e holes
are drilled, else the volume of air used will be too great for
the compressor economically to supply. Steam may also be employed
for like service.

[Illustration: FIGS. 102 to 108.--Protective devices for furnace
fronts.]

The latest shields of this type are all made double, as illustrated,
with an inner sheet of metal an inch or two inside of the front.
In the illustration, _A_, Fig. 102, this inner sheet is smaller,
but some are now built the same size as the front and bolted to
it with pipe spacers between. The advantage of the double sheet
is that the inner one bears the brunt of the flame, and, if needs
be, burns up before the outer; while, if due to a heavy fire it
should be heated red at any point, the outer sheet will still be
much cooler and act as an additional shield to the furnace man.

HEAVY FORGING PRACTICE.--In heavy forging practice where the metal
is being worked at a welding heat, the amount of flame that will
issue from an open-front furnace is so great that a plain, sheet-steel
front will neither afford sufficient protection nor stand up in
service. For such a place a water-cooled front is often used. The
general type of this front is illustrated in Fig. 103, and appears to
have found considerable favor, for numbers of its kind are scattered
throughout the country.

In this case the shield is placed at a slight angle from the vertical,
and along the top edge is a water pipe with a row of small holes
through which sprays of water are thrown against it. This water runs
down in a thin sheet over the shield, cooling it, and is collected
in a trough connected with a run-off pipe at the bottom. The lower
blast-pipe arrangement is similar to the one first described.

There are several serious objections to this form of shield that
should lead to its replacement by a better type; the first is that
with a very hot fire, portions in the center may become so rapidly
heated that the steam generated will part the sheet of water and
cause it to flow from that point in an inverted V, and that section
will then quickly become red hot. Another feature is that after
the water and fire are shut down for the night the heat of the
furnace can be great enough to cause serious warping of the surface
of the shield so that the water will no longer cover it in a thin,
uniform sheet.

After rigging up a big furnace with a shield of this type several
years ago, its most serious object was found in the increase