254 



Prof. C. V. Boys. 



[June 20, 



devised by the Rev. John Mitchell, F.R.S., was first carried out by 

 Cavendish,* and has been since performed by Reich, + Baily,J and 

 Comu and Baille,§ who have all followed very closely the arrangement 

 of Cavendish. 



Owing to the very small value of the constant of gravitation, all 

 these experimentalists have aimed at increasing the sensibility as 

 much as possible. With this object, a long beam carrying at its ends 

 considerable masses has been suspended by a very long and very fine 

 wire. The attracting masses have been made as large as possible, and 

 they have been brought almost into contact with the sides of the long 

 box in which the beam is suspended. Cornu, it is true, has reduced 

 the dimensions of all the parts to about one-quarter of the original 

 amount. His beam, an aluminium tube, is only half a metre long, and it 

 carries at its ends masses of J lb. each, instead of about 2 lb. as used 

 by Cavendish. This reduction of the dimensions to about one-quarter 

 of those used previously is considered by Cornu to be one of the 

 advantages of his apparatus, because, as he says, if the period of 

 oscillation is unchanged, then the sensibility is independent of the 

 mass of the suspended balls, and is inversely as the linear dimensions. 

 I do not quite follow this, because, as I shall show, if all the dimensions 

 are increased or diminished together the sensibility will be unchanged. 

 If only the length of the beam is altered and the positions of the 

 large attracting masses, so that they remain opposite to and the same 

 distance from the ends of the beam, then the sensibility is inversely 

 as the length. 



The other improvements introduced by Cornu are the use of 

 mercury for the attracting masses which can be drawn from one pair 

 of vessels to the other without coming near the apparatus, the use of 

 a metal case connected with the earth to prevent electrical disturb- 

 ances, and the electrical registration of the movements of the index 

 on the scale which was placed 560 cm. from the mirror. The period 

 of oscillation which has been used has varied between 398 seconds 

 (Cornu) and 840 seconds (Cavendish). Cavendish found that with 

 the very inconvenient period of 1800 seconds the balls knocked 

 against the side of the case. 



The difficulty that has been met with has been the perpetual 

 shifting of the position of rest, due partly to the imperfect elasticity 

 or fatigue of the torsion wires, and partly, as Cavendish proved 

 experimentally, to the enormous effects of air currents set up by 

 temperature differences in the box, which with large apparatus it is 

 impossible to prevent. In every case the power of observing was in 



* ' Phil. Trans.,' 1798, p. 469. 



t ' Comptes Eendus,' 1837, p. 697. 



X ' Phil. Mag.,' vol. 21, 1842, p. 111. 



§ ' Comptes TCendiis,' vol. 76, p. 954 ; vol. 86, pp. 571, 699, 1001. 



