Vibrating Dams. 369 
apparent vibration is no doubt caused by the shape of the surface 
of the water. Standing on this breastwork, my clothes shook 
and kept exact time with the vibrations. Standing on the canal 
bank of earth a few rods below, the effect on my clothes was 
perceptible, but much less. I was told that the vibrations were 
about two per second, but I found them very different. Others 
had not accurately observed the time, or the time is variable. 
There are three distinct vibrations that appear to be equidistant 
in time, and then a space nearly equal to two, and this in con- 
stant succession as observed from my station at the north end of 
the dam. The three vibrations with the space, occupied a sec- 
ond as near as I could measure. But then there appeared to be 
a finer set of vibrations between these, but how many is past 
human skill to ascertain with precision. But there is another 
very curious phenomenon that I observed. Standing twenty or 
thirty rods below (east of) the dam, where you look at the sheet 
of water nearly at a right angle, it seems to flash like lightning 
in dry clouds, or northern lights when there is a brilliant display 
of electrical flashes. 'This is evidently owing to the sheet of 
water partially breaking, and consequently looking white, while 
some part at the same time remained unbroken. I cannot de- 
scribe this without a figure. 
B 
| | } LL 
NTH 
AB represents the top of the sheet of water; C D the bottom; 
the curved line above H, a kind of arch; that above F', one some- 
what shorter, and at G, a part of an arch. Standing down the 
river and looking at the sheet, that part at G turns white or par- 
tially breaks, and the change begins at the bottom and runs up 
nearly to the top of the sheet. Then the part at F changes in 
the same manner, running rapidly from the bottom to the top, 
and then the part at E in the same manner. Then commencing 
again at G, the same order is observed without the least apparent 
variation. ‘There is some change observed in that part above 
the curved line, but small, especially in that part near the middle 
of the dam, or between Hand F. The changes at G, F, and E, 
correspond in time precisely with the vibrations. I mean by 
this, that the water breaks or changes white once in a second at 
Vol. xiv, No, 2.—July-Sept. 1843. 47 
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