also make gas-fired
furnaces.

Several types of furnaces for various purposes are illustrated
in Fig. 100 and 101. The first is a gas-fired hardening furnace
of the surface-combustion type.

A large gas-fired annealing furnace of the Maxon system is shown
in Fig. 101. This is large enough for a flat car to be run into
as can be seen. It shows the arrangement of the burners, the track
for the car and the way in which it fits into the furnace. These
are from the designs of the Industrial Furnace Corporation.

Before deciding upon the use of gas or oil, all sides of the problem
should be considered. Gas is perhaps the nearest ideal but is as a
rule more expensive. The tables compiled by the Gilbert & Barker
Manufacturing Company and shown herewith, may help in deciding
the question.

TABLE 27.--SHOWING COMPARISON OF OIL FUEL WITH VARIOUS GASEOUS FUELS
Heat units
per thousand
cubic feet
Natural gas 1,000,000
Air gas (gas machine) 20 cp 815,500
Public illuminating gas, average 650,000
Water gas (from bituminous coal) 377,000
Water and producer gas, mixed 175,000
Producer gas 150,000

Since a gallon of fuel oil (7 lb.) contains 133,000 heat units, the
following comparisons may evidently be made. At 5 cts. a gallon,
the equivalent heat units in oil would equal:

Per thousand
cubic feet
Natural gas at $0.375
Air gas, 20 cp at 0.307
Public illuminating gas, average at 0.244
Water gas (from bituminous coal) at 0.142
Water and producer gas, mixed at 0.065
Producer gas at 0.057

Comparing oil and coal is not always simple as it depends on the
work to be done and the construction of the furnaces. The variation
rises from 75 to 200 gal. of oil to a ton of coal. For forging
and similar work it is probably safe to consider 100 gal. of oil
as equivalent to a ton of coal.

Then there is the saving of labor in handling both coal and ashes,
the waiting for fires to come up, the banking of fires and the dirt
and nuisance generally. The continuous operation possible with
oil adds to the output.

When comparing oil and gas it is generally considered that 4-1/2