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CHAPTER V. 



HEARING. 



1. Acoustic Principles. 



THE knowledge acquired by animals of the presence and 

 movements of distant objects is derived almost wholly from 

 the senses of hearing and of sight; and the apparatus, ne- 

 cessary for the exercise of these senses, being more elabo- 

 rate and refined than any of the organs we have yet exa- 

 mined, exhibit still more irrefragable evidence of those pro- 

 found designs, and that infinite intelligence, which have 

 guided the construction of every part of the animal frame. 



Sound results from certain tremulous or vibratory motions 

 of the particles of an elastic medium, such as air or water, 

 excited by any sudden impulse or concussion given to those 

 particles by the movements of the sounding body. These 

 sonorous vibrations are transmitted with great velocity 

 through these fluids, till they strike upon the external ear; 

 and, then, after being concentrated in the internal passages 

 of the organ, they are made to act on the filaments of a par- 

 ticular nerve called the acoustic, or auditory nerve, of 

 which the structure is adapted to receive these peculiar im- 

 pressions, and to communicate them to the brain, where 

 they produce changes, which are immediately followed by 

 the sensation of sound. Sound cannot traverse a void space, 

 as light does; but always requires a ponderable material ve- 

 hicle for its transmission; and, accordingly, a bell suspended 

 in the vacuum of an air-pump, gives, when struck, no audi- 



