154 NEWTON S PRINCIPIA. 



accelerated, sometimes retarded, but apparently round 

 one another, or rather round their common centres of gra 

 vity. A course of observations continued for above 

 twenty years, led Herschel to this important conclusion 

 about the year 1803 ; his son has greatly added to our 

 knowledge of these motions ; and Professor Struve, of 

 Dorpat, applying geometrical reasoning to the subject, cal 

 culated the orbits in which some of the bodies appear 

 to move. One of the most remarkable is the star y Vir- 

 ginis, on which Cassini had made observations in 1720.* 

 It has now been found that one of the stars of which 

 it is composed is smaller than the other ; that the revolv 

 ing motions of the two during the first 25 years had a 

 mean annual velocity of 31 23&quot; ; during the next 

 21 years, of 29 17&quot; ; during the next 17 years, of 

 only 2 42&quot;; and during the last two years (1822, 

 23) of no less than 52 51&quot;. The elder Herschel 

 calculated the time of their whole revolution, the pe 

 riodic times of those distant suns, at 708 years; it is 

 now supposed not to exceed 629. Another pair of stars 

 are found to revolve round one another in between 43 

 and 44 years, while a third pair take 12 centuries to 

 accomplish their revolution.! Although our observations 

 are far too scanty to lay as yet the ground of a system 

 atic theory of these motions, they appear to warrant us in 

 assuming that the law of attraction which governs our 

 solar system extends to those remote regions, and as their 

 suns revolve round one another, each probably carrying 

 about with it planets that form separate systems, we shall 

 probably one day find that equal areas are there as here 

 described in equal times, and that the orbits are ellip 

 tical ; or, which would come to the same thing, that the 



* Mem. Acad. ties Sciences, 1720. 



f Phil. Trans. 1803, p. 339; ib. 1824, Part III. 



