176 THE PHILOSOPHY 



through a vacuum, or a fpace deprived of air. « I took,* 

 fays he, « a flrong receiver, armed with a brafs hoop at the 

 ^ bottom, in which I included a bell as large as it could well 



< contain. This receiver I fcrewed flrongly down to a brafs 



< plate with a wet leather between, 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 contained between 



* the outward and inward receivers was exhaufted. Now 

 « here I was fure, when the clapper fliould be made to ftrike 



< the bell, there would be actually found produced in the in- 



< ward receiver ; the air in which was of the fame denlity as 



* common air, could fufFer no alteration by the vacuum on 



* its outfide, fo ftrongly was it fecured on all parts. Thus, all 



< being ready for trial, the clapper was made to ilrike the 



< bell ; but I found that there was no tranfmiffion of it 

 « through the vacuum, though I was fure there was a£lual 

 ^ found produced in the inward receiver.' 



To enable us to under ftand the manner in which founds 

 are propagated through the air, philofophers have had re- 

 courfe to the undulations produced by a ftone thrown into a 

 pond of ftagnating water. Thefe undulations alTume the 

 form of circular waves, which fucceffively proceed from the 

 place where the ftone ftruck the water, as from a center, and 

 continually dilate, and become greater and greater as they 

 recede from that center, till they reach the banks of the wa- 

 ter, where they either vanifli or are reflecSled. Now, as air 

 is likevvife a fluid, iimilar undulations, though to us invilible, 

 are produced in it by the vibrations of fonorous bodies, and 

 are alfo propagated to great diftances in fucceflive waves or 

 rings. Thefe undulations of the air, when they come into 

 contact with our organs of hearing, make fuch a tremulous 

 impreflion upon them as excites in our minds the fenfation 

 pf found. This analogy, though not altogether perfed, is 



