DOUBLE STARS. 287 



tive positions has yet been observed. 28 In the ternary com- 

 binations of Librae and Cancri, the periodical movement 

 of the two companions has been recognized with great cer- 

 tainty. The latter system consists of three stars of the 

 3rd magnitude, differing very little in brightness, and the 

 nearer companion appears to have a motion ten times more 

 rapid than the remoter one. 



The number of the double stars, the elements of whose 

 orbits it has been found possible to determine, is at present 

 stated at from fourteen to sixteen. 29 Of these Herculis 

 has twice completed its orbit since the epoch of its first 

 discovery, and during this period has twice (1802 and 1831) 

 presented the phenomenon of the apparent occultation of 

 one fixed star by another. For the earliest calculations of 

 the orbits of double stars, we are indebted to the industry of 

 Savary ( Ursae Maj.), Encke (70 Ophiuchi), and Sir John 

 Herschel. These have been subsequently followed by Bessel, 

 Struve, Madler, Hind, Smyth, and Captain Jacob. Savary's 

 and Encke's methods require four complete observations, 

 taken at sufficient intervals from each other. The shortest 

 periods of revolution are thirty, forty-two, fifty-eight, and 

 seventy-seven years ; consequently, intermediate between the 

 periods of Saturn and Uranus ; the longest that have been 

 determined with any degree of certainty exceed five hundred 

 years, that is to say, are nearly equal to three times the period 

 of Le Verrier's Neptune. The eccentricity of the elliptical 

 orbits of the double stars, according to the investigations 

 hitherto made, is extremely considerable ; resembling that of 

 comets, increasing from 0'62 (a- Coronaa), up to 0'95 (a Cen- 

 tauri). The least eccentric interior comet that of Faye 



' M Madler, Astr., s. 517. Sir John Herschel, Outl., p. 568. 

 29 Compare Madler, Untersuch. uber die Fixstern-Systeme, 

 th. i. s. 225-275; th. ii. s. 235-240; and his Astr., s. 541. 

 Sir John Herschel, Outl., p. 573. 



