THE NECESSITY FOR GOOD INSTRUMENTS.

It is to be hoped the reader who intends to profit by this treatise has
fitted up such a pair of dividers as those we have described, because it
is only with accurate instruments he can hope to produce drawings on
which any reliance can be placed. The drawing of a ratchet-tooth lever
escapement of eight and one-half degrees pallet action will now be
resumed. In the drawing at Fig. 18 is shown a complete delineation of
such an escapement with eight and one-half degrees of pallet action and
equidistant locking faces. It is, of course, understood the escape wheel
is to be drawn ten inches in diameter, and that the degree arcs shown in
Fig. 1 will be used.

We commence by carefully placing on the drawing-board a sheet of paper
about fifteen inches square, and then vertically through the center
draw the line _a' a''_. At some convenient position on this line is
established the point _a_, which represents the center of the escape
wheel. In this drawing it is not important that the entire escape wheel
be shown, inasmuch as we have really to do with but a little over sixty
degrees of the periphery of the escape wheel. With the dividers
carefully set at five inches, from _a_, as a center, we sweep the arc
_n n_, and from the intersection of the perpendicular line _a' a''_ with
the arc _n_ we lay off on each side thirty degrees from the brass degree
arc, and through the points thus established are drawn the radial lines
_a b'_ and _a d'_.


[Illustration: Fig. 18]