THE TRANSMISSION OF SOUND. 21
The various experimental methods that have been employed for determining it, will be given later on. The following are some of the principal determinations made by Wertheim.
The superior conducting power of elastic solids may be illustrated by a variety of experiments. Thus, strike a tuning-fork and place the stem against the end of a rod of wood 12 or 15 feet long. So perfect is the transmission, that if a person applies his ear to the other end, the sound will appear to come from that part of the rod. A still better result is obtained by placing one end of the rod against a door panel, and applying the vibrating tuning fork to the other. Again, place a watch well between the teeth, without however touching them, and note the loudness of its tick. Now gently bite the watch, and observe how much more plainly it can be heard. In the first case, the vibrations pass through the air to the ears, in the second case, through the solid bones of the skull. This is the principle of the audiphone.
Sound cannot be transmitted through a vacuum.
The transmission of sound is a particular case of wave motion, of which, water waves and rope waves are other examples. The peculiar characteristic of a wave motion is, that the material particles through which the wave is passing, do not move onwards with the wave, but simply oscillate about their position of rest. In the rope wave, for example, the particles of the rope oscillate at right angles to the direction in which the wave is advancing; while, on the other hand, in the sound wave, the air particles oscillate in the same direction as the wave is moving.
Just as a water or rope wave consists of two parts, a crest and a trough, so a sound wave is made up of two portions, viz., a condensation and a rarefaction.