Vibrating Dams. 367 
Some effect is felt when the depth of water is less than four 
or five inches. When the water commences pouring over the 
dam ina thin sheet, the tremulous motion is perceived in the 
falling water. ‘This motion increases as the water descends, and 
a very thin sheet is soon broken and falls irregularly. As the 
depth of water increases, it is less and less scattered in its fall, 
until at length it forms an unbroken sheet. It is not until this 
takes place that much effect is produced on the surrounding at- 
mosphere. "The strength of the vibrations increases with the 
depth of water up to four or five inches; beyond this, the vibra- 
tions diminish, until at length they cease to be felt in the atmos- 
phere, and cannot be perceived in the sheet of falling water. The 
tremulous motion of the water, which is greatest at the bottom 
of the sheet, is not confined to the falling water, but may be seen 
for a short distance back of the edge of the dam. 
The vibrations are not at all times very perceptible to the ear. 
They give a fluttering sensation, like that produced by a partridge 
while “drumming.” ‘The number of vibrations is about five 
per second. A window will commence vibrating, and increase 
in force for five, ten or fifteen minutes, or perhaps longer, when 
it will gradually cease, and sometimes remain at rest for a short 
time. Owing, as is supposed, to the unequal thickness of the 
sheet of water, caused by the settling of some portions of the 
dam, and the breaking of some planks, the vibrations have not 
been experienced for the last two years, until since some recent 
repairs. ‘They occur now only occasionally, and the effect is but 
slight in comparison with what has formerly been witnessed. 
The same cause has prevented them before, within the memory 
of Mr. Osborn. 'The dam has constantly been built on the same 
plan for fifty years or more. The most powerful vibrations ever 
witnessed occurred when about twenty feet of one end of the dam 
was about two inches higher than the rest of it. They appa- 
rently began at that end of the dam, from which they extended 
the whole length, when the rest of the sheet was too thick to vi- 
brate of itself. At that time they would continue to grow more 
powerful until the motion was communicated to the water in the 
dam near to the edge, when it would cease fora few minutes. The 
vibrations have been known to affect windows in a house nearly 
one fourth of a mile distant in a northwest direction, and anoth- 
era little more than one fourth of a mile ina southeast direc- 
