Ixxii NOTES. 



" Nous observames avec beaucoup de soin, M. Mathieu et moi, pendant le 

 mois d'aout 1812, et pendant le mois de novembre suivant, la hauteur angu- 

 laire de 1'etoile au-dessus de 1'horizon de Paris. Cette hauteur, a la seconde 

 epoque, ne surpasse la hauteur angulaire de 1'etoile au-dessus de 1'horizon de 

 Paris. Cette hauteur, a la seconde epoque, ne surpasse la hauteur angulaire a 

 la premiere que de 0"-66. Une parallaxe absolue d'une seule seconde aurait 

 necessairement amene entre ces deux hauteurs une difference de l"-2. Nos 

 observations n'indiquent done pas que le rayon de 1'orbite terrestre, que 39 

 millions de lieues soient vus de la 61 e e <J U Cygne sous un angle de plus 

 d'une demi-seconde. Mais une base vue perpendiculairement soutend un 

 angle d'une demi-seconde quand on en est eloigne de 412 mille fois sa 

 longueur. Done la 61 e du Cygne est au mains a une distance de la Terre 

 egale a 412 mille fois 39 millions de lieues." 



( 309 ) p. 188. Bessel published in Schum. Jahrb. 1839, S. 39-49, and in 

 the Astr. Nachr. No. 366, the result 0"'3136, as a first approximation. His 

 subsequent final result was 0^3483 (Astr. Nachr. No. 402 in Bel. xvii. 

 S. 274). Peters found by his own observations, the almost identical result 

 of 0"-3490. (Struve, Astr. stell. p. 99.) The alterations which, after Bessel's 

 death, Professor Peters made in that astronomer's calculation of the angular 

 measurements obtained with the Konigsberg Heliometer, were founded on the 

 circumstance that Bessel himself (Astr. Nachr. Bd. xvii. S. 267) had promised 

 to subject the influence of temperature on the results with the heliometer to a 

 fresh examination, which intention he executed partially in the 1st Vol. of his 

 " Astronomischen Untersuchungen," but did not apply the temperature cor- 

 rection to the observations of parallax. This application was made by Peters 

 (Erganzungsheft zu den Astr. Nachr. 1849, S. 56), and in consequence this 

 distinguished astronomer found 0"'3744, instead of 0"-3483. 



( 31 ) p. 188. This result of 0"'3744 gives, according to Argelander, the 

 distance of the double star 61 Cygni from the Sun = 550900 mean distances 

 of the Earth from the Sun, or 11394000 German (45576000 English) geo- 

 graphical miles ; a distance which light traverses in 3177 mean days. By 

 the three successive assignments of parallax given by Bessel, 0"'3136, 

 0"'3483, and 0'3744, this star has come (apparently) gradually nearer to us ; 

 they correspond respectively to light-passages of 10, 9, and 8 T 7 3 years. 



( 311 ) p. 188. Sir John Herschel, Outlines, p. 545 and 551. Madler 

 (Astr. S. 425) gives for a Centauri, instead of 0"'9128, the parallax 0^9213. 



( 312 ) p. 189. Struve, Stell. compos. Mens. microm. p. clxix. clxxii. 

 Airy considers the parallax of a Lyrsc, which Peters has already diminished 



