This secondary cylinder also serves for another purpose; it draws in a certain volume of air directly after the explosion, which is afterwards driven through the motive cylinder, effectively clearing out the waste gases. The advantage which this arrangement of double cylinders has over the Otto cycle, lies in the fact that one explosion can take place during each revolution of the crank, and consequently such very heavy fly-wheels as are used for the Otto type of engine are not necessary. The great disadvantage which the Dugald-Clerk motor possesses is the extreme suddenness of the explosion, which is practically complete before the piston begins to move. In spite of this defect the machines have a fairly high efficiency. The gases are ignited by a gas-jet situated in a sliding valve. A water jacket is used for cooling the cylinder, and in some of these motors a mechanism is attached for converting the engine from simple to compound. The compression cylinder then becomes double-acting, and the gases are further expanded in what was previously the motor cylinder only. [Illustration: FIG. 5.—Plan of Dugald-Clerk Engine.] Owing to the fact that this motor was not as efficient as those of the Otto type it never became a commercial success, and it is doubtful if any are working at the present date. _Early Stockport gas engine._—The working of this motor affords a good example of British ingenuity. The compression cylinder is situated behind the motive cylinder, being a prolongation of it on the same axis; the two are firmly bolted together end on. The active piston is in front and connected to the crank, drawing behind it the piston of the compression chamber. Each cylinder has a separate sliding valve. The rear cylinder having aspirated and compressed a volume of the explosive gases, they are passed into the motive cylinder through a sliding valve which also serves to ignite them. The waste gas escapes into the air by a special valve. The aspiration and compression take place in the auxiliary cylinder once in every revolution of the crank, and besides, the motive piston also compresses the gases a trifle before the explosion takes place. The exhaust valve is made to open slightly before the piston reaches the end of its stroke. The motor was rather inefficient, and since its appearance a new type has been brought out with an Otto cycle which we will describe later on.