Mrs. B. Not absolutely necessary, though by far the most common vehicle of sound. Liquids, as well as air, are capable of conveying the vibratory motion of a sonorous body, to the organ of hearing; as sound can be heard under water. Solid bodies also, convey sound, as I can soon convince you by a very simple experiment. I shall fasten this string by the middle, round the poker; now raise the poker from the ground, by the two ends of the string, and hold one to each of your ears:—I shall now strike the poker, with a key, and you will find that the sound is conveyed to the ear by means of the strings, in a much more perfect manner, than if it had no other vehicle than the air.
Caroline. That it is, certainly, for I am almost stunned by the noise. But what is a sonorous body, Mrs. B.? for all bodies are capable of producing some kind of sound, by the motion they communicate to the air.
Mrs. B. Those bodies are called sonorous, which produce clear, distinct, regular, and durable sounds, such as a bell, a drum, musical strings, wind instruments, &c. They owe this property to their elasticity; for an elastic body, after having been struck, not only returns to its former situation, but having acquired momentum by its velocity, like the pendulum, it springs out on the opposite side. If I draw the string A B, ([fig. 6, plate 14],) which is made fast at both ends, to C, it will not only return to its original position, but proceed onwards, to D.
This is its first vibration; at the end of which, it will retain sufficient velocity to bring it to E, and back again to F, which constitutes its second vibration; the third vibration, will carry it only to G and H, and so on, till the resistance of the air destroys its motion.
The vibration of a sonorous body, gives a tremulous motion to the air around it, very similar to the motion communicated to smooth water, when a stone is thrown into it. This, first produces a small circular wave, around the spot in which the stone falls; the wave spreads, and gradually communicates its motion to the adjacent waters, producing similar waves to a considerable extent. The same kind of waves are produced in the air, by the motion of a sonorous body, but with this difference, that as air, is an elastic fluid, the motion does not consist of regularly extending waves, but of vibrations; and are composed of a motion, forwards and backwards, similar to those of the sonorous body. They differ also, in the one taking place in a plane, the other, in all directions: the ærial undulations, being spherical.
Emily. But if the air moves backwards, as well as forwards, how can its motion extend so as to convey sound to a distance?
Mrs. B. The first sphere of undulations, which are produced immediately around the sonorous body, by pressing against the contiguous air, condenses it. The condensed air, though impelled forward by the pressure, reacts on the first set of undulations, driving them back again. The second set of undulations which have been put in motion, in their turn, communicate their motion, and are themselves driven back, by reaction. Thus, there is a succession of waves in the air, corresponding with the succession of waves in the water.
Caroline. The vibrations of sound, must extend much further than the circular waves in water, since sound is conveyed to a great distance.
Mrs. B. The air is a fluid so much less dense than water, that motion is more easily communicated to it. The report of a cannon produces vibrations of the air, which extend to several miles around.
Emily. Distant sound takes some time to reach us, since it is produced at the moment the cannon is fired; and we see the light of the flash, long before we hear the report.