NOTES. 



from us, and that the light from them can only reach our eyes at intervals ot 

 time which differ several minutes from each other. I will not attempt to dec! Je 

 whether what you saw on the banks of the Orinoco, not at intervals of seconds, 

 but of minutes, were actual coruscations of the zodiacal light, or whether they 

 belonged solely to the upper strata of our atmosphere. Nor can I explain the 

 remarkable lightness of entire nights, or the anomalous increase and prolon- 

 gation of twilight in the year 1831, particularly if, as it has been said, the 

 lightest part of these singular twilights did not coincide with the place of the 

 sun below the horizon." (Extract from a letter from Dr. Olbers to myself, 

 written from Bremen, March 26, 1833.) 



( 10 ) p. 133. Biot, Traite d'Astron. physique, 3 e ed. 1841, T. i. pp. 171, 

 238, and 312. 



( 101 ) p. 131 Bessel, in Schum. Jahrb. for 1839, S. 51; perhaps four 

 millions of geographical miles in a day, in relative velocity at least 3336000 

 miles, or more than double the velocity of revolution of the earth in her orbit. 



( 102 ) p. 135. On the proper motion of the solar system, according to 

 Bradley, Tobias Mayer, Lambert, Lalande, and William Herschel, see Arago, 

 Ammaire, 1842, p. 388399 ; Argelander, in Schum. Astr. Nachr. Nr. 363, 

 364, and 398 : and on Perseus as the central body of our sidereal stratum, in 

 the treatise, Von der eigenen Bewegung des Sonnensystems, ] 837, S. 43 ; 

 also Otho Struve, in the Bull, de 1'Acad. de St.-Petersb. 1842, T. x. No. 9, 

 p. 137 139. By a more recent combination, Otho Struve found, for the 

 direction of the movement of the solar system, 261 23' A.R., + 37 36' Decl. ; 

 and, uni'ung his result with Argelander's, we find, by a combination of 797 

 stars, 259 9' A.R., + 34 36' Decl. 



( m ) p. 136.- Aristot. de Ccelo, iii. 2, p. 301 ; Bekker, Phys. viii. 5, 

 p. 256. 



O p. 137. Savary, in the Connaissance des Terns, 1830, p. 56 and 163 ; 

 Encke, Berl. Jahrb. 1832, S. 253 ff. ; Arago, Anuuaire, 1834, p. 260295 ; 

 John Herschel, in Mem. of the Astron. Soc. Vol. v. p. 171. 



( 10S ) p. 137. Bessel, Untersuchung des Theils der planetarischen Sto- 

 rungen welche aus der Bewegung der Sonne eutstehen, in Abh. der Berl. Akad. 

 der Wissensch. 1824 (Mathem. Classe) S. 26. The question has been 

 raised by Johann Tobias Mayer, in Comment. Soc. Reg. Getting. 1804, 108 ; 

 Vol. xvi. p. 3168. 



O p. 138. Phil. Trans. 1803, p. 225. Arago, Annuaire, 1842, p. 375. 

 Some idea of the distance assigned in the text to the nearest fixed stars may 

 be obtained by considering, that if we take one French foot as the Earth's 



