360 E. Loomis on vibrating Water-falls. 
Observations at Lawrence, Mass. 
The dam at Lawrence is 900 feet long, and has an average 
height of 84 feet. It is in the form of an are of a circle whose 
radius is 7488 feet; and the versed sine of the arc is therefore 
nearly 14 feet. It is built of hewn stone, consisting of large 
blocks, accurately fitted to each other. 
ixty-five feet from the south side of the river, is a fish way 
80 feet wide, which effectually divides the falling sheet of water, 
cutting off 95 feet of the dam. The portion of the sheet of 
water which is entirely free, cannot exceed 805 feet in length. 
Stout bars of iron about three feet long, are inserted in holes 
drilled in the top stones, at intervals of 3 or 4 feet, and these 
support a row of planks called. flash boards, which raise the 
water 18 inches higher than the stone work of the dam. These 
planks are put up in June, and remain until they are carried off 
by the spring or winter freshets. Until these planks are carried 
away, the vibrations are never noticed. 
I have obtained the codperation of an excellent observer at 
wrence, viz: Mr. Benjamin Coolidge, engineer of the Essex 
Manufacturing Company, who has watched the dam for several 
as made careful observations at my request. The 
following is a summary of his results: 
In the spring of 1860, there was no freshet, and no vibrations 
were noticed upon the sheet of water. 
In the spring of 1861, the vibrations were observed as follows: 
March 5th. 100 vibrations in 87} seconds. Depth of water 
3°63 feet. 
March 11th. 100 vibrations'in 374 seconds. Depth of water 
3°41 feet. 
April 1st. 100 vibrations in 374 seconds. Depth of water 
3°12 feet. 
The flash boards were on the north end of the dam, for about 
60 feet; thence for 40 feet mostly gone. At the south end of 
the 40 feet, the vibrations began ind continued to the fish weir, 
which gives about 700 feet of vibrating fall. The vibrations 
made a distinct noise that could be heard pulsating above the 
roar of the fall. 
April 16th. 100 vibrations in 51 seconds. The depth of 
water on the edge of the crest-stone was 5 feet. Perhaps there 
were — 50 feet more of vibrating over-fall than on the Ist 
cect 
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