THE SALTS OF THE SEA. I57 



is one which exists in reaUty, and, therefore, need not be regard- 

 ed as hypothetical: 



304. Let us next call in evaporation and precipitation, with 

 heat and cold — more powerful agents. Suppose the evaporation 

 to commence from this imaginary fresh-water ocean, and to go on 

 as it does from the seas as they are. In those regions, as in the 

 trade-wind regions, where evaporation is in excess of precipitation 

 (§ 126), the general level of this supposed sea would be altered, 

 and, immediately, as much water as is carried off by evaporation 

 would commence to flow in from north and south toward the 

 trade-wind or evaporating region, to restore the level. 



305. On the other hand, the winds have taken this vapor, borne 

 it off to the extra-tropical regions, and precipitated it (^ 129), we 

 w'ill suppose, where precipitation is in excess of evaporation. Here 

 is another alteration of sea level by elevation instead of by depres' 

 sion ; and hence we have the motive power for a surface current 

 from each pole toward the equator, the object of which is only to 

 supply the demand for evaporation in the trade-wind regions — de- 

 mand for evaporation being taken here to mean the difference be- 

 tween evaporation and precipitation for any part of the sea. 



306. Now imagine this sea of uniform temperature (§ 301) to 

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

 its w^aters 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 w^aters, which would de- 

 stroy the equilibrium of the whole ocean, upon w^hich (§ 275) a 

 set of currents (§ 277) would immediately commence to flow, viz., 

 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. 



307. We have now traced (^ 303 and 306) 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 different from those which w^e find in the salt sea. 

 One of these agents would be employed (§ 305) in restoring, by 

 means of one or more polar currents, the w^ater that is taken from 

 one part of the ocean by evaporation, and deposited in another by 



