162 ZOOLOGICAL GARDENS, 
so swelled by the eflects of a heavy thaw as to sweep 
away not only their magazine of provisions, but some- 
times even their habitations. 
When the water in the stream is not sufficiently deep 
for their purpose, or is lable to be diminished by the 
failure of the supply from above in consequence of frost, 
they commence their operations by throwmg a dam 
across it below the part which they intend to occupy. 
In slow rivulets this is made nearly straight; but 
where the current is strong, it is formed with a curve 
of greater or less extent, the convexity of which is 
turned towards the stream. The materials of which 
this dam is constructed consist of drift wood, and the 
branches of willows, birch, and poplars, compacted 
together by mud and stones, The work is raised in 
the form of a mound, of considerable thickness at the 
base, and gradually narrowing towards the summit, 
which is made perfectly level, and of the exact height 
of the body of water which it is intended to keep up. 
Cartwright adds that he has frequently crossed the 
rivers and creeks upon these dams, with only slightly 
wetting his shoes. The sticks which are used in their 
construction vary in size from the thickness of a man’s 
finger to that of his ancle; but are seldom larger unless 
where no others are to be procured. They are mostly 
obtained from the neighbourmg woods, where they 
are cut with a dexterity truly astonishing. A Beaver, 
according to Cartwright, will lop off with its teeth at 
a single effort a stem of the thickness of a common 
walking-stick, as cleanly as if it had been done by a 
gardener’s pruning-knife. When compelled to have 
recourse to the larger trunks they gnaw them round 
and round; always taking care that they shall fall im 
the direction of the water, in order as much as possible 
to save themselves carriage. Judging from the number 
