o useful where sufficient power can be applied only at one part of
the sweep. By means of pawls which engage in the ratchet-wheel, the
bit can be turned in either direction at the will of the user. The
size of the brace is indicated by the "sweep," that is, the diameter
of the circle thru which the swinging handle turns. To insert a bit
or other tool, Fig. 186, grasp firmly with one hand the sleeve of the
chuck pointing it upward, and revolve the handle with the other hand,
unscrewing the sleeve until the jaws open enough to admit the whole
tang of the bit. Then reverse the motion and the bit will be held
tightly in place. Various hand-, breast-, bench-, bow-drills and
automatic drills are of use in doing quick work and for boring small
holes, Fig. 187.

[Illustration: Fig. 179. Bench-Hook.]

[Illustration: Fig. 180. Miter-Box.]

[Illustration: Fig. 181. Iron Miter-Box.]

The _screwdriver_, Fig. 188, is a sort of holding tool for turning,
and so driving screws. Various devices have been tried to prevent the
twisting in the handle. This is now practically assured in various
makes. The other important matter in a screwdriver is that the point
be of the right temper, so as neither to bend nor to break. If the
corners break they can be reground, but care should be taken not to
make the angle too obtuse or the driver will slip out of the slot in
the screw-head. The bevel should have a long taper. A shop should be
equipped with different sizes of screwdrivers to fit the different
sizes of screws. Screwdrivers vary in size, the shank ranging in
length from 2-1/2" to 18". A long screwdriver is more powerful than a
short one, for the screwdriver is rarely exactly in line with the
axis of the screw, but the handle revolves in a circle. This means an
increased leverage, so that the longer the screwdriver, the greater
the leverage.

[Illustration: Fig. 182. Pliers.]

[Illustration: Fig. 183. Pincers.]

[Illustration: Fig. 184. Nippers.]

[Illustration: Fig. 185. Ratchet-Brace.]

For heavy work, screwdriver-bits, Fig. 189, in a bit-stock are useful,
and for quick work, the spiral screwdriver, Fig. 190, and for small
work, the ratchet-screwdriver.

7. MEASURING AND MARKING TOOLS.

It is a long step from the time when one inch meant the width of the
thumb, and one foot meant the length of the foot, to the measuring
of distances and of angles which vary almost infinitesimally. No such
accuracy is necessary in measuring