A locomotive has two steam-cylinders, either side by side within the frame, and immediately beneath the forward end of the boiler, or on each side and exterior to the frame. The engines are non-condensing, and of the simplest possible construction. The whole machine is carried upon strong but flexible steel springs. The steam-pressure is usually more than 100 pounds. The pulling-power is generally about one-fifth the weight under most favorable conditions, and becomes as low as one-tenth on wet rails. The fuel employed is wood in new countries, coke in bituminous coal districts, and anthracite coal in the eastern part of the United States. The general arrangement and the proportions of locomotives differ somewhat in different localities. In Fig. 127, a British express-engine, _O_ is the boiler, _N_ the fire-box, _X_ the grate, _G_ the smoke-box, and _P_ the chimney. _S_ is a spring and _R_ a lever safety-valve, _T_ is the whistle, _L_ the throttle or regulator valve, _E_ the steam-cylinder, and _W_ the driving-wheel. The force-pump, _B C_, is driven from the cross-head, _D_. The frame is the base of the whole system, and all other parts are firmly secured to it. The boiler is made fast at one end, and provision is made for its expansion when heated. Adhesion is secured by throwing a proper proportion of the weight upon the driving-wheel, _W_. This is from about 6,000 pounds on standard freight-engines, having several pairs of drivers, to 10,000 pounds on passenger-engines, per axle. The peculiarities of the American type (Fig. 128) are the truck, _I J_, or bogie, supporting the forward part of the engine, the system of equalizers, or beams which distribute the weight of the machine equally over the several axles, and minor differences of detail. The cab or house, _r_, protecting the engine-driver and fireman, is an American device, which is gradually coming into use abroad also. The American locomotive is distinguished by its flexibility and ease of action upon even roughly-laid roads. In the sketch, which shows a standard American engine in section, _A B_ is the boiler, _C_ one of the steam-cylinders, _D_ the piston, _E_ the cross-head, connected to the crank-shaft, _F_, by the connecting-rod, _G H_ the driving-wheels, _I J_ the truck-wheels, carrying the truck, _K L_; _N N_ is the fire-box, _O O_ the tubes, of which but four are shown. The steam-pipe, _R S_, leads the steam to the valve-chest, _T_, in which is seen the valve, moved by the valve-gear, _U V_, and the link, _W_. The link is raised or depressed by a lever, _X_, moved from the cab. The safety-valve is seen at the top of the dome, at _Y_, and the spring-balance by which the load is adjusted is shown at _Z_. At _a_ is the cone-shaped exhaust-pipe, by which a good draught is secured. The attachments _b_, _c_, _d_, _e_, _f_, _g_--whistle, steam-gauge, sand-box, bell, head-light, and "cow-catcher"--are nearly all peculiar, either in construction or location, to the American