JJ(^ ON DIVIDING ASTRONOMICAL INSTRUMENTS. 



sions; i. ^. he has supposed one of the divisions which bound 

 the erroneous extent, to be too much to the right, and the 

 ©thertoo much to the left, >ind that by equal quantities: 

 This is certainly a oood-natured way of stating the errours 

 of work ; and perhaps not unjustly so, where the worst part 

 has been selected ; but, in the other three instances, namely, 

 in General Koy's, Mr. Aubert's, and his own standard, he 

 has charged the whole errour of the most erroneous extent 

 to one of the bounding- lines. r 

 Cfeiieral accu- I was well contirmed in my high opinion of the general 

 tacy of B.ro's accuracy of Bird's dividing, wiien, last wanter*, 1 measured 

 Tblf Green- the chords of many arcs of the Greenwich quadrant, 

 wich. quadrant That instrument has indeed suffered both from a change in 

 its figure, and from the v. earing of its centre ; but the gra- 

 duation, considering the time when it was done, I found to 

 be very good. Sir George in his paper upon the equatorial 

 (Phil. Trans, for 1793), after some compliments paid to the 

 divider of his instrument, says, " the late Mr. John Bird 

 " seems to have admitted a probable disc!;epancy in the di- 

 *' visions of his eight feet quadrant amounting to 3" ;" and 

 he refers to Bird ou the construction of the Greenwich qua-p 

 " drant. This quantity being three times as great as any 

 errours that 1 met with, I was lately induced to inquire 

 how the matter stood. Bird, in the paper referred to, says, 

 « in dividing this instrument 1 never met with an iriequality 

 '• that exceeded one second. I will suppose, that in the §0 

 '* arch this errour lay toward the left hand, and in the 96 

 " arch that it lay towards the right, it will cause a differ- 

 " ence between the two arches of two seconds; and, if an 

 «' errour of one second be allowed to the observer in read- 

 " ing oft' his observation, the whole amount is no more than 

 " three seconds, which is agreeable to what I have heard, 

 co'upared wlih " &c." Sir George's examination of his own equatorial 

 Sir G. S.Eve- furnishes me with the means of a direct comparison: in hia 

 I'ai? '''*''"'^' account of the declination circle, we iiiul aa errour + %"-25, 

 and another — l"-5; to these add an errour of half a second 

 in each, for reading oil", wiiich Sir George also admits, we 

 shall then- have a discrepancy of 4"-85 ; but, as the errours 



* This [laper wa^s vvntten ia June, 1808. 



of 



