418 BINAEY SYSTEMS. [SECT, xxxvir. 



cally by means of the angles of position and the correspond- 

 ing times of observation. The angular velocities of the 

 stars are obtained by drawing tangents to this curve at 

 stated intervals, whence the apparent distances, or radii 

 vectores of the revolving star, become known for each angle 

 of position, because, by the laws of elliptical motion, they 

 are equal to the square roots of the apparent angular ve- 

 locities. 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 repre- 

 sent on paper the actual orbit of the star projected on the 

 visible surface of the heavens ; so that the elliptical ele- 

 ments of the true orbit, and its position 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 re- 

 gions, which could not be known a priori, it must be com- 

 pared with as many observations as can be obtained, to 

 ascertain how far the computed ellipse agrees with the 

 curve actually described by the star. 



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

 tude ; they were so far apart in the beginning and middle 

 of last century, that they were mentioned by Bradley, 

 and marked in Mayer's catalogue as two distinct stars. 

 Since that time they have been continually approaching each 

 other, till in January, 1836, one star was seen to eclipse the 

 other, by Captain Smyth at his Observatory at Bedford, 

 and by Sir John Herschel at the Cape of Good Hope. A 

 series of observations since the beginning of the present 

 century has enabled Sir John to determine the form and 

 position of the elliptical orbit of the revolving star with ex- 

 traordinary truth by the preceding method. According to 

 his calculation, it came to its perihelion on the 18th of 

 August of the year 1834. Its previous velocity was so great 

 that the revolving star described an angle of 68 in one 

 year. By the laws of elliptical motion its angular ve- 



