^5 Mr. Hersciiel's Catalogue 



oth^r. I iiare adopted thefe meafures on finding that I could 

 procure threads fine enough to lubtend only an angle of about 

 i^' 1^1'' -i and that by this means there was no longer any 

 great difficulty of judging when the flars were centrally co- 

 vered by the threads. However, I do net know whether thele 

 meafures, with ftars at a confiderable diftance, may not be 

 liable to an additional error of perhaps one fecond, owing to 

 the remaining uncertainty in judging of their cxad: central 

 pofition while the meafure is taking. 



The pofitions have all been meafured (unlefs marked other- 

 wife) with a power of 460, adapted to an excellent microme- 

 ter, executed by Mefl'. Nairne and Blunt, according to 

 the model given in the PhilofophicalTranfa6tions, vol. LXXI. 

 pnge 500-. fig. IV. ; but with a great and neceflary improve- 

 ment of 'making the wheel d^ d^ of that figure perform its 

 whole revolution ; bv which means the two filk-worms- 

 threads may be adjufted to a greater degree of exadlnefs ; for if 

 they are not placed fo as perfectly to bife6l the circle, the two 

 threads will not coincide exactly after having performed one 

 femi-revolution, which they muft be made to do with the 

 utmoft rigour. I found the abfolute neceflity of this precau- 

 tion when' I came critically to examine the pofitions of the 

 Georgium Sidus, as they are given in table III. Phil. Tranf. vol, 

 LXXI. p. 497. The meafures were afFeiSled with a fmall and 

 pretty regular error, which I was at a lofs to account for ; and 

 the diftance of this ft:ar being then totally unknown, I looked 

 for the caufe of the deviation at firft in a diurnal parallax of 

 that heavenly body ; but foon found it owing to the incon- 

 venience before-mentioned, of not being able experimentally 

 to adjuft the moveable thread to that critical nicety which I 



have 



