J72 On dividing Instruments. 



which became the subject of comparison with the new 

 measure of France, before tlie National Ins'.itute ; and their 

 report, drawn up by Mr. Pictel, has been ably re-stated 

 and corrected by Dr. Young, as published in the Journals 

 of the Royal Institution. 1 made a third for the magis- 

 trates of Aberdeen. I notice the two latter, principally to 

 give myself an opportunity of saying that, if those three 

 scales were to be con)pared together, notwithstanding they 

 were divided at distant periods of time, and at different 

 seasons of the year, they would be found to agree with 

 each other, as nearly as the dlflcrent parts of the same scale 

 agree. 



I hope I may here be allowed to allude to an inadver- 

 tency which has been conmnitled in the paper mentioned 

 above ; and which sir George intended to have corrected, 

 had he lived to conclude his useful endeavours to harmonize 

 the discordant weights and measures of this country. The 

 instruments which he has brought into comparison are, his 

 own five-feet standard measure and equatorial; general 

 Roy's forty- two inch scale; the standard ofMr. Aubert; 

 and that of the Royal Society. The inadvertency is this : 

 in his equatorial, and the standard of tlie Royal Society, 

 he has charged the error of the n)0st erroneous extent, 

 when compared with the mean extent, alike to both divi- 

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

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

 other too much to the left, and that by equal quaatities. 

 This is certainly a good-natured way of stating the errors 

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

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

 namely, in general Roy's, Mr. Aubert's, and his own 

 standard, he has charged the whole error of the most er- 

 roneous extent to (me of the bounding lines. 



I was well confirmed m my high opinion of the general 

 accuracy of Bird'.s dividing, when, hist winter*, I mea- 

 sured the chords of many arcs of the Greenwich quadrant : 

 that instrument has Indeed suffered both from a change in 

 its figure, and from the wearing of its centre; but the 

 graduation, considering the time when it was done, I found 

 to be verv good. Sir George in his paper upon the Equa- 

 torial (Phdosophlcal Transactions for 1793), after some 

 compliments paid to the divider of his instrument, says, 

 *' The late Mr. John Bird seems to have admitted a proba- 

 }jle discrepancy in the divisions of his eight- feet quadrant 



* This paper was written in June 1S08. 



amountintr 



