70 OX 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 velo- 

 cities of the waves of sound, considered apart. This single motion 

 of the air produced by the simultaneous action of various sound- 

 ing bodies, has now to be analysed by the air into the separate 

 paits which correspond to their separate effects. For doing this 

 the ear is much more unfavourably situated than the eye. The 

 latter surveys the whole und ulat ing 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 grea test 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, especially stringed 

 instruments. The string of a pianoforte when the damper is 

 raised begins to vibrata as soon as its proper tone is produced 

 in its neighbourhood with sufficient 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 arid its sounding-board. 



Each separate wave-crest (or condensation) of air which 

 passes by the string is, of course, too weak to produce a sensible 

 motion in it. But when a long series of wave-crests (or con- 

 densations) strike the string in such a manner that each succeed- 

 ing one increases the slight tremor which resulted from the 

 action of its predecessors, the effect finally becomes sensible. 

 It is a process of exactly the same nature as the swinging of a 



