IV NOTES. 



( 10 ) p. 24. Kosmos, Bd. i. S. 175 (English edition, p. 158). " II est tres- 

 remarquable qu'un astronome, sans sortir de son observatoire, en comparant 

 seulement ses observations a 1'analyse, eut pu de'terminer exactement la gran- 

 deur et I'aplatissement de la terre, et sa distance au soleil et a la lune, e'le'mens 

 dont la connaissance a e'te' le fruit de longs et pe*nibles voyages dans les deux 

 hemispheres. Ainsi la lune, par 1'observation de ses mouvements, rend sensible 

 a 1'astronomie perfectionne'e Tellipticite de la terre, dont elle fit connaftre la 

 rondeur aux premiers astronomes par ses ellipses." (Laplace, Expos, du Syst. 

 du Monde, p. 230.) We have already noticed, in Bd. iii. S. 498 and 540 (En- 

 glish edition, p. 355 and cxxvii), an almost analogous optical proposal of Arago, 

 founded on the remark that the intensity of the ash-coloured light (i. e. of the 

 " earthlight" on the moon) might inform us as to the mean state of transparency 

 of the whole of our atmosphere. Compare also Airy, in the Encycl. Metrop. p. 

 189 and 236, on the determination of the Earth's ellipticity from the moon's 

 movements, and p. 231235, on deductions of the Earth's ellipticity from the 

 observed magnitudes of precession and nutation. According to Biot, procession 

 and nutation could only give for the ellipticity limitary values (^ and 3^), very 

 distant from each other. (Astron. physique, 3 e e'd. t. ii. 1844, p. 463.) 



(") p. 24. Laplace, Me'canique celeste, e'd. de 1846, t. v. p. 16 and 53. 



( 12 ) p. 24. Kosmos, Bd. i. S. 421. Anm. 1 (English edition, p. xliii. Note 

 131). The application of the isochronism of the vibrations of a pendulum in 

 Arabian astronomical writings was first made known by Edward Bernard in 

 England. See his letter from Oxford, April, 1683, to Dr. Robert Huntington, in 

 Dublin. (Phil. Trans, vol. xii. p. 567.) 



( 13 ) p. 24. FreVet, de 1'^ltude de la Philosophic ancienne, in the Mem. de 

 1'Acad. des Inscr. t. xviii. (1753) p. 100. 



( 14 ) p. 25. Picard, Mesure de la Terre, 1671, art. 4. It is hardly probable 

 that the conjecture expressed at the Paris Academy before 1671, of a variation 

 of the intensity of gravity with the latitude (Lalande, Astronornie, t. iii. p. 20, 

 2668), should have belonged to the great/ Huygens. Huygens had indeed pre- 

 sented his " Discours sur la cause de la Gravitd" to the Academy in 1669 ; it is 

 not, however, in this memoir, but in the " additamentis," one of which must have 

 been completed after the appearance of Newton's Principia, as it refers to it 

 therefore, after 1687 that Huygens speaks of the shortening of the second's pen- 

 dulum observed by Richer at Cayenne. He says: "Maxima pars hujus libelli 

 scripta est, cum Lutetian degerem(to 1681), ad cum usque locum, ubi de altera- 

 tione, qua3 pendulis accidit e motu Terra;." Compare the explanation given by 

 me in Kosmos, Bd. ii. S. 520, Anm. 2 (English edition, p. cxxv. Note 542). 

 The observations made by Richer at Cayenne were not published until fully six 



