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"G IVE a light blow to a wine-glass. The glass will ring, giving forth a sound, weaker or stronger, lower or higher, according to the quality and size of the glass. The sound lasts but a moment, and then ceases. Strike the glass once more and while it is still ringing touch the rim with your finger. Instantly all is still again; not a sound from the glass. Why does it ring when struck, and why does it stop ringing at the touch of a finger? Before replying let us experiment with other resonant objects.
"A violin-string twangs on being scraped by the bow or plucked with the finger, and while it thus gives forth its note it is seen to vibrate rapidly. So rapid, indeed, is its vibration that it appears to fill the entire space between its extreme positions, with the result that it presents a swollen appearance in the middle, after the manner of a spindle. With the cessation of its vibration it becomes silent. It also falls silent immediately at the touch of a finger.
"A bell rings on being struck by its clapper, and if observed closely the substance of the bell will be seen to tremble in an unmistakable manner. Place your hand on the bronze and you will experience a disagreeable sensation almost amounting to pain, due to the vibration of the metal. Finally, if your hand continues to rest on the bell, the vibration will cease and with it the sound.
"Let us try something still more remarkable. Take a pin by its pointed end and bring the head very close to a ringing wine-glass or bell. You will hear a rapid succession of little taps. Whence do they come? From the glass or the bell-metal striking the pin a series of quick blows as long as the ringing continues. They come from the lively trembling of the sonorous object.
"It is unnecessary to cite other examples; these three will suffice. They show that, in order to give forth sound, a wine-glass or a bell or a violin-string—in short, any object whatever—must be made to tremble or vibrate with great rapidity. The sound is heard as long as the vibration continues, and ceases when the vibrating object returns to a state of rest. That explains why the wine-glass and the violin-string stop sounding at the touch of a finger, and why the bell will not ring if you rest your hand against it. The finger in the one instance, the hand in the other, stop the resonant trembling, and in so doing arrest the sound. Motion causes the sound; rest brings silence.
"To this rapid motion back and forth is given the name I have already used, vibration. An object from which comes a sound is in vibration; it vibrates. Each of its backward and forward movements, too rapid for the eye to follow, is a vibration; and the quicker these vibrations, the higher the note sounded; the slower they are, the lower the note. In a word, sound is motion and its place in the scale measures the rapidity of that motion.
"In order to be heard, this sound, this motion, must reach us. The hand detects it in its own way when, resting on the vibrating bell, it experiences a very disagreeable thrill; the finger becomes conscious of it in a peculiar manner when, touching the violin-string, it feels a ticking sensation. But how does the ear contrive to receive the sound when it is at a distance from the resonant object and apparently in no sort of communication with it?
"At this point let me invite you to join me in a sport very familiar to you all. We will take a big stone and drop it into a calm sheet of water. Around the place where the stone struck the water there is instantly formed a circle, then another and another, and so on indefinitely; and all these circles, described about the same center and as regular as if drawn with a pair of compasses, grow larger and larger, in successive rings, until they die out at a long distance from their common center, if the sheet of water is large enough.
"Now, do those circles really chase one another over the water's surface as they appear to? One would certainly think so. You remember how fast they go, one ring after another, each in apparent haste to catch up with its predecessor. But the pursued always keep clear of the pursuers, and the distance between the rings remains the same. And so the fact is they are not really chasing one another; they are not, in fact, moving at all; but they have that deceptive appearance, and it will not be hard for us to understand why.
"Let us drop a straw or a dry leaf upon the surface of the water. When one of these concentric circles passes like a wave, the straw or leaf is lifted up, after which the wave goes on and leaves it, and it sinks down again, remaining exactly where it was in the first place. Thus it is proved that the water does not move forward at all, for if it did it would carry with it the straw or leaf on its surface.
"What, then, are those waves? Mere palpitations of the water as, without changing its place, it gently rises and falls, thus producing a succession of alternate billows and furrows which appear to go chasing after one another. Watch a field of wheat when the wind blows. Its surface undulates in waves that seem to move forward although the wheat-stalks remain firmly rooted in the ground. Of like sort is the apparent movement of a sheet of smooth water when disturbed by the fall of a stone.
"The circles on the water and the undulations of the wheat-field explain to us the nature of sound. Every object emitting a sound is in rapid vibration, and each of these vibrations causes a shock to the surrounding air and produces a wave which spreads out in all directions, immediately followed by a second, a third, and countless others, all resulting from as many successive vibrations.
"In the air thus shocked by the vibrating body there takes place exactly what we see in the sheet of smooth water disturbed by a falling stone and in the wheat-field ruffled by the wind. Without changing its place the air undergoes an undulatory movement which is transmitted to great distances. In other words, air-waves are formed which propagate themselves in every direction at once through the atmosphere, thus taking the form, not of circles, but rather of hollow spheres, all having a common center.
"These air-waves are not visible to us, because air itself is invisible; but they are none the less real, just as real as waves of water and undulations of a wheat-field. If the eye cannot see them, the ear can hear them, for it is from them that sound comes. Hence they are called sound-waves. The ear hears when sound-waves reach it from any vibrating object."