2^C) ^AUSE Of THl CHANGES ©P RELATIVE 



I Bootis, 



Obfervations and This beautiful double ftar, on account of the different colours 

 infcrcnccrs re- ©f the ftars of which it is compofed, has much the appearance 

 changes of rela- of ^ planet and its fateUite, both ftiniing with innate but diifer- 

 tivc ikuation in gntly coloured light. 



»ear MchoTher. There has been a very gradual change in the diftance of the 

 two ftars ; and the refult of more than 1 20 obfervations, with 

 different powers, is, that with the ftandard magnifier, 460, and 

 the aperture of 6,3 inches, the vacancy between the two ftars, 

 in the year 1781, was \\ diameter of the large ftar, and that it 

 now is 1|. By fome earlier obfervations, the vacancy was found 

 to be confiderably lefs in 1779 and 1780; but the7-feet mirror 

 then in ufe was not fo perfed as it Ihould have been, for the 

 purpofe of fuch delicate obfervations. By many eftimations oC 

 the apparent fi:^e of the liars, I have fixed the proportion of 

 the diameter of £ to that of r, as 3 to 2. Auguft31, 1780, the 

 fir ft angle of pofition meafured 32** 19' north preceding*; 

 / and, March 16, 1803, I found it 44.** 52', alfo north pre- 



ceding : the motion, therefore, in 22 years and 207 days, is 

 12^ 33'. It fliould alfo be noticed, that while the apparent 

 motion of a Qeminorum, and of y Leonis, is retrograde, that 

 of « Bootis is diredl. 



A proper motion in this ftar, if it has any, is iiill unknown ; 

 our former arguments, therefore, cannot be applied toil, with- 

 out fome additional confiderations ; and, as many others of my 

 double ftars will ftand in the fame predicament, I fhall give an 

 outline of what may be faid, to fliow that this, and probably 

 many of the reft, are alfo binary fyftems. 



Single Motions, 



(a — e) If I Bootis is a ftar in which no proper motion can 

 be perceived, we may infer, from the highly probable motion of 

 the folar fyftem, that this ftar, which is of the third magnitude, 

 and on that account within the reach of parallax, muft have a 

 real motion, to keep up with the fun, in order to prevent an 



* The angle of pofition, in my fiifl Catalogue of double Stars, 

 Phil. Tianf. for 1782, page 115, is 31° 34' (it fliould be 54) 

 ftorth preceding. This will be found to be a mean of the three firft 

 meafuies hereafter given in a table of portions. 



apparent 



