10 



always a tendency to the production of liarmonic vibrations. If 

 movable particles of matter come within the range of such 

 vibrations, they will collect around the nodes or points of com- 

 parative rest. This the Lecturer exemplified by a series of 

 beautiful experiments with Chladni's plates. On drawing various 

 notes from these plates (which were of brass), sand which had 

 previously been strewed over the plates was found to rapidly 

 arrange itself in most perfect and symmetrical figures, every note 

 having its own definite sand-pattern. 



Sound-waves move through the air from a centre of disturb- 

 ance spherically in all directions, just as ripples spread in concentric 

 circles when a stone is thrown into calm water. The existence of 

 such waves was illustrated in two ways : first, by causing the waves 

 to impinge upon a gas flame in the so-called " sensitive " condition, 

 which flame was thereby affected in a very remarkable manner; 

 and secondly, by causing the air itself to take up and reinforce the 

 vibrations of a sounding body — an effect known as resonance. 

 The length of a column of air which has a maximum resonance for 

 a given note is just one-fourth of the length of the sound-wave. 

 Thus, a tube which resounds to a fork giving the note C with 

 512 vibrations per second, would be 6^ inches in length, because 

 the sound-waves produced by the sounding-fork are four times 

 6| inches, or 2 feet 2 inches long. The impact of sound-waves 

 upon the ear causes the tympanic membrane, which is situated in 

 front of the drum, to vibrate, and the vibrations are ultimately 

 communicated, through the auditory nerve, to the brain, and there 

 interpreted as sound. Hence sound, as such, has no existence 

 outside of ourselves, it merely consists of an aerial disturbance, 

 which is only differentiated as sound when affecting our organs of 

 hearing. 



Any other tense membrane will vibrate when struck by sound- 

 waves, and if a conductor, such as a rod or string, be attached to 

 two such membranes, sounds uttered near one may be conveyed to, 

 and heard in the vicinity of, the other. This is the principle of 

 the mechanical telephone, which, in its simplest form, has been 

 known for very many years. The use, however, of a thin metal 

 plate for a diaphragm, which, vibrated by sound-waves in front of 



