160 COSMOS. 



melaphyre instead of cavities, render it difficult, notwith- 

 standing the admirable simplicity of the method, to arrive at 

 any great result regarding the figure of the Earth from obser- 

 vation of the oscillations of the pendulum. In the astrono- 

 mical part of the determination of degrees of latitude, moun- 

 tain chains, or the denser strata of the Earth, likewise exercise, 

 although in a less degree, an unfavourable influence on the 

 measurement. 



As the form of the Eartn exerts a powerful influence on 

 the motions of other cosmical bodies, and especially on that of 

 its own neighbouring satellite, a more perfect knowledge of 

 the motion of the latter will enable us reciprocally to draw an 

 inference regarding the figure of the Earth. Thus, as Laplace 

 ably remarks,* " An astronomer, without leaving his obser- 

 vatory, may by a comparison of lunar theory with true obser- 

 vations, not only be enabled to determine the form and size of 

 the Pjarth, but also its distance from the Sun and Moon -results 

 that otherwise could only be arrived at by long and arduous 



Plana, in Lombardy, differences ranging from 20" to 47"'8 have been 

 found between direct observations of latitude and the results of these 

 operations. (See the instances of Andrate and Mondovi, and those of 

 Milan and Padua, in the Operations Geodes. et Astron. pour la Mesure 

 d'un Arc du Parallel Moyen, i. ii. p. 347 ; Effemeridi Astron. di Mi- 

 lano, 1842, p. 57.) The latitude of Milan, deduced from that of Berne, 

 according to the French triangulation, is 45 27' 52", while, according 

 to direct astronomical observations, it is 45 27' 35". As the perturba- 

 tions extend in the plain of Lombardy to Parma, which is far south of 

 the Po, (Plana, Operat. Geod., i. ii. p. 847,) it is probable that there are 

 deflecting causes concealed beneath the soil of the plain itself. Struve 

 has made similar experiments [with corresponding results] in the most 

 level parts of eastern Europe. (Schumacher, Astron. Nachrichten, 1 830, 

 Nr. 164, s. 399.) Regarding the influence of dense masses supposed to 

 lie at a small depth, equal to the mean height of the Alps, see the ana- 

 lytical expressions given by Hossard and Rozet, in the Comptes Rendus, 

 t. xviii., 1844, p. 292, and compare them with Poisson, Traite de M&- 

 canique, (2me ed.) t. i. p. 482. The earliest observations on the influence 

 which different kinds of rocks exercise on the vibration of the pendulum 

 are those of Thomas Young, in the Philos. Transactions for 181 9, pp. 

 70_96. In. drawing conclusions regarding the Earth's curvature from 

 the length of the pendulum, we ought not to overlook the possibility 

 that its crust may have undergone a process of hardening, previously to 

 metallic and dense basaltic masses having penetrated from great depth* 

 through open clefts, and approached near the surface. 

 * Laplace, Exp*s. du Syat. du Monde, p. 231. 



