370 Vibrating Dams. 
G, and the same at each of the other places, making three dis- 
tinct vibrations in a little more than half a second, and then a 
longer space as before observed. ‘The doors and windows shake 
in Springfield, two and a half miles from the dam. 
IV, V. Two dams in Northampton, Mass. 
The following information was furnished by Professor Hitch- 
cock. ‘The vibrations and jarring have occurred at two dams in 
Northampton, about a hundred rods apart on the same stream, 
running along the south side of the village. Both of them are 
built of wood; that is, of timber with planks pinned to them, and 
the top is straight. The upper one is two hundred feet long, and 
twelve feet high; the lower one about the same length, and six 
or seven feet high. Both of them rest on solid strata of red sand- 
stone, which have an easterly dip of 15° or 20°. The vibrations 
occur only at a particular height of the water. If the sheet is 
not continuous they do not take place; nor if it be quite thick at 
high water. As the water was not at the proper pitch either 
time when I visited the spot, I have not witnessed the effects. 
I was assured that the jarring of houses was sensible through 
the whole village, which must be over a distance of half a mile. 
Some of the near neighbors found the effect so annoying from 
the lower fall, that they threatened prosecution. The proprie- 
tors knew not what to do; when happily a log floated against 
the dam and stopped the vibrations. ‘The hint was improved, 
and a piece of plank fastened to the top of the dam, so as to break 
the sheet, and since that time they have not been noticed. They 
are still common at the upper dam, and I was told by an intelli- 
gent man who lived upon the bank, that the vibrations could be 
distinctly seen, not only on the curve of the descending sheet, but 
extending as waves a considerable way up stream. He informed 
me also that he first noticed these vibrations in a mill-dam of a 
similar kind, built upon a rock in Brattleborough, Vt. Your inqui- 
ries have also quickened my own memory, so that I distinctly 
recollect having frequently noticed a singular vibration in the 
descending sheet of water over a dam, so that I can hardly doubt 
but it is common. I regret that my time does not permit me to 
hunt up other facts of a similar character. 
