Vibrating Dams. 365 
buildings themselves (stone as well as wood) would vibrate with 
the doors and windows. This might be felt and also seen; as 
for example, a slender branch of a grape vine trained up against 
the side of a stone building, was seen to vibrate in exact time with 
the doors and windows. 
The vibrations ceased entirely when water ceased to pour over 
the dam; they were also inconsiderable when the depth of water 
was eighteen or twenty four inches. A depth of five or six inches 
produced the greatest effect. The number of vibrations per second 
was thought to be about constant, but no accurate experiments on 
this point were ever made. From the best estimate I could ob- 
tain, they amounted to twelve or fifteen per second. A heavy 
log resting against the dam materially impaired the effect. ‘The 
vibrations were seldom noticed in summer, as the water rarely 
run six inches over the dam; I however formed a plan during the 
summer of 1842, for a series of experiments during the ensuing 
winter and spring, to determine particularly the number of vibra- 
tions per second. [had several methods in mind to be tried if 
others should fail, but the one with which I expected most success 
was with a monochord ; being simply a cat-gut, one end of which 
passed over a pulley and was stretched by a variable weight. 
The number of vibrations was too small for a musical sound, but 
by holding a small slip of paper near the string when vibrated, a 
succession of rattles is produced, which I hoped might be tuned 
to unison with the rattling of the windows. ‘The vibrations of 
the string could be easily determined by the principles of acous- 
tics. Iwas however never allowed the opportunity of testing 
my methods. In my first communication to the Ohio Observer, 
I had stated as a test of my theory, ‘if this dam were filled up 
to the top with earth it would probably cease vibrating.” To 
my great regret the experiment was immediately tried. During 
the season of 1842 a large amount of rock, estimated at two hun- 
dred and fifty tons, was deposited upon the embankment. ‘This 
raised the bank fully up to the level of the dam for a depth of six 
feet or more. Since that time the river has passed through every 
stage of elevation, known in ten years; from that in which you 
might walk with dry shoes over the entire length of the dam, to a 
depth of six feet on the break of the dam, which happened June 
5, 1843, the greatest rise known since 1832. The result is that 
the vibrations have entirely ceased. 
