What sound is.
What is sound? If you look at a large bell when it is struck, you can see a quivering or shaking in it. If you put your hand on it, you can feel the quivering. It is this that makes the sound that we hear. You can see the same thing in the strings of a piano when they are struck, and in the strings of a violin as the bow is drawn over them. The wind makes the music on the Æolian harp in the window by shaking its strings. And when you speak or sing, the sound is made, as I have told you before, by the quivering of two flat cords in your throat.
But when a bell is struck, how does the sound get to our ears? The quivering or vibration, as it is called, of the bell makes a vibration in the air, and this vibration is continued along through the air to our ears.
The experiment of scratching on a log with a pin.
The vibration can go through other things besides the air. It will go through something solid better than it will through air. Put your ear at the end of a long log, and let some one scratch with a pin on the other end, you can hear it very plainly. The vibration made by the pin travels through the whole length of the log to your ear; but if you take away your ear from the log you can not hear it, for the vibration or sound can not come to you so far through the air.
Dying away of sound.
The nearer you are to where the sound is made, the louder it is;and the farther sound goes, the fainter it is. It is said to die away as it goes; that is, the vibration becomes less and less, till, after a while, it is all lost. It is like this: if you drop a stone into water, it makes little waves or ripples in all directions. These become less and less the farther they go from where the stone was dropped. It is just so with the waves or vibrations of sound in the air.
What is an echo? It is when a sound that you make comes back to you again. It is done in this way. The vibration strikes against some rock, or house, or something else, and then bounds back to you, just as a wave striking against a rock bounds back.
Speaking tubes.
Why is it that a person speaking in a building can be heard more easily than one speaking in the open air? It is because the vibrations are shut in by the walls. It is for the same reason that you can hear a whisper so far through a speaking tube extending from one part of a building to another. The vibrations are shut in within the tube. They have no chance to spread out in all directions, and they go right straight on through the tube.
I have thus told you how sound is made, and how it goes through the air and through other things; but how is it that we hear sound when it comes to our ears? How does the mind know any thing about the vibration of the air? This vibration does not go into the brain, where the mind is; it only goes a little way into the ear, and there it stops. It comes against the drum of the ear, and can go no farther. How, then, can the mind know any thing about it? This I will tell you.
The vibration of the air goes into the ear to a membrane fastenedto a rim of bone, and called the drum, and shakes it, and this shakes a chain of little bones that are the other side of this drum-head. The last of these bones is fastened to another little drum, and, of course, this is shaken. This drum covers an opening to some winding passages in bone. These passages are filled with a watery fluid. Now the shaking of the second little drum makes this fluid shake. The nerve of hearing feels this shaking of the fluid, and tells the mind in the brain.
The bones of the ear.
Here are the four little bones that make the chain of bones in the ear. They are curiously shaped. The one markedais called the hammer, andbis called the anvil. The little bone markedcis the smallest bone in the body. That markeddis called the stirrup. This is the bone that is fastened to the second drum—the one that covers the opening into the winding passages. The vibration that comes to the first drum is passed on by this chain of bones to the second drum.
The different vibrations in hearing.
See, now, how many different shakings there are for every sound that you hear. First, the bell, or whatever it is that makes the sound, shakes. Then there is a shaking of the air. This shakes the drum of the ear. Then the chain of bones is shaken. The farthest one of them shakes another drum, and this shakes the fluid in the bony passages. All this happens every time that you hear a sound; and when you hear one sound after another coming very quickly, how the vibrations chase each other, as we may say, as they go into the ear! But they are not jumbled together.They do not overtake one another. Every vibration goes by itself, and so each sound is heard distinct from the others, unless the vibrations come very fast indeed. Then they make one continued sound. Each puff of a locomotive, when it starts, is heard by itself. The vibration of one puff gets into the fluid in the bony passages before the one that follows it; but as the locomotive goes on, the puffs get nearer and nearer together, and when it goes very fast, they are so near together that the vibrations do not go separate into the ear, and they make a continued sound.
Different sizes of ears in animals.
Ear-trumpet.
Sound, I have told you, spreads in all directions in vibrations or waves. Now the more of these waves the ear can catch, the more distinct is the hearing. Some animals that need to hear very well have very large ears. Here is one, the long-eared bat. He must hear very well indeed, for his monstrous ears must catch a great many of the waves of sound. We could hear better if our ears were larger; but large ears would not look well on our heads; and we hear well enough commonly. Sometimes, when we do not hear as distinctly as we wish to, we put up the hand to the ear, as you see represented on the opposite page. This helps the hearing by stopping the waves of sound, and turning them into the ear. Those who are very deaf sometimes have an ear-trumpet, as it is called. In using it, the largetrumpet end is turned toward the person speaking, so as to catch the vibrations, while the tube part of it is in the ear.
Ears of rabbits, deers, etc.
Some animals can turn their ears so as to hear well from different directions. How quickly the horse pricks up his ears when he sees or hears something that he wants to know more about; and then he can turn his ears backward when he wants to do so. It is in such timorous animals as the hare, the rabbit, and the deer, that we see the ears most movable. They are on the watch all the time for danger, and the least sound that they hear they turn their ears in the direction of it. Their ears, too, are large, so that they hear very easily.
How the ear is guarded.
I have told you how the eye is guarded. The ear is well guarded also. I do not mean its outer part: it is the inner parts, where the hearing is really done, that are so well guarded. You remember that I told you that there are passages filled with a fluid. The nerve of hearing has its fine, delicate fibres in these passages. They feel the shaking of the fluid, and tell the mind of it. Now it is necessary that this part of the hearing apparatus should be well guarded; for this reason, these passages are inclosed in the very hardest bone in the body.
How the ear-wax guards the ear.
Then, too, the very entrance into the ear is well guarded, and in a curious way. The pipe that leads into the drum of the earis always open, and you know bugs are very apt to crawl into such holes. What do you suppose is the reason that they do not often crawl into the ear? There is something there to prevent them. It is the wax. They probably do not like the smell of it, and so, if they come to the entrance, they turn about. Once in a while one goes in, and then he is prevented from doing much harm by the wax. He is soon covered with this, and it is so sticky that it keeps him from kicking very hard. And, after all, though he may cause some pain, he can not get at the delicate part of the machinery of the ear. He dies after a while, if he is not got out, and perhaps the bitterness of the wax has something to do with killing him.
Questions.—How is sound made? How does it get to our ears? Tell about the vibration of sound in a log. What is said about the dying away of sound? What is this like? What is an echo? What is said about speaking in a building? What about speaking through a tube? Tell how we hear sound. Tell about the little bones in the ear. What do these bones do? Tell what the different vibrations are in hearing. What is said about the puffing of a locomotive? Why do some animals have large ears? Why are our ears so small? What animals can turn their ears different ways, and why? How is the inner part of the ear guarded? Tell what is said about the wax.
Questions.—How is sound made? How does it get to our ears? Tell about the vibration of sound in a log. What is said about the dying away of sound? What is this like? What is an echo? What is said about speaking in a building? What about speaking through a tube? Tell how we hear sound. Tell about the little bones in the ear. What do these bones do? Tell what the different vibrations are in hearing. What is said about the puffing of a locomotive? Why do some animals have large ears? Why are our ears so small? What animals can turn their ears different ways, and why? How is the inner part of the ear guarded? Tell what is said about the wax.