158 THE PHYSICAL GEOGRAPHY OF THE SEA. 



precipitation* The other agent would be employed in restoring, 

 by the forces due difference of specific gravity (§ 306), the equi- 

 librium, which has been disturbed by heating, and of course ex- 

 panding, the waters of the torrid zone on one hand, and by cool- 

 ing, and consequently contracting, those of the frigid zone on the 

 other. This agency would, if it were not modified by others, find 

 expression in a system of currents and counter currents, or rather 

 in a set of surface currents of warm and light water, from the 

 equator toward the poles, and in another set of under currents of 

 cooler, dense, and heavy water from the poles toward the equator. 



308. Such, keeping out of view the influence of the winds, 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 impart a peculiar feature to the system of oce- 

 anic circulation were the waters all fresh, which it is not neces- 

 sary to notice further than to say it can not exist in seas of salt 

 water, for salt water (§ 31) contracts as its temperature is lower- 

 ed to its freezing point. Hence, in consequence of its salts, 

 changes of temperature derive increased power to disturb the equi- 

 librium of the ocean. 



309. 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 ; arid that, were the sea not salt, we should 

 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 regarded as the pri- 

 mum mobile in any system of marine currents. 



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 this imaginary 

 ocean of fresh water suddenly to become that which we have, viz., 

 an ocean of salt water, which contracts as its temperature is low- 

 ered (§ 308) till it reaches 28° or thereabout. 



