CURRENTS OF THE SEA. I33 



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 w^ater, 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 w^ater runs deep." Moreover, we 

 know it is not such a skimming current as the geologist would 

 make, w^hich runs from one lake to another ; for wherever above 

 the Niagara Falls the water is deep, there we are sure to find the 

 current sluggish, in comparison wdth 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." 



