The common condensing pump is the kind used for inflating tires. (See Fig. 41.) In this, a loosely fitting metal piston is attached to a disc of leather somewhat larger than the cylinder. This device is called a _cup valve_. On raising the piston, air rushes in from the top past the valve, but on pushing the piston down, the valve is pressed tightly against the sides of the cylinder and prevents the escape of any air. The compressed air pushes open a valve on the tire and enters it. This valve closes as soon as the pressure is lessened from outside. It is well to notice in all of these pumps that _two_ valves are used. One holds the air already secured while the other opens for a new supply. Both valves are never open at the same time. [Illustration: FIG. 41.--Condensing pump used in inflating tires.] =62. Water Pumps.--The Common Lift Pump.= This, the simplest pump for raising water, consists of a cylinder _C_ (Fig. 42) connected by a pipe _R_ to a supply of water as a cistern or well. A valve opening upward is placed at the bottom of the cylinder over the entrance to the pipe. In the cylinder is a tightly fitting piston connected by a rod to a lever for ease in action. The piston contains a valve opening upward. In operating this pump water is usually first poured into the cylinder to "prime" it. This helps to close the valves and prevents air leaking past them. When the piston is lowered the lower valve closes, the air in the cylinder being compressed pushes the upper valve open and passes above the piston. On raising the piston the upper valve closes. This forms a partial vacuum in the cylinder. The air pressing on the surface of the water below forces the water and air that may be in the tube upward through the lower valve to fill this partial vacuum. When the cylinder becomes filled with water, this is lifted out on the up-stroke, whence its name, "lift pump." Since the atmospheric pressure at sea-level can only support a column of water about 34 ft. high, the lower valve must be within this distance of the water surface. In actual practice the limit is about 27 ft. In deeper wells, the cylinder and valves are placed so that they are within 25 or 27 ft. of the surface of the water in the well, a long piston rod reaching above the surface of the ground and connected to a pump handle operates the piston. A discharge pipe extends from the cylinder to the surface of the ground above. [Illustration: FIG. 42.--The common lift pump.] [Illustration: FIG. 43.--A force pump with an air chamber (_A_).] =63. The Force Pump.=--The force pump is used to deliver water under pressure either for spraying or to an elevated reservoir. The piston is solid, the second valve being placed at the entrance of the discharge pipe. (See Fig. 43.) The action is the same as that of the lift pump, with this exception; the piston in its down stroke forces the water out through the discharge pipe, the velocity depending upon the pressure exerted.