Vibrating Dams. 371 
VI. Dam at Gardiner, Maine. 
The following is a letter from Mr. R. H. Gardiner. The Cob- 
bossee Contee river, a fine mill stream, empties into the Kenne- 
beck at this village. In the last mile of its course it falls a hun- 
dred and twenty seven feet. ‘There are now six dams across the 
stream, all of them built of stone, and make a fall of from twelve 
to twenty two feet each. ‘This however is not the height that the 
water falls perpendicularly, which varies from eight to sixteen 
feet. All the dams are of the same construction, that is, two 
walls of split stone are laid and filled in between with small 
stones and coarse gravel, and covered on the top with flat stones 
seven or eight feet long, making that the width of the dam at 
the top, which varies at the bottom from ten to fourteen feet ac- 
cording to the height. The upper side is filled with earth and 
gravel to within two feet of the top.’ The wall on the lower 
side is nearly perpendicular, and the water falls from seven to 
fifteen feet on a wooden apron of timber. These dams have 
been erecting during the last twenty years; but what is remark- 
able, the vibrations were never observed until last year, although 
the oldest dam, and the one which has the highest perpendicular 
fall, is in the midst of the village. In all these dams, the portion 
over which the water falls is a perfectly straight line, and varies 
from eighty to a hundred and sixty feet in length, and the water 
where it runs over the dam in freshets is from three to six feet 
deep. 
We have had no freshet this year and no vibrations. I regret- 
ted that I did not give more attention to them last year; but 
living more than a mile from the village, and being particularly 
occupied at the time, I omitted the opportunity. The vibrations 
were seldom heard in the day time, but regularly in the night. 
The only circumstances that Iam aware of that were pecu- 
liar to the last season were, Ist, the dam immediately above the 
lowest was built the summer preceding. The roll of this dam, 
over which the water falls on the apron, is the shortest in the river. 
Of course the water falls here with more violence. 2nd. The 
freshet occurred when the frost was coming out of the ground, 
and the earth was full of water. I could not learn that the vi- 
brations were felt when there was much wind. ‘The strongest 
vibrations were felt not close to the dams, but in some buildings 
