The Otto gas engine is a marvel of simplicity from a mechanical point of view, very much more so than a Corliss steam engine for instance. The admission and exhaust valves are worked by cams, and the ignition takes place under pressure. The governor is sometimes of the centrifugal type, and at others of the inertia type, but in both it is a case of all or nothing, the supply being completely shut off if the engine is going too fast. The connecting rod joins the crank to the piston rod by a cross-head running into a bored out-guide. It is necessary to have a heavy fly-wheel, because, as only one explosion takes place per two revolutions, the fly-wheel must store up enough energy during that explosion to carry it through the rest of the cycle. Many different types of Otto gas engines now exist, some having two cylinders and a single crank, and others two fly-wheels, in order to ensure constancy of speed for driving dynamos. Dr. Otto devised a compound gas engine, but it did not succeed, and also a cheaper vertical type (Fig. 8), which is very convenient for small workshops. Since the invention of carburetted air the creator of the Otto cycle has devised another motor for use with gasoline instead of coal gas. Otto devised the first practical gas engine and opened up the path for others, who, following in his footsteps, have confined their attention to improvement of detail. Some have undoubtedly succeeded, and by avoiding waste of heat, and by raising the initial temperature of the gases, they have considerably reduced the consumption of fuel. We shall now discuss different types of motors which have appeared during the last fifteen years, confining ourselves to the really successful ones. [Illustration: FIG. 8.—Otto Gas Engine (vertical type).] _Second Lenoir motor._—Twenty-five years separated the appearances of the first and second Lenoir motors, and during this time M. Lenoir gained a great deal of practical experience, so that if reference be made to Figs. 1 and 9 they will be seen to have very little in common. In the later type the cylinder projects over the back of the bed-plate, and is provided with deep circular grooves on the outside to increase the cooling surface. Ignition is obtained by the spark from a coil supplied by a battery as in the early form. The consumption of gas is about 800 litres per horse-power hour. Later on we shall discuss a petroleum motor for motor cars, and also a stationary petroleum engine by the same inventor. These engines were originally constructed by Mignon and Rouart, but later by the Compagnie Parisienne du Gaz, and are very well designed and constructed. [Illustration: FIG. 9.—Lenoir Gas Engine (second type).]