8 ON DIVIDING INSTIIUMENTS. 



part of his paper, as it contains valuable information, which 



perhaps no one else could have written. 



London prac- . The only method of dividinjif larjre instruments now prac- 



tiet; of dividing .• j • t i .i t i ,. , ■ ■, , 



large mstru- tised in London, that I know of, beside in y own, has not 



merits. yet, I believe, been made [ju'olic. It consists in dividing by 



hand with beam compasses and spiing 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 tine conical point, 

 those dots which appear erroneous; and thus adjusting them 

 to tbejr proper places. The method admits of considerable 

 accuracy, provided the operator has a steady hand and good 



Its defect?. eye; but his work will ever be irregular and inelegant. Hs 

 must have a circular line passing through the middle of his 

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

 distance from the centre. The bisectional arcs, also, which 

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

 the dots which require correction (about two thirds perhaps 

 of the whole) will become larger than the rest, and un^? 

 equally 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 grow insufferably too 

 large, and it becomes necessary to i educe them to an equal- 

 ity with their neighbours. This is done with the burnisher, 

 and causes a hollow in the surface, which has a very disr 

 agreeable appearance. Moreover, dots which have bcfen 

 burnished up ar« always ill defined, and oi a bad figure. 

 Sir George Shuckburi^h Evelyn, in his paper on the Equa- 

 torial*, denominates these " doubtful or bad points;" and 

 (considering the few places which he examines) they bear 

 no inconsiderable proportion to the whole. In my opinion, 

 it would be a great improvement of this method, to divide 

 the whole by hand at once, and afterward to correct the 

 whole; for a dot forced to its place, as above, will seldom al- 

 low the compass-point to rest in the centre of its apparent 

 area; therefore other dots made from hese nill sc ircely 

 ever be found in their true places. This improvement also 

 prevents the corrected dots from being injured, or moved, 



♦ Phil. Trans for 1793. 



by 



