PREMONITIONS OF ISOSTASY 173 



From the unexpectedly small observed deflection of the vertical he in- 

 ferred, just as a modern geodesist would have done, that the volcano 

 must contain cavities. On tiie other hand, Charles Hutton" perverted 

 I lie attraction of Schehallien to a determination of the density of the 

 earth and entertained a poor opinion'^ of Cavendish's method,* now be- 

 come the standard means of determining this constant. Tliitton, how- 

 ever, introduced the method of dissecting a mountain mass into elements 

 bounded by two horizontal planes, tM-o vertical cylindrical surfaces, and 

 I wo vertical planes, radiating from tlio station, wliicli is still in use 

 much as he developed it. 



Laplace seems to have been the first to grasp the problem of isostasy. 

 In 1818 he pointed out that Bouguer's pendulum experiments at Quito 

 demonstrate that the Cordillera is of very low density, far smaller than 

 the mean density of the earth, which, following C^avendish, he takes at 

 5.5. This is a distinct recognition of compensation. Between the years 

 1735 and 1818 a considerable uimiber of observations in both hemispheres 

 had been made on the length of the pendulum beating seconds, and dis- 

 cussion of these, together Avith measurements of degrees and lunar obser- 

 vations, led Laplace to conclusions which may be expressed as follows: 

 The earth was once liquid and shells of equal density were approximately 

 spherical; it solidified throughout, for the most part with very slight 

 changes of configuration or disturbances of isostasy, and the irregularities 

 manifest at the surface then extended and still extend to a very small 

 depth compared with the earth's radius. The argument for the super- 

 ficiality of these irregularities, as I understand it, is substantially that 

 if Legendre's law of density (often referred to as Laplace's law) is as- 

 sumed, the computed attractions agree with those observed extremely 

 well, l)etter than they could agree if such irregularities in the distribution 

 of mass as are observahle at tlie surface prevailed at great depths.^ I 

 find nothing like a rigorous demonstration of this probable thesis. 



«PhiI. Trans., London, 1778. Button's Abridgment, vol. 14, p. 408. 



■^ Phil. Trans.. London. 1821, Part I, p. 276. 



" Phil. Trans.. London, 1798, p. 469. 



" Laplace's memoir on the figure of the earth appeared in the M^moires de I'Acad^mle 

 for 1817, printed in 1819. It is reprinted in his complete works, vol. 12, 1807. It Is 

 only partially reproduced in the chapter on the figure of the earth in Book XI of the 

 M<5canique r<?leste. A summary Is given in the memoir, but not in the magnum opus. 



His mathematical analysis he says : 



. . "compared with pendulum determinations, with the measurements of degrees, 

 and with lunar observations leads to these results : 



"1. The density of the shells of the terrestrial spheroid Increases from the surface to 

 (he center. 



"2. These shells are to a close appro\-iniaH<)n symniotrlcal with reference to the renter 

 of gravity. 



".S. The surface of this spheroid, a part of which Is covered by the sea, has a flgure 



