CHAPTER IV.

THE PRESSURE OF THE AIR.

[Sidenote: Air presses in wherever room is made for it.]

The air is every where. It is always ready to go where there is room made for it. If we move a bureau or any thing out of a room, the air fills up all the place where it stood. If you make a hole in any thing, the air at once presses in to fill it up. Every crack and crevice is filled with air.

You know how much water a sponge will hold. There are a great many little cells or spaces in it that hold the water. Now squeeze the water out, and as the water goes out of these cells, the air presses into them and fills them up. So, too, if you have any liquid in a barrel, just so fast as you draw it off, the air goes in to take its place.

When you pull the handles of a pair of bellows apart, as represented here, you make more space in the bellows, and the air rushes in to fill up this space. It is the same with breathing. When you breathe in, or draw a breath, as we say, the air goes down into your lungs through the windpipe. This is because the chest is made larger as it heaves, and so there is more room in the lungs; and the air goes in to fill up this room, just as it does in the bellows.

When the air moves very fast, it is, you know, often very inconvenient, and sometimes does much harm, as when houses are blown down, or when ships are driven upon a rocky shore. But commonly it is very accommodating. It is so easily moved out of the way that we do not think of its being in the way at all. When you are walking, your body pushes the air one way and the other, just as a man pushes persons to the one side and the other when he goes through a crowd; and as the people close up behind him as he moves along, so the air closes up behind you as you walk through it. Now, if the crowd were facing him, and should push against him, he would find it slow and hard work to get through. So, when the wind blows strongly in your face, it is hard walking, and you get along slowly, because the air presses against you so hard.

[Sidenote: Air easily moved out of the way.]

[Sidenote: Why it is easier to walk in air than in water.]

The air is pushed out of the way easily because it is so light. This is the reason that it is easier to walk in air than in water. The water, as you wade in it, is pushed to the one side and the other, as the air is when you walk in it; but it is not done so quickly and easily; and, as it is easier to walk with the wind than against it, so it is easier, in a running stream, to wade down stream than up against the current.

The air is so light a thing that you hardly think of it as pressing on any thing; but it does press on every thing. Let us see what this pressure does.

See this glass tube. It is open at the end which is in the vessel of water, but it is closed at the other end. It is full of water. But water is apt to run down whenever it can get a chance to do it. Now what makes it stay up in this tube? It is kept up by the air that presses on the water in the vessel. If you could take away the air from all about the vessel, the water in the tube would come down into the vessel, because there would be nothing there to hold it up.

[Sidenote: Experiments showing the pressure of the air.]

There is another way in which the water in the tube can be made to run down into the vessel. Let a little hole be made in the top of the tube, and the air will go into it, and make the water run down by pressing on it. Even if it be only a pin-hole, the air, ready to go in every where, will rush in, and down the water will all go. Now you can not very well make a hole in the top of the tube, but you can try the experiment in another way, so as to show what letting the air in will do. The experiment is represented here. You take a glass tube open at both ends. Covering one end tight with the palm of your hand, you fill the tube with water. Then carefully put the other end under water, and hold it as you see here. The water will stay up in the tube as long as you keep the palm of your hand tight over the top of it; but loosen your hand, and the air will go in and push down the water into the vessel.

You can see, from what I have told you, why a vent-hole is needed in a barrel from which we draw any liquid. If the barrel be tapped, the liquid will not run out, unless the air can get in above so as to press it out. Till the vent-hole is made, the liquid will stay in, just as the water stays up in the tube in the experiment. When we make the vent-hole, we do the same to the barrel as we should do to the tube if we should make a little hole in the top of it, or as you do to the tube in the second experiment when you loosen your hand at the top of it to let the air in.

[Sidenote: Experiment showing that the air presses upward as much as downward.]

This pressure of the air that I have told you about is in every direction. It is upward and sideways as well as downward. This may be shown by another experiment with a glass tube, as represented here. Fill the tube with water, and then place carefully over its open end a smooth slip of paper. You can then turn it over so that the open end shall be downward, as seen in the figure, and the water will not run out. What is the reason of this? It is because the pressure of the air on the paper keeps the water in. We can often succeed with this experiment with a wine-glass, or even a common tumbler, though we can do it more easily with something that has a smaller opening.

[Sidenote: How bubbles of air rush in among the particles of a liquid.]

But you will ask, perhaps, this question: If it be the pressure of the air that keeps the water from running out, what need is there of the paper? The paper merely serves to keep the surface of the water smooth and whole. If the paper were not there, the air would get in between the parts of the water, and would rush up and force the water out. For the same reason, if, instead of the small hole commonly made in tapping, a large hole be made in the barrel, the liquid will run out without any vent-hole. In this case, the air has a chance to work itself in among the parts or particles[A3] of the liquid, and go in bubbles up into the upper part of the barrel. A mere slip of paper put on the hole would keep the liquid in, as in the case of the tube or the wine-glass, and for the same reason. You know that there is a gurgling sound made when a liquid is poured from a jug or a bottle. This is caused by the bubbles of air that pass in while the liquid is coming out.

_Questions._--What is said about the air’s being every where? Tell about the sponge and the barrel. How is breathing like using a pair of bellows? What is said about the ease with which air is moved out of the way? Give the comparison about going through a crowd. Why is the air pushed out of the way so easily? What is said about wading in water? Tell about the experiment with the glass tube open at one end. Why is a vent-hole needed in a barrel when we want to draw off what is in it? Give the comparison to the experiments with the tube. How can you show that the air passes upward and sideways as well as downward? What does the paper do in this experiment? Why is there no need of a vent-hole when a large opening is made in a barrel? What makes the gurgling when a liquid is poured from a jug or a bottle?

[Footnote A3: I explain about the particles of water farther on, in the 16th and 17th chapters.]