280 PLEASANT WAYS IN SCIENCE. 



as to give forth a particular series of tones (the so-called 

 fundamental tone of the disc and other tones combined 

 with it which belong to its series of overtones), the various 

 parts of the disc are vibrating to and fro in a direction square 

 to the face of the disc, except certain points at which there 

 is no vibration, these points together forming curves of 

 special forms along the substance of the disc. 



When, on the other hand, tones of various kinds are 

 sounded in the neighbourhood of a disc or of a stretched 

 circular membrane, we may assume that the different parts o\ 

 the disc are set in vibration after a manner at least equally 

 complicated. If the tones belong to the series which could 

 be emitted by the diaphragm when struck, we can understand 

 that the vibrations of the diaphragm would resemble those 

 which would result from a blow struck under special condi- 

 tions. When other tones are sounded, it may be assumed 

 that the sound-waves which reach the diaphragm cause it to 

 vibrate as though not the circumference (only) but a circle 

 in the substance of the diaphragm concentric, of course, 

 with the circumference, and corresponding in dimensions 

 with the tone of the sounds were fixed. If a drum of given 

 size is struck, we hear a note of particular tone. If we heard, 

 as the result of a blow on the same drum, a much higher 

 tone, we should know that in some way or other the effective 

 dimensions of the drum-skin had been reduced as for 

 instance, by a ring firmly pressed against the inside of the 

 skin. So when a diaphragm is responding to tones other 

 than those corresponding to its size, tension, etc, we infer 

 that the sound-waves reaching it cause it to behave, so far as 

 its effective vibrating portion is concerned, as though its 

 conformation had altered. When several tones are responded 

 to by such a diaphragm, we may infer that the vibrations of 

 the diaphragm are remarkably complicated. 



Now the varieties of vibratory motion to which the 

 diaphragm of the telephone has been made to respond have 

 been multitudinous. Not only have all orders of sound singly 

 and together been responded to, but vocal sounds which in 



