VI NOTES. 



(obtained at earlier periods by Bouguer, Legentil, Lacaille, Maupertius, and La 

 Croyere) could not be looked upon as having nearly the same degree of exact- 

 ness. 



( 19 ) p. 28. Biot and Arago, Recueil d'Observ. ge'ode'siques et astronomiques, 

 1821, p. 526540, and Biot, Traitd d'Astr. phys. t. ii. 1844, p. 465473. 



( 20 ) p. 28. Idem, p. 488. Sabine (Exper. for determining the Variation in 

 the Length of the Pendulum vibrating Seconds, 1825, p. 352) found from his 

 thirteen stations, so widely dispersed over the northern hemisphere, ^5 ; and 

 from the same with the addition of all the pendulum stations of the British Sur- 

 vey and of the French arc of the meridian (from Formentera to Dunkirk) from 

 the comparison therefore, in all, of 25 points of observation almost the same re- 

 sult, viz. ~^. At a considerable distance from the above stations (situated 

 on the two sides of the Atlantic), the pendulum lengths found in the meridians of 

 Petropaulowski and Sitka, give, as Liitke has remarked, a still much higher 

 ellipticity : 5^. Bessel, with his peculiar lucidity, has shown analytically, in 

 his investigations on the length of the simple second's pendulum (S. 32, 63, and 

 126 129), how the theory of the influence of the air surrounding the pendulum 

 which was previously generally received and employed, leads to an error in cal- 

 culation, and renders a correction necessary, on account of the difference of the loss 

 of weight of solid bodies when they are immersed in a fluid, either in a state of 

 repose or in vibration. Such a correction was somewhat obscurely pointed to in 

 1 786, by the Chevalier de Buat, Bessel says, " If a body moves in a fluid (air), 

 the latter becomes part of the moving system ; and the moving force must be im- 

 parted, not only to the particles of mass of the solid body put in motion, but also 

 to all the particles of mass of the fluid which are put in motion." On the experi- 

 ments of Sabine and Baily, to which Bessel's practically important pendulum 

 correction (reduction to a vacuum) gave occasion, see John Herschel in his 

 Memoir of Francis Baily, 1845, p. 17 21. 



( 21 ) p. 28. Kosmos, Bd. i. S. 175 and 422, Anm. 2 (English edition, 

 p. 158 ; Note 132). Compare for the phenomena in islands, Sabine, Pendulum 

 Exper. 1825, p. 237 ; and Lutke, Obs. du Pendule invariable, exe'cute'es de 

 1826 1829, p. 241. The same work contains a remarkable table of the kinds 

 of rocks at sixteen pendulum stations (p. 239) from Spitsbergen (79 50' N. 

 lat.) to Valparaiso (in 33 2' S. lat.). [See also Sabine, Pend. Exper. 1825, 

 p. 338 ; in which is the first table of this description, containing thirteen sta- 

 tions. Ed.] 



( K ) p. 29. Kosmos, Bd. i. S. 424, Anm. 5 (English edition, Note 

 135). Eduard Schmidt (Mathem. und phys. Geographic, Th. i. S. 394) 

 has taken from the many pendulum observations made in the corvettes Descu- 



