185 



Mr. Briggs referred to a subject which had been dis- 

 cussed at a previous meeting of the Society, (see Proceed- 

 ings of May 1, 1850, Vol. iii. p. 287,) namely, the oscilla- 

 tion of the sheet of water at Hadley Falls, which is accom- 

 panied by a loud noise and a jarring sensation, perceptible 

 at a great distance. It had been attributed in part to the 

 vibration of the timber of which the dam is constructed. 

 He had recently observed the same phenomena at Trenton, 

 where there is a dam of sixty feet in length, with a fall of 

 twelve feet. Here the sheet, at certain stages of water, un- 

 dulates forward and backward through a distance of three 

 or four feet, and causes, by the jarring which it produces, 

 great annoyance to the dwellers in houses in its vicinity. 

 The dam is built of stone on a stone foundation up to 

 within three feet of the top, which is constructed of timber. 

 In this case, therefore, the oscillation cannot be properly 

 attributed to the vibration of the dam. The phenome- 

 non occurs when the water is about four inches deep on 

 the dam, ceasing as it becomes deeper. Mr. Briggs found 

 that by inserting a board at one end of the Fall, thus 

 diminishing the width of the sheet, the oscillation immedi- 

 ately ceased. In fact, it was evident that it depended 

 upon a relation between the width of the sheet, its thick- 

 ness, and the air beneath it. 



February 4, 1852. 



The President in the Chair. 



The Secretary presented, in the name of Mr. Girard, 

 descriptions of two new genera and two species of Nemer- 

 tes, as follows : — 



Hecate Girard. General form elongated, linear, subcylin- 

 drical, tapering posteriorly. Head oblong, rounded, anteriorly 



