Common Science Part 3

SECTION 2. "_Water seeks its own level._"

Why does a spring bubble up from the ground?

What makes the water come up through the pipe into your house?

Why is a fire engine needed to pump water up high?

You remember that up where the pull of the earth and the sun balance each other, water could not flow or flatten out. Let us try to imagine that water, here on the earth, has lost its habit of flattening out whenever possible--that, like clay, it keeps whatever shape it is given.

First you notice that the water fails to run out of the faucets. (For in most places in the world as it really is, the water that comes through faucets is simply flowing down from some high reservoir.) People all begin to search for water to drink. They rush to the rivers and begin to dig the water out of them. It looks queer to see a hole left in the water wherever a person has scooped up a pailful. If some one slips into the river while getting water, he does not drown, because the water cannot close in over his head; there is just a deep hole where he has fallen through, and he breathes the air that comes down to him at the bottom of the hole. If you try to row on the water, each stroke of the oars piles up the water, and the boat makes a deep furrow wherever it goes so that the whole river begins to look like a rough, plowed field.

When the rivers are used up, people search in vain for springs. (No springs could flow in our everyday world if water did not seek its own level; for the waters of the springs come from hills or mountains, and the higher water, in trying to flatten out, forces the lower water up through the ground on the hillsides or in the valleys.) So people have to get their water from underground or go to lakes for it. And these lakes are strange sights. Storms toss up huge waves, which remain as ridges and furrows until another storm tears them down and throws up new ones.

But with no rivers flowing into them, the lakes also are used up in time. The only fresh water to be had is what is caught from the rain.

Even wells soon become useless; because as soon as you pump up the water surrounding the pump, no more water flows in around it; and if you use a bucket to raise the water, the well goes dry as soon as the supply of water standing in it has been drawn.

You will understand more about water seeking its own level if you do this experiment:

EXPERIMENT 1. Put one end of a rubber tube over the narrow neck of a funnel (a gla.s.s funnel is best), and put the other end of the tube over a piece of gla.s.s tubing not less than 5 or 6 inches long. Hold up the gla.s.s tube and the funnel, letting the rubber tube sag down between them as in Figure 1. Now fill the funnel three fourths full of water. Raise the gla.s.s tube higher if the water starts to flow out of it. If no water shows in the gla.s.s tube, lower it until it does.

Gradually raise and lower the tube, and notice how high the water goes in it whenever it is held still.

This same thing would happen with any shape of tube or funnel. You have another example of it when you fill a teakettle: the water rises in the spout just as high as it does in the kettle.

[Ill.u.s.tration: FIG. 1. The water in the tube rises to the level of the water in the funnel.]

WHY WATER FLOWS UP INTO YOUR HOUSE. It is because water seeks its own level that it comes up through the pipes in your house. Usually the water for a city is pumped into a reservoir that is as high as the highest house in the city. When it flows down from the reservoir, it tends to rise in any pipe through which it flows, to the height at which the water in the reservoir stands. If a house is higher than the surface of the water in the reservoir, of course that house will get no running water.

WHY FIRE ENGINES ARE NEEDED TO FORCE WATER HIGH. In putting out a fire, the firemen often want to throw the water with a good deal of force. The tendency of the water to seek its own level does not always give a high enough or powerful enough stream from the fire hose; so a fire engine is used to pump the water through the hose, and the stream flows with much more force than if it were not pumped.

_APPLICATION 2._ A. C. Wheeler of Chicago bought a little farm in Indiana, and had a windmill put up to supply the place with water. But at first he was not sure where he should put the tank into which the windmill was to pump the water and from which the water should flow into the kitchen, bathroom, and barn. The barn was on a knoll, so that its floor was almost as high as the roof of the house. Which would have been the best place for the tank: high up on the windmill (which stood on the knoll by the barn), or the bas.e.m.e.nt of the house, or the attic of the house?

[Ill.u.s.tration: FIG. 2. Where is the best location for the tank?]

_APPLICATION 3._ A man was about to open a garage in San Francisco. He had a large oil tank and wanted a simple way of telling at a glance how full it was. One of his workmen suggested that he attach a long piece of gla.s.s tubing to the side of the tank, connecting it with an extra faucet near the bottom of the tank. A second workman said, "No, that won't do.

Your tank holds ever so much more than the tube would hold, so the oil in the tank would force the oil up over the top of the tube, even when the tank was not full." Who was right?

[Ill.u.s.tration: FIG. 3. When the tank is full, will the oil overflow the top of the tube?]

SECTION 3. _The sea of compressed air in which we live: Air pressure._

Does a balloon explode if it goes high in the air?

What is suction?

Why does soda water run up a straw when you draw on the straw?

Why will evaporated milk not flow freely out of a can in which there is only one hole?

Why does water gurgle when you pour it out of a bottle?

We are living in a sea of compressed air. Every square inch of our bodies has about 15 pounds of pressure against it. The only reason we are not crushed is that there is as strong pressure inside of our bodies pus.h.i.+ng out as there is outside pus.h.i.+ng in. There is compressed air in the blood and all through the body. If you were to lie down on the ground and have all the air pumped out from under you, the air above would crush you as flat as a pancake. You might as well let a dozen big farm horses trample on you, or let a huge elephant roll over you, as let the air press down on you if there were no air underneath and inside your body to resist the pressure from above. It is hard to believe that the air and liquids in our bodies are pressing out with a force great enough to resist this crus.h.i.+ng weight of air. But if you were suddenly to go up above the earth's atmosphere, or if you were to stay down here and go into a room from which the air were to be pumped all at once, your body would explode like a torpedo.

When you suck the air out of a bottle, the surrounding air pressure forces the bottle against your tongue; if the bottle is a small one, it will stick there. And the pressure of the air and blood in your tongue will force your tongue down into the neck of the bottle from which part of the air has been taken.

In the same way, when you force the air out of a rubber suction cap, such as is used to fasten reading lamps to the head of a bed, the air pressure outside holds the suction cap tightly to the object against which you first pressed it, making it stick there.

We can easily experiment with air pressure because we can get almost entirely rid of it in places and can then watch what happens. A place from which the air is practically all pumped out is called a _vacuum_.

Here are some interesting experiments that will show what air pressure does:

[Ill.u.s.tration: FIG. 4. When the point is knocked off the electric lamp, the water is forced into the vacuum.]

EXPERIMENT 2. Hold a burned-out electric lamp in a basin of water, break its point off, and see what happens.

All the common electric lamps (less than 70 watts) are made with vacuums inside. The reason for this is that the fine wire would burn up if there were any air in the lamps. When you knock the point off the globe, it leaves a s.p.a.ce into which the water can be pushed. Since the air is pressing hard on the surface of the water except in the one place where the vacuum in the lamp globe is, the water is forced violently into this empty s.p.a.ce.

It really is a good deal like the way mud comes up between your toes when you are barefoot. Your foot is pressing on the mud all around except in the s.p.a.ces between your toes, and so the mud is forced up into these s.p.a.ces. The air pressure on the water is like your foot on the mud, and the s.p.a.ce in the lamp globe is like the s.p.a.ce between your toes. Since wherever there is air it is pressing hard, the only s.p.a.ce into which it can force water or anything else is into a place from which all the air has been removed, like the inside of the lamp globe.

The reason that the water does not run out of the globe is this: the hole is too small to let the air squeeze up past the water, and therefore no air can take the place of the water that might otherwise run out. In order to flow out, then, the water would have to leave an empty s.p.a.ce or vacuum behind it, and the air pressure would not allow this.

WHY WATER GURGLES WHEN IT POURS OUT OF A BOTTLE. You have often noticed that when you pour water out of a bottle it gurgles and gulps instead of flowing out evenly. The reason for this is that when a little water gets out and leaves an empty s.p.a.ce behind, the air pus.h.i.+ng against the water starts to force it back up; but since the mouth of the bottle is fairly wide, the air itself squeezes past the water and bubbles up to the top.

EXPERIMENT 3. Put a straw or a piece of gla.s.s tube down into a gla.s.s of water. Hold your finger tightly over the upper end, and lift the tube out of the water. Notice how the water stays in the tube. Now remove your finger from the upper end.

The air holds the water up in the tube because there is no room for it to bubble up into the tube to take the place of the water; and the water, to flow out of the tube, would have to leave a vacuum, which the air outside does not allow. But when you take your finger off the top of the straw or tube, the air from above takes the place of the water as rapidly as it flows out; so there is no tendency to form a vacuum, and the water leaves the tube. Now do you see why you make two holes in the top of a can of evaporated milk when you wish to pour the milk out evenly?

[Ill.u.s.tration: FIG. 5. The water is held in the tube by air pressure.]

EXPERIMENT 4. Push a rubber suction cap firmly against the inside of the bell jar of an air pump. Try to pull the suction cap off. If it comes off, press it on again; place the bell jar on the plate of the air pump, and pump the air out of the jar. What must have been holding the suction cap against the inside of the jar? Does air press up and sidewise as well as down? Test this further in the following experiment:

[Ill.u.s.tration: FIG. 6. An air pump.]

EXPERIMENT 5. Put a cork into an empty bottle. Do not use a new cork, but one that has been fitted into the bottle many times and has become shaped to the neck. Press the cork in rather firmly, so that it is air-tight, but do not jam it in.

Set the bottle on the plate of the air pump, put the bell jar over it, and pump the air out of the jar. What makes the cork fly out of the bottle? What was really in the "empty" bottle?

Why could it not push the cork out until you had pumped the air out of the jar?

EXPERIMENT 6. Wax the rims of the two Magdeburg hemispheres (see Fig. 7). Screw the lower section into the hole in the plate of the air pump. Be sure that the stop valve in the neck of the hemisphere is open. (The little handle should be vertical.) Fit the other section on to the first, and pump out as much air as you can. _Close_ the stop valve. Unscrew the hemispheres from the air pump. Try to pull them apart--pull straight out, taking care not to slide the parts. If you wish, let some one else take one handle, and see if the two of you can pull it apart.

[Ill.u.s.tration: FIG. 7. The experiment with the Magdeburg hemispheres.]

Before you pumped the air out of the hemisphere, the compressed air inside of them (you remember all the air down here is compressed) was pus.h.i.+ng them apart just as hard as the air outside of them was pus.h.i.+ng them together. When you pumped the air out, however, there was hardly any air left inside of them to push outward. So the strong pressure of the outside air against the hemispheres had nothing to oppose it. It therefore pressed them very tightly together and held them that way.

This experiment was first tried by a man living in Magdeburg, Germany.

The first set of hemispheres he used proved too weak, and when the air in them was partly pumped out, the pressure of the outside air crushed them like an egg sh.e.l.l. The second set was over a foot in diameter and much stronger. After he had pumped the air out, it took sixteen horses, eight pulling one way and eight the opposite way, to pull the hemispheres apart.

EXPERIMENT 7. Fill a bottle (or flask) half full of water.

Through a one-hole stopper that will fit the bottle, put a bent piece of gla.s.s tubing that will reach down to the bottom of the bottle. Set the bottle, thus stoppered, on the plate of the air pump, with a beaker or tumbler under the outer end of the gla.s.s tube. Put the bell jar over the bottle and gla.s.s, and pump the air out of the jar. What is it that forces the water up and out of the bottle? Why could it do this when the air was pumped out of the bell jar and not before?

HOW A SELTZER SIPHON WORKS. A seltzer siphon works on the same principle. But instead of the ordinary compressed air that is all around us, there is in the seltzer siphon a gas (carbon dioxid) which has been much more compressed than ordinary air. This strongly compressed gas forces the seltzer water out into the less compressed air, exactly as the compressed air in the upper part of the bottle forced the water out into the comparative vacuum of the bell jar in Experiment 7.