cover the point, so that when the actual soldering takes place
the solder will not creep away from the tool.

By a little care and attention to these details, the work of uniting
metals will be a pleasure. It is so often the case, however, that the
apparatus for doing this work is neglected in a shop; the acid is
allowed to become dirty and full or foreign matter, and the different
parts separated.

CHAPTER X

ON GEARING AND HOW ORDERED

The technical name for gears, the manner of measuring them, their pitch
and like terms, are most confusing to the novice. As an aid to the
understanding on this subject, the wheels are illustrated, showing the
application of these terms.

SPUR AND PINION.--When a gear is ordered a specification is necessary.
The manufacturer will know what you mean if you use the proper terms,
and you should learn the distinctions between spur and pinion, and why a
bevel differs from a miter gear.

If the gears on two parallel shafts mesh with each other, they both may
be of the same diameter, or one may be larger than the other. In the
latter case, the small one is the pinion, and the larger one the spur
wheel.

Some manufacturers use the word "gear" for "pinion," so that, in
ordering, they call them _gear_ and _pinion_, in speaking of the large
and small wheels.

MEASURING A GEAR.--The first thing to specify would be the diameter. Now
a spur gear, as well as a pinion, has three diameters; one measure
across the outer extremities of the teeth; one measure across the wheel
from the base of the teeth; and the distance across the wheel at a point
midway between the base and end of the teeth.

These three measurements are called, respectively, "outside diameter,"
"inside diameter," and "pitch diameter." When the word _diameter_ is
used, as applied to a gear wheel, it is always understood to mean the
"pitch diameter."

[Illustration: _Fig. 121. Spur Gears_]

PITCH.--This term is the most difficult to understand. When two gears of
equal size mesh together, the pitch line, or the _pitch circle_, as it
is also called, is exactly midway between the centers of the two
wheels.

Now the number of teeth in a gear is calculated on the pitch line, and
this is called:

[Illustration: _Fig. 122. Miter Gear Pitch_]

DIAMETRAL PITCH.--To illustrate: If a gear has 40 teeth, and the pitch
diameter of the wheel is 4 inches, there are 10 teeth to each inch of
the pitch diameter, and the gear is th