1880.] On a Simplified Form of Torsion-Gravimeter. 



141 



December 9, 1880. 



THE TREASURER in the Chair. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



The Bishop of Limerick and Professor Asa Gray (Foreign Member) 

 were admitted into the Society. 



The Chairman announced that the President had appointed as Vice- 

 Presidents : — 



The Treasurer. 

 Mr. W. H. Barlow. 

 Dr. Hirst. 

 Sir James Paget. 

 General Strachey. 



The following Papers were read : — 



I. " On a Simplified Form of the Torsion-Gravimeters of Broun 

 and Babinet." By Major J. Hersghel, R.E., F.R.S., 

 Deputy Superintendent, Great Trigonometrical Survey of 

 India. Received October 31, 1880. 



The present communication anticipates one of greater length and 

 extent, which I hope to be enabled to offer for publication in the 

 "Proceedings" of the Society, the subject of which is the gravi- 

 meters mentioned in the above title. In case the account which I 

 there give of these instruments should be delayed, it seems advisable 

 to hasten, if possible, the time when a serviceable gravimeter shall be 

 available to geodesists, by pointing out a method by which the same 

 principle may be embodied in a simpler instrumental shape. It is 

 not necessary for this purpose that a prior knowledge of the existing 

 designs should be presumed. I hope to be able to show all that 

 is essential without drawing upon any source but a moderate know- 

 ledge of physics. 



Let us imagine a weight, P, suspended by two* parallel lines, 

 flexible, but inelastic, and offering no resistance on their own account 

 to torsion. Let their length be R, and their distance asunder 2r. 

 Also let be the angle through which P is turned by some external 

 force. Then it may be shown that the force which P exerts in a 



* There is no necessary restriction to two suspenders. 

 VOL. XXXI. M 



