544 THE SENSE OF HEARING. 



vibrations, or seven or eight impulses in a second ; and sounds 

 still deeper might probably be heard, if the individual im- 

 pulses could be sufficiently prolonged. 



By removing one or several teeth from the toothed wheel 

 before mentioned, M. Savart was also enabled to satisfy himself 

 of the fact, that in the case of the auditory nerve, as in that 

 of the optic nerve, the sensation continues longer than the 

 impression which causes it ; for the removal of a tooth from 

 the wheel produced no interruption of the sound. The gradual 

 cessation of the sensation of sound renders it difficult, how- 

 ever, to determine its exact duration beyond that of the im- 

 pression of the sonorous impulses. 



The power of perceiving the direction of sounds is not a 

 faculty of the sense of hearing itself, but is an act of the mind 

 judging on experience previously acquired. From the modifi- 

 cations which the sensation of sound undergoes according to 

 the direction in which the sound reaches us, the mind infers 

 the position of the sounding body. The only true guide for 

 this inference is the more intense action of the sound upon one 

 than upon the other ear. But even here there is room for 

 much deception, by the influence of reflection or resonance, 

 and by the propagation of sound from a distance, without loss 

 of intensity, through curved conducting-tubes filled with air. 

 By means of such tubes, or of solid conductors, which convey 

 the sonorous vibrations from their source to a distant resonant 

 body, sounds may be made to appear to originate in a new 

 situation. 



The direction of sound may also be judged of by means of 

 one ear only ; the position of the ear and head being varied, 

 so that the sonorous undulations at one moment fall upon the 

 ear in a perpendicular direction, at another moment obliquely. 

 But when neither of these circumstances can guide us in dis- 

 tinguishing the direction of sound, as when it falls equally 

 upon both ears, its source being, for example, either directly 

 in front or behind us, it becomes impossible to determine 

 whence the sound comes. 



Ventriloquists take advantage of the difficulty with which 

 the direction of sound is recognized, and also the influence of 

 the imagination over our judgment, when they direct their 

 voice in a certain direction, and at the same time pretend 

 themselves to hear the sounds as coming from thence. 



The distance of the source of sounds is not recognized by the 

 sense itself, but is inferred from their intensity. The sound 

 itself is always seated but in one place, namely, in our ear ; 

 but it is interpreted as coming from an exterior soniferous 

 body. When the intensity of the voice is modified in imita- 



