242 Mr. J. Croll on the Physical Cause of Ocean-currents. 



ration is in excess of precipitation, the general level of this sup- 

 posed sea would be altered, and immediately as much water as 

 is carried off by evaporation would commence to now in from 

 north and south toward the trade-wind or evaporation region, 

 to restore the level " (§ 509) . " On the other hand, the winds 

 have taken this vapour, borne it off to the extratropical regions, 

 and precipitated it, we will suppose, where precipitation is in 

 excess of evaporation. Here is another alteration of sea-level, by 

 elevation instead of by depression ; and hence we have the motive 

 power for a surface -current from each pole towards the equator, 

 the object of which is only to supply the demand for evaporation 

 in the trade-wind regions" (§ 510). 



The above result would follow, supposing the ocean to be 

 fresh. He then proceeds to consider an additional result that 

 follows in consequence of the saltness of the ocean. 



" Let evaporation now commence 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 vapour of salt 

 water is fresh, or nearly so, fresh water only is taken up from 

 the ocean; that which remains behind is therefore more salt. 

 Thus, while the level is lowered in the salt sea, the equilibrium 

 is destroyed because of the saltness of the water ; for the water 

 that remains after evaporation 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 vapour" (§ 517). 



" The vapour is taken from the surface-water ; the surface- 

 water thereby becomes more salt, and, under certain conditions, 

 heavier. When it becomes heavier, it sinks ; and hence we 

 have, due to the salts of the sea, a vertical circulation, namely, a 

 descent of heavier — because Salter and cooler — water from the 

 surface, and an ascent of water that is lighter — because it is not 

 so salt — from the depths below" (§ 518). 



In section 519 he goes on to show that this vapour removed 

 from the intertropical region is precipitated in the polar regions, 

 where precipitation is in excess of evaporation. " In the preci- 

 pitating regions, therefore, the level is destroyed, as before ex- 

 plained, by elevation, and in the evaporating regions by depres- 

 sion \ which, as already- stated, gives rise to a system of surface- 

 currents, moved by gravity alone, from the poles towards the 

 equator" (§520). 



"This fresh water being emptied into the Polar Sea and agi- 

 tated by the winds, becomes mixed with the salt; but as the agita- 

 tion of the sea by the winds is supposed to extend to no great 

 depth, it is only the upper layer of salt water, and that to a mo- 

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



