OF NATURAL HISTORY. 175 



OF HEARING. 

 THE fenfation of hearing is conveyed to the mind by un« 

 dulations of air ftriking the ear, an organ of a very delicate 

 and complex flru^lure. In man and quadrupeds, the exter- 

 nal ears are large, and provided with mufcles by wliich they 

 can erect and move them from fide to fide, in order to catch 

 the undulations produced in the air by the vibrations of fono- 

 rous bodies, or to diftinguifh with greater accuracy the fpe- 

 cies of found, and the nature and fituation of the animal or 

 object from which it proceeds. Though the human ears, 

 ^ like thofe of quadrupeds,^ are furnifhed with mufcles, evi- 

 dently intended for fimilar movements, yet, I know not for 

 what reafon, there is not one man in a million who has the 

 power of moving his ears. "When we liften to a feeble 

 found, we are confcious of an exertion ; but that exertion, 

 and the motions produced by it, are confined to the internal 

 parts of the organ. 



The canals or paiTages to the internal parts of the eai: 

 are cylindrical, fomewhat contorted, and become gradually 

 fmaller till they reach the memhrana tympanic which covers 

 what is called the drum of the ear. This membrane, which 

 is extremely fenfible, when a<Sl:ed upon by indulations of air, 

 however excited, conveys, by means of a complex apparatus 

 of bones, nerves, &c. the fenfation of found to the brain or 

 fentient principle. 



That air is the medium by which all founds are propagat- 

 ed, has been eftablifhed by repeated experiments. The 

 found of a bell, fufpended in the receiver of an air-pump, 

 gradually diminifhes as the air is exhaufted, till it almofl en- 

 tirely ceafes to be heard. On the other hand, when the 

 quantity of air is increafed by a condenfer, the intenfity of 

 the found is proportionally augmented. Mr. Haufkbee, in a 

 paper publifhed in the Philofophical Tranfa^lions, has prov- 

 ed, that founds actually produced cannot be tranfmitted 



