§ 470. THE SALTS OF THE SEA. 24:S 



ther than to say it can not exist in seas of salt water, for salt wa- 

 ter (§ 405) contracts as its temperature is lowered, and until it 

 passes its freezing point. Hence, in consequence of its salts, 

 changes of temperature derive increased power to disturb the 

 equilibrium of the ocean. 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 wa- 

 ters are all fresh, would be quite feeble ; and that, were the sea 

 not salt, we should (§ 462) probably have no such current in it as 

 the Gulf Stream. So far we have been reasoning hypothetically, 

 to show what would be the chief agents, exclusive of the winds, in 

 disturbing the equilibrium of the ocean were its waters fresh and 

 not salt. And whatever disturbs equilibrium there may be re- 

 garded as the jprimum mobile in any system of marine currents. 



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

 influence of salts and plaluiug aud illustrating the effect of the salts of the 

 evaporation. g^^ l^^ ^I^q systcm of occaulc circulatlou. To this 



end, let us suppose the imaginary ocean of fresh water suddenly 

 to become that which we have, namely, an ocean of salt water, 

 which contracts as its temperature is lowered (§ 441) till it reach- 

 es 25°. 6. Let evaporation now commence in the trade-wind re- 

 gion, as it was supposed to do (§ 468) in the case of the fresh- wa- ■ 

 ter 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 equilib- 

 rium 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 before any portion of it was .converted into vapor. The va- 

 por is taken from the surface water; the surface water thereby 

 becomes more salt (§ 463), 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, or, being as salt, 

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

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



