9€ Messrs Herschel and South on the Double Stars 



nected in a binary system by their mutual gravitation, and re- 

 volving round their common centre of gravity with a motion so 

 rapid as to admit of being traced and measured from month to 

 month, must be allowed to be a phenomenon of no common inte- 

 rest, and deserving every attention from the theoretical and 

 practical astronomer. Sir W. Herschel first pointed out its ra- 

 pid change of position, and the observations of M. Struve and 

 Messrs Herschel and South indicate a remarkable alteration 

 in its velocity, which can only be accounted for by supposing 

 the relative orbit to be one of great ellipticity. The following 

 are the different angles of position. 



Sir W. Herschel, 



Mean of Struve's 17 observations, 

 Mean of Struve's 7 observations. 

 Mean of Messsrs Herschel and South's, 

 Mean of Mr South's, 



The mean distance of the two stars at the last of these 

 epochs Mr South found to be 2."442. 



^' In the first interval,'" says Mr Herschel, " of 21.11 years 

 48°.72, were described, giving an annual motion of 2°.309. In 

 the next interval of 16°.93 years, 177°-75 ^^^^^ described, being 

 at the mean rate of 10". 5 in the year. In the next period of 1.74 

 years, the angle described was 12°.03, or 6°.914 per annum, 

 while in the succeeding short period of 1°.54 years the motion 

 amounted only to 4*.442. It is therefore at present rapidly 

 diecreasing, and the maximum annual motion must at some 

 period between 1803 and 1820 have greatly exceeded 10°.5, 

 and perhaps may have amounted to 20° or 30°. This consi- 

 deration would lead us to place the perihelion of the orbit in 

 the north preceding quadrant, between the 30th and 60th de- 

 gree from the parallel, and to suppose its plane greatly in- 

 clined to the visual hne in a plane not far from that passing 

 through the eye, and the major axis of the orbit ; and this 

 agrees well with the change of distance, which is certainly less 

 at present than in 1782, though the estimation by diameters 

 is necessarily very uncertain. 



