Oliver Evans, the celebrated American mechanic, projected an engine, which he termed a volcanic engine, wherein he proposed to combine the expansive force of the heated gases and other products of combustion with that of steam. Subsequently the idea has been embodied in several patents in this country: we select one of the most recent for illustration. The patent was taken out in the name of Mr. Hawkins, the patentee's agent in this country.
The annexed figure represents a section of the boiler, a a is a vertical cylinder constituting the shell of the boiler, b b a smaller cylinder placed within the former, and forming the furnace and ash pit: this is entirely surrounded with water, c is a tube connected with a blowing machine, and having two branches d and e, the former of which admits a portion of ah above the fuel, and the latter a portion in to the ash-pit, below the fire-bars: two throttle valves or dampersff are provided to regulate the draft through each branch, g is a short cylindrical neck, through which the smoke and heated air pass into the steam-chamber, where they mix with the steam, and with it pass to the working cylinders. The neck g is covered with a valve h, opening upwards, the sides of which are turned down, to cause the heated air to pass through the water, and thereby give out a portion of its heat to the water: i the safety valve, k valve by which the pipe that conveys the steam to the engine can be closed when required, l the pipe by which the water is conveyed to the boiler, from the feed pump; the end of this pipe enters the boiler, and delivers the water on to the top of the valve h; this is with a view to prevent the valve getting excessively heated by the action of the fire. m, is the chimney, or rather passage by which the fuel is introduced into the furnace; on it is bolted a hopper n n, having at its upper end a flat plate or sliding valve o, and another valve p at its lower end; these valves slide in grooves, and are moved by means of a rack and pinion; they are ground on their seats so as to form an air-tight joint, and during the whole time the engine is in operation, the chimney is kept closed by one or other of these valves.
To kindle the fire before starting, the valves o and p are both opened, and a quantity of lighted fuel put in first through the hopper, and fresh fuel heaped upon it, and when the whole is thoroughly ignited, the valve p is closed, and the blowers set in operation. When the engine is set to work, it forces a quantity of air into the furnace both above and below the fuel at each stroke, which having no vent to escape but the valve A, accumulates in the furnace until its pressure somewhat exceeds that of the steam upon the valve h, when it lifts the valve, and rising up through the water mixes with the steam, and passes along with it to the engines, t is a slider, by opening which the ashes from the furnace can be withdrawn; when this is requisite the dampersffmust be first closed, v is the blow-off cock by which the water can be discharged from the boiler when required, and w is a hole covered by a door, for removing any mud, etc. which may have accumulated: at x is a glass gauge to show the height of the water in the boiler, and at y is a glass eyepiece, through which the state of the fire can be ascertained, and z is the man-hole by which admission is obtained into the boiler.