183.  The Common Pump or Lifting Pump.  Place a tube containing a close-fitting piston in a vessel of water, as shown in Figure 132.  Then raise the piston with the hand and notice that the water rises in the piston tube.  The rise of water in the piston tube is similar to the raising of lemonade through a straw (Section 77).  The atmosphere presses with a force of 15 pounds upon every square inch of water in the large vessel, and forces some of it into the space left vacant by the retreating piston.  The common pump works in a similar manner.  It consists of a piston or plunger which moves back and forth in an air-tight cylinder, and contains an outward opening valve through which water and air can pass.  From the bottom of the cylinder a tube runs down into the well or reservoir, and water from the well has access to the cylinder through another outward-moving valve.  In practice the tube is known as the suction pipe, and its valve as the suction valve.

In order to understand the action of a pump, we will suppose that no water is in the pump, and we will pump until a stream issues from the spout.  The various stages are represented diagrammatically by Figure 133.  In (1) the entire pump is empty of water but full of air at atmospheric pressure, and both valves are closed.  In (2) the plunger is being raised and is lifting the column of air that rests on it.  The air and water in the inlet pipe, being thus partially relieved of downward pressure, are pushed up by the atmospheric pressure on the surface of the water in the well.  When the piston moves downward as in (3), the valve in the pipe closes by its own weight, and the air in the cylinder escapes through the valve in the plunger.  In (4) the piston is again rising, repeating the process of (2).  In (5) the process of (3) is being repeated, but water instead of air is escaping through the valve in the plunger.  In (6) the process of (2) is being repeated, but the water has reached the spout and is flowing out.

[Illustration:  FIG. 133.  Diagram of the process of pumping.]

After the pump is in condition (6), motion of the plunger is followed by a more or less regular discharge of water through the spout, and the quantity of water which gushes forth depends upon the speed with which the piston is moved.  A strong man giving quick strokes can produce a large flow; a child, on the other hand, is able to produce only a thin stream.  Whoever pumps must exert sufficient force to lift the water from the surface of the well to the spout exit.  For this reason the pump has received the name of lifting pump.

[Illustration:  FIG. 134.—­Force pump.]

184.  The Force Pump.  In the common pump, water cannot not be raised higher than the spout.  In many cases it is desirable to force water considerably above the pump itself, as, for instance, in the fire hose; under such circumstances a type of pump is employed which has received the name of force pump.  This differs but little from the ordinary lift pump, as a reference to Figure 134 will show.  Here both valves are placed in the cylinder, and the piston is solid, but the principle is the same as in the lifting pump.