_Bénier gas engine._—This motor was the first conceived by the inventors of the combined gas plant and engine which we will describe later, and is extremely simple. This piston rod is connected to the crank in a manner similar to a beam engine (Fig. 3). Both the admission of the gases and their ignition are accomplished by a single spring valve worked by a cam on the crank-shaft. The cylinder, which is vertical and inverted, draws in the gases for half the forward stroke, and then the valve, which has moved still further forward, brings a flame opposite the admission port and ignites the mixture; a small auxiliary gas-jet re-ignites the flame at each stroke. The gases escape from the cylinder by a second port with a special valve and cam. A water jacket for the cylinder is provided to carry off the surplus heat. The consumption is high, being about 1400 litres per horse-power hour, but owing to the extreme simplicity of the working parts this motor met with a certain amount of success about 1880. [Illustration: FIG. 3.—Bénier Motor.] _Forest gas engine_ (Fig. 4).—This motor, being of the single-acting type without compression, had at one time a considerable sale, being used where only a small power was required. The rectilinear motion is changed into a rotary one by means of an Oliver Evans beam, and a connecting rod which returned alongside of the cylinder to the crank-shaft and fly-wheel, which are placed at the back. Ignition is obtained by a burner which is re-lit by a smaller one at each stroke, and the cylinder is cooled not by a water jacket but by a helical groove, which increases the surface. This helix is formed by a thin plate cast on the cylinder. The fuel consumed was about 1400 litres of gas per horse-power hour, which may be considered good for such an engine. [Illustration: FIG. 4.—Forest Motor.] _Economic motor._—Constructed in New York. This engine is another example of the early attempts to obtain economy without compression. As a rule they were not constructed of more than half horse-power size, and the general arrangement is ingenious, but rather more complicated than those which we have so far spoken of. The piston rod is guided by being attached to one end of a lever, connected with crank by means of a vertical connecting rod. The cylinder is grooved, and cooled by the circulation of the air round it, and constancy of speed is obtained by a centrifugal governor, which cuts off the supply of gas when the speed is too high. The engine seems to have given some very fair results.