440 LECTURE XLVI. 



The internal constitution of the earth is little known from actual 

 observation, for the depths to which we have penetrated are comparatively 

 very inconsiderable, the deepest mine scarcely descending half a mile per- 

 pendicularly. It appears that the strata are more commonly in a direction 

 nearly horizontal than in any other ; and their thickness is usually almost 

 equable for some little distance ; but they are not disposed in the order of 

 their specific gravity, and the opinion of their following each other in a 

 similar series, throughout the greater part of the globe, appears to rest on 

 very slight foundations. 



From observations on the attraction of the mountain Shehallion, Dr. 

 Maskelyne* inferred the actual mean density of the earth to be to that of 

 water as 4^ to 1, judging from the probable density of the internal sub- 

 stance of the mountain, which he supposed to be a solid rock. Mr. 

 Cavendish f has concluded more directly, from experiments on a mass of 

 lead, that the mean density of the earth is to that of water as 5^ to 1. 

 Mr. Cavendish's experiments, which were performed with the apparatus 

 invented and procured by the late Mr. Michell, appear to have been con- 

 ducted with all possible accuracy, and must undoubtedly be preferred to 

 conclusions drawn from the attraction of a mountain, of which the internal 

 parts are perfectly unknown to us, except by conjectures founded on its 

 external appearance. Supposing both series of experiments and calcula- 

 tions free from error, it will only follow that the internal parts of Shehal- 

 lion are denser, and perhaps more metallic, than was before imagined. 

 The density assigned by Mr. Cavendish is not at all greater than might be 

 conjectured from observations on the vibrations of pendulums ; Newton 

 had long ago advanced it as a probable supposition that the mean density 

 of the earth might be about 5 or 6 times as great as that of water, and the 

 perfect agreement of the result of many modern experiments with this con- 

 jecture affords us a new proof, in addition to many others, of the accuracy 

 and penetration of that illustrious philosopher. $ 



LECT. XLVI. ADDITIONAL AUTHORITIES. 



Figure of the Earth. Snellius, Eratosthenes Batavus de Terrse Ambitus Quanti- 

 tate, Lugd. Bat. 1617. Norwood, The Seaman's Practice, 4to, Lond. 1637. Ric- 

 cioli, Geog. et Hydrog. fol. Bon. 1661. Cassini, De la Figure de laTerre, 12mo, 

 Amst. 1723. Hist, et Mem. 1735, p. 255 ; 1736, p. 64, H. 80. Maupertuis, Ph. 

 Tr. 1733, 1736, p. 302. Hist, et Mem. 1737, p. 389, H. 90. La Figure de la 

 Terre determinee, Paris, 1738. Examen des Ouvrages faits pour cet Objet, Amst. 

 1741. Clairaut on the Figure of Planets of unequal density, Ph. Tr. 1738, p. 277. 

 Celsius, De Figura Telluris, 4to, Upsal, 1738. Bouguer, La Fig. de la Terre, 4to, 

 Paris, 1749. Justification de do. 4to, Paris, 1752. Lettre sur do. 1754. War- 

 gentin, Schwed. Abhand. 1749, p. 243 ; 1750, pp. 3, 83 ; Ph. Tr. 1777, p. 162. La 



* Ph. Tr. 1775, p. 501. Button's Calculations, ibid. 1778, p. 689. See also 

 Zach, L' Attraction des Montagnes determinee par des Observations faites en 1810, 

 pres de Marseilles, 2 vols. Avignon, 1814. 



t Ph.Tr. 1798, p. 469. 



I Cavendish's Experiment has been repeated by Reich, Versuche iiber die Mitt- 

 lere Dichtigkeit der Erde, Freiburg, 1838, and by Baily, Memoirs of the Astrono- 

 mical Society, vol. xiv. who concludes that the mean density of the earth is 5-6747 

 times that of water. 



