The engine, it will be seen, is single-acting, and no compression of the explosive charge is employed. An explosive mixture of combustible gas and air is drawn through the valves, h2 and h6, and exploded behind the piston once in a revolution; but by a duplication of the valve and igniting apparatus, placed also at the front end of the cylinder, the engine may be constructed double-acting. At the proper time, when the piston has proceeded far enough to draw in through the mixing chamber, h, into the igniting chamber, g, the requisite amount of gas and air, the ratchet plate, j, is pushed into such a position by the pawl, j3, that the flame from the igniting jet, l, passes through one of the slots or holes, j1, and explodes the charge when opposite j6, which is the only aperture in the end of the working cylinder (see Fig. 7 and Fig. 2), thus driving the piston on to the end of its forward stroke. The exhaust valve, Fig. 9, though not exactly of the form shown, is kept open during the whole of this return stroke by means of the eccentric, e3, on the shaft working the ratchet, and thus allowing the products of combustion to escape through the exhaust pipe, i7, in the direction of the arrow. Between the ratchet disk and the igniting flame a small plate not shown is affixed to the pipe, its edge being just above the burner top. The flame is thus not blown out by the inrushing air when the slots in ratchet plate and valve face are opposite. This ratchet plate or ignition valve, the most important in any engine, has so very small a range of motion per revolution of the engine that it cannot get out of order, and it appears to require no lubrication or attention whatever. The engines are working very successfully, and their simplicity enables them to be made at low cost. They cost for gas from ½d. to 1½d. per hour for the sizes mentioned. [Illustration: Fig.9.] * * * * * GAS FURNACE FOR BAKING REFRACTORY PRODUCTS. In order that small establishments may put to profit the advantages derived from the use of annular furnaces heated with gas, smaller dimensions have been given the baking chambers of such furnaces. The accompanying figure gives a section of a furnace of this kind, set into the ground, and the height of whose baking chamber is only one and a half meters. The chamber is not vaulted, but is covered by slabs of refractory clay, D, that may be displaced by the aid of a small car running on a movable track. This car is drawn over the compartment that is to be emptied, and the slab or cover, D, is taken off and carried over the newly filled compartment and deposited thereon.