265 



Society, by Messrs. Briggs and Desor in 1850, (Proc. vol. 3, 

 p. 287,) and by Mr. Briggs in 1852, (vol. 4, p. 185.) 



The only cause for the phenomenon that I have seen assigned 

 is the agitation of the air, behind tlie falling sheet of water. Tliis 

 theory is effectually disproved — if there were no other reasons 

 against it — by the fact cited by Mr. Briggs, of the dam at Lewis- 

 ton, where the water falls over an inclined plane, leaving no 

 space for air under it ; yet the vibrations are very decided. 



The case of Hadley Falls seems to have attracted more atten- 

 tion than elsewhere, as probably the vibrations are moi-e power- 

 ful, and have been noticed at greater distances than in other 

 places. I expect to show that the causes of the vibrations are 

 there found nearer a maximum, and the intensity and force of 

 the vibrations, as well as the distance at which they are felt, 

 ought to be greater there than at any of the other places which 

 have been referred to. 



The dam at Hadley is 1,000 feet long, at nearly right angles to 

 the current of the river, and causes a vertical fall of thirty or 

 thirty two feet. The water does not fall in an even stream from the 

 summit of the dam to the surface of the water below, but the upper 

 surface in section presents to the eye a waved or curved outline. 

 This appearance I have noticed at Hadley, Nashua, Lawrence, 

 and at every other vertical fall which I have ever seen, when 

 under the proper conditions to exhibit it. If we could see the 

 under side of the sheet, we should find, undoubtedly, correspond- 

 ing appearances on that side. This phenomenon is caused by the 

 property of falling fluids, by which they assume the globular 

 form, which may be seen in the Kauterskill Falls, on the Cats- 

 kill Mountains, where the whole body of falling water is broken 

 into spray and drops — in the fall of water from the jet of a foun- 

 tain, and in water flowing from a vessel. In all cases, if the 

 water falls a certain distance, proportioned to its mass, the whole 

 will assume the globular form, and become drops. Applying 

 this principle to the fall over an artificial dam, the water at the 

 very commencement of its descent begins to assume that form, 

 and the further it descends, the nearer it approaches it. In pass- 

 ing over an artificial dam, like that at Hadley, the water presents 

 a uniform depth throughout the whole length of the dam ; and if 

 we imagine the current of water to be an infinitude of small 



