increase of temperature about the equatorial, and IIjc decrease of il about (lie Polar regions can only produce a 

 certain effect, which, like the effectof the cenliipedal force upon tlie figure of the Earth, in elevating the sea-level 

 at the equator, becomes constant, and which like every other constant in nature, is compensated : whereas the 

 process of evaporation and precipitation being continued, the difference of level created by these in different 

 parts of the ocean is accumulative and not constant. Il, therefore, remains for currents to restore. 



We iiave now traced from their princii)les of action 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, would be quite different from those which we find in the salt 

 se.i. 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 

 atrent 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 Ton id Zone on one hand, a.nd by 

 cooling, and consequently contracting, those of the Frigid Zone on the other. This agency would, it 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 towards the poles, and in another set of under currents of 

 cooler, dense, and heavy water, from the poles towards the equator. 



Such, keeping out of viewtheinfluence of the winds which we may suppose would be the same whether thesea 

 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 il reaches the freezing 

 point and ceases to be fluid. This law of expansion by cooling, would impart a peculiar feature to the system of 

 oceanic circulation were'the waters all fresh. Water at the temperature of 40° would be at its maximum of 

 density. Raise or lower the temperature from that and the water would expand ; of course, then, it would grow 

 lighter and ascend to the surface. Therefore, when the warm waters of the Torrid Zone, by flowing North and 

 cooling down to 40°, for instance, should meet the cold current coming from the Polar basin with a tempera- 

 ture of 34°, the current from the equator being of denser water would sink, and the current from the Frigid 

 Zone would then appear as a surface current until the temperature should rise to 40° for example. Here the 

 ( urrent fiom the equator would be 50° we may suppose, and there would be another node in the system of 

 fresh water circulation ; for here, at this latter place of meeting, the current from the Polar regions, which all 

 along had been of the lighter water, and Iheiefore on the surface, would now become the heavier, disappear 

 from the suiface, sink and continue its course as an under current. 



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 

 wfif the sea not salt, we should probably have no such current in it as the Gulf Stream. 



So tar we have been reasoning hypothetically 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 iipiilibrium there, may be regarded as the prlmum mobile in the system of marine currents. 



