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motion. Here again we use the forward and backward motion of the piston to get motion in a circle, such as is needed to turn the wheels of the car. What device does this in the steam engine?

Have you ever thought why automobiles have come into such general use in the last few years, or why the airplane has just recently become a success? The

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FIG. 227. — A “tank,” or “caterpillar”; first used to do the heavy work of

the farm, but afterwards changed into a war engine.

answer is that both of these wonderful devices of man were made possible because the gasoline engine was made a success. The gasoline engine is very light for the a . amount of power it can produce, and its fuel, the gasoline, is also very light (see Appendix III). Thus it is that one of man's inventions generally depends upon another.

Did you realize that the gasoline tractor, used instead of horses to do the heavy work of the farm, is practically

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the same as an automobile, and that the tank,” or caterpillar (Fig. 227), used in the later years of the Great War, is really an armored automobile, with a peculiar endless chain instead of tires, by means of which it crawls over rough ground with great force ?

295. Exercises. 1. How is a cannon like a steam engine? What takes the place of the piston? Is it a one-stroke or a two-stroke piston ? What corresponds to the steam chest ?

2. Find out why an old-fashioned kite needs a tail. Why does not a box kite need one?

3. What kind of simple machine is the oar with which you row a boat?

4. Why must an airplane travel at such great speed?

5. What machines are run by falling water? What force gives falling water its energy?

6. Ask at home what are some of the different stitches made by sewing machines. Which of these ravels out most easily? What is the difference between stitches used for basting and those used for permanent sewing ?

7. What other fuel besides gasoline may be used in automobiles and airplanes? Is there any limit to the world's supply of gasoline?

8. Why is cream “whipped," and how? How is ice cream irozen ?

9. Is the dirt collected by a vacuum cleaner drawn in, or forced in? Explain the difference.

10. Find out what was used to set powder on fire in the flintlock muskets of Revolutionary days (see § 22).

11. What is the difference in principle between the screw propeller and the paddle wheel of steamboats?

12. Why is a toy balloon filled with air unable to rise from the ground?

Summary. — A sailboat is a machine in which we can exchange rapid motion before the wind for slow motion against the wind. Its sail uses the principle of the inclined plane.

In a kite part of the force of a horizontal wind striking the inclined surface raises the kite into the air.

Airplanes are kites depending upon their own forward motion to raise themselves into the air.

Windmills have inclined planes by which the force of a horizontal wind is used to produce motion in a circle.

In waterwheels the water may flow under, or over, the wheel; or it may strike curved paddles, or turbines.

In a sewing machine the seamstress does harder work than by hand, but does it more rapidly.

Cream is separated from milk by means of centrifugal force: the milk is heavier than the cream, and is thrown out farther when whirled.

In a vacuum cleaner the force of the inrushing air carries the dirt with it.

The electric washer moves the clothes through the water, or the water through the clothes.

Gases cause motion by their rapid expansion from a compressed condition. In a steam engine steam under great pressure is allowed to expand alternately on each side of a piston, so as to produce rapid forward and backward motion.

In an automobile the explosion of a mixture of gasoline and air produces hot gases at high pressure; the escape of these produces motion in the engine.

PART VI

LIVING THINGS AND THEIR RELATION

TO US

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