The operation of a pump explained.
I will show by some figures how a pump operates. In the first figure the hand is raising the handle, as you know we always do when we begin to pump. The raising of the handle, you see, makes the piston, as it is called, go down in the pump. Here it is going down through air, for the water has not as yet got up as far as the piston. Now, if this piston were a whole solid piece of wood, it would do no good, for it would press the air down before it. But it is not solid. It has a hole through it, and a sort of clapper or valve on the hole. Therefore, as the piston goes down, the air pushes up the valve, and goes up through the hole. You see that this air is shut in between the piston and the water; and when the piston presses down, the only way for it to get out of the way is to press upon that little door, and go up above the piston.
Explanation of the pump continued.
Well, the handle is up. The next thing is to bring it down, as represented in this picture. As the handle goes down, the piston goes up, as you see. You remember that I told you that, as the piston was going down, as seen in the first figure, some of the air went up through the hole and got above the piston. Now this air can not get down again, for the moment that the piston begins to move up, the air, pressing on the valve, shuts it down. Now, as the piston goes up, there is room made below it. How is this room filled? The air that is there, as you see, rises up to fill it, and the water follows the air.
The next moving of the piston down will carry it below all the air and down into the water; and the water will go up through the little door, just as the air has done before it. Then the moving of the piston up will carry this water so high as to make it run out of the mouth of the pump, as seen in this figure.
But there is a valve in the pump that I have said nothing about as yet. This lower valve operates in this way: As the air or the water goes up in the pump, the valve is pushed open by it, as you see in the second figure and in the last one; but when the piston works down, as seen in the first figure, this valve is shut, so that all the water that gets above it is safe, and can not go back.
What is it that makes the air and the water rise in the pump? All that gets above the piston is lifted up by the piston, as you see. But what makes that rise which is below the piston? It is the pressure of the air on the water in the well or cistern. This pushes up the water as fast as there is room made for it.
If a cistern were full of water, and were air-tight also, you could not pump up the water from it. You must have air there to push up the water, or it will not come up when you make room for it by working the pump.