Mr. Troughton on dividing Instruments. 115 
apparatus has undergone during a long course of years,* and 
the various manner of its application, before I brought it to its 
present state of improvement ; but I think I may save myself 
that trouble, for truly I do not see its use : I will, therefore, 
proceed immediately to a disclosure of the method, as prac- 
tised on a late occasion, in the dividing of a four feet meridian 
circle, now the property of Stephen Groombridge, Esq. of 
Blackheath. 
The surface of the circle which is to receive the divisions, 
as well as its inner and outer edges, but especially the latter, 
should be turned in the most exact and careful manner ; the 
reason for which will be better understood, when we come to 
describe the mode of applying the roller : and, as no projection 
can be admitted beyond the limb, if the telescope, as is gene- 
rally the case, be longer than the diameter, those parts which 
extend further must be so applied, that they may be removed 
during the operation of dividing. Fig. 1 and 2 represent the 
principal parts of the apparatus ; Fig. 1 shewingthe plan, and 
Fig. 2 the elevation ; in both of which the same letters of re- 
ference are affixed to corresponding parts, and both are drawn 
to a scale of half dimensions. A A is a part of the circle, the 
surface of which is seen in the plan, and the edge is seen in 
the elevation. B B B is the main plate of the apparatus, rest- 
ing with its four feet aaaa upon the surface of the arc ; these 
feet, being screws, may be adjusted so as to take equal shares 
of the weight, and then are fastened by nuts below the plate, 
* The full conception of the method had occupied my mind in the year 1778 ; but 
as my brother could not be readily persuaded to relinquish a branch of the business 
to me in which he himself excelled, it was not until September 1785 that I produced 
my first specimen, by dividing an astronomical quadrant of two feet radius. 
