_De Ravel motors._—The first motor constructed by M. de Ravel was exhibited in Paris in 1878, and was of the oscillating cylinder type, with a variable centre of gravity. The explosion drove up a heavy piston whose rod was directly connected to the crank-pin. The revolution of the crank-pin caused the whole cylinder to move in the same manner as that of the early oscillating steam engines. This movement was used as a means of opening and closing the ports. The efficiency of the engine was low, using some 600 to 700 litres of gas per horse-power hour; and besides the motor had, owing to faulty mechanism, the unhappy knack of suddenly stopping dead. These defects caused M. de Ravel to abandon this type and to bring out a second motor in 1885, performing one cycle per revolution. This new motor only had one cylinder, whose rear half acted as a compression chamber during the backward stroke of the piston, whilst the explosion took place in the front end of the cylinder. The consumption of gas was slightly more than an Otto engine, and the motor ran exceedingly silently and evenly, but this advantage was not of much service in the struggle against the all-conquering Otto motor. _Midland motor (Taylor)._—Constructed in Nottingham, this engine is of the horizontal double-cylinder type. One cylinder compresses the explosive mixture and passes it on to the other, where it is ignited and does work. The cranks connected to the two pistons are placed 65° apart, and a complete cycle in the cylinders is performed every revolution. The makers of this engine claim a consumption of only 600 litres of gas per horse-power hour. _Campbell gas engine._—The mechanism of this engine is very much like that of the Dugald-Clerk motor, two cylinders being placed side by side. The utilization of the heat is, however, far superior, and only about 500 litres of coal gas per horse-power hour are required. A result which the inventor of this type of cycle never succeeded in obtaining, but as far as we know the motor has never had any official trial, and the above figures are only taken from the makers prospectus. However, there is only a hardly perceptible shock from the explosion, and the motor can be safely recommended to anybody requiring a silent gas engine. There still exist gas engines worked on this same cycle per revolution principle, such as the Conelly, Day, De Ravel motors, descriptions of which must be sought elsewhere. The limited space at our disposal prevents us from discussing all those types which have only obtained a very small measure of success. The only remaining engine based on this principle worth mentioning is the Bénier motor, which is exceedingly instructive, but we shall speak of it later under the head of poor-gas engines. We shall at present pass on to the consideration of engines based in principle on the patent of Beau de Rochas, and first practically realized in 1877 by Dr. Otto.