368 ORBITS OF DOUBLE STARS. SECT. XXXVII. 



have been extreme, and the apparent angular velocity 

 so great that it might have described an angle of 68 in 

 a single year. Observations made at the Cape of Good 

 Hope, by Sir John Herschel, as well as those of Captain 

 Smyth, R. N., at home, correspond in proving an aug- 

 mentation of velocity as the star was approaching its 

 shortest distance from its primary. By the laws of el- 

 liptical motion, the angular velocity of the revolving star 

 must now gradually diminish, till it comes to its aphelion 

 some 314 years hence. The satellite star of a Coronae 

 attained its perihelion in 1835, and that of Castor will do 

 the same some time in 1855. 



It sometimes happens that the edge of the orbit of a 

 revolving star is presented to the earth, as in TT Serpen- 

 tarii. Then the star seems to move in a straight line, 

 and to oscillate on each side of its primary. Five ob- 

 servations are requisite in this case for the determina- 

 tion of its orbit, provided they be accurate. At the time 

 Sir William Herschel observed the system in question, 

 the two stars were distinctly separate : at present, one 

 is so completely projected on the other, that M. Struve, 

 with his great telescope, cannot perceive the smallest 

 separation. On the contrary, the two stars of C Orionis, 

 which appeared to be one in the time of Sir William 

 Herschel, are now separated. Were this lib ration owing 

 to parallax, it would be annual, from the revolution of the 

 earth ; but as years elapse before it amounts to a sensi- 

 ble quantity, it can only arise from a real orbitual motion 

 seen obliquely. Among the triple stars, two of the stars of 

 Cancri revolve about the third. There are also quadru- 

 ple stars, and there are even assemblages of five and six 

 stars, as 6 and or of Orion. It is remarked that, in gen- 

 eral, the ellipses in which the revolving stars of binary 

 systems move, are much more elongated than the orbits 

 of the planets. Sir John Herschel, Sir James South, 

 and Professor Struve of Dorpat, have increased Sir 

 William Herschel's original catalogue of double stars to 

 more than 6000, of which thirty or forty are known to 

 form revolving or binary systems : and Mr. Dunlop has 

 formed a catalogue of 253 double stars in the southern 

 hemisphere. To this Sir John Herschel has added 

 many ; but he has found that the southern hemisphere 



