176. =Pumps.=--In Fig. 123 you have a plan of a common pump. A tube, C D, extends down into the well, W. Above this is the barrel of the pump, A B, in which the piston works up and down. There is a valve, F, in the piston, and another, E, at the bottom of the barrel. Both of them open upward. We will suppose that the pump is entirely empty of water. If, now, the piston descend, the valve E shuts down, and F opens, letting the pressed air between the piston and E pass upward. See what will happen when the piston rises. The air above the piston can not get below, for its pressure will shut the valve F. But there will be a tendency to a vacuum below the piston as it rises, and the air will go up through the valve E to fill up the space. But why does the air rise? Because of the pressure of the air upon the surface of the water in the well. This forces up in the pump the water and the air above it, just in proportion as the downward pressure in the pump is lessened. If the pumping be continued all the air will soon be expelled, the water following it and flowing out at the opening, G. It is obvious that the pump will be useless if the valve E be over 34 feet above the surface of the water in the well, as the pressure of the atmosphere will not sustain a column of water higher than this. 177. =Suction.=--In common language, the operation of the pump is attributed to what is called a principle of suction, as if there was a drawing up of the water. But the water, you see, is not drawn, but forced, up. So it is with all operations of a similar character. In sucking up a fluid through a tube the fluid is forced up, because the pressure downward in the tube is removed. But how is it removed? It is done by a movement of the tongue downward from the roof of the mouth, thus causing a tendency to a vacuum, as the upward movement of the piston in the pump causes this tendency under it. To fill the space made by the movement of the tongue the air is forced up the tube, the liquid following; and, as in the case of the pump when the air is all expelled, the liquid will begin to discharge into the mouth. [Illustration: Fig. 124.] 178. =Forcing-Pump.=--The forcing-pump is constructed differently from the common pump. Its plan is given in Fig. 124. It has a pipe, C D, and a barrel, A B, like the common pump. It has also the valve E at the bottom of the barrel. But it has no valve in the piston. Connected with the barrel is another pipe, F G, from which the water issues. This has a valve, H, opening upward. The operation of the pump is obvious. As the piston is drawn up E opens and H shuts, and when it is forced down E shuts and H opens. [Illustration: Fig. 125.]