372 Vibrating Dams. 
(and one of them a brick hotel) erected on boggy ground near 
the river, but thirty rods below the lower dam, and ninety below 
the one most recently built. I intended to have made careful 
observations this season had the opportunity been afforded. 
VIL. Dam at Hartford, Conn. 
The following is from Mr. J. P. Brace. About nine years ago, 
I lived near the mill-dam called Imlay’s dam, and was much an- 
noyed by the rattling of the windows of my house, when any 
quantity of water was passing over the dam. It was some time 
before ] ascertained the cause, but after some investigation was 
fully satisfied it was produced by the passage of the water. The 
reason of my hesitation at first was the variation in the noise 
when the state of the dam was the same; but I soon discovered 
that the direction of the wind would account for this variation. 
‘The noise was produced by a rapid vibratory motion of the win- 
dow sash, if slightly loose. Ihave stood by the dam and per- 
ceived that the impulse of the falling water gave the same motion 
to the contiguous air, and that the noise of the waterfall was not 
continuous. I never ascertained how much water was necessary 
to produce the effect, but I have the impression left upon my 
mind, that in a great freshet the vibratory motion ceased. Those 
who lived nearer the. dam than I did were often very much an- 
noyed. Within a few years, the dam has been taken down and 
built anew from its foundations. Whether the same effect now 
takes place, | cannot inform you. 
Besides the seven cases here described, and an eighth alluded 
to at Brattleborough, IT have heard, of a ninth at Putnain in Ohio, 
and have also received vague intimations of several others. 
Remarks. 
In all these cases it is sufficiently obvious that the running 
water is the prime cause of the vibrations, for the vibrations in- 
variably cease when the water ceases running. But how is the 
effect produced? By friction upon the dam? by collision with 
the air in its fall? by impulse upon the rock beneath? or in 
some other way ? 
1. The dam itself vibrates.—The experiment tried at Cuyahoga 
Falls admits I apprehend of no other explanation. Last year when 
the dam was comparatively free, the rattling of windows was 
