The piston is pulled forward with the application of some outside force, which in practice is supplied by the inertia of the fly-wheel, or in starting the engine by the action of a crank with which every user of an automobile is familiar. In being pulled forward, the piston draws gas into the cylinder; as the piston returns, this gas is compressed; the compressed gas, constituting an explosive mixture, is then ignited by a piece of incandescent metal or by an electric spark; the exploding gas expands, pushing the piston forward, this being the only thrust during which work is done; the returning piston expels the expanded gas, completing the cycle. Thus there are three ineffective piston thrusts to one effective thrust. Nevertheless, the engine has proved a useful one for many purposes. This so-called Otto cycle has been adopted in almost all gas and oil engines, the later improvements being in the direction of still higher compression, and in the substitution of lift for slide valves. There has been a steady increase in the size and power of such engines, the large ones usually introducing two or more working cylinders so as to secure uniform driving. Cheap forms of gas have been employed such as those made by decomposing water by incandescent fuel, and it has been proved possible thus to operate gas-power plants on a commercial scale in competition with the most economical steam installations. A practical modification of vast importance was introduced when it was suggested that a volatile oil be employed to supply the gas for operation in an internal combustion engine. There was no new principle involved in this idea, and the Otto cycle was still employed as before; but the use of the volatile oil--either a petroleum product or alcohol--made possible the compact portable engine with which everyone is nowadays familiar through its use in automobiles and motor boats. The oil commonly used is gasoline which is supplied to the cylinder through a so-called carburettor in which the vapors of gasoline are combined with ordinary air to make an explosive mixture. The introduction of this now familiar type of motor is to a large extent due to Herr G. Daimler, who in 1884 brought out a light and compact high-speed oil engine. About ten years later Messrs. Panhard and Levassor devised the form of motor which has since been generally adopted. Few other forms of mechanisms are better known to the general public than the oil engine with its two, four, six, or even eight cylinders, as used in the modern automobile. As everyone is aware, it furnishes the favorite type of motor, combining extraordinary power with relative lightness, and making it feasible to carry fuel for a long journey in a receptacle of small compass.