engine is carried off by the water jacket, was made use of, as in the old Hugon motor, to vaporize a spray of cold water, and thus adding to the total force behind the piston. This process was so effective, that on shutting off the supply of gas the motor continued to revolve for a considerable period by means of the vaporized water. About 800 litres of gas were consumed and four litres of water per horse-power hour, a very good result. The Ravel motor used even less, about 500 or 600 litres only, but owing to the bad arrangement of the parts the mechanical efficiency was very low. Such was the position of the gas engine in 1878. A standard type had been adopted and worked excellently. It merely required to be perfected in detail and simplified in order to make it still more economic, and capable of holding its own against its powerful rival the steam engine. Many modifications of the Otto gas engine have appeared since that date, among the most important being those by—Dugald-Clerk, in 1879, a motor which compressed and exploded the gases once in every revolution; Lenoir, in 1833, the cylinder being cooled by currents of air; and in the same year appeared the Griffin gas engine, with a complete cycle of operations every three revolutions. At the Antwerp Exhibition of 1884 several new types appeared, among them the Stockport engine by Andrews, and others by Koerting, Bénier, and Benz. In the same year a very good motor appeared, called the _Simplex_, constructed by Powell of Rouen (now Matter et Cie.), according to the plans of MM. Delamare-Deboutteville and L. Malandin. This engine was the subject of some litigation, the Otto people considering it an infringement of their patents, but the improvements in the design of the working part and the novelty of several details being apparent, the _Simplex_ gas engine gained the day. In 1885, after the appearance of the _Simplex_ and the new Lenoir motors, most makers made use of the Otto cycle, and about this time appeared the first carburetted gas motors, that is to say, using volatile spirits and products of petroleum for their source of energy. Such motors have been devised by Tenting, Koerting-Boulet, Diedrichs, Gotendorf, Noël, Forest, Ragot, Rollason, Atkinson, etc. At the International Exhibition in 1889 there were thirty-one exhibitors and fifty-three machines, with a total power of 1000 horse-power. All except four used the Otto cycle, and for the first time a motor was to be seen using a gas other than coal gas, namely a poor gas produced at a very low cost in a special gas-producing plant. The motor itself was of the single-cylinder _Simplex_ type of 100 horse-power, opening up a new horizon to inventors, and demonstrating the possibility of using large gas engines supplied with poor gas.