I'KOrEK MOTION OF THE STARS, 185 



their distances, have, by leadini^ to the improvement and 

 perfcctiuu of arc-graduation and optical iustniiuonts in con- 

 nection \vith luicronietric appliances, contributed more than 

 any thing else to raise the science of observation to the 

 height which, by the ingenious employment of great merid- 

 ian-circles, refractors, and heliometers, it has attained, espe- 

 cially since the year 1830. *• 



The quantity of the measured proper motions of the stars 

 varies, as we intimated at the commencement of the present 

 section, from the twentieth part of a second almost to eight 

 seconds. The more luminous stars have in general a slower 

 motion than stars from the fifth to the sixth and seventh mag- 

 nitudes.* Seven stars have revealed an unusually great 

 motion, namely : Areturus, first magnitude (2""25) ; a Cen- 

 tauri, first magnitude (3"-58) ;t ^i Cassiopeiae, sixth magni- 

 tude (3"- 74) ; the double star, 6 Eridani, 5-4 magnitude 

 (4"-08) ; the double star 61 Cygni, 5-6 maguitude (5"- 123), 

 discovered by Bessel in 1812, by means of a comparison with 

 Bradley's observations ; a star in the confines of the Canes 

 Venatici,$ and the Great Bear, No. 1830 of the catalogue of 

 the circumpolar stars by Groombridge, seventh magnitude 

 (according to Argelander, 6"-974) ; e Indi (7"-74, according 

 to D' Arrest) \k 2151 Puppis, sixth magnitude (7" -871). The 

 arithmeticalil mean of the several proper motions of the fixed 

 stars in all the zones into which the sidereal sphere has been 

 divided by Miidler would scarcely exceed 0""102. 



An important inquiry into the "Variability of the proper 

 motions of Procyon and Sirius," in the year 1844, a short 



* Bessel, iii the Jahrlmch von Schumacher fur 1839, s. 38. Arago 

 Annuaire pour 1842, p. 389. 



t a Centauii, see Henderson and Maclear, in the Memoirs of the 

 Astron. Soc, vol. xi., p. 61 ; and Piazzi Smyth, in the Edinburgh 

 Transact., vol. xvi., p. 447. The proper motion of Areturus, 2"-'-25 

 (Baily, in the same Memoirs, vol. v., p. 165), considered as that of a 

 very bi'ight star, may be called veiy large in comparison with Akleba 

 ran, 0"*185 (Madler, Centralsonne, s. 11), and a Lyrue, 0"'400. Among 

 the stars of the first magnitude, a Centauri, with its great proper motion 

 of 3"*58, forms a very remarkable exception. The proper motion of 

 the binary system of Cygnus amounts, according to Bessel (Schuni 

 Astr. Nachr., bd. xvi., s. 93), to 5"-123. 



X Schumacher's Astr. Nachr., No. 4.5.5. 



§ Op. cit.. No. 618, s. 276. D'Arest founds this result on compansons 

 of Lacaille (1750) with Brisbane (1825), and of Brisbane with Taylor 

 (1835). The star 2151, Puppis, has a proper motion of 7"*871, and is 

 of the sixth magnitude. (Maclear, in Madler's Unters. uher die Fix- 

 ttern-Systeme, th. ii., s. 5.) 



II Schum., Astr Nachr., No. 661, s. 201 



