_Answer._ The ordinary single-acting force pump, fig. 66, used on locomotive and other steam engines consists of a _pump-barrel_, _A A_, which is a cast-iron or brass cylinder in which a tight-fitting piston, _B B_, called the _pump-plunger_, works. This piston or plunger is simply a round rod which works air-tight through what is called a _stuffing box_, _C_, whose construction will be fully explained hereafter. The plunger receives a reciprocating motion, usually from the piston-rod of the engine, but is sometimes worked by a small crank attached to one of the crank-pins, or by an eccentric on one of the axles. The pump-barrel is connected with the water-tank of the tender by the _suction-pipe_, _D_, and with the water-space of the boiler by the feed-pipe, _E E_. Over the suction-pipe _D_ is a valve, _F_, called the _suction-valve_, which opens upward, and below the feed-pipe, _E_, is another valve, _G_, called the _pressure-valve_. These valves are cylindrical and made of brass, and rest on brass seats, _g_, _g_, to which they are fitted so as to be water-tight. They work in guides, _k_, _k_, called _cages_, the form of which is more clearly shown in the section, fig. 67, and the plan, fig. 68. When the plunger is drawn out of the pump-cylinder it creates a vacuum behind it, and the pressure above the valve _G_ closes it, while the atmospheric pressure on the water in the tank forces it into the suction-pipe, opens the valve _F_, and fills the pump-cylinder. When the plunger is forced back again the force with which it presses against the water in the pump-barrel, _A_, closes the valve _F_, and opens the pressure-valve _G_, and the water is then forced through the feed-pipe into the boiler. In order to be certain that the water in the boiler will not flow back into the pump, and also to prevent all the water and steam in the boiler from escaping in case of accident to either the feed-pipe or pump, another valve, _H_, fig. 66, called a _check-valve_, is placed between the feed-pipe and the boiler. The construction of this valve is similar to that of the pressure and suction valves. It is inclosed in a cast-iron or brass case, _I I_. All of these valves have cages in which they work and which also act as stops, which prevent them from rising from their seats further than a certain distance. This distance is called their _lift_, and the successful working of the pumps depends very much on the amount of lift which the valves have. This is usually from ³⁄₁₆ to ¹⁄₂ inch. [Illustration: _Fig. 66._] [Illustration: _Fig. 67._] [Illustration: _Fig. 68._]