of cold air and is frozen. Frequently the small hail stone thus formed is carried about by currents of air and more layers of frozen water are added to it. This process may continue for some time, and as a result the large hail stones with which we are all familiar are occasionally formed. Much damage to windows and crops may be caused by the larger hail stones.

Questions

1. What is the height to which the atmosphere extends?

What is meant by "our environment"?

3. Of what importance is atmosphere to us? 4. What changes may take place in the atmosphere?

5. Is air a mechanical mixture or a chemical compound? What are the reasons for your answer? 6. What gases are found in the air?

7.

What per cent of each of these gases is present? 8. Are there any advantages in having fourfifths of the air composed of an inactive gas like nitrogen?

9. Explain in what way alfalfa or clover makes soil more fertile.

10. Of what use is the oxygen in the air?

11. Why does not the supply of oxygen become exhausted?

12.

air?

What are the sources of carbon dioxide in the

13. How is an equilibrium of carbon dioxide maintained?

14. Under what conditions may dust be harmful? 15. How is the atmosphere heated?

16. What are the sources of water vapor in the atmosphere?

17. What is meant by the expression "air is saturated"?

18. Define humidity. Dew point.

19. Under what conditions is dew formed? Frost? 20. Why does covering plants with a piece of paper or cloth help to keep them from freezing?

21.

What are clouds? What is their distance from the surface of the earth?

22. Under what conditions is rain formed? Hail? Snow?

23. How can you account for the fact that a large hailstone is made of concentric layers of ice?

CHAPTER XIX

ATMOSPHERIC PRESSURE

Introduction. It is a well known fact that water has weight and exerts pressure. It is more difficult for us to believe that the air around us is constantly exerting pressure, first, because we do not feel conscious of it; second, because air is invisible.

Imagine for a moment that extending for over two hundred miles above us there were water.

We can casily appreciate that it would have weight. Similarly, the air above us has weight, and hence exerts pressure. Suppose we consider a column of air one inch wide and one inch thick and over two hundred miles high. If this column of air could be placed upon one pan of a balance, it would be found to weigh about 15 pounds. This weight would vary from time to time, and from place to place. If we weighed the column of air extending above one square inch on top of a mountain, its weight would be less than 15 pounds, because the column would not be so high.

We are practically never conscious of this really enormous pressure of the atmosphere, which is exerted over every inch of our bodies. We are not conscious of it because the pressure is exerted equally over the

inside and the outside. We are so constructed as to be most healthy when under this pressure. Without it we feel uncomfortable. If this pressure is suddenly changed outside of our bodies, we are at once conscious of it. On tops of high mountains breathing becomes more difficult, headaches and other results follow.

Aeronauts have never ascended much higher than seven miles. At that height the pressure outside of the body is reduced to about one-fifth of what it is at sea level. As a result of the high internal pressure the blood is forced to the surface, the walls of the blood vessels frequently rupture, and other physical difficulties result.

Fish living at the bottom of the sea are subjected to enormous pressure. Nevertheless they are adapted to those great depths. Were the pressure to which they are accustomed diminished to any great extent, they would suffer great pain, and possibly death. Alexander Agassiz says: "In fish brought up from the deep water, the swimming bladder often protrudes from the mouth, the eyes are forced out of their sockets, the scales fall off, and they present a most disreputable appearance."

Just as they are adapted to the pressure at the bottom of a sea of water, so we are best adapted to the pressure near the bottom of a sea of air.

Air presses in all directions. Place a piece of pasteboard, or blotting paper, over the mouth of a tumbler filled with water. Shake the tumbler until the paper

is thoroughly moistened. Then invert the tumbler. The water does not run out because the pressure of the

A

B

FIG. 46. The pasteboard does not fall off from the tumbler because the pressure of the air on the outside of it is greater than that of the water in the tumbler. In A, the air presses from below; in B, from the side.

air outside of the pasteboard is at least equal to the downward pressure of the water in the tumbler.

If the tumbler is held so

that the mouth faces sidewise, the water does not run out, because the air also presses upon the pasteboard from the sides.

Tie a piece of sheet rubber over the large end of a belljar. Now exhaust the air from the belljar by means of an air pump. As