Let i!S now proceed anotlier step in the process of explaining and illustrating the effect of the salts of the 

 Sea in the system of oceanic circulation. To lliis end, let us suppose this imaginary ocean of fresh water sud- 

 denly to beconu' that which we have, viz : nn ocean of .salt water which contracts as its temperature is lowered, 

 till it reaches 28° or thereahout. 



Let evaporation now cornnieiice in the trade wind region, as it was supposed to do in the case of the fresh water 

 seas, and as it actually goes on in nature — and what takes place ? Why a lowering of the sea level as before. But 

 as the vapor of sail water is fresh or nearly so, fresh water only is taken up from the ocean : that which remains 

 behind is thcrulore more salt ; thus while the level is lowered in the salt sea, the equilibrium is destroyed because 

 of tlu' saltncss of the water, tor the water that remains after the evi^poration takes place, is, on account of the solid 

 matter held in solution, specifically heavier than it was before any portion of it was converted into vapor. 



■j'he vapor is taken from the surface water ; the surface water thereby becomes more salt and consequently 

 heavier: it therefore sinks; and hence we have due to the salts of the sea, a vertical circulation, viz: a descent 

 iiea\ier — because salter and cooler — water from the surface, and an ascent of water that is lighter — because it 

 i-- not h.0 j^alt — from the depths below. 



This vapor then which is taken up from the evaporating regions — by wiiieli is meant those regions whert 



the evaporation is greater than the precipitation, — is carried by the winds through their channels of circulation 



:(1 pouted back into the ocean where the regions of precipitation are; — and by the regions of precipitation 



J mean those parts of the ocean, as in the polar basins, where the ocean receives more fresh water in the 



shape of rain, snow, &c., 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, gives rise to a system of surface cizrrents moved 

 by gravity alone from the poles towards the equator. 



IJut we are now considering the effects of evaporation and pi<;i:ii)itaiion in giving impulse to the rirrula- 

 ti.iii 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 

 Aliich, for illustration merely, we will locale 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 irjto it by the rivers of the great hydrographical basins of Arctic- 

 Kutope, Asia, and Ameiica. 



This fresh water, being emptied into the Polar sea, and agitated by ihe witids, becomes mixed with the salt r 

 • It as ilie agitation of the sea by the winds extends to no great depth, it is only the upper layer of salt water, 

 ni\d that to a moderate depth, which becomes mixed with the frt sh. The specific gravity of this upper layer 

 tlierelore is diminished just as much as the specific gravity of the sea water in the evaporating regions was 

 increa>ed. 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 towards the poles. This under current supplies 

 in a great measure the salt which the upper current, freighted with fresh water tVom the clouds and rivers, 

 carries back. 



