234 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



precipitated it, we will suppose, where precipitation is in excess 

 of evaporation. Here is another alteration of sea level by elevation 

 instead of by depression ; and hence we have the motive power for 

 a surface current from each pole towards the equator, the object of 

 which is only to supply the demand for evaporation in the trade- 

 wind regions — demand for evaporation being taken here to mean 

 the difference between evaporation and precipitation for any part of 

 the sea. Now imagine this sea of uniform temperature to be sud- 

 denly 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 equili- 

 brium of the whole ocean ; upon this a set of currents would imme- 

 diately 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 diflerence of specific gravity arising from differ- 

 ence of temperature in fresh water. We have now traced the effect 

 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, 

 woiffd be quite feeble in comparison ^vith those which we fimd 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 deposited in another 

 by precipitation. The other agent would be employed in restoring, 

 by the forces due to 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 conse- 

 quently 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 sur- 

 face currents of warm and lighter water, from the equator towards 

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

 heavy water from the poles towards the equator. 



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

 Currents without whicli WO 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°,* 



* 390.5. 



