adopted by the French navy, who at the present date have a number of small vessels propelled by this motive power. [Illustration: FIG. 18. FIG. 19. De Forest Gas Engines.] [Illustration: FIG. 20.—De Forest Double Piston Motor.] _Cuinat gas engines._—These engines are constructed in four types. The A type is vertical with the cylinder above and fly-wheel and shaft below. This is a more stable arrangement than placing them in the reversed order, as is more frequently done. For small engines the vertical type is undoubtedly the best, space, or rather lack of space, being very often an important consideration to the purchaser. The B type is similar to the previous one, except that two fly-wheels are provided so as to make it suitable for small electric light installations. The C type is horizontal, having all the valves placed vertically, which plan seems to work better for engines of power greater than 10 horse-power. The D type is also horizontal, but has two fly-wheels, having been specially designed for electric lighting purposes. In order to take up as little space as possible the dynamo is situated underneath the projecting cylinder, both the engine and dynamo being bolted down on to a special bed-plate. This arrangement is more stable than placing them on separate foundations. In the petroleum engines an electric spark is used to ignite the gases, and in the gas engines both electricity and a gas flame. It is best not to use animal or vegetable oils for lubricating the inside of the cylinder, because they decompose, forming fatty acids which have a corrosive action. Besides this, when they have fulfilled their function of lubrication they settle down to a thick paste, which has a most injurious effect on the working of the engine. It is best, therefore, to use nothing but perfectly pure mineral oils and to avoid all others. This does not only apply to the type of engine we have just been describing, but to all gas engines. The Cuinat gas and petroleum motors do not mix a portion of their exhausted gases with the fresh charge as is frequently done, but completely sweep away the products of the explosion before admitting a new charge. The result is, that combustion is more complete but at the same time rather more violent. An examination of an indicator diagram taken from one of these engines shows that the combustion takes place at once as an explosion, and that the final expanded pressure is as low as it is possible to get it. These being the conditions necessary for a high efficiency, it is needless to state that the consumption of fuel in these engines is as low as in any other engine.