THE NATURE OF SOUND.
Sound is a form of movement. This may te shown in various ways. We all know that tightened string may be caused to give out a nusical sound by being pulled strongly to one lide and then let go. The string makes a rapid o-and-fro motion, which is accompanied by he sound. As the motion becomes less vigor nis the sound becomes feebler, and when the novement stops, the sound also ceases. The imbs of a tuning-fork are in rapid motion when it is sounded. In the case of very large heavy forks the to-and-fro movement can be wen; but in the case of small forks, such as those used by musicians, the motion is so fine tud rapid that it is not visible. When a bell mounds, after being struck, it can be shown to be in motion ; not moving as a whole, but the particles of which its mass consists being in rapid vibration. Now suppose we have a small bell the hammer of which is worked by clock work ; let us place it on the plate of an air pump, resting on a thick cushion of felt; and let it be covered with the glass bell-jar of the air-pump. If the clockwork be working we still hear the sound through the glass. Now let the pump be worked so as to remove the air from the chamber, and as soon as a consider able quantity of air has been removed the sound becomes very feeble. When the glass jar has been as completely exhausted of air as possible the sound is no longer heard, though the hammer may still be seen to strike the bell. The stroke still causes vibrations of the particles forming the substance of the bell, but owing to the absence of air there are no means of communicating the vibrations to the ear of anyone in the neighbourhood. This shows, then, that sound is a movement, that it is only when the movement is conveyed to the ear and affects the nerves of hearing that the sensation of sound is produced, and that it is usually the atmospheric air that acts in conveying the motion to the ears.
The sound movements, or vibrations as it is better to call them, are conveyed through the air in a wave-like fashion, represented in Fig. 187. The figure shows a tuning-fork, supposed to be sounding. Its limbs are, therefore, in a state of rapid vibration. In the figure what occurs is shown only on one side. The limb a moves to and fro, now in the position a, and now in the position a". When the limb moves from a to a' the air in contact with it receives a shock, and the particles of the air are crowded together by the blow. When the'limb moves
from a to a" the particles that were crowded together have now more space at their disposal, and are less crowded than before the shock of the limb. But with great speed the limb moves back from a" to a', and again the particles are crowded, and then it rapidly returns to a", where the greater space is again created. Thus, while the fork continues to vibrate, the par ticles of air in its immediate neighbourhood are at one moment crowded together, at another moment the opposite is the case, and this goes on on each side of the fork as long as its move ments continue. The crowding together of the particles of air by the shock of the fork is called a condensation, and the opposite movement a rarefaction. To use these terms, then, the air on each side of the limb of the fork is at one moment in a state of condensation and at another moment in a state of rarefaction, in time with the vibrations of the fork. But this peculiar agitation is not confined to the air that is in immediate contact with the fork.
The condensation travels outwards from the fork through the atmosphere, and the rarefaction likewise ; and as long as the fork keeps going these two conditions are passed along through the atmosphere from the sound ing body, which thus becomes a centre of dis turbance, just as a stone thrown into still water imparts a shock to the water, and from the plea where it struck waves pass outward on all sides Fig. 187 represents by the difference in the shad. ing the alternate condensation and rarefaction Now we all know that a person in a boat or the water will become aware of the agitation of the water, even though at a distance fron the centre of disturbance, by the waves rippling up to him. Even so a person becomes conscioul of a disturbance in the atmosphere, though he be at a distance from the place where it is pro duced, because the waves of condensation and rarefaction, spreading outwards on all sides from the vibrating body, at length reach him and beat upon him. They affect his ears, they irritate his nerves of hearing, and so he becomes aware of what he calls sound. If we could see air, as we see water, we would perceive the dis turbance, caused by a sounding body, passing through it.