538 BUILDING INSTINCT OF BEAVER. 
a uniform height; this they form of branches interlaced one 
with another, the intervals between them being filled with 
stones and mud, with which materials they give a coat of 
rough-cast to the exterior also. When the dam passes across 
a running stream, they make it convex towards the current ; 
by which it is caused to possess much greater strength than 
if it were straight. ‘This dam is usually eleven or twelve feet 
across at its base, and it is enlarged every year; and it fre- 
quently becomes covered with vegetation, so as to form a kind 
of hedge. 
707. When the dam is completed, the community separates 
into a certain number of families ; and the Beavers then em- 
ploy themselves in constructing huts, or in repairing those of 
a preceding year. These cabins are built on the margin of 
the water ; they have usually an oval form, and an internal 
diameter of six or seven feet. Their walls are constructed, 
like the dam, of branches of trees ; and they are covered on 
two of their sides with a coating of mud. Each has two 
chambers, one above the other, separated by a floor; the 
upper one serves as the habitation of the Beavers; and the 
lower one as the magazine for the store of bark which they 
lay up for their provision. These chambers have no other 
opening than one by which they pass out into the water. It 
has been said that the flat oval tail of the Beavers serves them 
as a trowel, and is used by them in laying-on the mud of 
which their erections are partly composed ; but it does not 
appear that they use any other implements than their incisor 
teeth and fore-feet. With their strong incisors they cut-down 
the branches and even the trunks of trees which may be 
suitable ; and by the aid of their mouths and fore-feet i 
drag these from one place to another. When they establi 
themselves on the banks of a running stream, they cut-down 
trees above the point where they intend to construct thei 
dwellings, set them afloat, and, profiting by the current, direet 
them to the required spot. It is also with their feet that 
they dig-up the earth they require for mortar, from the banks 
or from the bottom of the water. These operations are exe 
cuted with extraordinary rapidity, although they are only 
carried-on during the night. When the neighbourhood of 
Man prevents the Beavers from multiplying to the degree 
necessary to form such associations, and from possessing the 
