CONCERNING SCIENTIFIC APPAEATUS. 513 



prism is accurately ground to a straight line, and rests on a plane horizontal 

 surface of agate. 



The relative motion in this case is one of rolling contact. 



In another class of instruments sliding and rolling are entirely done away 

 with, and sufficient freedom of motion is secured by the pliability of certain 

 solid parts. 



Thus many pendulums are hung, not on knife-edges, but on pieces of 

 watch-spring, and torsion balances are suspended by metallic wires or by silk 

 fibres. The motion of the piece is then affected by the elastic force of the 

 suspension apparatus, but this force is much more regular in its action than 

 friction, and its effects can be accurately taken account of, and a proper cor- 

 rection applied to the observed result. 



12. THE TORSION ROD, OR BALANCE OF TORSION. 



The balance of torsion has been of the greatest benefit to modern science 

 in the measurement of small forces. The first instrument of the kind was that 

 constructed by the Rev. John Michell, formerly Woodwardian Professor of Geology 

 at Cambridge, 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 mean- 

 time, however, independently of Michell, and before Cavendish had actually 

 used the instrument, Coulomb J had invented a torsion balance, by which he 

 established the laws of the attraction and repulsion of electrified and magnetic 

 bodies. 



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

 t Philosophical Transactions, 1798. 



* Mem. de I' Academic, 1784, tfcc. 



VOL. IL 



