Xll NOTES. 



gression from the surface to the centre, and with the certainly erroneous assump- 

 tion that the density of the outermost strata = 3, found, for the mean density 

 of the whole Earth, 4'7647, which differs considerably from the results of Reich, 

 5'577, and Baily, 5*660; far more so than the probable errors of observation could 

 at all account for. By a new discussion of Laplace's hypothesis, in an interest- 

 ing memoir which is soon to appear in Schumacher's Astr. Nachrichten, Plana 

 has arrived at the result that it is possible, by an alteration in the treatment 

 of this hypothesis, to represent very approximately Eeich's mean density of the 

 Earth, together with my estimate of 1/6 as the mean density of the outermost 

 stratum or surface of both land and sea, and the Earth's ellipticity, within the 

 probable limits of the last-named element. He says : " Si la compressibilite 

 des substances dont la Terre est forme'e a ete' la cause qui a donne' a ses couches 

 des formes re'gulieres, a peu pres elliptiques, avec une densite croissante depuis 

 la surface jusqu'au centre, il est permis de penser que ces couches, en se conso- 

 lidant, ont subi des modifications, a la verite fort petites, mais assez grandes 

 pour nous empecher de pouvoir driver, avec toute 1'exactitude que Ton pourrait 

 souhaiter, 1'e'tat de la Terre solide de son e'tat ante'rieur de fluidite. Cette re'- 

 flexion m'a fait appre'cier davantage la premiere hypothese, propose'e par 1'auteur 

 de la Mecanique celeste, et je me suis de'cide & la soumettre ^ une nouvelle dis- 

 cussion." 



( a ) p. 33. Compare Petit, " Sur la latitude de 1'Observatoire de Toulouse, la 

 densite' moyenne de la chaine des Pyre'ne'es, et la probabilite' qu'il existe un vide 

 sous cette chaine," in the Comptes rendus de 1'Acad. des Sc. t. xxix. 1849, p. 

 730. 



( M ) p. 34. Kosmos, Bd. i. S. 183 and 427, Anm. 10 (English edition, 

 p. 166, and Note 140). 



( 31 ) p. 34. Hopkins (Physical Geology), Reports of the British Association 

 for 1838, p. 92; Phil. Trans. 1839, Pt. II. p. 381; and 1840, Pt. I. p. 193; 

 Henry Hennessey (Terrestrial Physics), in the Phil. Trans. 1851, Pt. II. p. 504 

 and 525. 



(32) p> 34. Kosmos, Bd. i. S. 249 and 450452, Anm. 95 (English edi- 

 tion, p. 227, and Note 225). 



( 33 ) p. 35. The observations communicated by Walferdin are of the autumn 

 1847. They differ very little from the results (Kosmos, Bd. i. S. 181, and 

 Anm. 8 (English edition, p. 163, and Note 138); Comptes rendus, t. xi. 1840, 

 p. 707), which Arago obtained also with Walferdin's apparatus in 1840, at a 

 depth of 505 metres, just as the borer had passed through the chalk and began 

 to penetrate the gault. 



(**) p. 36. According to manuscript communications from the mine captain 



