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 staymg. 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 
