ing to his cii'cuits" as dry atmosphere. And here we can quote Scrip- 

 tm'e again : " The north wind driveth away rain." This is a 

 meterological fact of high authority, and one of great significance 

 too. 



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

 The trade-winds the foTced to the conclusiou that OUT rivors are supphed 

 evaporating winds. ^^]^ ^]^gjj, wators principally from the trade-wind 

 regions — the extra-tropical northern rivers fi'om the southern 

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

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



293. Taking for om- guide such faint glimmerings of Hght as 

 The saitest part of ^6 can catch from theso facts, and supposing these 

 the sea. viows to be correct, then the saitest portion of the 

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

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

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

 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 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 



