THE LIFE OF MAMMALS 
untrue as the fable that they construct five-story houses, and make wattle- 
work like an Indian fish weir. They do nothing of the sort, but continue 
to heap up old sticks, earth, and stones anyhow, until they have formed 
some kind of a barrier,— always moving the materials with hands or 
teeth. 
Now this dam is always begun in the center of the stream. If the cur- 
rent is slight, it will run straight across, but where the current is fast the dam 
is likely to show a decided curve on its upstream side. This has been 
represented as an intelligent employment of the strength of the arch form 
against the current; but more probably it is simply the result of the action 
of the stream pressing down the wings of the first, central, obstruction. 
The water does not pour over the top of these dams, but filters through 
them, and its washing necessitates constant repairs, which are made on 
the inner (or upper) face, leaving the outer front a mere tangle of sticks 
and poles. Sometimes, however, a short high dam will get so filled in with 
earth that it becomes a solid, tree-grown dike; and some relics of this kind 
may be several centuries old. The length of a dam, of course, depends 
upon its site. In a flat valley, the most advantageous situation for a beaver 
colony, a pond will broaden rapidly, and the original channel-barrier must 
be extended to prevent the water running out beyond its wings; and thus 
a few old dams in the level swampy woods about the sources of the Mis- 
sissippi exceeded one hundred and twenty-five yards in length. 
The young beavers of which I have spoken probably spent their 
first winter in a burrow, but when, next May, half a dozen young 
ones arrived, they would begin to build a lodge above ground. 
A beaver lodge is a hollow mound on the bank or on some islet, 
three or four feet high and eight or ten in outer diameter. It is 
erected in the same way as is the dam, of sticks, earth, and stones 
heaped around a hollow center, the floor of which is a little 
higher than the surface of the pond. As the dam grows and 
the pond expands and rises, the floor of the chamber is raised 
by hollowing out the interior overhead, and piling more stuff 
on the roof to equal the loss. It has two entrances, always 
beneath the water, for an opening into the air would admit 
both cold and enemies. The walls of such a lodge may be 
three feet thick and as solid as masonry. The beavers heap 
on patted mud, and drag over it astonishingly big limbs, so 
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