S*cr. XXXVH. BINARY SYSTEMS. 367 



are equal to the square roots of the apparent angular 

 velocities. Now that the angles of position estimated 

 from a given line, and the corresponding distances of the 

 two stars, are known, another curve may be drawn, 

 which will represent on paper the actual orbit of the 

 star projected on the visible surface of the heavens ; so 

 that the elliptical elements of the true orbit and its posi- 

 tion in space may be determined by a combined system 

 of measurements and computation. But as this orbit 

 has been obtained on the hypothesis that gravitation 

 prevails in these distant regions, which could not be 

 known d priori, it must be compared with as many 

 observations as can be obtained, to ascertain how far the 

 computed ellipse agrees with the curve actually described 

 by the star. 



By this process Sir John Herschel has discovered 

 that several of these systems of stars are subject to the 

 same laws of motion with our system of planets : he has 

 determined the elements of their elliptical orbits, and 

 computed the periods of their revolution. One of the 

 stars of y Virginis revolves about the other hi 629 years ; 

 the periodic time of a Corona? is 287 years ; that of 

 Castor is 253 years; that of t Bootes is 1600 ; that of 

 70 Ophiuchi is ascertained by Professor Encke to be 80 

 years ; Professor Bessel has ascertained the period of 

 61 Cygni to be 540 years ; and M. Savary, who has the 

 merit of having first determined the elliptical elements 

 of the orbit of a binary star from observation, has shown 

 that the revolution of f Ursae is completed in 58 years. 

 y Virginis consists of two stars of nearly the same mag- 

 nitude. They were so far apart in the beginning and 

 middle of the last century, that they were mentioned by 

 Bradley and marked in Mayer's catalogue as two distinct 

 stars. Now, they are so near to one another, that even 

 with good telescopes they look like a single star some- 

 what elongated. A series of observations, since the 

 beginning of the present century, has enabled Sir John 

 Herschel to determine the form and position of the el- 

 liptical orbit of the revolving star with extraordinary 

 truth. According to his computation, it must have ar- 

 rived at its perihelion on the 18th of August of the year 

 3 834. The actual proximity of the two stars must then 



