_Charon motor_ (Fig. 23).—M. Charon has attempted to obtain a prolonged combustion without the complicated devices resorted to by Atkinson. He has obtained the same result by means of a regulator controlling a double cam, one half of which actuates the admission valve and the other half an auxiliary valve, which opens into a tube forming a sort of reservoir, into which part of the explosive mixture passes during the compression. After the explosion this stored-up mixture is gradually allowed to re-enter the cylinder and prolong the combustion. By this means a considerable gain in economy is obtained, so that engines of this type only use about 500 litres of coal gas per horse-power hour. In appearance the motor much resembles the Otto motor, the crank-shaft and valve-shaft and valves being placed in the same relative positions. Although these motors are rather more expensive than others working with the Otto cycle, they are nevertheless widely used. This is due to the fact that if cost of coal gas in a particular district is high it is cheaper in the end to pay a higher price for an efficient engine than to buy a less expensive and at the same time less economical machine. [Illustration: FIG. 23.—Charon Motor (latest type).] _Roger motor._—This excellent little engine (Fig. 24) was especially designed for small workshops. The extreme simplicity of the working parts in no way lessens the efficiency, for the two horse-power only burns 700 litres of gas per horse-power hour. The simple design also keeps the cost of construction low, and the price averages about £30 per horse-power. [Illustration: FIG. 24.—Roger Vertical Gas Engine.] The governor controls the admission valve. Ignition is obtained by means of an incandescent tube; the cylinder is cooled by a water jacket; the mean speed is about 200 revolutions per minute, and this speed is constant enough to allow the engine to drive a dynamo for electric lighting. The motor has been very favourably received abroad, but is not much known in this country. _Motor of the Compagnie Parisienne du Gaz_ (Fig. 25).—Owing to its very large consumption of gas the engine can only be used if the price of gas is low, but it has several advantages which to a certain extent neutralize this defect. It runs at a high speed of 400 revolutions per minute, and the parts are arranged so as to be easily accessible. [Illustration: FIG. 25.—Vertical Gas Engine of the Compagnie Parisienne du Gaz.] _Letombe motor._—Constructed by the firm of Mollet-Fontaine of Lille, this engine presents several interesting features which we will briefly enumerate. The cylinder is double-acting, giving one impulse to the piston during every revolution. The speed is therefore maintained fairly constant. The efficiency is high owing to the gases in the cylinder being made to burn slowly as in the Charon motor. The machine on the whole works exceedingly satisfactorily and reflects great credit on its inventors.