THE SALTS OF THE SEA. 235 



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 circulation 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 distui'b 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 waters are all fresh, 

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

 (§ 462) probably have no such cm-rent in it as the GruK Stream. 

 So far we have been reasoning hypothetically, to show what would 

 be the chief agents, exclusive of the winds, in disturbing the equi- 

 Hbrium of the ocean were its waters fresh and not salt. And what- 

 ever disturbs equilibrium there may be regarded as the jprimum 

 mohile in any system of marine currents. 



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

 influence of salts plaining and illustrating the effect of the salts of the 

 and evaporation. ^^^ :^ ^YiQ systcm of oceauic circulatiou. 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 temperatm^e is lowered (§ 441) tiU it reaches 

 25^.6. Let evaporation now commence in the trade-mnd region, 

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

 seas, and as it actually goes on in natm^e — 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 equi- 

 librium 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 iu solution, specifically heavier than it was 

 before any portion of it was converted into vapour. The vapour 

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

 more salt (§ 463), and, under certaiu 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, 



