236 PHYSIC-\X. GEOGKAPHY OF THE SEA, AND ITS METEOROLOGY. 



is not so cool (§ 404) — from the deptlis below. This vapour, then, 

 which is taken up from the evaporating regions (§ 293), is carried 

 by the winds through their channels of circulation, and poured 

 back into the ocean where the regions of precipitation are; and 

 by the regions of ^precipitation I mean those parts of the ocean, 

 as in the polar basins, where the ocean receives more fresh water 

 in the shape of rain, snow, etc., than it returns to the atmosphere 

 in the shape of vapour. In the precipitating regions, therefore, 

 the level is destroyed, as before explained, by elevation ; and in 

 the evaporating regions, by depression; which, as already stated 

 (§ 468), gives rise to a system of surface currents, moving on an 

 inclined plane, from the poles towards the equator. But we are 

 now considering the effects of evaporation and precipitation in 

 giving impulse to the circulation of the ocean where its waters are 

 salt. The fresh water that has been taken from the evaporating 

 regions is deposited upon those of precipitation, v\^hich, for illustra- 

 tion merely, we will locate in the north polar basin. Among the 

 sources of supply of fresh water for this basin, we must include not 

 only the precipitation which takes place over the basin itself, but 

 also the amomit of fresh water discharged into it by the rivers of 

 the great hydrographical basins of Arctic Europe, Asia, and America. 

 This fresh water, being emptied into the Polar Sea and agitated by 

 the winds, becomes mixed with the salt ; but as the agitation of 

 the sea by the winds is supposed to extend to no great depth 

 (§ 468), it is only the upper layer of salt water, and that to a 

 moderate depth, which becomes mixed with the fresh. The specific 

 gravity of this upper layer, therefore, is diminished just as much as 

 the specific gravity of the sea-water in the evaporating regions was 

 increased. And thus we have a surface current of saltish water 

 from the poles towards the equator, and an under current of water 

 Salter and heavier from the equator to the poles. This under 

 cmTent of brine supplies, in a great measure, the salt which the 

 upper current, freighted with fresh water from the clouds and 

 rivers, carries back. 



471. Thus it is to the salts of the sea that we owe that feature 

 The under currents in the systcm of occanic circulatiou which causes 

 2iTt"of sea^4ter. ^^ au uudcr currcut to flow from the Mediterranean 

 into the Atlantic (§ 385), and another (§ 377) from the Eed Sea 

 into the Indian Ocean. And it is evident, since neither of these 

 seas is salting up, that just as much, or nearly just as much salt 

 as the under current brings out, just so much the upper cmTents 



