BEAVER DAMS. 89 



solid bank of earth bound together by a mass of 

 sticks imbedded and concealed from view. A trans- 

 verse section, therefore, is a triangle with the base 

 longer than either side. We thus have a section of 

 a dam about a foot high, constructed with the least 

 amount of materials, but holding the water securely, 

 and yet so fragile that the weight of a man would 

 sink it below the surface of the water. 



At the great curve, near the centre of the dam, the 

 minute as well as general structure of a large beaver 

 dam can be seen to the highest advantage. The en- 

 graving (Plate VII.) represents a section, upwards of 

 one hundred feet wide, through the centre of the dam, 

 including the great curve. It is engraved three-fourths 

 the size of the photograph. Small sticks are no longer 

 used, but billets of wood and poles trimmed of their 

 branches and stripped of their bark, and varying in size 

 from one to three inches in diameter, and from three to 

 seven and ten feet in length. These short cuttings and 

 poles, which are interlaced and arranged in every con- 

 ceivable way, form a sloping bank at an angle of from 

 35° to 40°. Their main direction is from the ground 

 upward toward the water face of the dam. They are 

 neither parallel with each other, nor in courses, but 

 are banked together in an irregular but compact mass, 

 and are so adjusted as to form an innumerable series 

 of props or braces, with their lower ends against the 

 ground, and their upper ends incorporated in the em- 

 bankment which forms the water face of the dam. 

 These poles, however, formed no part of the original 

 structure, but were added from year to year to repair 

 the waste of the dam from settlement and decay, and 

 to increase its height. We may therefore conceive 



