158 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
precipitation. The other agent would be employed in restoring, 
by the forces due difference of specific gravity 306), the equi- 
librium, which has been disturbed by heating, and of course ex- 
panding, the waters of the torrid zone on one hand, and by cool- 
ing, and consequently 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 surface currents of warm and light water, from the 
equator toward the poles, and in another set of under currents of 
cooler, dense, and heavy water from the poles towar^l the equator, 
308. 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 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 impart a peculiar feature to the system of oce- 
anic circulation were the waters all fresh, which it is not neces- 
sary to notice further than to say it can not exist in seas of salt 
water, for salt water (§ 31) contracts as its temperature is lower- 
ed to its freezing point. Hence, in consequence of its salts, 
changes of temperature derive increased power to disturb the equi- 
librium of the ocean. 
309. If this train of reasoning be good, we may infer that, in a 
system of oceanic circulation, the dynamical force to be derived 
from difference of temperature, where the waters are all fresh, 
would be quite feeble ; and that, were the sea not salt, we should 
probably have no such current in it as the Gulf Stream. 
So far we have been reasoning hypothetic ally, to show what 
would be the chief agents, exclusive of the winds, in disturbing the 
equilibrium of the ocean, were its waters fresh and not salt. And 
whatever disturbs equilibrium there may be regarded as the jpri- 
mum mobile in any system of marine currents. 
Let us now proceed another step in the process of explaining 
and illustrating the effect of the salts of the sea in the system of 
oceanic circulation. To this end, let us suppose this imaginary 
ocean of fresh water suddenly to become that which we have, viz., 
an ocean of salt water, which contracts as its temperature is low- 
ered 308) till it reaches 28° or thereabout. 
