Vibrating Dams. 375 
body of air. The dam vibrating before this air is like a tuning 
fork before the open mouth of a tube or cubical box, the opposite 
end of which is closed by a stretched membrane. 'The box does 
not originate the vibrations, but reinforces them, so that effects 
are produced upon distant objects to which the dam itself would 
be entirely incompetent. This effect is impaired by every divis- 
ion of the sheet of water. It is arent in the membrane. It is 
not to be supposed that the vibrations of this confined column of 
air correspond to the fundamental note of a tube of the same 
length. A segment of the dam at Springfield makes but one vi- 
bration per second, which would require an open tube eleven 
hundred feet in length to vibrate in unison with it. 
5. Are the vibrations transmitted to distant objects by the earth 
or atmosphere ?—Air is the more common vehicle of sound, yet 
as most of the dams here mentioned rest on solid rock, which is 
a good conductor, it has been supposed that this might be the 
principal conductor. ‘There are some facts which seem to indi- 
cate that these vibrations are transmitted chiefly by the atmos- 
phere. (1.) The two buildings mentioned p. 367, at Scantic, 
as being near the limit of this influence, both rest on a bed of 
sand, which is a very poor conductor of musical vibrations. (2.) 
The vibrations are chiefly heard on the side of buildings next the 
dam. ‘This is asserted of Scantic, p. 368, and has also been assert- 
ed of Cuyahoga Falls. (3.) It is probable that the vibrations are 
chiefly confined to the upper edge of the dam, where the friction 
is applied and where the dam is more free. (4.) The facts men- 
tioned at Springfield, pp. 368, 369, show that the stone breastwork 
vibrates, but the canal bank of earth a few rods below hardly at 
all. This shows how poor a conductor is sand, and renders it im- 
probable that it should be the vehicle for transmitting the vibra- 
tions to the buildings mentioned p. 367. (5.) The fact men- 
tioned at Gardiner, p. 371, respecting the place where the strong- 
est vibrations were felt, may seem inconsistent with my position ; 
but it is presumed that this statement was derived not from Mr. 
Gardiner’s own observation but from the testimony of others, and 
it is possible that a general conclusion may have been drawn too 
hastily. 
6. Why is not the rattling of windows continuous, instead of 
being subject to frequent interruption ?—I do not understand that 
the dam ceases to vibrate whenever the windows cease to rattle. 
