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A sand blast is a current of air set free from pressure and carrying sharp sand with great speed. It is used to roughen the surface of glass, producing "ground glass." The sand particles chip the surface. The wind of a sandy desert is a natural sand blast; it not only roughens window glass, but wears away the surface of rocks (cf. § 151).
15. Exercises. 1. Is it easy to get ketchup out of a small-mouth bottle? How do you do it? Why?
2. How do we get machine oil out of its can? Why?
3. Why is there a vent, or hole, near the top of a vinegar or kerosene barrel? Why is there one in a steam radiator?
4. Examine a kerosene or gasoline can. How does the liquid come out of the spout when the top of the can is closed? How, when the top is open ? Why?
5. If you bring an inflated toy balloon from the cold, outer air into a warm room, what will happen to it? Why?
6. Set an empty” glass, mouth down, into à saucer or basin of hot water; what happens, and why? Have you ever noticed any phenomenon like this in the rinsing of dishes with hot water?
7. Suppose you balance two dry, empty" flasks or tomato cans on the two pans of the scales, and then heat one of the flasks, or cans, and put it back on the scales; what will happen, and why?
8. Do you think the gas in an inflated toy balloon is under pressure? Why?
9. Suppose that the inside space of a basketball holds ș of a cubic foot. How much does this volume of air weigh? Suppose that three times this volume of air is packed into the ball, what does the air in the ball weigh?
PRESSURE OF THE ATMOSPHERE
16. Does the Atmosphere Have Pressure ? — If we were living at the bottom of an ocean of water, the weight of the water above us would cause a great pressure upon our bodies and upon all the objects in the water. Is the same true of an ocean of air ? Let us perform a few experiments to find out.
(a) Look back at § 9, Fig. 4. Why does not the water fall out of the bottle that is set, upside down, in the pan of water?
(6) Fill a drinking glass or a large-mouth bottle completely with water, close the mouth of the vessel entirely with a wet piece of cardboard or stiff paper; a stiff envelope will do. Now hold the covering in place with your hand, turn the vessel upside down, and remove your hand. Why does not the water fall out? In what direction does the atmosphere exert pressure in this case ?
(c) Hold a clean glass vial or other small-mouth bottle. between your lips and remove as much as possible of the air by suction. Then, before the air reënters, close the mouth of the vial or bottle with your tongue or the inside of your lips or cheek. What evidence do you get of the pressure of the atmosphere? What happens when you pull the bottle away?
(d) Over the mouth of a clay pipe tie tightly a piece of thin sheet rubber and then, by suction through the stem, remove some of the air. Explain what happens. Does this show anything of air pressure?
If your laboratory has an air pump, perform the same experiment as shown in Fig. 12. (e) Blow a paper bag full of air.
The pressure of the air inside and outside the bag will then be the
Now make the mouth of the bag small and remove the air by suction. Why does the bag collapse ?
Can you tell why you can drink lemonade or soda water through a straw ?
Do any of these experiments show that the atmosphere's pressure pushes upwards? Downwards ? Sidewise?
17. Can the Atmosphere's Pressure Be Measured ? Suppose that you had a tube or pipe 10, 20, 30, or 40 feet long and that your lungs were strong enough, could you, by suction,
raise a liquid, such as CESS
water or lemonade, up to
your mouth? Your lungs Fig. 12. — As the air is removed from the bell-shaped jar by the air pump, the are not strong enough, pressure of the outside air forces the sheet but you could use a sucrubber into the jar.
tion, or lift, pump for the purpose. This is just what we usually do when we pump water from a well or cistern.
Nearly 300 years ago a landowner in Italy was trying to raise water from a well. No pump he could get would raise the water more than about 34 feet. He asked the scientist Galileo (pronounced Găl-1-lē'ō) to tell him why; but Galileo did not know. Torricelli (pronounced Torry-tchěll'y), a pupil of Galileo, gave the answer in 1643. He believed that water rose into a pump because the atmosphere's pressure forced it up. He said to himself that if the pressure of the atmosphere was just great enough to push up a column of water 34 feet, it could not push up a heavier liquid to so great a height. He decided to try mercury, which is 13.6 times as heavy as water. If Torricelli's guess was correct, the air ought to be able to hold up a column of mercury only 34:13.6, or about 2.5 feet high. This would be about 30 inches.
18. The Barometer. - To test his guess, Torricelli used a straight glass tube about 3 feet long (Fig. 13) and closed at one end. He filled the tube entirely with mercury, closed the open end with his finger, and turned the tube upside down, so that its open end was below the surface of some mercury in a dish. When he removed his finger, some of the mercury ran
FIG. 13. — A barom
The pressure of the down into the dish, but not all. It atmosphere holds up the stopped when the column was about column of mercury. 30 inches high. So Torricelli's guess was correct. The space above the mercury did not contain air ; we call it a vacuum, which means empty space. Torricelli's apparatus is called a barometer.
If the opening of a barometer tube has an area of 1 square inch, the mercury column 30 inches high weighs 14.7 pounds. This is the reason why we say that the air pressure on every square inch of surface is about 15 pounds.
19. Is the Atmosphere's Pressure Always the Same ? — If we examine a barometer frequently, we shall find that the height of the mercury column is not always the same; it varies from day to day and sometimes even from hour to hour. These changes in the “barometer height,” as it is called, are caused by changes in the atmosphere's pressure. A study of these changes is of
great importance to the weather observer, for they help him to foretell changes of weather. How he does this we shall learn later (cf. § 81).
What do you think would
be the effect of carrying a 334
barometer down into a deep mine or up a mountain? At the bottom of
a haystack the hay is more FIG. 14. — The balloon and the air- compact than at the middle plane are still in the lower part of the ocean of air.
or the top, because all
the hay above is pressing upon that at the bottom. The same would be true of a pile of sofa pillows or of any other material that is easily compressed. Is this true of air? If it is, then the pressure should become greater if we go down into a mine, and should grow less if we go to the top of a high building, or up a mountain. This has been found to be true (Fig. 14). At the height of 4 miles above sea level the barometer height is about 15 inches; at about