When the vaporizing cartridge has been sufficiently heated a rapid turn of the fly-wheel by hand, so as to produce the first explosion, is sufficient to start it, after which the vaporizer keeps itself warm. The pump which keeps up the supply of oil will be seen in the right-hand bottom corner of Fig. 35. It is of the plunger pattern, and at every stroke of the piston draws in the right quantity of oil and injects it into the vaporizer through a valve. The illustration on page 91 shows the other side of the engine with the valve-shaft, which revolves once for every two revolutions of the crank, also driving the centrifugal Porter governor. If the speed should happen to exceed the normal, a steel finger moved by the governor closes the valve admitting oil to the vaporizer, which then returns to the reservoir, having failed to accomplish its object. As the ordinary Otto cycle is used, a heavy fly-wheel is necessary to maintain the speed constant between the explosions. After being once started very little attention is necessary, everything working automatically. A water jacket W cools the cylinder. [Illustration: FIG. 36.—Exterior View of the Hornsby-Akroyd Oil Engine.] The specific gravity of the oil is usually about ·854, and of this the engine consumes about one pint per hour per B.H.-P., at a cost of 1½_d._ An additional advantage of the high speed is, that there is less chance of the cylinder becoming fouled by tarry products. _Vulcan motor._—This engine is in many respects a counterpart of the Akroyd motor. The cylinder is horizontal, and projects over the end of the bed-plate. Two fly-wheels are provided if very steady running is required. The lamp which vaporizes the petroleum at starting is removed when once the normal speed is attained. A water jacket prevents the cylinder from getting too hot. The most noticeable feature is the rather curious arrangement for governing by means of bent oscillating levers, which control both the admission and escape valves. The governor itself is situated inside the transmission pulley and keeps the speed within 1% of the normal; 600 grammes of petroleum per hour per horse-power is the usual consumption. The motor is rather too complicated, and can scarcely be said to have justified the expectations formed of it.