DWELLINGS 1645 



water; sometimes the trees will shoot up forming 

 a hedge. The dam has a thickness of from three to 

 four metres at the base, and about sixty centimetres 

 at the upper part. The wall facing up-stream is 

 sloping, that directed down-stream is vertical ; this is 

 the best arrangement for supporting the pressure of 

 the mass of water which is thus expended on an 

 inclined surface. In certain cases beavers carry 

 hydraulic science still further. If the course of the 

 water is not very rapid, they generally make an 

 almost straight dike, perpendicular to the two banks, 

 as this is then sufficient; but if the current is strong, 

 they curve it so that the convexity is turned up- 

 stream. In this way it is much better fitted to resist. 

 Thus they do not always act in the same way, but 

 arrange their actions so as to adapt them to the con- 

 ditions of the environment. 



The embankment being completed, the animals 

 construct their lodges. Fragments of wood, deprived 

 of the bark, are arranged and united by clay or mud 

 which the beavers take from the river-side, transport, 

 mix, and work with their forepaws. During a single 

 night they can collect as much mud at their houses as 

 amounts to some thousands of their small handfuls. 

 They thus plaster their houses with mud every 

 autumn ; in the winter this freezes as hard as a stone 

 and protects them from enemies. These cabins form 

 domes from three to four metres in diameter at the 

 base, and from two to two and a half metres in 

 height. The floor is on a level with the surface of 

 the artificial pond. A passage sinks in the earth and 

 opens about one and a half metres below the level of 



