_Noël motor._—This type, constructed at Provins, has the advantage of being exceedingly simple. The entries to the cylinder are controlled by spring-valves, and the gases are ignited by electricity. From the ¼ up to the 2 horse-power size the cylinder is not provided with a water jacket, and the general arrangement is either vertical or horizontal. These engines work equally well with coal gas or carburetted air; in the latter case the carburator is placed inside the cast-iron frame of the motor. The guaranteed consumption of fuel is about 900 litres of gas or 500 grammes of gasoline per horse-power hour, which is quite satisfactory. _Tenting motor._—This is a horizontal motor with the cylinder cooled atmospherically, and the gases ignited by electricity. The governor acts upon the exhaust valve; the products of the explosion remain in the cylinder if the speed is too great, and then the admission valve, which is automatic, no longer rises to admit a fresh charge until the speed has once more fallen to the normal. This little motor is one of the most practical small-power engines existing, partly because of the great simplicity of the mechanism. It works well with carburetted air, and the vertical type has been successfully applied to the propulsion of small pleasure-boats. _Atkinson motor_ (Figs. 21, 22).—This apparatus, constructed by the British Gas Engine Company, was until recently the most efficient heat engine in existence, its indicated efficiency being 22·8%. Mr. Atkinson, the inventor, has arrived at this result by making the gases burn gradually and by shortening the compression stroke. The discharge of the residual gases is complete, this being directly opposite to Dr. Otto’s procedure, for he diluted his explosive mixture largely with the exhausted gases. In order, however, to make the piston execute two strokes relative to the cylinder of different lengths, the inventor has had to devise a rather complicated mechanism (Fig. 21). The result, however, quite neutralizes this slight disadvantage. [Illustration: FIG. 21.—Diagram of Atkinson Mechanism.] [Illustration: FIG. 22.—Section of Charon Motor.]