The attention of instrument makers is now being given to the devising of
means for ensuring the smooth and even working of the slide in the stock
of the rule. In some cases very good results are obtained by slitting
the back of the stock to give more elasticity.
In the rules made by Messrs. John Davis & Son, a metal strip, slightly
curved in cross section as shown at A (Fig. 30), runs for the full
length of the stock to which it is fastened at intervals. Near each end
of the rule, openings about 1 in. long are made in the metal backing
through which the scales on the back of the slide can be read. To
prevent warping under varying climatic conditions both the stock of the
rule and the slide are of composite construction. The base of the stock
is of mahogany, while the grooved sides, firmly secured to the base, are
of boxwood. Similarly the centre portion of the slide is of mahogany and
the tongued sides of boxwood. Celluloid also enters into the
construction, a strip of this material being laid along the bottom of
the groove in the stock. A fine groove runs along the centre of this
strip in order to give elasticity and to allow the sides of the stock to
be pressed together slightly to adjust the fitting of the slide. As a
further means of adjustment the makers fit metal clips at each end of
the rule, so that by tightening two small screws the stock can be closed
on the slide when necessary.
[Illustration: FIG. 30.]
[Illustration: FIG. 32.]
[Illustration: FIG. 31.]
In the rule made by the Keuffel and Esser Company of New York, one strip
is made adjustable (Fig. 32).