80 ON THE PHYSIOLOGICAL CAUSES OF 



It is the same with the waves of sound. They, too, are 

 added together at every point of the mass of air, as well 

 as in contact with the listener's ear. For them also the 

 degree of condensation and the velocity of the particles of 

 air in the passages of the organ of hearing are equal to the 

 algebraical sums of the separate degrees of condensation 

 and of the velocities of the waves of sound, considered 

 apart. This single motion of the air produced by the 

 simultaneous action of various sounding bodies, has now* 

 to be analysed by the ear into the separate parts which 

 correspond to their separate effects. For doing this the 

 ear is much more unfavourably situated than the eye. 

 The latter surveys the whole undulating surface at a 

 glance. But the ear can, of course, only perceive the 

 motion of the particles of air which impinge upon it. 

 And yet the ear solves its problem with the greatest 

 exactness, certainty, and determinacy. This power of the 

 ear is of supreme importance for hearing. Were it not 

 present it would be impossible to distinguish different 

 tones. 



Some recent anatomical discoveries appear to give a 

 clue to the explanation of this important power of the 

 ear. 



You will all have observed the phenomena of the sym- 

 pathetic production of tones in musical instruments, espe- 

 cially stringed instruments. The string of a pianoforte 

 when the damper is raised begins to vibrate as soon as its 

 proper tone is produced in its neighbourhood with suffi- 

 cient force by some other means. When this foreign tone 

 ceases the tone of the string will be heard to continue 

 some little time longer. If we put little paper riders on 

 the string they will be jerked off when its tone is thus 

 produced in the neighbourhood. This sympathetic action 

 of the string depends on the impact of the vibrating 

 particles of air against the string and its sounding-board. 



