Common Science. Carleton Washburne
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.
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 glass 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 space; 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 space 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 glass 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 pushing 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 space above the water and press down a little on the water within the pump.
Fig. 9. A glass model suction pump.
If you had a straw over 33 feet long, and if some one held a glass 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 rushing from one place to another is called wind.
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 glass. 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 glass to drink the lemonade, the surface of the lemonade does not tip with the glass, but remains horizontal.
Section 4. Sinking and floating: Displacement.
What keeps