FIG. 71.

among the most wonderful things that the microscope has revealed to us. We admire the skill and power of the Creator as we look at the construction of the human eye; but His skill and power appear vastly more wonderful, when we think of the eye of a mere common insect, as made up of thousands of optical instruments, each, though so minute, being more perfect than any instruments that man can make.

CHAPTER XI.

THE EAR.

1. THE mind acquires the knowledge of sounds by the apparatus of hearing. This apparatus is very complicated, and some of it is exceedingly delicate. Before describing it I will say something of sound, in order that you may better understand the operation of this apparatus.

2. Sound is caused by a vibration or shaking of some substance. You can perceive this vibration in a bell if you touch it after it has been struck. If the bell is quite large you can see as well as feel the vibration. You can see it in the string of a piano or a violin. It is the vibration of the cords in the larynx that produces the sound of the voice. It is not solid bodies alone that produce sound by their vibration.

Why are the compound eyes of insects more wonderful than the human eye? How is sound produced? Give examples of sound made by the vibration of air?

It is often produced by the vibration of the air. This is the case in whistling. In the flute it is the vibration of the air in the instrument that produces the sound. And so of other similar instruments.

3. When the vibrations are equal, the sound is a musical one. But when they are irregular, the sound is a noise, that is, a confused sound.

4. Sound passes through the air by vibrations. It may be said to pass by waves in all directions, just as waves go in all directions on the surface of water when a stone is dropped into it. And as these waves in the water lessen as they extend from the spot where they begin, so the waves of sound lessen the farther they are from where the sound is produced. That is, the sound dies away in the distance, as it is expressed. 5. That sound is transmitted in this way through the air can be proved by experiment. If a bell be set to ringing under the glass receiver of an air-pump, as you pump the air out of the receiver, the sound (f the bell becomes more and more faint, till at length you cannot hear it at all. The reason is, that the vibrations of the air lessen as the air itself lessens and pecomes thin; and when the air is all pumped out, there are no vibrations to convey the sound of the bell. So, too, sounds made on the top of a very high mountain are not as loud as when made in the valley below, because the air at so great a height is very

thin.

What makes the difference between a musical sound and a noise! How does sound pass through the air? Give the comparison in regard to the diffusion of sound. How can you prove that sound passes through air by vibrations?

170

6. Other substances besides air transmit the vibrations or motions of sound. If you put your head under water, and let some one strike two stones together under the water at some distance from you, you will hear the sound. That is, the vibration will come to your ear through the water. If you place a watch between your teeth, you hear its ticking quite as distinctly as when you put it to your ear. In this case the vibration goes to the nerve of hearing by the teeth and the bones, and does not go round by the air into the tube of the ear.

7. The vibration of sound passes more readily through solids than through the air. If you put your ear upon the end of a long log you can hear the scratch of a pin made at the other end. And yet you cannot hear it through the air at the distance of only a few feet. A deaf gentleman, as he rested the bowl of his long pipe upon his daughter's piano, found that he could hear the music much more distinctly than he could through the air. In this case the vibration went through the pipe to the teeth, and then through the bone to the nerve of hearing.

8. The vibrations or waves of sound are reflected by objects against which they strike. For this reason a sound can be heard further along a wall than in an open field. If one speaks in an open field, the sound is scattered in all directions. But the wall keeps it from being thus scattered. For the same reason, a

Illustrate the fact that other substances besides air transmit the vibrations of sound. What is said of the transmission of sound through solids compared with its transmission through air? Illustrate in various ways the reflection of sound.

speaker can be better heard in a building than in the open air. In this case the walls shut in the waves of sound. So, also, a speaker can be heard better when the ceiling is low than when it is very high. When the ceiling is high much of the sound of the voice is lost in the space above. In a speaking-tube, even a whisper can be heard at a great distance, because the waves of sound are so shut in by the tube.

9. In hearing, the waves of sound are caught by the outer ear, as it is called, and they go into the tube which you see there. The purpose of this outer ear is to collect these vibrations and direct them into this tube. It is well shaped on the whole for this purpose, but the ridges and prominences that you see on it do not render any assistance in this respect. They merely serve to make the ear a comely organ. Some animals have ears which answer much better in collecting the waves of sound than the ear of man does, because they need them. Man could hear more easily if his ears were larger, and were shaped more like the open end of a trumpet, but such ill-looking appendages are not necessary in his case. He sometimes assists the ear in collecting the vibrations of sound by putting his hand up behind it. Very deaf persons often use an ear-trumpet. The broad trumpetshaped end is turned towards the speaker, so as to catch the waves of sound and direct them into the tube of the ear by the pipe of the instrument.

10. The vibrations of sound in the air, entering the

What is the purpose of the outer ear? What is said of its shape and the irregularities on its surface? What is said of the ears of some animals? In what way is the ear sometimes assisted}

tube of the ear, strike upon a drum at the end of the tube. This drum of the ear is a membrane fastened to the bone, just as the drum-head of a common drum is fastened to its wooden rim. The vibrations that thus enter this tube as they strike the drum make it to vibrate.

11. The vibration does not stop here. It is communicated to a chain of little bones on the other side of the drum. The farther one of this chain of bones rests on another membrane or drum. The vibration is therefore communicated to this second drum. And this drum covers an opening into some winding passages in solid bone. These passages are filled with a fluid, and the vibration of the drum over the opening makes this fluid to vibrate or shake.

12. The fine delicate fibres of the nerve of hearing are in the midst of the fluid in the winding passages. They feel the vibration of the fluid there, and an impression goes by them through the trunk of the nerve to the brain, and is received there by the mind. And this completes the process of hearing. These winding passages, where the nervous fibres are at their post ready to feel the vibrations that come there, are the real halls of audience, as we may call them. I will now describe some of these parts more particularly.

13. The little bones in the ear are four in number. They are connected together, and are commonly spoken of as a chain of bones. In Fig. 72 they are re

Upon what do the waves of sound entering the ear strike? Trace the transmission of the vibration inward from the drum of the ear. Where are the fibres of the nerve of hearing, and how are they affected What completes the process of hearing?