_Root motor._—These engines work with the ordinary four-cycle, and are interesting because of the double-ignition apparatus. A lateral chamber is attached to the cylinder, communicating with it by an orifice situated at the back end, which is covered when the piston is at the beginning of its stroke. During the compression stroke a portion of the mixture is enclosed in this chamber and separated from the rest of the charge, which is compressed and ignited in the ordinary way. The explosion drives forward the piston, and when it has moved forward some distance it unmasks the above-mentioned orifice, igniting the gas contained in the lateral chamber, and helping to maintain the pressure behind the piston. This addition to the pressure is noticeable in a diagram taken from the cylinder, by sudden rise of pressure. We don’t quite see the utility of this arrangement, which seems to merely complicate matters. The first ignition is effected by a red-hot tube. The Root vaporizer utilizes the heat of the exhaust gases; the air is heated in a spiral coil wound round the tube through which the vaporized oil passes. The two become mixed in a small vessel, and the vaporizer is supplied by a chamber placed directly round the exit of the exhaust gases. Strictly speaking, there is no oil pump, but a plunger in a small auxiliary cylinder serves to direct the flow of oil into the right channels. Admission takes place through an automatic valve. The escape valve is actuated by the same mechanism which drives the piston governing the flow of oil. [Illustration: FIG. 39.—Carburator of the Ragot Oil Engine.] _Koerting-Boulet motor._—In this four-cycle engine we shall only describe the vaporizer and spray-maker. The oil is stored in a reservoir placed about six feet from the ground, and a pipe conducts it into a space between two discs with a valve at the top. This circular space, about half a millimetre in thickness, is traversed by the air which pulverizes the oil; the spray thus formed is vaporized by the high temperature developed by a lamp situated underneath the tube which conducts the carburetted air to the cylinder. In addition, this lamp is used for igniting the gases through a porcelain tube kept at red heat by it. An auxiliary spirit-lamp is used for starting the engine, which takes about fifteen minutes to acquire a sufficiently high temperature.