190 PHYSICAL GEOGRAPEIY OF THE SEA, AND ITS METEOROLOGY. 



tlie water at tlie bottom of the great American lakes ought to 

 remain there for over, for the bottom of Erie is far below the 

 barrier which separates this lake from the Falls of Niagara, and 

 so is the bottom of every one of the lakes below the shallows in 

 the straits or rivers that connect them as a chain. 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 

 for ever we have abundant proof. If so the Niagara Kiver 

 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. Con- 

 sequently, 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 water is 

 deep, there we are sure to find the current sluggish, in com- 

 parison with the rate it assumes as it approaches the Falls ; and 

 it is sluggish in deep places, rapid in shallow ones, because it is 

 fed from below. The common " wastes" in our canals teach us 

 this fact. 



388 Tlie bars at the mouths of the Mississippi an illustration. — The 

 reasoning of this celebrated geologist appears to be founded 

 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, so to speak, 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 j)assage of the great rivers 

 into the sea: 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 var3'ingfrom twenty to one hundred yards a j^ear. In the 

 place where that bar was when it was one thousand yards nearer 

 to New Orleans than it now is, whether it were fifteen years ago 

 or a century ago, with only fifteen or sixteen feet of water on it, 

 we have now four or five times that depth. As new bars were 



