14 SOCIAL HABITATIONS. 
the logs that are washed down by the stream, after the 
Beavers have nibbled off all the bark. 
That this position is untenable is evident from the 
acknowledged fact that the dam is by no means placed 
at random in the stream, just where a few logs may have 
happened to lodge, but is set exactly where it is wanted, 
and is made so as to suit the force of the current. In 
those places where the stream runs slowly, the dam is 
carried straight across the river, but in those where the 
water has much power, the barrier is made in a convex 
shape, so as to resist the force of the rushing water. 
The power of the stream can, therefore, always be in- 
ferred from the shape of the dam which the Beavers 
have built across it. 
Some of these dams are of very great size, measuring 
two or three hundred yards in length, and ten or twelve 
feet in thickness, and their form exactly corresponds with 
the force of the stream, being straight in some parts, and 
more or less convex in others. 
The dam is formed, not by forcing the ends of the 
logs into the bed of the river, but by laying them hori- 
zontally, and covering them with stones and earth until 
they can resist the force of the water. Vast numbers of 
logs are thus laid, and as fast as the water rises, fresh 
materials are added, being obtained mostly from the 
trunks and branches of trees which have been stripped 
of their bark by the Beavers. 
The reader will remember that many persons have 
thought that the dam of the Beaver is only an accidental 
agglomeration of loose logs and branches, without any 
engineering skill on the part of the animals. There is 
some truth in this statement, though the assertion is too 
sweeping. For, after the Beavers have completed their 
dam, it obstructs the course of the stream so completely 
