420 BINARY SYSTEMS. [SECT, xxxvn. 



the southern heavens, yet he has published a catalogue of 

 1081, the angles' distances of which he measured. Many of 

 these have very rapid orbital motions. Of all the double 

 stars, a Centauri is the most beautiful. Both stars are 

 of a bright ruddy colour, and, together, are equal, or even 

 superior, to Arcturus in brightness. The orbit of the re- 

 volving star is longer than that of Uranus, and its plane, 

 like that of 44 Bootis, passes through the solar system ; for 

 it sometimes happens that the edge of the orbit of a re- 

 volving star is presented to the earth, as in TT Serpentarii, 

 and those mentioned. Then the star seems to move in a 

 straight line, and to oscillate on each side of its primary. 

 Five observations are requisite in this case for the deter- 

 mination of the orbit, provided they be accurate. At the 

 time when Sir William Herschel observed the system in 

 question, the two stars were distinctly separate; in 1846 

 one was so completely projected on the other, that M. Struve, 

 with his great telescope, could not perceive the smallest 

 separation; on the contrary, the two stars of Orionis, 

 which appeared to be one in the time of Sir William 

 Herschel, have since separated. Another occultation of 

 a Centauri will take place in 1867. 



Were this libration owing to parallax, it would be annual, 

 from the revolution of the earth about the sun ; but, as 

 years elapse before it amounts to a sensible quantity, it can 

 only arise from a real orbital motion seen obliquely. 



Among the triple systems, two of the stars of Cancri 

 revolve about the third. -There are also quadruple stars, 

 and even assemblages of five and six, as 6 and <r of Orion. 

 It appears, from the calculations of Mr. Dunlop, that <r 

 Eridani accomplishes a revolution in little more than 30 

 years. The motion of Mercury is more rapid than that 

 of any of the planets, being at the rate of 107,000 miles an 

 hour. The perihelion velocity of the comet of 1680 was 

 880,000 miles an hour ; but, if the two stars of a- Eridani, or 

 of Ursse Majoris, be as remote from one another as the 



