i68 THE PHILOSOPHY 



The canals or paffages to the Internal parts of the ear are cylin- 

 drical, fomewhat contorted, and become gradually fmaller till they 

 reach the mtmbrana tympani, which covers what is called the drum 

 of the ear. This membrane, which is extremely fenfible, when 

 adled upon by inductions 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 propagated, has 

 been eftablifhed by repeated experiments. The found of a bell, fu- 

 fpended in the receiver of an air-pump, gradually diminiflies as 

 the air is exhaufted, till it almofl: entirely ceafes to be heard. On 

 the other hand, when the quantity of air is increafed by a con- 

 denfer, the intenfity of the found is proportionally augmented. Mr 

 Haukfbee, in a paper publifhed in the Philofophical Tranfadions, 

 has proved, that founds adually produced cannot be tranfmitted 

 through a 'vacuum, or a fpace deprived of air. ' I took,' fays he, 

 ' a ftrong receiver, armed with a brafs hoop at the bottom, in which 

 ' 1 included a bell as large as it could well contain. This receiver 

 ' I fcrewed ftrongly down to a brafs plate with a wet leather be- 

 ' tween, and it was full of common air, which could nowife make 



* its efcape. Thus fecured, it was fet on the pump, where it was 

 ' covered with another large receiver. In this manner, the air con- 



* tained between the outward and inward receivers was exhaufted. 

 ' Now here I was fure, when the clapper fhould be made to ftrike 

 ' the bell, there would be adually found produced in the inward 

 ' receiver ; the air in which was of the fame denfity as common air, 



* could fuffer no alteration by the vacuum on its outfide, fo ftrong- 



* ly was it fecured on all parts. Thus, all being ready for trial, the 



* clapper was made to ftrike the bell ; but I found that there was no 

 ' tranfmiffion of it through the ■vacuum, though I was fure there 



* was adual found produced in the inward receiver.' 



To 



