122 SENSES. 



Of Hearing. 



The sensation of hearing is conveyed to the mind by undu- 

 lations of air striking the ear, an organ of a very delicate and 

 complex structure. In quadrupeds, the external ears are 

 large, and provided with muscles, by which they can erect 

 and move them from side to side, in order to catch the undu- 

 lations produced in the air by the vibrations of sonorous 

 bodies, or to distinguish with greater accuracy the species of 

 sound, and the nature and situation of the animal or object 

 from which it proceeds. Though the human ears, like those 

 of quadrupeds, are furnished with muscles, evidently intended 

 for similar movements, yet, I know not for what reasons, 

 there is not one man in a million who has the power of mov- 

 ing his ears. When we listen to a feeble sound, we are con- 

 scious of an exertion; but that exertion, arid the motions 

 produced by it, are confined to the internal parts of the organ. 



The canals or passages to the internal parts of the ear 

 are cylindrical, somewhat contorted, and become gradually 

 smaller till they reach the membrane which covers what is 

 called the drum of the ear. This membrane, which is ex- 

 tremely sensible when acted upon by undulations of air, how- 

 ever excited, conveys, by means of a complex apparatus of 

 bones, nerves, &c., the sensation of sound to the brain or 

 sentient principle. 



Infants hear bluntly, because the bones of their ears are 

 soft and cartilaginous; and, of course, the tremulations ex- 

 cited in them by the motions of the air, are comparatively 

 weak. Young children, accordingly, are extremely fond of 

 noise. It rouses their attention, and conveys to them the 

 agreeable sensation of sound ; but feeble sounds are not per- 

 ceivod, which gives infants, like deaf persons, the appearance 

 of inattention, or rather of stupidity. 



' That air is the medium by which sounds are propagated, 

 has been established by repeated experiments. The sound 

 of a bell suspended in the receiver of an air-pump, gradually 

 diminishes as the air is exhausted, till it almost entirely 

 ceases to be heard. On the other hand, when the quantity 

 of air is increased by a condenser, the intensity of the sound 

 is proportionally augmented. It has also been proved that 

 sounds actually produced, cannot be transmitted through a 

 vacuum, or a space deprived of air. 



' The undulations of the air by which sounds are produced 



