_Knight gas engine._—In his endeavour to completely drive out the products of combustion from the cylinder, Mr. Knight has had recourse to the Griffin cycle and to a double-acting cylinder: we have already discussed the relative merits of this system, and think it is sufficient to point out the greater constancy of speed obtained by it. The vaporizer is placed behind the motor-cylinder, and is separated from it by a steel plate, so that it is maintained at a high temperature; this vaporizer is made of bronze, and is provided with radiating ribs. A pump injects oil into it, and it is also supplied with compressed air by a special pump, which furnishes in addition a jet for producing a blow-pipe flame. This flame white-heats a platinum wire, which ignites the gases. The wire is situated in an opening in the ignition valve; the valve moves forward, and the white-hot wire is brought into contact with the explosive mixture at the right moment. The admission valve is automatic, and situated in the steel plate separating the vaporizer from the cylinder. The other organs of the motor are not of any special interest. _Crossley-Holt oil engine_ (Fig. 40).—This is practically the same engine which we described in the chapter on gas engines; a vaporizer is added in the oil motor to enable it to work with petroleum. It consists of a chamber divided into four channels by vertical partitions, and heated by a lamp placed underneath. The flame traverses these channels before reaching the chimney placed at the top. The petroleum is converted into a spray, and drawn off by a jet of air warmed by passing it through a spiral coil placed round the lamp chimney. This jet of air is supplied by a small pump worked by a lever. The oil lamp has no wick, which is rather a novel arrangement: it is supplied by a small pump similar to that which provides the air. Since its first appearance this engine has undergone certain changes which have considerably improved it. Messrs. Crossley Bros. have especially devoted themselves to shortening the length of time necessary to heat and start the engine. Besides this they have paid great attention to simplicity of the working parts. [Illustration: FIG. 40.—Crossley-Holt Petroleum Engine.]