BEAVER DAMS. 93 



number of these dams, constructed in very dissimilar 

 situations, tends to show that their curvature is purely 

 accidental. 



The remainder of this dam is nearly as remarkable as 

 the central portion, and much longer as well as larger 

 than the engraving represents (Plate VI.), unless due 

 allowance is made for perspective. The focal point 

 occupied by the instrument was so near the struc- 

 ture as to depreciate quite rapidly its extreme parts. 

 Throughout its entire extent of two hundred and 

 sixty feet the face of the dam is composed, as at the 

 centre, of interlaced sticks and poles, and presents 

 the same general appearance, with a gradual abate- 

 ment in height. 



On the water face of the dam neither a stick nor 

 a pole is seen, but a regular sloping embankment of 

 earth, from the crest downward, under the waters of 

 the pond. This face of the dam is precisely in the 

 form of the shelving bank of a stream. 



There is no opening in the top of the dam, in any 

 part of it, for the discharge of the surplus Avater; 

 neither does it pass over its crest; but it percolates 

 through the thin bank of earth near its crest in nu- 

 merous places along its entire length. The dams of 

 this class all agree in this respect. In the most of 

 these dams the rapidity or slowness with which this 

 surplus is discharged, is undoubtedly regulated by the 

 beavers, otherwise the level of the pond would con- 

 tinually vary. There must be a constant tendency to 

 enlarge the orifices through which the water passes, 

 which, if left to itself, would in due time draw down 

 the pond, and expose the entrances to their lodges and 

 burrows; on the other hand, if the embankment was 



