by Dr Thomas Young 443 



18. Experiments to determine the Density of the Earth. (Phil. Trans. 1798, 

 p. 469.) The apparatus, with which this highly important investigation was 

 conducted, had been invented and constructed many years before by the 

 Reverend John Michell, who did not live to perform the experiments for which 

 he intended it. Mr Cavendish, however, by the accuracy and perseverance with 

 which he carried on a course of observations of so delicate a nature, as well as 

 by the skill and judgment with which he obviated the many unforeseen diffi- 

 culties that occurred in its progress, and determined the corrections of various 

 kinds which it was necessary to apply to the results, has deserved no less 

 gratitude from the cultivators of astronomy and geography, than if the idea 

 had originally been his own. The method employed was to suspend, by a vertical 

 wire, a horizontal bar, having a leaden ball at each end; to determine the magni- 

 tude of the force of torsion by the time occupied in the lateral vibrations of the 

 bar; and to measure the extent of the change produced in its situation by the 

 attraction of two large masses of lead, placed on opposite sides of the case 

 containing the apparatus, so that this attraction might be compared with the 

 weight of the balls, or, in other words, with the attraction of the earth. In 

 this manner the mean density of the earth was found to be 5^ times as great as 

 that of water; and although this is considerably more than had been inferred 

 from Dr Maskelyne's observations on the attraction of Schehallion, yet the 

 experiments agree so well with each other, that we can scarcely suppose any 

 material error to have affected them. Mr MichelTs apparatus resembled that 

 which M. Coulomb had employed in his experiments on magnetism, but he 

 appears to have invented it before the publication of M. Coulomb's Memoirs. 



19. On an Improved Method of Dividing Astronomical Instruments. (Phil. 

 Trans. 1809, p. 221.) The merits of this improvement have not been very 

 highly appreciated by those who are in the habit of executing the divisions of 

 circular arcs. It consists in a mode of employing a microscope, with its cross 

 wires, as a substitute for one of the points of a beam compass, while another 

 point draws a faint line on the face of the instrument in the usual manner. The 

 Duke de Chaulnes had before used microscopical sights for dividing circles ; but 

 his method more nearly resembled that which has been brought forwards in an 

 improved form by Captain Kater; and Mr Cavendish, by using a single micro- 

 scope only, seems to have sacrificed some advantages which the other methods 

 appear to possess : but none of them has been very fairly tried ; and our artists 

 have hitherto continued to adhere to the modes which they had previously 

 adopted, and which it would perhaps have been difficult for them to abandon, 

 even if they had been convinced of the advantages to be gained by some partial 

 improvements. 



Such were the diversified labours of a philosopher, who possessed a clearness 

 of comprehension and an acuteness of reasoning which had been the lot of very 

 few of his predecessors since the days of Newton*. Maclaurin and Waring, 

 perhaps also Stirling and Landen, were incomparably greater mathematicians; 



* [As regards Cavendish's dynamical manuscripts, see the end of the second 

 volume of this edition.] 



