Engineers 



structure varies according to the force of the current in 

 the stream to be dammed. When the stream is slow- 

 running the dam is built straight across from bank to 

 bank ; in faster-running streams the dam is somewhat 

 V-shaped against the stream to break the force of the 

 water. 



The dams vary in size from quite insignificant little 

 structures to solid masonry, two to three hundred yards 

 in length and ten or a dozen feet thick. The first care of 

 the little engineers is to remove all the bark from the 

 felled logs ; this is hidden away in safe keeping and serves 

 as food when other provender is scarce. The bare logs 

 are carried by the beavers to the bed of the stream and 

 there they are covered with mud and stones. Hundreds 

 of logs are used, all of them being laid horizontally 

 beneath the water, and rendered more secure by having 

 slender branches from the felled trees twined round them. 

 Mud, logs and branches form a barrier which effectually 

 stems the current and maintains the water at a constant 

 level. In addition, much floating material, carried down 

 by the stream, is arrested by the dam and adds to its 

 stability. 



The reason of all this activity is not far to seek when 

 we examine a beaver lodge. Now the lodge, as we have 

 said, covers one entrance to the beaver's burrow ; the other 

 end opens into the water some distance below its surface. 

 We can easily realise the importance to the beaver of 

 always having the water at constant level. Should the 

 height of the water vary, it is likely that, in dry weather, 

 the normally sub-aquatic entrance to the burrow would 

 become exposed and thus form an open invitation to all 

 and sundry to enter the lodge. 



Again, the country inhabited by beavers is cold in the 

 extreme at certain periods of the year. The streams 

 freeze, so that, if its inmates are to have free access to 

 their lodges, it is highly essential that the entrances 

 should not be frozen, and this is brought about by their 



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