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7 miles some aëronauts found it to be only 7 inches. They had thus left of the air behind them.

The pressure of the blood in our bodies is about 17 pounds to the square inch; hence there is danger of hemorrhage, or bursting of the blood vessels, when the atmosphere's pressure suddenly becomes too small. So it happens that many persons have nosebleed when they are carried quickly up a mountain.

The barometer column falls about an inch for every 900 feet we ascend above sea level ; what would its height be at an elevation of 1800 feet? Of 4500 feet?

20. Exercises. — 1. If your body has an area of 2000 square inches and the atmosphere's pressure upon it is 15 pounds for every square inch, your body must support a pressure of 30,000 pounds. Why do you not feel so great a pressure?

2. When the cover of a Mason jar is hard to remove, we sometimes push a knife blade under the edge of the cover until we hear a hissing noise. What causes the noise? Why is it easier afterwards to remove the cover?

3. Alcohol is only as heavy as water; would a barometer column consisting of alcohol be higher or lower than one made of water?

4. Why is mercury used in the barometer instead of water or alcohol or any other liquid ?

5. How could a barometer be used to tell the height of a mountain top above sea level?

6. Can you think of any reason why the atmosphere's pressure should vary from day to day?

7. Could you prove, by the use of a clay pipe with sheet rubber tied over the bowl, that the air presses in all directions ? Try it.

CHAPTER IV

FIRE

21. What Does Fire Mean to Us? - What a number of pictures come to our minds as we hear the word "Fire"! Perhaps you think first of the fire that destroys some house, of the uncontrolled prairie fire and forest fire, of the destructive fires in coal mines and petroleum wells, of the great conflagrations that have swept over cities like Rome, London, Moscow, Chicago, Boston, and San Francisco. We ought also to think of the quiet fires of the home: a burning match, burning wood or coal in the open fireplace or in the stove, the gas jet and gas stove, the flame of a candle, the kerosene lamp, the furnace fire. We think, too, of the great fires of industry and commerce : the blast furnace, that gives us iron ; the copper, zinc, and lead smelters in which, by the aid of fire, these metals are prepared for our use; the furnace fires that produce the steam of the locomotive and steamship. Then, too, we think of the joyous bonfire of our celebrations and of the camp fire over which we cook our picnic meal.

It is true that fire destroys many precious lives every year and enormous sums in property, yet without it man would be poor indeed. A proverb says: “Fire is a good servant, but a bad master.” The poet has the same idea when he says:

“How kindly is the fire's might,

When tamed by man and watched aright.” What is fire and how did man come to use it ?

22. How Are Fires Started ? — How do you suppose man ever learned to start a fire? Nowadays we do this in a very matter-of-fact way. We strike a match ; its tip bursts into flame; then the match stick burns and we use its flame to kindle our fire. But suppose we were off in the woods or the mountains, without matches; tow could we kindle a fire then? Fig. 15. Kindling fire by

means of a flint-and-steel.' A hundred years ago there were no matches; men lighted their fires by means of a flint-andsteel. That is, they struck a piece of flint (a kind of rock) with a short bar of steel and produced hot sparks. They caught the sparks in some tinder and thus set the tinder on fire. We often see such sparks when a horse's hoofs strike a stone pavement. The tinder consisted of dried moss, bark, pitch, or some other substance that was easily set on fire.

Our forefathers did not use the flint-and-steel every time they wanted a fire, but saved the hot coals each evening for the next day's fire. If the coals “ went out,” as they sometimes did, some one (usually a child) had to go out in the frosty morning to “ borrow fire " of a neighbor. The glowing coals were carried in little covered iron pails or kettles.

But the flint-and-steel was not the earliest way of making fire. At a still earlier time men started their fires by rubbing one piece of wood against another, until the wood, or some tinder placed near it,

was heated to the kindling temperature. Some barbarous peoples do this today (see Fig. 16).

[graphic]

23. How Does a Fire Burn? — Watch a fire and see how the flame or glow travels from one part of the burning body to another. We see this in the burning of an incense stick, a match, or a stick of wood, as well as in the burning of a house or a city block. The part al

ready afire heats up the (Copyright, 1912, dy Frederick Starr.) part next to it to the

- How a Batua of the Congo point of burning," or Free State kindles his fire by turning a drill rapidly between his hands.

kindling temperature.

A number of other things happen in a fire. There is usually a flame, which gives off light. We use lamps for the light of their flames. Lincoln learned to read by the light of a burning pine knot. A third thing about a fire is that it gives off heat; a fourth, that there is usually some smoke. A fifth thing is that after solid fuels, like coal and wood, no longer burn with a flame,

Fig. 16.

they burn with a glow. Finally, when the coal or wood is entirely through burning, gray ashes remain. Many substances do not seem to burn at all; such are the iron of a stove and the bricks of a fireplace. Why does not the nail in a stick of wood burn up with the wood ?

24. Experiments with Burning.-In order to find out more about burning, or fire, let us carry out a few experiments. First we must have a bottle and a cork to close it, also a pine splinter that can be fastened into the under side of the cork (Fig. 17). Let us light the splinter and hold it in the bottle, and press the stopper into the bottle's mouth. At first the flame burns brightly, then faintly; finally it goes out." Suppose we remove the cork,

Fig. 17. —A relight the splinter, and thrust the splinter lighted splinter once more into the bottle. The splinter burns for a time

in the inclosed will not burn. Except, possibly, for a little air of the bottle smoke, the inside of the bottle looks just and then “ goes as it did before we burned the wood in it.

Let us remove the splinter, fill the bottle entirely with water, pour out the water, and put the burning splinter back into the bottle. What happens ? The splinter burns once more. What did the water force out of the bottle? The used, spent air. What entered the bottle when we poured the water out? Fresh air. The experiment shows that burning wood spoils the air for further burning and that we must provide fresh supplies of air, if burning is to go on. The same is true of all ordinary burning.

out."

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