4$ Mr. Hersciiei/s Catalogue 
other. I have adopted thefe meafures on finding that 1 could 
procure threads fine enough to fubtend only an angle of about 
i" and that by this means there was no longer any 
great difficulty of judging when the ftars were centrally co- 
vered by the threads. However, I do net know whether thefe 
meafures, with flars 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 exadt central 
politico while the meafure is taking. 
The pofinons have all been meafured (unlefs marked other- 
wife) with a power of 460, adapted to an excellent microme- 
ter, executed by Meffi Nairne and Blunt, according to 
the model given in the Philofophical Tranfadlions, vol. LX XI. 
page 500. fig. iv. ; but with a great and neceffary improve- 
ment of making the wheel d , d, of that figure perform its 
whole revolution ; by which means the two filk-worms- 
threads may be adj ufted to a greater degree of exadtnefs ; for if 
they are not placed fo as perfedlly to bifedt the circle, the two 
threads will not coincide exadlly after having performed one 
femi* revolution, which they mull: be made to do with the 
utmofl rigour. I found the abfolute neceffity of this precau- 
tion when I came critically to examine the pofitions of the 
Geotgium Sidus, as they are given in table HI. Phil. Tranf. vol. 
LXXI. p. 497. The meafures were affedled with a fmall and 
pretty regular error, which I was at a lofs to account for ; and 
the diflance of this ftar being then totally unknown, I looked 
for the caufe of the deviation at fir ft 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 
