[Illustration: Fig. 1]

We shall, in addition, need an arc of degrees, which we can best make
for ourselves. To construct one, we procure a piece of No. 24 brass,
about 5½" long by 1¼" wide. We show such a piece of brass at _A_,
Fig. 1. On this piece of brass we sweep two arcs with a pair of dividers
set at precisely 5", as shown (reduced) at _a a_ and _b b_. On these
arcs we set off the space held in our dividers--that is 5"--as shown at
the short radial lines at each end of the two arcs. Now it is a
well-known fact that the space embraced by our dividers contains exactly
sixty degrees of the arcs _a a_ and _b b_, or one-sixth of the entire
circle; consequently, we divide the arcs _a a_ and _b b_ into sixty
equal parts, to represent degrees, and at one end of these arcs we
halve five spaces so we can get at half degrees.

[Illustration: Fig. 2]

Before we take up the details of drawing an escapement we will say a few
words about "degrees," as this seems to be something difficult to
understand by most pupils in horology when learning to draw parts of
watches to scale. At Fig. 2 we show several short arcs of fifteen
degrees, all having the common center _g_. Most learners seem to have an
idea that a degree must be a specific space, like an inch or a foot. Now
the first thing in learning to draw an escapement is to fix in our minds
the fact that the extent of a degree depends entirely on the radius of
the arc we employ. To aid in this explanation we refer to Fig. 2. Here
the arcs _c_, _d_, _e_ and _f_ are all fifteen degrees, although the
linear extent of the degree on the arc _c_ is twice that of the degree
on the arc _f_. When we speak of a degree in connection with a circle we
mean the one-three-hundred-and-sixtieth part of the periphery of such a