ii 6 Mr. Troughton on dividing Instruments. 
as shewn in Fig. 2. C C and D D are two similar plates, each 
attached to the main plate, one above and the other below, 
by four pillars ; and in them are centred the ends of the axis 
of the roller E. F and G are two friction wheels, the latter 
firmly fastened to B, but the former is fixed in an adjustable 
frame, by means of which adjustment these wheels and the 
roller E, may be made to press ; the former on the interior, 
and the latter on the exterior edge of the circle, with an equal 
and convenient force.* At the extremities of the axis of the 
roller, and attached to the middle of the plates C and D, are 
two bridges, having a screw in each * by means of which an 
adjustment is procured for raising or lowering the roller re- 
specting the edge of the circle, whereby the former, having its 
diameter at the upper edge about .001 of an inch greater 
than at the lower edge (being, as before described, a little 
conical), it may easily be brought to the position where it will 
measure the proper portion of the circle. 
Much experience and thought upon the subject have taught 
me, that the roller should be equal to one sixteenth part of 
the circle to be divided, or that it should revolve once in 22 a 
50' ; and that the roller itself should be divided into sixteen 
parts ; no matter whether with absolute truth, for accuracy is 
not at all essential here. Each of such divisions of the roller 
will correspond with an angle upon the circle of i° 24' 2 2, "5, 
or 2^6^ P art circle This number of principal divisions 
was chosen, on account of its being capable of continual bisec- 
* Sufficient spring for keeping the roller in close and uniform contact with the 
edge of the circle, is found in the apparatus, without any particular contrivance for 
that purpose ; the bending of the pillars of the secondary frames and of the axis of 
the roller, chiefly supplies this property. 
