327 



The President read a letter from Mr. William Edwards, 

 of South Natick, Mass., upon the Cause of the Vibra- 

 tions in water falling over dams. 



Mr. Edwards says that his attention was called to tliis subject 

 by the report of the Proceedings of the American Association 

 for the Advancement of Science at Montreal, and by recent dis- 

 cussions at the Boston Society of Natural History. The dam 

 across Charles River in the village of South Natick is nine feet 

 high and two hundred 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, the water being at its most 

 favorable point for the production of vibratory movement, in con- 

 nection with his brother, a series of experiments upon the subject 

 was undertaken by him. By erecting a flashboard, three feet in 

 length, on the top of the dam, the water was shut oif, 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. Snell, 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 dis- 

 charge of a pistol produced no perceptible effect upon the water, 

 although the report nearly stunned them. It having been sug- 

 gested 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 efll"ect 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 corner 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 increased, with the velocity of the water, to the extent of 

 fourteen inches. An aperture in the water, made by the passage 



