How the tongue in sucking acts like the piston of a pump.
You see, then, that sucking and pumping are very much alike. In the pump the piston makes the room for the air and the water to be pushed up into. Now, when you suck, there is a piston that operates very much as the piston of a pump does. Your tongue is the piston. See how this is. When you suck through a tube held in water, you move your tongue in such a way as to make a space in the mouth, and the air in the tube is pushed in to fill up this space; and when the air is all pushed in, the water is pushed in after it. Both are pushed in, as I have before told you, by the air pressing on the water in the vessel. It is just as water is pushed up into a squirt-gun when you draw the piston. This piston does in the gun, when you draw it, the same thing that your tongue does in your mouth when you move it in sucking. It makes space, and the water is pushed into the gun, as it is into the mouth, to fill up this space. The way in which the space is made in the mouth in sucking is this. Before you begin to suck, the tongue fills the mouth, so as to be up against its roof; but when you suck, you move the tongue down from the roof of the mouth, and this makes a space there; and whatever is in the tube, whether it be air or water, is pushed in to fill this space.
The common language about sucking and pumping incorrect.
The common language, then, which is used about sucking and pumping is not exactly correct. When we suck or pump, it seems to us as if the liquid was drawn up, and so we use the word draw in regard to it. So, too, we talk about the suction or drawing power. But, as I have showed you, the liquid is pushed up instead of being drawn. All that the piston in a pump does is to make room. It does not draw the water into that room, but the pressure of the air forces it in. Whenever there is any room made, the air is always ready either to go in itself or to force something else in.
Questions.—Explain the operation of sucking up water through a tube. Why does the water in the tube run down into the vessel when you stop sucking and take your mouth away? Why is it that you commonly have to suck several times before the water reaches your mouth? How is pumping like sucking? What is shown by the first figure? What by the second? What by the third? Explain the operation of the lower valve of the pump. What makes the air and the water rise in the pump? Why would they not rise if the cistern were full and were air-tight? Explain how the tongue acts as a piston in sucking. Give the comparison about the squirt-gun. What is said about the language used about sucking and pumping?
CHAPTER VI.
THE BAROMETER.
Water can be raised in a pump only to a certain height, and the mistake has sometimes been made of getting the pump so long that it would not work. If it be more than about thirty-four feet from the water up to the piston, the water can not be made to go up so high. What is the reason? It is because the air, pressing on the surface of the water in the cistern or well, will raise it only to the height of thirty-four feet. It does not press hard enough to force it up any higher.
Suppose you had a glass tube over thirty-four feet long, with one end open, and used it as represented in the first experiment in Chapter IV., on page 27. The water would be kept up in it only the thirty-four feet. The weight of a column of water of that height just balances the pressure or weight of the air. Above that height in the tube there would be a space in which there would not be any thing.
Pressure of the air holds up water in the pump and mercury in the barometer.