NATURAL PHILOSOPHY. 181 



OX THE VIBEATIOXS IX WATER FALLING OVER DAMS. 



The continuance of the discussion on this subject has been kept up during 

 the past year, in the Boston Society of Natural History, and at a recent meet- 

 ing the following communication respecting experiments made at South 

 Natick, Mass., by Mr. William Edwards, was presented. The dam at this 

 place across Charles River, is nine feet high and 200 feet long, and at certain 

 stages of the water, the vibrations are so powerful as to agitate bodies three- 

 quarters of a mile distant. During the month of December, 18-37, the water 

 being at its most favorable point for the production of vibratory movement, 

 Mr. E. undertook a series of experiments by erecting a flashboard, three feet 

 in length, on the top of the dam ; the water was shut off, and a dry entrance 

 obtained behind the falling sheet. Ample room was found to walk back and 

 forth. At the place of entrance, the flame of a lamp was unagitated, a fact 

 unexpected by them, as they had favored the hypothesis of Prof. Sncll, and 

 had supposed that air sufficient to cause the vibrations would have found an 

 exit at this opening. 



Twenty-five or thirty feet from the entrance, the flame was slightly agitated 

 simultaneously with the vibratory motions. A falling feather descended as 

 quietly as in a close room. The discharge of a pistol produced no perceptible 

 effect upon the water, although the report nearly stunned them. It having 

 been suggested that the vibrations are produced at the bottom of the sheet 

 and continued upward to the top of the dam, they endeavored to produce 

 such an effect by placing obstructions at various points, but without success. 

 Whilst holding the ends of the fingers in the current, two or three inches 

 from the comer of the timber over which the water breaks, it was evident 

 that the water passed the fingers in ridges, apparently one-half or three- 

 quarters of an inch apart. By following with the eye, in their descent, these 

 ridges or vibrations, it was found that the interval between them inci'eased, 

 with the velocity of the water, to the extent of fourteen inches. An aperture 

 in the water, made by the passage of a stone of the size of a hen's egg, at the 

 top of the fall, increased in size just in proportion to the increasing distance 

 of the vibrations, retaining its circular form, and finally expanded to fourteen 

 inches in diameter. Upon the top of the dam they could see distinctly 

 through the current to the edge of the timber over which the water breaks, 

 and they found that the water at this point acquires a tremulous motion, giving 

 origin to the ridges or vibrations alluded to above, which here follow each other 

 at intervals not exceeding one quarter of an inch. The sheet of water at the 

 top of the dam is six inches in thickness; at the bottom tw r o and a half 

 inches. The ridges are evident on the inside the whole length of the fall, but 

 upon the outside they do not make their appearance for the space of ten or 

 twelve inches. They increase in size relatively with the distance. 



At a subsequent meeting,' Mr. E. stated that he had counted, as nearly as 

 possible, the number of vibrations, at some distance from the dam, and the 

 number of the Avaves, and, although their rapidity made it very difficult to 

 count them, ranging as they did from 280 to 325 per minute, he found that 

 they coincided. This fact was rendered still more conclusive by assuming a 

 position at one end of the dam where the vibrations could be seen, heai'd, 

 and felt, all at the same time. Every poi'tion of the timber over which the 

 water flows, produces vibrations of greater or less distinctness, and, occa- 

 sionally, the, waves of a certain portion of the dam fall in the wave intervals 



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