188 THE PHYSICAL GEOGRAPHY OF THE SEA. 



is not necessary to notice farther than to say it can not exist in 

 seas of salt water, for salt water (§31) contracts as its tempera- 

 ture is lowered to its freezing point. Hence, in consequence of 

 its salts, changes of temperature derive increased power to disturb 

 the equilibrium of the ocean. 



514. If this train of reasoning be good, we may infer that, in a 

 system of oceanic circulation, the dynamical force to be derived 

 from difference of temperature, where the waters are all fresh, 

 would be quite feeble ; and that, were the sea not salt, we should 

 probably have no such current in it as the Gulf Stream. 



515. So far we have been reasoning hypothetically, to show 

 what would be the chief agents, exclusive of the winds, in disturb- 

 ing the equilibrium of the ocean, were its waters fresh and not salt. 

 And whatever disturbs equilibrium there may be regarded as the 

 jprimum mobile in any system of marine currents. 



516. Let us now proceed another step in the process of ex- 

 plaining and illustrating the effect of the salts of the sea in the sys- 

 tem of oceanic circulation. To this end, let us suppose the im- 

 aginary ocean of fresh water suddenly to become that which we 

 have, namely, an ocean of salt water, which contracts as its tem- 

 perature is lowered (§ 513) till it reaches 28° or thereabout. 



517. Let evaporation now commence in the trade- wind region, 

 as it was supposed to do (§ 509) 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 

 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 the evaporation takes place is, on account of the 

 solid matter held in solution, specifically heavier than it was be- 

 fore any portion of it was converted into vapor. 



518. The vapor 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 



