Page:Natural History (1848).djvu/227

instinctive association for the purpose of building structures of considerable size and durability. These are habitations for winter-residence, and dams for maintaining an equable depth of water. The latter are not needed when the beaver-house is erected in a pond or lake, or broad river; but these situations are less frequently chosen than a narrow stream, as in the latter case the current enables the animals to float down materials, besides affording them additional security. If there is a probability of the water diminishing by reason of the freezing of the source, they display wonderful sagacity in forming a dam quite across the river at some distance from their house. If the current be sluggish, the dam is nearly straight, but if it be rapid, additional strength is imparted to the structure by making it convex towards the stream. The materials used are drift-wood, or young trees cut down by the sharp cutting teeth of the Beavers, and gnawed into lengths, with mud and stones dragged from the banks and bottom, the whole intermixed without any regularity, except that which preserves the general sweep of the dam. The sticks are not forced into the bottom as has been pretended, but are laid horizontally, and are kept in their places simply by the weight of the stones and mud laid over them. "In places,” says Hearne, "which have been long frequented by Beavers undisturbed, their dams, by frequent repairing, become a solid bank, capable of resisting a great force both of water and ice; and as the willow, poplar, and birch, generally take root and shoot up, they by degrees form a kind of regular planted hedge, which I have seen in some places