am 



is placed upstream, and the small bushy end 

 downstream. If in a current these sometimes are 

 weighed down with mud or stones. Short, stout 

 sticks and long, slender poles are deftly mingled 

 in the dam as it rises. The poles overlie, and 

 many completed dams appear as though made 

 of gigantic inclined half-closed shears and com- 

 passes of poles. Thus a dam is doubly braced. The 

 weight against it is resisted both by the end-on 

 poles that are parallel to the flow and by those 

 set at an angle to it. 



The shape and the material of a dam are de- 

 pendent on a number of things : the nature of 

 the place where built, the kind of materials avail- 

 able for its building, the purpose it is intended 

 to serve, and the relation it may have to dams 

 already constructed. Sometimes a small dam will 

 be made that may ultimately become a big one 

 by simply digging a ditch across the stream 

 or basin and piling the excavated material into a 

 dam. 



Beaver, like men, are unequal in their skill, 

 both in planning and in doing work, and the work 

 of most beaver falls short of perfection. Errors are 

 67 



