242 THE PHYSICAL GEOGRAPHY OF THE SEA. 



sea. Now imagine this sea of uniform temperature to be sudden- 

 ly stricken with the invisible wand of heat and cold, bringing its 

 waters 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 destroy the equilib- 

 rium of the whole ocean ; upon this a set of currents would im- 

 mediately commence to flow, namely, a current of cold and heavy 

 water to the place of the warm, and a current of warm and lighter 

 to the place of the cold. The motive power of the currents thus 

 created would be difference of specific gravity arising from differ- 

 ence of temperature in fresh water. "We have now traced the ef- 

 fect of two agents, which, in a sea of fresh water, would tend to 

 create currents, 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 

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

 that is taken from one part of the ocean by evaporation, and de- 

 posited in another by precipitation. The other agent would be 

 employed in restoring, by the forces due difference of specific 

 gravity, 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 modi- 

 fied by others, find expression in a system of currents and coun- 

 ter currents, or rather in a set of surface currents of warm and 

 lighter 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. 



469. Such, keeping out of view the influence of the winds, 

 Currents without which wc 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 wa- 

 ter, 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 circulation were 

 the waters all fresh, which it is not necessary here to notice far- 



♦ 39°.5. 



