I 



On dividing Instniments. 173 



amounting fo 3";" and he refers to Bird on the construc- 

 tion of the Greenwiclj quadrant.' Thiso,uantity being three 

 times as great as any errors that I met with, I was lately 

 induced to inquire how the matter stood. Bird, in the 

 paper referred to, says, "In dividing this instrument I 

 never met with an inequality that exceeded one second. I 

 will suppose that in the 90 arch this error lay towards the 

 left hand, and in the 96 aich that i-t lay towards the right, 

 it will cause a difl'ercnce between the two arches of two 

 seconds; and if an error of one second be allowed to the 

 observer in reading off his observation, the whole amount 

 is no more than three seconds, which is agreeable to what 

 I have heard, &c." Sir George's examination of his own 

 equatorial furnisljes me with the means of a direct com- 

 parison : in his account of the declination circle, we find 

 an error + <2."-35, and another — V'-5 ; to these add an 

 error of half a second in each, for reading off, which sir 

 George also admits ; we shall then have a discrepancy of 

 4"'85 ; but as the errors of reading off are not errors oF 

 division, let them be discharged from both, and the errors 

 will then stand, — for the quadrant 2", and for the circle 

 3"'Q5. As the radius of the former, hovv'ever, is four times 

 greater than that of the latter, it will ap])ear, bv this mode 

 of trial, that the equatorial is rather more than twice as ac- 

 curately divided as the quadrant. In doing justice to Bird 

 in this instance, I have only done as I would be done bv ; 

 for, should any future writer set me back a century on the 

 "chronological scale of progressive improvement, I hope 

 some one will be found to restore me to my proper niche. 

 I now subjoin a re-statement of the greatest error of each 

 of the instruments that are brought uito comparison by sir 

 George, after having reduced them all by one rule ; viz, 

 allowing each of the two points which bound the most er- 

 roneous extent to divide the apparent error equally between 

 them. They are expressed in parts of an inch, and follow 

 each other in the order of their accuracy. 



Sir George Shuckburg's 5-fcct standard - '0001 65 

 General Roy's scale of 42 inches - - '000240 

 Sir George's equatorial, 2-feet radius - "000273 

 'I'he Greenwich quadrant, 8 feet radius - •000465 

 Mr. Aubcrt'$ standard, 5 feet long - - '000700 

 The lioyal Society's standard, 92 inches long* -000795 

 For the justness of the above statement Iconsidirmv 

 name as pledged ; requesting the permission to say, that if 



• This is the same wliich Mr. J3ird used in dividing his ciglit-fect mur^l 

 quadrants, and was piesentcd to the Royal Society bv Bird's executors. 



on 



