DIRECTION AND DISTANCE OF SOUNDS. 695 



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 circum- 

 stances can guide us in distinguishing 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 recognised 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 imitation of the effect of distance, it excites 

 the idea of its originating at a distance ; and this is also 

 taken advantage of by ventriloquists. 



The experiments of Savart, already referred to, prove 

 that the effect of the action of sonorous undulations upon 

 the nerve of hearing, endures somewhat longer than the 

 period during which the undulations are passing through 

 the ear. If, however, the impression of the same sound 

 be very long continued, or constantly repeated for a long 

 time, then the sensation produced may continue for a very 

 long time, more than twelve or twenty-four hours even, 

 after the original cause of the sound has ceased. This 



