244 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



surface currents of warm and lighter water, from tlie equatc»r 

 towards the poles, and in another set of under currents of cooler, 

 dense, and heavy water from the poles towards the equator. 



469. Currents without wind. — Such, keeping out of view the 

 influence of the Avinds, which we may suppose would be the 

 same whether the sea were salt or fresh, would be the system of 

 oceanic circulation were the sea all of fresh water. But fresh 

 water, in cooling, begins to expand near the temperature of 40°,* 

 and expands more and more till it reaches the freezing-point, and 

 ceases to be fluid. This law of expansion by cooling would im- 

 part a peculiar feature to the system of oceanic circvdation were 

 the waters all fresh, which is not necessary here to notice farther 

 than to say it cannot exist in seas of salt water, for salt water 

 (§ 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, w^here the w'aters 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 equi- 

 librium of the ocean were its waters fresh and not salt. And 

 whatever disturbs equilibrium there may be regarded as the 

 primum mobile in any system of marine currents. 



470. Influence of salts and evaporation. — Let us now proceed 

 another step in the process of explaining and illustrating the 

 effect of the salts of the sea in the system of oceanic circulation. 

 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 

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

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

 water seas, and as it actually goes on in nature — and what 

 takes place ? AVhy, 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 



* 39° 5. 



