Part 8

When men had studied light thoroughly, however, they came to the conclusion that it must have the power of pressure, which, from the standpoint of results, would amount to the same thing as having weight. They reasoned that if you had a perfect balance and let sunlight shine down on one of the sides of the balance, that side should go down under the pressure of light. In their first experiments along this line men failed to show that under such conditions the side of the balance on which the light shone did go down, but by continuous experiments it was proved finally that the light did exert a sufficient pressure to cause the scales to go down, and in effect this is the same as having weight; but this has been found to be a common property of rays of various kinds, including heat, and we, therefore, do not speak of this quality as weight, but as the power of radiating pressure.

Why Does a Stick Seem to Bend When Put in Water?

When light passes from one medium to another, as for example from glass or water to air, or from air or glass to water, the rays of light change their course, thus making them seem to be bent or broken. The rays of light from the part of the stick in the water take a different direction from the rays from the part which is out of the water, giving the appearance of breaking or bending at the place where the air and water meet. It is, of course, the light rays which are bent and not the object itself.

This bending or changing of the path of light rays is called refraction. If you place a coin in a glass of water so that it may be viewed obliquely, you can apparently see two coins, a small one through the surface of the water and another apparently magnified through the side of the glass.

This is due only to the absolute principle that rays of light change their direction in passing from one thing to another, and on this principle of the rays of light our optical instruments, including the microscope, the telescope, the camera and eyeglasses are based.

What Makes the Stars Twinkle?

I might tell you, just to show how clever I am, that stars do not twinkle at all, and leave you with that for an answer. But since they really do seem to twinkle, and that is what causes your question, I will tell you. As we have already learned in our talks about the stars and the sky in general, the stars are suns which are constantly throwing off light, just as our sun gives us light, and when this light strikes the air which surrounds the earth it meets many objects--little particles of dust and other things always floating about in it. The light comes to us in the form of rays from the stars and some of these rays strike particles of various kinds in the air and are thus interfered with. If you are looking at a lighted window some distance away and there are a lot of boys and girls or men and women running past the window, one after the other, rapidly, it will make the light in the window appear to twinkle. The twinkling is due to the interference which the rays of light encounter while traveling toward the eye.

Why Does an Onion Make the Tears Come?

That is nature’s way of protecting the eyes from the smarting which the onion would cause in your eyes if the tears did not come quickly and overcome the bad effect so produced. Tears are provided for washing the ball of your eyes. Every time you wink a little tear is released from under the eyelid, and the wink spreads it all over the eyeball. This washes down the front of the eyeball and cleanses it of all dust and other things that fly at the eye from the air. Then the tear runs along a little channel, much like a trough, at the lower part of the eye, and out through a little hole in the eye, and in this case the tear is really only an eye-wash. Many things, but more often sadness or injured feelings, start the tears coming so fast from under the eyelid that the little trough at the bottom and the hole in the corner of the eye are too small to hold them or carry them off, so they roll over the edge of the lower eyelid and down the face. These are what we call tears. Among other things that will cause tear-glands to cause an over-supply of eye-wash to come down, are onions. What they give off is very trying to the eyes, and so, just as soon as the something which an onion throws off hits the eyeball, the nerves of the eye telegraph the brain to turn on the tears quickly, and they come in a little deluge and counteract the bad effect of the onion.

The First Missile

~HOW MAN LEARNED TO SHOOT~

A naked savage found himself in the greatest danger. A wild beast, hungry and fierce was about to attack him. Escape was impossible. Retreat was cut off. He must fight for his life--but how?

Should he bite, scratch or kick? Should he strike with his fist? These were the natural defences of his body, but what were they against the teeth, the claws and the tremendous muscles of his enemy? Should he wrench a dead branch from a tree and use it for a club? That would bring him within striking distance to be torn to pieces before he could deal a second blow.

There was but a moment in which to act. Swiftly he seized a jagged fragment of rock from the ground and hurled it with all his force at the blazing eyes before him; then another, and another, until the beast, dazed and bleeding from the unexpected blows, fell back and gave him a chance to escape. He knew that he had saved his life, but there was something else which his dull brain failed to realize.

He had invented arms and ammunition!

In other words, he had needed to strike a harder blow than the blow of his fist, at a greater distance than the length of his arm, and his brain showed him how to do it. After all, what is a modern rifle but a device which man has made with his brain permitting him to strike an enormously hard blow at a wonderful distance? Firearms are really but a more perfect form of stone-throwing, and this early Cave Man took the first step that has led down the ages.

This strange story of a development has been taking place slowly through thousands and thousands of years, so that today you are able to take a swift shot at distant game instead of merely throwing stones.

PRACTICE DEVELOPED SOME WONDERFUL MARKSMEN AMONG THE USERS OF THIS PRIMITIVE WEAPON]

We do not know the name of the man who invented the sling. Possibly he did not even have a name, but in some way he hit upon a scheme for throwing stones farther, harder, and straighter than any of his ancestors.

The men and women in the Cave Colony suddenly found that one bright-eyed young fellow, with a little straighter forehead than the others, was beating them all at hunting. During weeks he had been going away mysteriously, for hours each day. Now, whenever he left the camp he was sure to bring home game, while the other men would straggle back for the most part empty-handed.

Was it witchcraft? They decided to investigate.

Accordingly, one morning several of them followed at a careful distance as he sought the shore of a stream where water-fowl might be found. Parting the leaves, they saw him pick up a pebble from the bank and then to their surprise, take off his girdle of skin and place the stone in its center, holding both ends with his right hand.

Stranger still, he whirled the girdle twice around his head, then released one end so that the leather strip flew out and the stone shot straight at a bird in the water.

The mystery was solved. They had seen the first slingman in action.

The new plan worked with great success, and a little practice made expert marksmen. We know that most of the early races used it for hunting and in war. We find it shown in pictures made many thousands of years ago in ancient Egypt and Assyria. We find it in the Roman Army where the slingman was called a “funditor.”

Surely, too, you remember the story of David and Goliath when the young shepherd “prevailed over the Philistine with a sling and with a stone.”

Yet slings had their drawbacks. A stone slung might kill a bird or even a man, but it was not very effective against big game.

What was wanted was a missile to pierce a thick hide.