i56 HISTORY OF PHYSICAL ASTRONOMY. 



cermine the amount of the oblateness. Laplace showed, at the same 

 timf, that along with this Inequality in Longitude there must be an 

 Inequality in Latitude ; and this assertion Burg confirmed by the dis- 

 cussion of observations. The two Inequalities, as shown in the obser- 

 vations, agree in assigning to the earth's form an Ellipticity of l-305th.] 



Sect, s. Confirmation of the Neiotonian Theory by Experiments on 



Attraction. 



THE attraction of all the parts of the earth to one another was, thus 

 proved by experiments, in which the whole mass of the earth is con- 

 cerned. But attempts have also been made to measure the attraction 

 of smaller portions ; as. mountains, or artificial masses. This is an ex- 

 periment of great difficulty ; for the attraction of such masses must be 

 compared with that of the earth, of which it is a scarcely percepti- 

 ble fraction ; and, moreover, in the case of mountains, the effect of 

 the mountain will be modified or disguised by unknown or unappre- 

 ciable circumstances. In many of the measurements of degrees, indi- 

 cations of the attraction of mountains had been perceived; but at the 

 suggestion of Maskclyne, the experiment was carefully made, in 1774, 

 upon the mountain Schehallien, in Scotland, the mountain being min- 

 eralogically surveyed by Playfair. The result obtained was, that the 

 attraction of the mountain drew the plumb-line about six seconds from 

 the vertical ; and it was deduced from this, by Hutton's calculations, 

 that the density of the earth was about once and four-fifths that of 

 Schehallieu, or four and a half times that of water. 



Cavendish, who had suggested many of the artifices in this calcula- 

 tion, himself made the experiment in the other form, by using leaden 

 balls, about nine inches diameter. This observation was conducted 

 with an extreme degree of ingenuity and delicacy, which could alone 

 make it valuable; and the result agreed very nearly with that of the 

 Schehallien experiment, giving for the density of the earth about five 

 and one-third times that of water. Nearly the same result was ob- 

 tained by Carliui, in 1824, from observations of the pendulum, made 

 at a point of the Alps (the Hospice, on Mount Cenis) at a considerable 

 slevation above the average surface of the earth. 



