On dividing Instrum&nts, 87 



tact used in Smeaion's pyrometer, which latter is performed 

 by the most delicate touch ; and is represented, 1 believe 

 justly, to be sensible to the fro-oo-o'^ P^''^ of an inch. Smea- 

 ton has, however, acquitted himself well, in describing and 

 improving the method of his friend ; and the world is par- 

 ticularly obliged to him for the historical part of his paper, 

 as it contains valuable information which perhaps no one 

 else could have written. 



The only method of dividing large instruments now prac- 

 tised in London, that I know of, besides my own, has no^ 

 yet, I believe, been made public. It consists in dividing by 

 hand with beam compasses and spring dividers, in the usual 

 way; with the addition of examining the work by micro- 

 scopes, and correcting it, as it proceeds, by pressing for- 

 wards or backwards by hand, with a fine conical point, those 

 dots which appear erroneous ; and thus adjusting them to 

 their proper places. The method admits of considerable ac- 

 curacy, provided the operator has a steady hand and good 

 eye; but his work will ever be irregular and inelegant. He 

 must have a circular line passing through the middle of his 

 dots, to enable him to make and keep them at an equal di- 

 stance from the centre. The bisect ional arcs, also, which 

 cut them across, deform them much ; ami, what is worse, 

 the dots which require correction (about two thuds perhaps 

 of the whole) will become larsier than the rest, a'^d une- 

 qually so in proportion to the number of attempts which 

 have been found necessary to adjust them. In the course 

 of which operation, some of them gro^v insufferably too 

 large, and it becomes uecessary to reduce :hem to an equality 

 with their neighbours. This is done wiih the burnisher, and 

 causes a hollow in the surface, which has a very disagreeable 

 appearance. Moreover, dots which have bten burnished up 

 are always ill-defined, and of a bad figure. Sir George 

 Shuckburgh Evclvn, in his paper on the Equatorial*, de- 

 nominates these " doul)lful or bad points;" and (consider- 

 ing the tew places which he examine-;) they bear no incon- 

 siderable proportion to the whole. In •iiy opinion, it would 

 be a great ntiprwvcmtiit of this method, to divide the whole 

 by hand at once, and afterwards to correct the whole; for a 

 dot forced to its place, as above, will seldom allow ihe com- 

 pass-point to rest in tiie centre of its apparent area: there- 

 fore other dots made from those will scarcely ever be found 

 in their true places. This improvement also prevents the 

 corrected dots from being injured, or moved, by the future 



• Philosophical Transactions for 1793, 



F 4 application 



