GENERAL CONSIDERATIONS. i, 



instrument of the kind was that constructed by the Rev. John 

 Michell, formerly Woodwardian Professor of -Geology at Cam- 

 bridge, in order to observe the effect of the attraction of a pair of 

 large lead balls on a pair of smaller balls hung from the extremities 

 of the rod of the balance. Michell, however, died before he had 

 opportunity to make the experiment, and his apparatus came into 

 the hands of Professor F. J. H. Wollaston, and was transmitted 

 by him to Henry Cavendish. Cavendish greatly improved the 

 apparatus,* and successfully measured the attraction of the balls, 

 and thus determined the density of the earth, f 



The experiment has since been repeated by Reich and Baily. 

 In the meantime, however, independently of Michell, and before 

 Cavendish had actually used the instrument, Coulomb J had in- 

 vented a torsion balance, by which he established the laws of the 

 attraction and repulsion of electrified and magnetic bodies. 



13. BIFILAR SUSPENSION. 



The elastic force of torsion of a wire, though much more regular 

 than the force of friction, is subject to alterations arising from 

 hitherto unknown causes, but probably depending on facts in the 

 previous history of the wire, such as its having been subjected to 

 twists and other strains before it was hung up. Hence it is 

 sometimes better to employ another mode of suspension, in which 

 the force of restitution depends principally on the weight of the 

 suspended parts. 



The body is suspended by two wires or fibres, which are close 

 together and nearly vertical, and are so connected by a pulley that 

 their tensions are equal. The body is in equilibrium when the 

 two fibres are in the same plane. When the body is turned about 

 a vertical axis, the tension of the fibres produces a force tending 

 to turn the body back towards its position of equilibrium ; and this 



* Cavendish's apparatus now belongs to the Royal Institution, 

 f "Philosophical Transactions," 1798. 

 J "Mem. de P Academic," 1784, &c. 



