Sound

Every source of sound is in a state of vibration.

Sometimes the vibration is of short duration as in the crack of a whip or the bang of a gun, in many instances the vibration is clearly visible, as in the case of a taut wire. The vibratory origin of many common sounds is not so obvious, often because the vibrating system is gaseous and therefore invisible - the moaning of wind, the note of a whistle, the thunderous noise which succeeds a lightning flash, but experiments have been devised which prove the vibratory nature of all such sounds.

Mere vibration, however, is not in itself sufficient to produce the sensation of sound; there must be some material medium to transmit the effects of the vibration to the ear of the listener - sound cannot travel through a vacuum. This can be proved by hanging an electric bell in a glass jar from which air can be withdrawn by 'an exhaust pump.