Common Science Part 4

EXPERIMENT 8. Fill a toy balloon partly full of air by blowing into it, and close the neck with a rubber band so that no air can escape. Lay a saucer over the hole in the plate of the air pump, so that the rubber of the balloon cannot be sucked down the hole. Lay the balloon on top of this saucer, put the bell jar over it, and pump the air out of the jar. What makes the balloon expand? What is in it? Why could it not expand before you pumped the air out from around it?

A toy balloon expands for the same reason when it goes high in the air. Up there the air pressure is not so strong outside the balloon, and so the gas inside makes the balloon expand until it bursts.

[Ill.u.s.tration: FIG. 8. A siphon. The air pushes the water over the side of the pan.]

EXPERIMENT 9. Lay a rubber tube flat in the bottom of a pan of water, so that the tube will be filled with water. Let one end stay under water, but pinch the other end tightly shut with your thumb and finger and lift it out of the pan. Lower this closed end into a sink or empty pan that is lower than the pan of water. Now stop pinching the tube shut. This device is called a _siphon_ (Fig. 8).

EXPERIMENT 10. Put the mouth of a small syringe, or better, of a gla.s.s model lift pump, under water. Draw the handle up. Does the water follow the plunger up, stand still, or go down in the pump?

When you pull up the plunger, you leave an empty s.p.a.ce; you shove the air out of the pump or syringe ahead of the plunger. The air outside, pressing on the water, forces it up into this empty s.p.a.ce from which the air has been pushed. But air pressure cannot force water up even into a perfect vacuum farther than about 33 feet. If your gla.s.s pump were, say, 40 feet long, the water would follow the plunger up for a little over 30 feet, but nothing could suck it higher; for by the time it reaches that height it is pus.h.i.+ng down with its own weight as hard as the air is pressing on the water below. No suction pump, or siphon, however perfect, will ever lift water more than about 33 feet, and it will do well if it draws water up 28 or 30 feet. This is because a perfect vacuum cannot be made. There is always some water vapor formed by the water evaporating a little, and there is always a small amount of air that has been dissolved in water, both of which partly fill the s.p.a.ce above the water and press down a little on the water within the pump.

[Ill.u.s.tration: FIG. 9. A gla.s.s model suction pump.]

If you had a straw over 33 feet long, and if some one held a gla.s.s of lemonade for you down near the sidewalk while you leaned over from the roof of a three-story building with your long straw, you could not possibly drink the lemonade. The air pressure would not be great enough to lift it so high, no matter how hard you sucked,--that is, no matter how perfect a vacuum you made in the upper part of the straw.

The lemonade would rise part way, and then your straw would be flattened by the pressure outside.

Some days the air can force water up farther in a tube than it can on other days. If it can force the water up 33 feet today, it will perhaps be able to force it up only 30 feet immediately before a storm. And if it forces water up 33 feet at sea level, it may force it up only 15 or 20 feet on a high mountain, for on a mountain there is much less air above to make pressure. The pressure of the air is different in different places; where the air is heavy and pressing hard, we say the pressure is _high_; where the air is light and not pressing so hard, we call the pressure _low_. A place where the air is heavy is called an area of high pressure; where it is light, an area of low pressure. (See Section 44.)

WHAT MAKES WINDS? It is because the air does not press equally all the time and everywhere that we have winds. Naturally, if the air is pressing harder in one place than in another, the lower air will be pushed sidewise in the areas of high pressure and will rush to the areas where there is less pressure. And air rus.h.i.+ng from one place to another is called _wind_.

[Ill.u.s.tration: FIG. 10.]

_APPLICATION 4._ A man had two water reservoirs, which stood at the same level, one on each side of a hill. The hill between them was about 50 feet high. One reservoir was full, and the other was empty. He wanted to get some of the water from the full reservoir into the empty one. He did not have a pump to force the water from one to the other, but he did have a long hose, and could have bought more. His hose was long enough to reach over the top of the hill, but not long enough to go around it. Could he have siphoned the water from one reservoir to the other? Would he have had to buy more hose?

_APPLICATION 5._ Two boys were out hiking and were very thirsty. They came to a deserted farm and found a deep well; it was about 40 feet down to the water. They had no pump, but there was a piece of hose about 50 feet long. One boy suggested that they drop one end of the hose down to the water and suck the water up, but the other said that that would not work--the only way would be to lower the hose into the water, close the upper end, pull the hose out and let the water pour out of the lower end of the hose into their mouths. A stranger came past while the boys were arguing, and said that neither way would work; that although the hose was long enough, the water was too far down to be raised in either way. He advised the boys to find a bucket and to use the hose as a rope for lowering it. Who was right?

INFERENCE EXERCISE

EXPLANATORY NOTE. In the inference exercises in this book, there is a group of facts for you to explain. They can always be explained by one or more of the principles studied, like gravitation, water seeking its own level, or air pressure. If asked to explain why sucking through a straw makes soda water come up into your mouth, for instance, you should not merely say "air pressure," but should tell why you think it is air pressure that causes the liquid to rise through the straw. The answer should be something like this: "The soda water comes up into your mouth because the sucking takes the air pressure away from the top of the soda water that is in the straw. This leaves the air pressing down only on the surface of the soda water in the gla.s.s. Therefore, the air pressure pushes the soda water up into the straw and into your mouth where the pressure has been removed by sucking." Sometimes, when you have shown that you understand the principles very well, the teacher may let you take a short cut and just name the principle, but this will be done only after you have proved by a number of full answers that you thoroughly understand each principle named.

Some of the following facts are accounted for by air pressure; some by water seeking its own level; others by gravitation.

See if you can tell which of the three principles explains each fact:

1. Rain falls from the clouds.

2. After rain has soaked into the sides of mountains it runs underground and rises, at lower levels, in springs.

3. When there are no springs near, people raise the water from underground with suction pumps.

4. As fast as the water is pumped away from around the bottom of a pump, more water flows in to replace it.

5. After you pump water up, it flows down into your pail from the spout of the pump.

6. You can drink lemonade through a straw.

7. If a lemon seed sticks to the bottom of your straw, the straw flattens out when you suck.

8. When you pull your straw out to remove the seed, there is no hole left in the lemonade; it closes right in after the straw.

9. If you drop the seed, it falls to the floor.

10. If you tip the gla.s.s to drink the lemonade, the surface of the lemonade does not tip with the gla.s.s, but remains horizontal.

SECTION 4. _Sinking and floating: Displacement._

What keeps a balloon up?

What makes an iceberg float?

Why does cork float on the water and why do heavier substances sink?

If iron sinks, why do iron s.h.i.+ps not sink?

Again let us imagine ourselves up in the place where gravitation has no effect. Suppose we lay a nail on the surface of a bowl of water. It stays there and does not sink. This does not seem at all surprising, of course, since the nail no longer has weight. But when we put a cork in the midst of the water, it stays there instead of floating to the surface. This seems peculiar, because the less a thing weighs the more easily it floats. So when the cork weighs nothing at all, it seems that it should float better than ever. Of course there is some difficulty in deciding whether it ought to float toward the part of the water nearest the floor or toward the part nearest the ceiling, since there is no up or down; but one would think that it ought somehow to get to the outside of the water and not stay exactly in the middle. If put on the outside, however, it stays there as well.

A toy balloon, in the same way, will not go toward either the ceiling or the floor, but just stays where it is put, no matter how light a gas it is filled with.

The explanation is as follows: For an object to float on the water or in the air, the water or air must be heavier than the object. It is the water or air being pulled under the object by gravity, that pushes it up. Therefore, if the air and water themselves weighed nothing, of course they would be no heavier than the balloon or the cork; the air or water would then not be pulled in under the balloon or cork by gravity, and so would not push them up, or aside.

[Ill.u.s.tration: FIG. 11. The battles.h.i.+p is made of steel, yet it does not sink.]

WHY IRON s.h.i.+PS FLOAT. When people first talked about building iron s.h.i.+ps, others laughed at them. "Iron sinks," they said, "and your boats will go to the bottom of the sea." If the boats were solid iron this would be true, for iron is certainly much heavier than water. But if the iron is bent up at the edges,--as it is in a dish pan,--it has to push much more water aside before it goes under than it would if it were flattened out. The water displaced, or pushed aside, would have to take up as much room as was taken up by the pan _and all the empty s.p.a.ce inside of it_, before the edge would go under. Naturally this amount of water would weigh a great deal more than the empty pan.

But suppose you should fill the dish pan with water, or suppose it leaked full. Then you would have the weight of all the water in it added to the weight of the pan, and that would be heavy enough to push aside the water in which it was floating and let the pan sink. This is why a s.h.i.+p sometimes sinks when it springs a leak.

You may be able to see more clearly why an iron s.h.i.+p floats by this example: Suppose your iron s.h.i.+p weighs 6000 tons and that the cargo and crew weigh another 1000 tons. The whole thing, then, weighs 7000 tons. Now that s.h.i.+p is a big, bulky affair and takes up more s.p.a.ce than 7000 tons of water does. As it settles into the water it pushes a great deal of water out of the way, and after it sinks a certain distance it has pushed 7000 tons of water out of the way. Since the s.h.i.+p weighs only 7000 tons, it evidently cannot push aside more than that weight of water; so part of the s.h.i.+p stays above the water, and all there is left for it to do is to float. If the s.h.i.+p should freeze solid in the water where it floated and then could be lifted out of the ice by a huge derrick, you would find that you could pour exactly 7000 tons of water into the hole where the s.h.i.+p had been.

But if you built your s.h.i.+p with so little air s.p.a.ce in it that it took less room than 7000 tons of water takes, it could go clear under the water without pus.h.i.+ng 7000 tons of water aside. Therefore a s.h.i.+p of this kind would sink.

The earth's gravity is pulling on the s.h.i.+p and on the water. If the s.h.i.+p has displaced (pushed aside) its own weight of water, gravity is pulling down on the water as hard as it is on the s.h.i.+p; so the s.h.i.+p cannot push any more water aside, and if there is enough air s.p.a.ce in it, the s.h.i.+p floats.

Perhaps the easiest way to say it is like this: Anything that is lighter than the same volume of water will float; since a cubic foot of wood weighs less than a cubic foot of water, the wood will float; since a quart of oil is lighter than a quart of water, the oil will float; since a pint of cream is lighter than a pint of milk, the cream will rise. In the same way, anything that is lighter than the same volume of air will be pushed up by the air. When a balloon with its pa.s.sengers weighs less than the amount of air that it takes the place of at any one time, it will go up. Since a quart of warm air weighs less than a quart of cold air, the warm air will rise.

You can see how a heavy substance like water pushes a lighter one, like oil, up out of its way, in the following experiment:

EXPERIMENT 11. Fill one test tube to the brim with kerosene slightly colored with a little iodine. Fill another test tube to the brim with water, colored with a little blueing. Put a small square of cardboard over the test tube of water, hold it in place, and turn the test tube upside down. You can let go of the cardboard now, as the air pressure will hold it up. Put the mouth of the test tube of water exactly over the mouth of the test tube of kerosene. Pull the cardboard out from between the two tubes, or have some one else do this while you hold the two tubes mouth to mouth. If you are careful, you will not spill a drop. If nothing happens when the cardboard is pulled away, gently rock the two tubes, holding their mouths tightly together.

[Ill.u.s.tration: FIG. 12. The upper tube is filled with water and the lower with oil. What will happen when she pulls the cardboard out?]

Oil is lighter than water, as you know, because you have seen a film of oil floating on water. When you have the two test tubes in such a position that the oil and water can change, the water is pulled down under the kerosene because gravity is pulling harder on the water than it is pulling on the kerosene. The water, therefore, goes to the bottom and this forces the kerosene up.

_APPLICATION 6._ Three men were making a raft. For floats they meant to use some air-tight galvanized iron cylinders. One of them wanted to fill the cylinders with cork, "because," he said, "cork is what you put in life preservers and it floats better than anything I know of." "They'd be better with nothing in them at all," said a second. "Pump all the air out and leave vacuums. They're air-tight and they are strong enough to resist the air pressure." But the third man said, "Why, you've got to have some air in them to buoy them up.

Cork would be all right, but it isn't as light as air; so air would be the best thing to fill them with."

Which way would the floats have worked best?

_APPLICATION 7._ A little girl was telling her cla.s.s about icebergs. "They are very dangerous," she said, "and s.h.i.+ps are often wrecked by running into them. You see, the sun melts the top off them so that all there is left is under water. The sailors can't see the ice under water, and so their s.h.i.+ps run into it and are sunk." Another girl objected to this; she said, "That couldn't be; the ice would bob up as fast as the top melted." "No, it wouldn't," said a boy. "If that lower part wasn't heavier than water, it never would have stayed under at all. And if it was heavier at the beginning, it would still be heavier after the top melted off."