permanently marked with a carefully-made prick punch. After the arc _a a_
is divided, the brass plate _A_ is cut back to this arc so the
divisions we have just made are on the edge. The object of having two
arcs on the plate _A_ is, if we desire to get at the number of degrees
contained in any arc of a 5" radius we lay the scale _A_ so the edge
agrees with the arc _a a_, and read off the number of degrees from the
scale. In setting dividers we employ the dotted spaces on the arc _b b_.


DELINEATING AN ESCAPE WHEEL.

[Illustration: Fig. 4]

We will now proceed to delineate an escape wheel for a detached lever.
We place a piece of good drawing-paper on our drawing-board and provide
ourselves with a very hard (HHH) drawing-pencil and a bottle of liquid
India ink. After placing our paper on the board, we draw, with the aid
of our T-square, a line through the center of the paper, as shown at
_m m_, Fig. 4. At 5½" from the lower margin of the paper we establish
the point _p_ and sweep the circle _n n_ with a radius of 5". We have
said nothing about stretching our paper on the drawing-board; still,
carefully-stretched paper is an important part of nice and correct
drawing. We shall subsequently give directions for properly stretching
paper, but for the present we will suppose the paper we are using is
nicely tacked to the face of the drawing-board with the smallest tacks
we can procure. The paper should not come quite to the edge of the
drawing-board, so as to interfere with the head of the T-square. We are
now ready to commence delineating our escape wheel and a set of pallets
to match.