Mr. Troughton on dividing Instruments. 113 
The only method of dividing large instrument 5 now prac- 
tised in London, that I know of, besides my own, has not 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 microscopes, and 
correcting it, as it proceeds, by pressing forwards or back- 
wards 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 accuracy, pro- 
vided 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 distance from the 
centre. The bisectional arcs, also, which cut them across, 
deform them much ; and, what is worse, the dots which re- 
quire correction ( about two thirds perhaps of the whole ) will 
become larger than the rest, and unequally 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 reduce them to an equality with their neighbours. This is 
done with the burnisher, and causes a hollow in the surface, 
which has a very disagreeable appearance. Moreover, dots 
which have been burnished up are always ill-defined, and of 
a bad figure. Sir George Shuckburgh Evelyn, in his paper 
on the Equatorial,* 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 
* Phil. Trans, for 1793. 
MDCCCIX. O 
