THE BEAVERS OF NORTH AMERICA 45 



weakness. The smallest hole is soon enlarged by 

 the pressure of water passing through, so that if it 

 is not speedily closed the destruction of the whole 

 structure is threatened ; rats, musquash and otters 

 sometimes burrow through the dams and cause 

 untold damage. 



How persistent the beaver is in repairing is well 

 illustrated by the following experiment made in 

 the Algonquin Reserve. Knowing that the best 

 way to secure flashlight photographs of the animals 

 at work was by making a break in their dam, I 

 selected one within convenient distance of where 

 I was staying. After arranging the cameras in 

 position, I made an opening in the dam about two 

 feet wide and laid a thread across. This was 

 attached to the electric switch which operated the 

 flashlight and shutters of the cameras. Scarcely 

 three hours elapsed before the animals, who had 

 found that the water was rapidly escaping from 

 their pond, visited the dam and repaired the breach. 

 Before midnight I returned and reset the cameras, 

 after again opening the dam. Before morning this 

 had again been mended. After that I continued 

 each night to repeat the operation, breaking open 

 the dam at least once each night, sometimes twice, 

 and on two occasions three times. During the 

 twenty-two nights, only twice did the beaver fail 

 to more or less completely fill in the openings I 

 had made, though they never once did so while I 

 remained to watch, even though I took every 



