CURRENTS OF THE SEA. 
133 
any of these lakes should, in consequence of its specific gravity, 
once sink below the level of the shoals in the rivers and straits 
which connect them, it never could flow out again, and conse- 
quently must remain there forever* — were this principle physi- 
cally correct, would not the water at the bottom of the lakes grad- 
ually have received salt sufficient, during the countless ages that 
they have been sending it off to the sea, to make this everlasting- 
ly pent-up water briny, or at least quite different in its constitu- 
ents from that of the surface ? We may presume that the water 
at the bottom of every extensive and quiet sheet of water, whether 
salt or fresh, is at the bottom by reason of specific gravity ; but 
that it does not remain there forever we have abundant proof. If 
so, the Niagara River would be fed by Lake Erie only from that 
layer of water which is above the level of the top of the rock at 
the Falls. Consequently, wherever the breadth of that river is no 
greater than it is at the Falls, we should have a current as rapid 
as it is at the moment of passing the top of the rock to make the 
leap. To see that such is not the way of Nature, we have but to 
look at any common mill-pond when the water is running over 
the dam. The current in the pond that feeds the overflow is 
scarcely perceptible, for " still water runs deep." Moreover, we 
know it is not such a skimming current as the geologist would 
make, which runs from one lake to another ; for wherever above 
the Niagara Falls the w^ater is deep, there we are sure to find the 
current sluggish, in comparison with the rate it assumes as it ap- 
proaches the Falls ; and it is sluggish in deep places, rapid in shal- 
low ones, because it is fed from below. The common "wastes" 
in our canals teach us this fact. 
The reasoning of this celebrated geologist appears to be found- 
ed upon the assumption that when water, in consequence of its 
specific gravity, once sinks below the bottom of a current where' 
it is shallowest, there is no force of traction in fluids, nor any other 
power, which can draw this heavy water up again. If such were 
the case, we could not have deep water immediately inside of the 
bars which obstruct the passage of the great rivers into the sea. 
Thus the bar at the mouth of the Mississippi, with only fifteen 
feet of water on it, is estimated to travel out to sea at rates vary- 
ing from one hundred to twenty yards a year. 
* See paragraph quoted 253) from " Lyell's Principles of Geology." 
