_Lentz gas engine._—It is difficult to conceive a more simple mechanism than is to be found in this motor. The supply of gas is drawn into the cylinder by an open valve, and a gas flame situated in this admission port ignites the explosive gases. The force of the explosion closes the admission valve, and on the return stroke a cam opens an exhaust port situated underneath the cylinder. There is no water jacket, but the cylinder is formed of two parts connected together by a non-conducting joint. In order to smooth down the jerk of the explosion the head of the connecting rod slides in a groove, and is kept pressed against the crank-pin by a spring, the result being that the connecting rod is longitudinally elastic and deadens the shock of the explosion. GROUP II., CLASS I.—ONE CYCLE PER REVOLUTION. _Dugald-Clerk gas engine._—In the ideal motor we should have at least one explosion per revolution of the fly-wheel, which is not the case in the Otto cycle. For this reason many inventors have tried to construct gas engines with one cycle per revolution, but experience has taught us that though they may be mechanically more simple, they lose in efficiency what they gain in simplicity, and in spite of many eminent inventors attempting to solve the problem. Even the best-designed motors of this type have been unable to hold their own against the Otto cycle because they are not as efficient. The first attempt was made by Dugald-Clerk in 1881. His engine is simple in the extreme, containing no gear-wheels, and working steadily and noiselessly at a fairly constant speed (Fig. 5). There are two cylinders of equal diameter placed side by side, and projecting over the end of a cast-iron bed-plate. The first of these is the motive cylinder in which the explosion takes place; the other is used for compressing the explosive mixture, this compression taking place in the Otto cycle in the motive cylinder itself.