they feel warmer or cooler? Also try the following: Put a few drops of ether or gasoline upon your hand and blow over it gently until the liquid has evaporated. What sensation do you feel?

From these experiments you can see that evaporation of liquids takes heat from bodies touching them and so causes the bodies to be cooled. This is what perspira

tion does for our bodies. It is constantly
evaporating from the skin, even when we
do not know it. If we are very hot from
exercise, the perspiration is formed more
rapidly than it can evaporate and we have
noticeable perspiration," or
or sweating.
Heat is needed to produce evaporation of
a liquid, just as it is to cause the boiling
of a liquid, although evaporation takes
place at a lower temperature. The heat
which is needed for the evaporation is
taken from the body. The liquid is
changed into the form of a vapor, or gas,
just as in boiling.

As water oozes through the po

rous jar, it evaporates and cools the water inside the jar.

In tropical countries a porous jar is used to cool water. As the water oozes out through the walls of the jar, it evaporates, and so cools the water that is left in the jar (cf. Fig. 45). Would you say that the jar should be left in a draft, or where the air is quiet? Where does perspiration evaporate most rapidly, in moving air or in a quiet room?

72. Exercises. 1. Why does the sprinkling of water over a porch floor cool the porch in hot weather?

2. If you have a cup of boiling water, what is the quickest way to cool it? What becomes of the heat?

FIG. 46. A "fireless cooker" is a box made of nonconducting materials; it does not allow the heat of cooking food to escape.

3. Do birds and other animals fluff out their feathers and fur in summer, or in winter? Why? 4. Why do we wear cotton and linen clothing in summer and woolen clothing in winter? 5. Is it correct to say that woolen clothing keeps out the cold"?

6. Do you suppose that snow and ice evaporate without melting? How do you account for the fact that the sharp edge of a piece of ice becomes rounded, even in very cold weather?

7. How is a fireless cooker constructed? Why? See Fig. 46.

8. Is a fireless cooker also a "heatless" cooker? What is the source of its heat?

9. Why does a thermos bottle keep hot food hot and cold food cold?

CHAPTER X

WEATHER

73. What is the Weather? All of us know what weather is. It is the temperature of the air outside our houses; the amount of moisture in the air; whether the sky is clear or cloudy; the amount of dew or frost, rain or snow; the direction of the wind and the rate at which it is moving. These are the things which we mean by "the weather." The average of the weather conditions at any given place makes the climate of that place. Inside our houses we try to control the conditions of the air, but outside our houses these largely control us.

The weather would not be so interesting to us if it were always the same, but it is continually changing. One day is fair and the next stormy; one is hot and the next cold; one has a strong wind from one direction, the next has a wind from nearly the opposite direction, and the third has no wind at all. So the expression, " changeable as the weather," is a common one. In thinking about the weather we should remember that we are surrounded on all sides by an ocean of air: the atmosphere. This covers the earth with a layer perhaps 200 miles thick. We should remember that the weather at the place where we live is the condition of only a very small part of the atmosphere. When we look at the

weather of our country as a whole, we see that the changes that seem to be haphazard and fickle are parts of great air movements and that there is a cause for every change.

74. Of What Is the Atmosphere Composed? - We have already learned what air is like. It has weight and takes up room, like a liquid or solid object; it can be expanded or compressed. We have also learned that it is made up of more than one kind of gas. Its active gas is oxygen, which is used in burning and in respiration; its inactive gas is chiefly nitrogen. Another gas that is present in the atmosphere is carbon dioxide (cf. § 42). Besides these gases there is water in the form of vapor. Water vapor is taken up by the atmosphere, as the water of rains, lakes, and rivers evaporates. Dust is also found in the atmosphere; it contains a multitude of particles, some living and some not living (cf. § 204).

75. What Causes Dew and Frost? Have you ever seen a pitcher, or glass, of water "sweat" on a hot day? Do you think the drops of water go through the glass? Suppose we perform this experiment (Fig. 47): Partly fill a polished metal cup, or small, smooth, metal pail, with water at room temperature and add small pieces of ice to it, one at a time. Add a new one only after the one before has melted. Stir the ice and water with the bulb end of the thermometer, noting carefully when the first moisture is collected on the outside of the metal vessel. Then read the thermometer. The temperature you find is the dew point of the air at that time. Find it on a day when the weather is clear, also on a rainy day. It varies according to the amount of moisture

in the air. The air that cannot be cooled at all without depositing dew is said to be saturated with moisture, or "at the dew point." Dew and frost are water deposited from the vapor of the air upon cold objects, such as grass blades and stones. These objects cool more rapidly than air; so they cool the air near them, causing some of its water to be condensed. If the dew

point of air is below 0° C., frost is formed instead of dew.

FIG. 47. The temperature shown by the thermometer when moisture begins to

A clear night favors the forming of dew and frost, because the earth and the air cool more rapidly in clear weather than when clouds are present. Clouds act as a blanket over the earth and prevent rapid cooling. A gentle breeze brings fresh supplies of air to the cool objects and so helps in the formation of dew and frost. A strong wind, however, does not leave the air near the cool object long enough to allow dew or frost to be formed. Orchards and vineyards in hollows are more likely to be injured by frost than those on a hillside or on the hilltop, because the heavier, cold air falls into the hollows and remains there. Should orchards be planted in hollows?

appear on the outside

of the metal cup is the "dew point."

76. How Are Clouds Formed? - Some people think that clouds are water vapor floating in the air. Is this true? In the first place, water vapor is an invisible gas, like the air itself. In the second place, the cloud particles are droplets of water and are heavier than the air. Why, then, don't they fall to earth? The answer is that they do fall, but that at a certain level, which forms the bottom of the cloud, they evaporate completely to