338 



Wm. Edwards, of Natick, upon the Vibrations caused 

 by Water falling over Dams. 



Mr. Edwards writes that, at the request of Mr. Stodder, he 

 counted, as nearly as possible, the number of vibrations, at some 

 distance from the dam, and the number of the waves, and, al- 

 though their rapidity made it very difficult to count them, ranging 

 as they did from 280 to 325 per minute, he found that they coin- 

 cided. This fact was rendered still more conclusive by assuming 

 a position at one end of the dam where the vibrations could be 

 seen, heard, and felt, all at the same time. Every portion of the 

 timber over which the water flows, produces vibrations of 

 greater or less distinctness, and, occasionally, the waves of a cer- 

 tain portion of the dam fall in the wave intervals of the other 

 end of the dam, and then the vibrations of the earth cease. 

 Standing in front of the dam, and placing a pole on the bed of 

 the river, directly under the fall, the jwle was violently agitated, 

 although there are two feet of back-water through which the 

 water must pass before it reaches the pole. In the falling sheet, 

 the wavelets are concavo-convex, and not double convex, that is, 

 the internal surface corresponding with an external convexity is 

 concave. 



Prof. W. B. Rogers remarked, that the wave-like divisions of 

 the descending sheet of water were probably referable to the 

 same general law wliich has been shown by Savart and Plateau 

 to obtain in the case of a stream flowing from an aperture in the 

 bottom or side of a vessel. These philosophers have proved that, 

 at a certain distance from the point of discharge, the stream, 

 although seemingly continuous, is in reality divided into separate 

 parts ; and Plateau explains this subdivision by the prepon- 

 derance of transverse cohesive forces in the column, when its 

 length exceeds its thickness by more than a determinate amount. 

 In this case the sides of the stream are drawn together at inter- 

 vals, and the mass is thus broken up into separate sections, 

 which, by further cohesive action, are moulded into drops. Thus 

 every such stream, at some distance below the aperture, loses its 

 straight outline, and assumes the form of a series of enlargements 

 and contractions, which, at a still greater distance, become visible 

 as a succession of drops. 



