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 51/2" 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.
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