The cylinders are surrounded by water jackets for cooling purposes, aided by air which can enter the front end when the piston is moving in its backward stroke. The hottest portion is therefore that situated at the valve end, and in which the explosion takes place. This portion is surrounded by a closed chamber forming a boiler, where steam is formed for use in the gas plant; the steam is superheated in a spiral coil placed on the top of the gasogene. As we have already stated, this system of gas production avoids the ammoniacal products, which are a constant source of trouble in other engines. The drawback lies in the compressor, which uses up about one-third of the indicated power, but the speed is exceedingly constant, and averages about 175 revolutions per minute. The consumption of gas is not very high, probably owing to the amount of heat which is absorbed from the waste gases, usually lost. A number of other engines exist of a similar type to the Gardie motor, such as the Shaw, Woodburg, Crowe, Buchett, and others, which cannot, however, consume poor gases, and which have not any distinguishing features worth mentioning. _Bénier motor gasogene_ (Figs. 49 and 50).—We have described the Bénier gasogene, and we will now complete the description by a short notice of the motor associated with it. This motor works on the Dugald-Clerk cycle, and in general arrangement is not unlike the Dugald-Clerk motors. A special compressing cylinder is provided cast in one with the motor cylinder, and the cranks are set at 90° degrees apart from one another. The pump is double, with two pistons coupled tandem-wise in the same cylinder. One of these draws in the air and the other forces the gas into the gasogene. The air and gas thus conducted by different paths reach the mixing chamber placed behind the motor cylinder and provided with a valve. The motor cylinder has exhaust ports perforating the walls, and which are uncovered by the piston when it has moved through five-sixths of the forward stroke. It remains open during the remaining sixth and the first sixth of the return stroke. At this moment the two piston pumps have forced into the cylinder the air and gas which they contain. The mixture enters through a valve and a perforated plate, which thoroughly mixes them in the cylinder. During the return stroke the piston, having closed the exhaust ports, compresses the explosive mixture till the end of the stroke. Ignition then takes place, and the explosion drives the piston forward, and the products of combustion escape to the air directly the exhaust ports are uncovered by the piston. [Illustration: FIGS. 49, 50.—Bénier Engine and Gas Plant (sectional plan and elevation).]