The Andrews gas engine is also constructed of a special type for consuming poor gas produced by the Dowson process, and gives very good results. As a rule, about 600 to 800 grammes of anthracite are necessary to produce one horse-power hour. In one particular plant generating electricity, the cost has been certified to be as low as one penny per kilowatt hour, including lubrication. _Fielding gas engine._—The characteristic point in these engines is the extreme simplicity of the valve gear, only one valve being ever subjected to pressure. Even in the small engines of this type all sliding valves are replaced by those of the spring pattern, in fact, the valve mechanism consists simply of two spring-valves, one of which fulfils two functions, controlling the admission and the escape of the gases. The two valves are moved by a double lever actuated by a single cam. The cycle is that of Dr. Otto. When the piston is starting on the return stroke after an explosion has taken place, the lever lifts one of the valves, and the products of the combustion escape into a circular space situated below. In the next forward stroke this valve is still further lifted, opening the admission port, while at the same moment the exhaust port is closed by the second valve; a new charge is therefore drawn into the cylinder. At the end of the admission stroke the valves are released by the lever, and the compression can now take place during the second return stroke. The movement causing the lifting of the valve continuously throughout one whole revolution is effected by means of a cam with two prominences on it, which act in succession on the lever. There is only one valve, therefore, which is subjected to pressure, and even this is well provided against risk of leakage by the second valve being placed behind it, acting as a double seating. A chamber is also provided in which the gases are first mixed, and this mixing is regulated by a second lever and a special valve. The governor is of the inertia type, and is attached to the upper side of the double lever. It consists of a straight rod with a ball at one end and a knife edge at the other, pivoted at its centre to the end of the valve lever. As this lever is thrown forward the knife edge strikes the end of the spring-valve admitting gas to the mixing chamber. If the speed is too great the sudden jerk on the ball at the other end of the knife edge causes it to miss the valve, and no gas is admitted till the speed is reduced to the normal number of revolutions per minute. This arrangement is very sensitive, and keeps the speed exceedingly constant. Mr. Fielding has constructed some large gas engines of 350 horse-power which are started by compressed air, this air having been previously stored up under pressure when the motor was stopping.