_Benz motor_ (Fig. 6).—In this motor the inventor attempts to drive out the whole of the exhaust gases before the second half of the backward stroke of the piston is reached. To do this he injects a certain volume of air under pressure, driving out the burnt gases and substituting itself in their place. Before the end of this return stroke a small auxiliary pump introduces the requisite amount of coal gas, which is then compressed during the rest of the stroke. The ignition is effected by a small magneto machine driven by the engine itself, the sparks being generated between two fine metallic points in the cylinder. The jet of air required to drive out the product of the combustion is furnished from a reservoir, pressure in it being maintained by using the other side of the piston as an air-pump. This arrangement is an advantage, because cold air is being continually drawn into the cylinder which keeps it cool, and enables the lubrification to take place more effectively. There is also an external cooling apparatus in the form of a water jacket, which uses about 40 litres of water per horse-power hour, and keeps the cylinder at about 75° C. The Benz motor is very well known on the Continent, and much of its success is due to the fact that it seems to work as well with gasoline as with coal gas! It is constructed by M. Roger of Paris, and in consequence of the extreme constancy of its speed, it has been successfully applied to driving dynamos, and also to launches, motor cars, etc. [Illustration: FIG. 6.—Benz Gas Engine.] _Baldwin gas engine._—The cycle of this engine is somewhat similar to that of the Benz motor, one side of the piston being used for expansion and the other side for compression. Part of the bed-plate casting is arranged so as to form a reservoir for the compressed gases. The coal gas is also admitted into this vessel, so that it contains an explosive mixture. As the vessel is only made of cast-iron this arrangement is rather dangerous. There are three valves, the admission valve being regulated by the governor. The power developed is, therefore, always kept proportional to the demand, and the constancy of speed is sufficient to warrant the use of these engines for running dynamos for electric light. The ignition is by an electric spark, and is generally obtained from some extra apparatus, such as accumulators or batteries, and an induction coil. The engine is constructed by Messrs. Otis Bros. of New York.