Q\Q tAVSE OF THE CHANGES OF RLLATlVK 



Cibfervationsanddiftance, I have known it to take up two or three nlonthsTy 

 fpeSrthr' ^^^^^^ ^'^6 ^ye vvas fafficiently acquainted with the objea, to 

 changes of re- judge with the requifite precifion. 



lative fituation ' Whatever may be the difficulties or uncertainties, attending 

 mftars extreme- ti,-! •• t n r ^ r n i 



ly near each the method of determining the diftance of two dole ftars by 



other. an eftiraation of the apparent diameter, it mufi: however be con- 



fefled, that we have no other way of obtaining the fame end 

 w^ith fo much precilion. Our prefent inftance of a Geminorum, 

 will ftiow the degree of accuracy of which fuch eftimations.are 

 capable, and at the fame time prove^ that the purpofe for which 

 I (hall ufe the eftimated interval between the two ftars will be 

 fufficiently anfwered. By an obfervation of the 10th of May, 

 1781, we have the diameter of the largefl; of the two ftars to 

 that of the fmalleft as 6 to 5 ; and according to feveral n:>ea- 

 fures I have taken with the micrometer, we may admit their 

 diftance, diameters included, to be five feconds. Then, as the 

 vacancy between the two ftars is nearly, but not quite two dia- 

 meters of the large one, I ftiall value it at 1|-. From this we 

 calculate, that the diameter of the large ftar, under the circum- 

 llances of our eftimation, is nearly V',S5 : fo that an error of 

 one, quarter of fuch a diameter, which is the moft we can admit, 

 .will not exceed 0",34'. Nor is it of much confequence, if the 

 meafure of 5^' ftiould not be extremely corred; as a fmall 

 miftake in that quantity will not materially affe<5t the error of 

 eftimation by the diameter, which, from what has been faid, if 

 the meafure was faulty to a fecond, would not amount to more 

 than one-fifteenth part of it. 



Having thus afcertained that no perceptible change in the 

 diftance of the ftars has taken place, we are now to examine 

 the angle of pofition. In the year 1779, it was 32° 47' north 

 preceding; and, by a mean of the three laft meafures I have 

 taken, it is now only 10° 3S'. In the fpace of about 23 years 

 and a half, therefore, the angle of pofition has manifeftly under- 

 gone a diminution, of nolefs than 21^ 54'; and, that this change 

 ha- been brought on by a regular and gradual decreafe of the 

 angle, will be feen when the reft of the meafures come to be 

 examined. 



The accuracy of the micrometer which has been ufed, when 

 the angles of pofition were taken, being of the utmoft im- 

 portance, it becomes neceflary to afcertain how far it will be 

 fafe to rely on the refult of the meafures. It might be eafily 



fliown 



