222 
Mr. Cavendish on the Manner of 
$, and a microscope m made to slide from one end to the other. 
This beam compass is supported at d, in such manner as to turn 
round on this point as a centre, without shake or tottering ; 
and at the end £ it rests on another support, which can readiiy 
be lowered, so as either to let the pcint rest on the circle, or 
to prevent its touching it. It must be observed, however, that 
as the distance of d from the center of the circle must be va- 
ried, according to the magnitude of the arch to be divided, the 
piece on which d is supported had best be made to slide nearer 
to, or further from, the center ; but the frame must be made 
to bear constantly against the edge of the circle to be divided, 
so that the distance of d from the center of this circle, shall 
not alter by sliding the frame. 
This being premised, we will first consider the manner of 
dividing by continued bisection. Let F and/ be two points on 
this limb which are to be bisected in <p. Take the distance of 
the microscope from the point nearly equal to the chord o f/<p, 
and place d so that the point and the axis of the microscope 
shall both be in the circle in which the divisions are to be cut. 
Then slide the frame B B B till the wire of the microscope 
bisects the point F ; and having lowered the support at make 
a faint scratch with the point. 
Having done this, turn the beam compass round on the 
center d till the point comes to D, where it must rest on a 
support similar to that at 3 ; and having slid the frame till the 
wire of the microscope bisects the point/, make another faint 
scratch with the point, which if the distance of the microscope 
from the point has been well taken, will be very near the 
former scratch ; and the point mid-way between them will be 
the accurate bisection of the arch F/; but it is unnecessary. 
