HOW BUILT. 155 



to pass under this, a tight calking of small twigs and mud- 

 mortar preventing the passage of a singe drop. 



The trunk of the tree takes all the scour of the water 

 passing over it, and the friction from the debris that comes 

 down in freshets. The means possessed by beavers of 

 estimating the height of a standing tree, is quite incom- 

 prehensible to me, since they are not able to climb one ; 

 but it is most rare to find an instance where they have 

 taken the trouble to cut down a larger tree than would 

 suffice to reach across from bank to bank of the stream, 

 excepting where they could get no other ; and as to their 

 cutting down too short a tree, I do not think they are ever 

 guilty of such a folly. I have sought for the stumps that 

 would show where they had made this mistake, but sought 

 in vain. That there is both skill and judgment required to 

 fell a tree of forty or fifty feet high, and a foot or more 

 through at its base, so that it shall fall just where it ^is 

 wanted, is obvious ; and that the perpendicular wall of 

 branches forming the dam, and the felled tree trunk should 

 come together, requires both calculation and ingenuity. On 

 the upper side the dam slopes gradually downwards, and 

 where the stream is liable to heavy freshets this slope is very 

 long, making an angle with the perpendicular side of the 

 dam of nearly sixty degrees ; but where no freshets of conse- 

 sequence are to be apprehended, the angle is sometimes 

 quite sudden. 



Nature has provided the beaver with a natural flexible 

 trowel, his tail, and he uses it as such, making a mortar by 

 puddling the earth of the banks of the stream, carrying it on 

 his tail to where it is required, and then with it spreading 



