158 TERRESTRIAL PHENOMENA. 



measured, this property of the pendulum, whereby, like a 

 sounding line, it searches unknown depths, and reveals in 

 volcanic islands, ( 132 ) or on the declivity of raised continental 

 mountain chains, ( 133 ) the presence of dense masses of basalt 

 and melaphyre, instead of subterranean cavities, renders a 

 general result in the deduction of the figure of the earth 

 difficult of attainment, in spite of the admirable simplicity 

 of the^onethod. In the astronomical part also of the mea- 

 surement of degrees, mountain chains and rocks, or strata 

 of great density, exert, though in a less degree, a prejudicial 

 influence on the determinations of latitude. 



As the figure of the Earth influences materially the 

 movements of other planetary bodies, and especially those 

 of her own satellite which is nearest to her, the more perfect 

 knowledge of the lunar movements enables us reciprocally 

 to infer from them the Earth's figure ; and thus, as Laplace 

 ingeniously remarked, ( 134 ) " an astronomer, without quit- 

 ting his observatory, may be able, from a comparison of the 

 lunar theory with actual observation, to determine not only 

 the form and magnitude of the Earth, but also its distance 

 from the Sun and Moon, results only to be otherwise ob- 

 tained by long and arduous undertakings in the remotest 

 regions of both hemispheres/' The ellipticity inferred from 

 the lunar inequalities has an advantage not afforded either 

 by measurements of degrees or by pendulum experiments, 

 in being independent of local accidents, and thus shewing 

 the mean ellipticity of the planet. The comparison of the 

 Earth's figure with its velocity of rotation serves also tj 

 establish the increase of density of the strata from the sur- 

 face to the center ; an increase which the comparison of the 

 ratios of the polar and equatorial axes of Jupiter anil Saturn 



