274 -^ Short History of Astronomy [LH. x. 



the mass of the whole earth is compared with that of a 

 globe of water of the same size, and, the size being known, 

 is expressible in pounds or tons. 



By a process of this sort Newton had in fact, with extra- 

 ordinary insight, estimated that the density of the earth 

 was between five and six times as great as that of water.* 



It was, however, clearly desirable to solve the problem 

 in a less conjectural manner, by a - direct comparison of 

 the gravitational attraction exerted by the earth with that 

 exerted by a known mass a method that would at the 

 same time afford a valuable test of Newton's theory of the 

 gravitating properties of portions of the earth, as distinguished 

 from the whole earth. In their Peruvian expedition ( 221), 

 Bouguer and La Condamine had noticed certain small deflec- 

 tions of the plumb-line, which indicated an attraction by 

 Chimborazo, near which they were working ; but the obser- 

 vations were too uncertain to be depended on. Maskelyne 

 selected for his purpose Schehallien in Perthshire, a narrow 

 ridge running east and west. The direction of the plumb- 

 line was observed (1774) on each side of the ridge, and 

 a change in direction amounting to about 12" was found 

 to be caused by the attraction of the mountain. As the 

 direction of the plumb-line depends on the attraction of 

 the earth as a whole and on that of the mountain, this 

 deflection at once led to a comparison of the two attrac- 

 tions. Hence an intricate calculation performed by Charles 

 Hutton (1737-1823) led to a comparison of the average 

 densities of the earth and mountain, and hence to the final 

 conclusion (published in 1778) that the earth's density was 

 about 4 1 times that of water. As Hutton's estimate of the 

 density of the mountain was avowedly almost conjectural, 

 this result was of course correspondingly uncertain. 



A few years \ater fa/in Michell (1724-1793) suggested, and 

 the famous chemist and electrician Henry Cavendish (1731- 

 1810) carried out (1798), an experiment in which the 

 mountain was replaced by a pair of heavy balls, and their 

 attraction on another body was compared with that of the 

 earth, the result being that the density of the earth was 

 found to be about 5^ times that of water. 



* Principia, Book III., proposition 10. 



