PHYSICS OF SOUND. 167 



Most of the points in acoustics which are essential to the 

 comprehension of the physiology of audition are definitely 

 settled. The theories of the propagation of sound involve 

 wave-action, concerning which there is no dispute among 

 physicists. For the conduction of sound, a ponderable me- 

 dium is essential ; and it is not necessary, as in the case of 

 the undulatory theory of light, to assume the existence of an 

 imponderable ether. The human ear, though perhaps not so 

 acute as the auditory apparatus of some of the inferior ani- 

 mals, not only appreciates irregular waves, such as produce 

 noise as distinguished from sounds called musical, but is ca- 

 pable of distinguishing regular waves, as in simple musical 

 sounds, and harmonious combinations. 



In music, certain successions of regular sounds are agree- 

 able to the ear and constitute what we call melody. Again, 

 we are able to appreciate, not only the intensity of sounds, 

 both noisy and musical, but we recognize different qualities, 

 particularly in musical tones. Still farther, we find that 

 tones may be resolved into certain invariable component 

 parts, such as the octave, the third, fifth, etc. These com- 

 ponents of what are usually called simple tones may be iso- 

 lated, in carefully-conducted experiments, but there is always 

 a tone heard which predominates over others, the pitch and 

 quality of which are readily appreciated by a practised musi- 

 cal ear. 



The quality of tones may be modified by the simulta- 

 neous production of other tones which correspond to certain 

 of the components of the predominating note. For example, 

 if we add to a single tone, the third, fifth, and octave, we 

 produce a major chord, the sound of which is very different 

 from that of a single note or of a note with its octave. If we 

 diminish the third by a semitone, we have a different quality, 

 which is peculiar to minor chords. In this way, we can 

 form an immense variety of musical sounds upon a single 

 instrument, as the piano. And still farther, by the har- 

 monious combinations of different notes of different instru- 



