CHAPTER V.
PUMPS.
Explanation of the operation of sucking.
You know that you can suck up water or any fluid through a straw or any other tube. Now what is it that makes the water go up through the tube into your mouth? I will tell you. When you put the tube into your mouth it is full of air, and so long as the air is there the water will be kept out; but when you suck you remove the air from the tube; and as the air goes out, the water comes in, following right on after the air. But what makes the water come in? Does it come in of itself because there is room made for it? No. Water can not move itself. It must be moved by something else. It is the air pressing on the water in the vessel you are sucking from that pushes it up into the tube. You do not really draw up the water. You get the air out of the way in the tube, and then the air that is all the time pressing on the water in the vessel pushes it up into your mouth. As soon as you stop sucking, and take your mouth from the tube, the water that is in the tube will run down into the vessel, because it is pressed down by the air that goes in at the top of the tube.
You know that you have to suck commonly several times before the water will reach your mouth. If the tube is a very large one, you suck a great many times to get all the air out of it. At first you suck out a little of the air in the tube, and the water is pushed up to take its place; then you suck a little more out, and more water is pushed up, and so on till it reaches the top of the tube. Here is a boy that has partly filled his tube, and one more suck would bring the fluid to his mouth.
How pumping is like sucking.
You can now see how we pump up water out of a well or cistern. The water is not drawn up, but it is pushed up just as it is in the tube when you suck. When you work the handle, you do the same thing for the pump that your mouth does for the tube in sucking any liquid; and when the pump has not been worked for some time, you have to move the handle up and down several times before the water comes, just as you have to suck several times to fill a tube of any length with water.
