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CHAPTER III.

ON THE NATURE OF SOUND.

Sound is the sensation peculiar to the ear.

This sensation is caused by rapidly succeeding to-and-fro motions of the air which touches the outside surface of the drum-skin of the ear. These to-and-fro motions may be given to the air by a distant body, like a string of a violin. The string moves to and fro, that is, it vibrates. These vibrations of the string act on the bridge of the violin, which rests on the belly or sounding-board of the instrument. The surface of the sounding-board is thus set trembling, and these tremors, or vibrations, spread through the air in all directions around the instrument, somewhat in the manner that water-waves spread around the place where a stone has been dropped into a quiet pond. These tremors of the air, however, are not sound, but the cause of sound. Sound, as we have said, is a sensation ; but, as the cause of this sensation is always vibration, we call those vibrations which give this sensation sonorous vibrations. Thus, if we examine attentively the vibrating string of the violin, we shall see that it looks like a shadowy spindle, showing that the string swings quickly to and fro; but, on closing the ears, the sensation of sound disappears, and there remains to us only the sight of the quick to-and-fro motion which, the moment before, caused the sound.

Behind the drum-skin of the ear is a jointed chain of three little bones. The one, H of Fig. 4, attached to the drum-skin, is called the hammer; the next, A, is called the anvil ; the third, S, has the exact form of a stirrup, and is called the stirrup-bone. This last bone of the chain is attached to an oval membrane, which is a little larger than the foot of the stirrup. This oval membrane closes

SC

FIG. 4.

a hole opening into the cavity forming the inner ear; a cavity tunneled out of the hardest bone of the head, and having a very complex form. The oval hole just spoken of opens into a globular portion of the cavity, known as the vestibule, and from this lead three semicircular canals, SC, and also a cavity, C, of such a marked resem

blance to a snail's shell that it is called cochlea, the Latin word for that object. The cavity of the inner ear is filled with a liquid, in which spread out the delicate fibres of the auditory nerve.

Let us consider how this wonderful little instrument acts when sonorous vibrations reach it. Imagine the violin-string vibrating 500 times in one second. The sounding-board also makes 500 vibrations in a second. The air touching the violin is set trembling with 500 tremors a second, and these tremors speed with a velocity of 1,100 feet in a second in all directions through the surrounding air. They soon reach the drum-skin of the ear. The latter, being elastic, moves in and out with the air which touches it. Then this membrane, in its turn, pushes and pulls the little ear-bones 500 times in a second. The last bone, the little stirrup, finally receives the vibrations sent from the violin-string, and sends them into the fluid of the inner ear, where they shake the fibres of the auditory nerve 500 times in a second. These tremors of the nerve-how we know not-so affect the brain that we have the sensation which we call sound. The description we have just given is not that of a picture created by the imagination, but is an account of what really exists, and of what can actually be seen by the aid of the proper instruments.

A body may vibrate more or less frequently in a second ; it may swing over a greater or less space; and it may

have several minute tremors while it makes its main swing. These differences in vibrations make sounds higher or lower in pitch, loud or soft, simple or compound. It is easy to say all this, but really, to understand it, one must make experiments and discover these facts for himself.

CHAPTER IV.

ON THE NATURE OF VIBRATORY MOTIONS.

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The character of a sound depends on the nature of the vibrations which cause it, therefore our first experiments will be with vibrations which are so slow that we can study the nature of these peculiar motions. These experiments will be followed by others on vibrations of the same kind, only differing in this—that they are so rapid and frequent that they cause sounds. A correct knowledge of the nature of these motions lies at the foundation of a clear understanding of the nature of sound. that the student will make these experiments with care, and keenly observe them.

EXPERIMENT 3.-At the toy-shops you can buy for a few cents a wooden ball having a piece of elastic rubber fastened to it. Take out the elastic and lay it aside, as we shall need it in another experiment. Get a piece of fine brass wire, about 2 feet (61 centimetres) long, and fasten it to the ball. The weight of the ball should pull the wire straight, and, if it does not, a finer wire must be used. Hold the end of the wire in the left hand, and with the right hand draw the ball to one side. Let it go, and it will swing backward and forward like the pendulum of a clock. This kind of movement we call a pendulous or transverse vibration.

EXPERIMENT 4.-Cut out a narrow triangle of paper, 4 inches (10 centimetres) long, and paste it to the bottom of the ball. Twist the wire which supports the ball by turning the latter half round, and watch the paper pointer as it swings first one way and then the other. Here we have another kind of vibration, a motion caused by the twisting and untwisting of the wire. Such a motion is called a torsional vibration. EXPERIMENT 5.-Take off the wire and the

paper

and put the elastic on the ball. Hold the end of the elastic in one hand, and with the other pull the ball gently downward, then let it go. It vibrates up and down in the direction of the length of the elastic. Hence we call this kind of motion a longitudinal vibration.

These experiments show us the three kinds of vibrations, transverse, torsional, and longitudinal. They differ in direction, but all have the same manner of moving; for the different kinds of vibration, transverse, longitudinal, and torsional, go through motions with the same changes in velocity as take place in the swings of an ordinary pendulum. These vibrations all start from a position of momentary rest. The motion begins slowly, and gets faster and faster till the body gains the position it naturally has when it is at rest-at this point it has its greatest velocity. Passing this point, it goes slower and slower till it again comes momentarily to rest, and then begins its backward motion, and repeats again the same changes in velocity.

It is now necessary that the student should gain clear ideas of the nature of this pendulous motion. It is the cause of sound. It exists throughout all the air in which a sound may be perceived, and, by the changes in the number, extent of swing, and combinations of these pendular motions, all the changes of pitch, of intensity, and of quality

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