but the very bright ones are also visible in the daytime. A comet has the appearance of a shining fog, or veil. It is rounded off at one end into a blunt " head," while a "tail" of light is spread out behind it. The tail always points away from the sun. Stars can be seen shining right through the material of a comet. The comets we see all revolve about the sun, but in paths that take them far out of our vision (see Fig. 58) for many years at a time.

Meteors are heavenly bodies that fall on or into the earth. When seen at night, a meteor is like a ball of fire followed by a stream of light. If we were near enough, we could hear a dull roar as it tears its way through the air. Usually it throws off sparks and fragments of matter during its passage. Sometimes the meteor disappears; sometimes we learn where it strikes. Usually, instead of falling to the earth as one body, it bursts into many pieces. Often tons of stones come from a single meteor. A few meteors consist of almost pure iron mixed with nickel.

Did you ever see a shooting star? Shooting stars are small fragments of matter that fall into the earth's atmosphere. They are very numerous and many may be seen on a clear night. They never reach the ground, except in the form of dust and ashes so fine that we cannot see them falling. Did you ever realize, before that matter comes to us all the time from the space through which the earth moves?

94. Exercises. 1. Have you read about "daylight saving"? Why was it carried out? How?

2. Why is a day divided into forenoon and afternoon instead of day and night?

3. The Indian said that an event was so many what did he mean?

"" moons ago;

4. What festival is still fixed by lunar, or moon, time?

5. Name the planets in the order of their distance from the sun. 6. Coat a piece of glass with soot and see if you can find sunspots" on the sun.

7. Does the moon revolve about the sun?

8. What causes the four "quarters" of the moon?

9. What is the horizon? The zenith? The zodiac? 10. When is wheat harvested in Iowa? In Argentina?

11. What do you suppose causes a ring around the moon"? Is the ring near the moon?

12. How do you imagine the earth would look if seen from the moon?

13. What daily path would a star have if you were at the earth's north pole? What path would the sun have?

14. On a sunny day measure the length of the shadow of a certain post, such as a fence post, in the morning, at noon, and in the late afternoon. When is it the shortest? Why?

Measure the shadow at noon every week or two for several months and keep an accurate record of the month and day on which the measurement was taken. What change do you notice? Explain it. On what date would the noonday shadow be the longest? Why?

15. In an almanac read the time of sunrise and sunset from autumn to spring. What day is shortest? In what month does the sun rise at almost the same hour and minute for several days in succession? Is this true at any other season?

CHAPTER XII

FORCE AND ENERGY

95. What Holds the Solar System Together? — Have you been asking yourself, as you have studied about the solar system, why the planets in their great orbits swing around the sun year after year, and what it is that holds the sun and all its planets together? Before we answer this question, let us think of simpler questions. In the first place, why does a body that is not held up, or supported in some way, fall to the earth? Why is it that a bullet shot upward from a rifle does not continue in its flight off into space, but always returns to the earth? Sir Isaac Newton gave the reason when he said that the earth pulls objects toward itself, or attracts other objects. Do you suppose so small an object as an apple hanging on a tree attracts the earth? Why not? In proportion to its weight it pulls the earth as much as the earth pulls it.

Does it seem impossible that an orange lying on a table attracts, or draws toward it, another orange, lying beside it? It is not easy to show this, because the earth pulls both oranges strongly toward itself; but by means of a celebrated experiment it was shown clearly that one body of matter attracts another near it. A large ball of lead (Fig. 61) and a small one of copper were hung side by side, and it was possible to see distinctly that the copper

ball moved over toward the lead ball. So we know that this pull which the earth has for bodies near it, is also present between two bodies on the

earth. We call this earth pull, gravity. Scientists believe that the same pull that the earth has for objects near it also exists between the sun, the earth, and other planets. When we speak of this pull between the bodies of the solar system, we call it gravitation. Therefore, it is gravitation which holds the solar system together.

96. Why Does a Body Have Weight?

FIG. 61. There is a pull, or attraction, be

tions of matter.

tween two por

§ 10). How do we weigh an object? If we use a spring balance (Fig. 62), we find that the balance consists of a spring which can be uncoiled or stretched by the object. When the object pulls the spring so far that the pointer is at the mark for 3 pounds, the object weighs 3 pounds. But why does the object stretch the spring at all? The answer is that the earth pulls the matter of the object toward itself with such a force that the spring is stretched to the 3-pound mark. So it is gravity, or the earth pull, that causes a body to have

FIG. 62. The spring is stretched

by the earth's pull upon the object

weighed.

(Copyright by International Stereograph Co.) FIG. 63. - Leaning Tower of Pisa.

weight.

Does it make

any difference, so far as the earth is concerned, whether the object weighing 3 pounds is feathers, or water, or lead? Of course not, for the earth has the same pull upon all objects that contain equal quantities of matter, of whatever sort the matter may be.

97. In What Direction Does the Earth Pull?

If you drop a stone from your hand, in what direction does it fall? It falls "straight down,' or vertically, because gravity acts in that direction. Bricklayers, masons, and carpenters use a string with a weight attached (a plumb line) to guide their work, so that their walls shall be

plumb," or vertical. In doing this they depend upon gravity to pull "straight down and to give them a reliable line.

FIG. 64.

In a vacuum

light bodies heavy

Before the time of Galileo there was a belief that heavy materials, such as metals, fall more rapidly than light materials, such as papers or feathers. Galileo let objects of different materials fall from the leaning tower of Pisa (Fig. 63), and decided that the earth pull gives the same speed to all falling bodies, but that the air interferes with rate.

and

bodies fall at the same