376 Vibrating Dams. 
Thus it is mentioned, p. 364, that a window when apparently at 
rest if put in motion by the hand, will continue to rattle ; showing 
that there was a power previously acting upon the window, but 
not quite sufficient to overcome all the resistances; yet with a 
little foreign assistance at starting, the vibrations are maintained. 
‘The seemingly capricious motions of the window indicate to me 
only slight changes of éntensify in the moving force, or in the 
amount of the resistances. Slight changes in the moving force 
may arise from achange in the depth of water upon the dam, 
due to the drain from the mills dependent upon it. Hence when 
the water is low, the phenomenon would more frequently happen 
by night than by day. Orit may be due toa real increase of 
water in the river, owing to showers at a distance. A change in 
the amount of the resistances may be due to the moisture of the 
atmosphere, temperature of the room, etc. Or finally, a slight 
change in the direction or force of the wind may occasion an 
appreciable difference in the strength of the transmitted vibra- 
tions ; and common impression would seem to ascribe nearly the 
whole effect to this circumstance. 
Hints to observers. 
As this phenomenon is not known to have been particularly 
studied hitherto, and as it seems intimately connected with an 
important branch of science, it is hoped that it may receive some 
attention. I therefore propose to direct observers to some points 
which seem to merit particular examination. 
1. The number of vibrations per second. ‘This should be de- 
termined with the utmost accuracy, and frequently verified to see 
if the number is invariable. When the number does not much 
exceed four or five vibrations per second, they may be directly 
counted. With a seconds watch in your hand, count the number 
for one minute. Repeat the process several times, and take the 
mean result. Do the same every day for a long period, and pre- 
serve the separate results. See if the discordances exceed the 
unavoidable errors of observation. If they do not, it may be pre- 
sumed the time is constant. When the number cf vibrations 
amounts to ten or fifteen per second, some artifice must be resort- 
ed to. Compare the rattle of the windows with that produced 
by some known movement; e. g. a wheel moving with a known 
velocity, and having projecting teeth which strike upon some 
