avri:^osiTioN OF d.ouble stars. 253 



\ 



Oh looking over the fourth column of this table, it will be Obfervatlonsand 



found, that the differences between the obferved and calculated fp/^i"g t^e ' 

 angles are not greater than may be expeded, confidering that changes of rela* 

 mod of the early meafures are fingle, and cannot have the ac- ^^^5 eSremely" 

 curacy which maybe obtained by repetition. Even as they near each other* 

 are, we muft acknowledge them fufficient to afcertain the gra- 

 dual change in the angle of pofition of the two flars. In one 

 place, the difference amounts to fix degrees; but it will foon 

 appear, that a more accurate annual motion gives a calculated 

 pofition which takes off much of the error of this meafure. 



In a converfation with my highly efteemed friend the Aftro- 

 nomer Royal, he happened fome time ago accidentally to men- 

 tion, that Dr. Bradley had formerly obferved the two ftars of a 

 Geminorum to ftand in the fame diredion with Caftor and 

 Pollux. It occurred to me immediately, that if the time of this 

 obfervation could be nearly afcertained, it would be of the 

 greateft importance to the fubjedt at prefent under conlider- 

 ation. For, (liould Dr. Bradley*s pofition be very different from 

 a calculated one, it would induce us at once to give up the idea 

 of a revolving ftar. The obfervation was made by Dr. Bradley 

 with a view to fee whether any change could be perceived in 

 the courfe of the year, by which the annual parallax of the ftars 

 might be difcovered. Dr. Matkelyne, who had this informa- 

 tion from Dr. Bradley in converfation, had made a memoran- 

 dum of it in his papers. He has been fo kind as to look for it ; 

 and, as foon as he found the note, he fent me the following 

 copy, which I have his permiflion to tranfcribe. 



" Double jiar Cajior. No change of pofition in the twojlars : 

 If* the line joining theniy at all times of the year, parallel to the line 

 *' joining Cajior and Pollux in the heavens, feen by the naked 

 ** eye." 



Dr. Matkelyne informs me, that (he obfervation muft have 

 been made about the year 1759 ; and alfo mentions, that he 

 himfelf verified the fa6l, as to the line joining the two ftars ap- 

 pearing through the telefcope parallel to the line joining Caftor 

 and Pollux, in 1760 or 1761 ; but that he did not examine it at 

 various times of the year. 



The advantage of having an angle of pofition obferved in 

 1759 by Dr. Bradley, and fo foon after verified by Dr. Maf- 

 kelyne, will give us an addition of 20 years to our period. On 

 calculating the right afcenfion and polar diftancs of Caftor and 



Pollux 



