Although this was clear to Mr. Wentworth, it may perhaps need explanation here. In modern furnaces, for the fusion of glass or other operations where great heat is necessary, the process of combustion of the fuel is carried on, not in one operation, as it is in the simple furnaces with which every one is familiar, but in two distinct, separate, progressive stages. The first stage takes place in a subsidiary furnace known as a “gas producer.” Here part of the heat which the fuel is capable of generating is utilized for the production of a combustible gas. In other words the fuel is changed into gaseous form, but only partly burned. A familiar example of this operation is seen in any ordinary fireplace when the fire is first lighted. There is at first an inadequate “draft.” This supplies the fire with an insufficient amount of oxygen, and although the fuel—paper, for instance—is entirely volatilized it is not entirely burned; there is smoke, which, if it could be mixed with more air, and at a sufficiently high temperature, would burst into flame. This was the process Jimmy proposed to carry on in the ground; that is, only partly to consume the coal by supplying it with an insufficient amount of oxygen. And it was the unburned coal gases—the combustible smoke, in other words—that he proposed to pipe up to the furnace at the surface—not the actual heat. The burning mine, hundreds of feet down in the ground, was in effect to be his subsidiary furnace—his gas-producer. These unburned gases, from the producer, pass, hot, into the furnace proper; either directly or sometimes after being conveyed a considerable distance—as they would have to be according to Jimmy’s plan. In this latter event they cool off, but are heated up again by the waste heat of the furnace. These hot gases, entering the main furnace, meet a current of hot air, also heated by the waste heat of the furnace. The combination of hot gas and hot air burns rapidly and completely, and yields very high temperatures if properly proportioned. To the layman it may seem surprising that when part of the combustion of the fuel takes place entirely away from the furnace—the heat of this combustion being completely wasted—that a far greater heat can subsequently be obtained. But it is a fact nevertheless. “How would you start the fire in the ground?” suggested Mr. Wentworth. “By dropping down incandescent coal,” Jimmy returned promptly. “And then blowing air to it. You see—” The president raised his hand. “That’s only a detail. Then you really think you could approximate a gas-producer with this burning mine of yours?” “Yes, sir. By forcing down the proper proportions of air and steam. You see, the hole in the burning coal-bed would gradually spread out. But that wouldn’t make any difference, because it would only have two outlets to the surface air, and both of them under control.” “The lower ends of your casing would melt,” said Mr. Wentworth.