[Illustration: FIG. 98.--Semi-muffle furnace.] [Illustration: FIG. 99.--Muffle furnace.] Furnaces of these types of oil-burning furnaces are shown in Figs. 97, 98, and 99; these being made by the Gilbert & Barker Manufacturing Company. The first has an air curtain formed by jets from the large pipe just below the opening, to protect the operator from heat. [Illustration: FIG. 100.--Gas fired furnace.] [Illustration: FIG. 101.--Car door type of annealing furnace.] Oil furnaces are also made for both high- and low-pressure air, each having its advocates. The same people 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.