DIFFERENTIAL GRAVITY METER COMMITTEE. 141 



Report of the Committee, consisting of G-eneral J. T. Walker, Sir 

 W. Thomson, Sir J. H. Lefroy, Greneral E. Strachey, Professor 

 A. S. Herschel, Professor Gr. Chrystal, Professor C. Niven, 

 Professor A. Schuster, o,nd Professor J. H. Poynting {Secretary), 

 appointed for the purpose of inviting designs for a good 

 Differential Gravity Meter in supersession of the, pendulum, 

 whereby satisfactory results may be obtained at each station of 

 observation in a feiu hours, instead of the many days over tvhich 

 it is iiecessary to extend, the pendtolum observations. 



The Committee have issued the following circular. They subsequently 

 learnt of the work of M. Mascart in this direction. An account of his 

 investigation is appended. 



Copy of the Circular. 



The Committee hereby invite designs for an instrument to fulfil the 

 above condition. It should aim to give some ' statical ' measure of 

 variation in the weight of a fixed mass in place of the present laborious 

 ' dynamical ' method by means of the pendulum. 



The principle of a statical differential gravity meter was very clearly 

 stated by Sir J. Herschel in his ' Outlines of Asti'onomy ' (§ 189 in 

 editions 1-4, § 234 in later editions). He suggested, in illustration of 

 the principle, a weight suspended by a spiral spring, the spring being 

 always stretched to the same length, whatever the variations of gravity, 

 by the addition or removal of small weights. There appear to have been 

 only three attempts to construct such an instrument, resulting in the 

 torsion Gravimeter of the late J. Allan Broun and the two bathometers 

 of the late Sir C. W. Siemens. A full account of Mr. Broun's instru- 

 ment, by Colonel Herschel, will be found in the ' Proceedings of the Royal 

 Society ' (vol. xxxii. p. 507). An account is also given there of a proposal 

 for a similar instrument by M. Babinet, though the proposal does not 

 seem to have been carried out. In Mr. Broun's Torsion Gravimeter a 

 mass is supported by a bifilar suspension ; a third single wire along the 

 axis of suspension is also attached to the mass, and this third wire is 

 twisted till the mass is turned through 90°. If the weight increases the 

 amount of torsion of the single wire required to keep the weight at 90° 

 from its original position is increased. For details, see Colonel Herschel's 

 paper, which contains a careful criticism of the instrument. 



Sir C. W. Siemens' Bathometers are shortly described by Colonel 

 Herschel {loc. cit. p. 515), but a full account of them will be found in 

 the ' Phil. Trans.' 1876. The first instrument was virtually a barometer 

 with the cistern at the bottom containing a considerable quantity of air 

 and closed. The temperature was kept at 0° C. Any variation in gravity 

 led to an alteration in the height of the mercury column requisite to 

 balance the pressure of the air. The alteration in height of the mercury 

 was magnified 300 times by the use of two other liquids, one over the 

 other, above the mercury. The junction of the middle liquid with the 

 mercury was in an enlargement of the tube, and its junction with the top 

 liquid, this junction being the one observed, was in a narrow part of the 

 tube. The top surface of the uppermost liquid was also in an enlar"-e- 

 ment of the tube. Above this was a vacuum . 



