THE SALTS OF THE SEA. 187 



be suddenly stricken with the invisible wand of heat and cold, 

 and its waters brought to the various temperatures at which they 

 at this instant are standing. This change of temperature would 

 make a change of specific gravity in the waters, which would de- 

 stroy the equilibrium of the whole ocean, upon which a set of cur- 

 rents would immediately commence to flow, namely, a current of 

 cold and heavy water to the warm, and a current of warm and 

 lighter to the cold. 



The motive power of these would be difference of specific grav- 

 ity due to difference of temperature in fresh water. 



512. We have now traced (§ 507 and 511) the effect of two 

 agents, which, in a sea of fresh water, would tend to create cur- 

 rents, and to beget a system of aqueous circulation ; but a set of 

 currents, and a system of circulation which, it is readily perceived, 

 would be quite feeble in comparison with those which we find in 

 the salt sea. One of these agents would be employed (§ 509) in 

 restoring, by means of one or more polar currents, the water that 

 is taken from one part of the ocean by evaporation, and deposited 

 in another by precipitation. The other agent would be employed 

 in restoring, by the forces due difference of specific gravity (§ 511), 

 the equilibrium, which has been disturbed by heating, and of 

 course expanding, the waters of the torrid zone on one hand, and 

 by cooling, and consequently contracting, those of the frigid zone 

 on the other. This agency would, if it were not modified by oth- 

 ers, find expression in a system of currents and counter currents, 

 or rather in a set of surface currents of warm and light Avater, 

 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. 



513. 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 ex- 

 pand near the temperature of 40°, and expands more and more 

 tiU 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 oceanic circulation were the waters all fresh, which it 



