_Robuste (Levasseur) motor._—The composition, admission, and ignition of explosive mixture are regulated by a sliding valve as in the Otto motor; in this case, however, it is a piston-valve and not a flat one. The valve-chest is at the back of the cylinder. The governor is of the inertia type, and suppresses the admission if the speed becomes higher than the normal. A double water jacket is provided for cooling the cylinder walls. Although not presenting any new features, this motor fully merits its name, and its solid construction enables it to withstand a surprising amount of bad handling. _Richardson and Norris gas engine._—Yet another high-speed engine, running at a speed of 230 revolutions per minute. Roby & Co. construct this machine especially for driving dynamos, and for this purpose two fly-wheels are provided in order to make it run smoothly without variation of speed. The gases are ignited through a valve with a double seating by means of a red-hot tube. The motor is reversible, which is an advantage under certain conditions. Poor gas can also be used instead of coal gas, consuming about 510 grammes of anthracite per horse-power hour for an 86 horse-power engine supplied with Dowson gas. This works out to a thermal efficiency of 21%, a result which places this engine above criticism. _H. C. motor._—This is an enclosed motor for use in mines or dusty places, the fly-wheel alone of the moving parts being visible. It works equally well with coal gas, carburetted air, or petroleum, and is constructed in sizes from ½ to 60 horse-power. In spite of its original features it has not met with much success up to the present time. _Le Marcel and Le Maurice motors (Cadiot)_ (Fig. 26).—The smallest types of the Marcel motors are of one man-power, and the largest of one horse-power, so that they are only suitable for small operations. One impulse is given to the piston every two revolutions, the cycle being that of Dr. Otto. The gas is compressed in a red-hot tube during half of the backward stroke. There are only two valves, one for the admission and the other for the escape of the gases, actuated by a single cam on the crank-shaft. The cylinder is cooled by a water jacket. The governor works on the all or nothing principle, the supply of gas being completely shut off if the speed rises above the normal of 350 revolutions per minute. [Illustration: FIG. 26.—Maurice Motor (de Cadiot).] The Maurice motors are somewhat similar in construction, but are designed for operating dynamos. For this purpose two fly-wheels are provided. Many of these little motors are to be seen about the country working fans, lathes, pumps, etc.