BEAVER DAMS. 99 



secure an exact representation of this great structure 

 while it was in a perfect state, althougli not then as 

 well assured, as at present, that it is not surpassed in 

 magnitude by any other beaver dam in North America. 



Two adjuncts of this dam remain to be noticed. 

 Of these, the first is a remarkable effort of engineering 

 skill, if from the end it subserves we are at liberty to 

 infer an intention on the part of the beaver to produce 

 that end. It is a second dam, in two sections, each 

 twenty-five feet long and two feet high, constructed 

 across the thread of the stream, and about one hun- 

 dred feet below the great curve. It is shown in 

 Plate VIII. At this point, the waters that flowed 

 through the dam above have again become collected 

 into a small running stream. This low dam forms a 

 shallow pond, in itself of no apparent use for beaver 

 occupation, but yet subserving the important purpose 

 of setting back water to the depth of twelve or fifteen 

 inches in the great curve. At this point the pressure 

 of the water in the pond against the dam is the 

 greatest, because here the bed of the channel is the 

 lowest, and the structure the highest; and the small 

 dam, by maintaining the water a foot deep below the 

 great dam, diminishes, to this extent, the difference 

 in level above and below; and neutralizes, to the 

 same extent, the pressure of the water in the jDond 

 above against the main structure. Whether the lower 

 dam was constructed with this motive, and for this 

 object, or is explainable on some other hypothesis, I 

 shall not venture an opinion. I have found the same 

 precise work repeated below other large dams. 



The second is also a dam which is constructed 

 across the outlet of Lake Diamond at the place where 



