THE SALTS OF THE SEA. 
157 
is one which exists in reality, 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 
will 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 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 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 jDerceived, 
would be quite different from those which we find in the salt sea. 
One of these agents would be employed (§ 305) 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 
