SECT, xxxvii.] ORBITS AND PERIODIC TIMES. 419 



locity must diminish till it arrives at its aphelion. The 

 accuracy with which the motions of the binary systems are 

 measured, and the skill employed in the deduction of the 

 elliptical elements is now so great, that the periodic time 

 of y Virginis, determined by Sir John Herschel and Captain 

 Smyth, R.K, from their respective observatories, combined 

 with those of Sir William Herschel, only differ by two years, 

 Sir John having obtained a period of 182 years, Captain 

 Smyth that of 180. y Virginis is not the only instance of 

 the occultation of one star by another. Sir William Herschel 

 saw it take place with the two stars of Herculis, in 1782 ; 

 since that time the revolving star has made more than one 

 revolution. 



It appears that the ellipses in which the revolving stars 

 move are much more elongated than those of the planets, 

 that many of them are much larger, and their periods some- 

 times very much longer. The revolving star of e Bootis 

 takes 980 years to complete its circuit. M. Bessel's com- 

 putation gives a period of 540 years to 61 Cygni, now more 

 accurately known to be only 514 years, and M. Encke 

 ascertained that 70 Ophiuchi takes 80 years. The extremes 

 at present known are f Herculis, which accomplishes its 

 revolution in 35 years, while the star 65 Piscium takes 

 3077 years. 



Though the orbits of so few are actually known, yet the 

 motions of many hundreds have been measured by Sir 

 John Herschel and Sir James South; and the catalogue 

 of the double stars in the northern hemisphere, whose 

 motions have been micrometrically measured, has been 

 increased to 6000 by MM. Bessel, Struve, and other 

 astronomers. 



By the astronomers in our colonial establishments, extenv 

 sive catalogues of the double stars on the southern hemi- 

 sphere have been published. To these Sir John Herschel 

 added 500 during his residence at the Cape of Good Hope ; 

 and, although the object of his voyage was the nebulae of 



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