244 THE PHYSICAL GEOGRAPHY OF THE SEA. 



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 vapor. 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 toward 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 evap- 

 orating regions is deposited upon those of precipitation, which, 

 for illustration 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 amount of fresh water discharged into 

 it by the rivers of the great hydrographical basins of Arctic Eu- 

 rope, 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 toward the equa- 

 tor, and an under current of water Salter and heavier from the 

 equator to the poles. This under current 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 

 Tiic under currents in thc systcm of occanlc circulation which causes 

 salts of gea water, an uudcr currcut to flow from the Mediterranean 

 into the Atlantic (§ 885), 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 currents 



