[Illustration: FIG. 16.—Pygmée Gas Engine.] _The “National” gas engine._—As a gas engine this machine is constructed in all sizes, from one horse-power up to large units requiring a gas plant of their own. As petroleum motors they range from one to ten horse-power. They are all provided with two fly-wheels, which keep them well balanced and steadies their speed. They have besides been especially designed with a view to economy of coal gas or petroleum. An idea of the general arrangement of the parts will be obtained by glancing at Fig. 17. The petroleum motors are provided in addition with a vaporizer and a petroleum lamp placed at the front end of the machine, and the oil reservoir is situated immediately above the cylinder. M. Herckenrath has specially devoted himself to simplifying the mechanism and making it more self-contained and less unsightly. Up to 50 horse-power only one cylinder is found necessary, and for larger powers two are provided. The patent governor is centrifugal and rather novel in construction, and to it is partly due the high efficiency of these engines. The large sizes are as easily started as the smaller ones, and the lubrication is perfectly automatic. Besides these advantages they require a minimum of attention, in fact, a skilled attendant can be dispensed with, a few explanations and instructions being all that is necessary to enable a boy or labourer to take competent charge of them. [Illustration: FIG. 17.—Herckenrath National Gas Engine.] _Forest motor_ (Figs. 18, 19, 20).—We have already described the earlier attempts of M. Forest to produce a practical gas engine. The idea embodied in the motor depicted in Fig. 20, if not particularly advantageous, is none the less highly original. The ends of the single cylinder are open, and within it are two pistons, between which the explosive mixture is introduced. The explosion drives these pistons out in different directions, but by means of a suitable mechanism they are each connected to one of two cranks on the same shaft. The whole forms a very neat and compact arrangement. In another type by the same inventor (Figs. 18, 19), also working on the Otto cycle, the cylinders are vertical, there being one, two, or four placed side by side. In the last case two explosions per revolution are obtained, ensuring a very constant torque throughout. M. Forest has, however, particularly applied himself in collaboration with M. Gallice to designing petroleum engines for small boats. These two inventors have conceived a highly ingenious arrangement for reversing the direction of rotation of the engine, which is absolutely necessary in the propulsion of boats. All the cams which actuate the valves are grouped together on a light shaft, and by a simple twisting of this shaft from one end by a handle the engine is reversed. The engines are self-starting, and are so especially suitable for the propulsion of boats that they have been