was tedious and great
care was necessary to avoid getting paraffin where it was not wanted.

[Illustration: FIG. 40.--Shaft to be coated with paraffin.]

To insure uniform coating the device shown in Fig. 41 was made.
Melted paraffin is poured in the well _A_ and kept liquid by setting
the device on a hot plate, the paraffin being kept high enough
to touch the bottoms of the rollers. The shaft to be coated is
laid between the rollers with one end against the gage _B_, when
a turn or two of the crank _C_ will cause it to be evenly coated.

[Illustration: FIG. 41.--Device for coating the shaft.]

THE PENETRATION OF CARBON

Carburized mild steel is used to a great extent in the manufacture
of automobile and other parts which are likely to be subjected to
rough usage. The strength and ability to withstand hard knocks
depend to a very considerable degree on the thoroughness with which
the carburizing process is conducted.

Many automobile manufacturers have at one time or another passed
through a period of unfortunate breakages, or have found that for
a certain period the parts turned out of their hardening shops
were not sufficiently hard to enable the rubbing surfaces to stand
up against the pressure to which they were subjected.

So many factors govern the success of hardening that often this
succession of bad work has been actually overcome without those
interested realizing what was the weak point in their system of
treatment. As the question is one that can create a bad reputation
for the product of any firm it is well to study the influential
factors minutely.

INTRODUCTION OF CARBON

The matter to which these notes are primarily directed is the
introduction of carbon into the case of the article to be hardened.
In the first place the chances of success are increased by selecting
as few brands of steel as practicable to cover the requirements of
each component of the mechanism. The hardener is then able to become
accustomed to the characteristics of that particular material, and
after determining the most suitable treatment for it no further
experimenting beyond the usual check-test pieces is necessary.

Although a certain make of material may vary in composition from
time to time the products of a manufacturer of good steel can be
generally relied upon, and it is better to deal directly with him
than with others.

In most cases the case-hardening steels can be chosen from the
following: (1) Case-harden