OF SOUND. 



When an elastic body is struck, its parts are put 

 into a tremulous or vibratory motion, which is 

 communicated to the surrounding air. This vi- 

 bration of the air, which may be compared to the 

 waves in water, occasioned by a stone thrown into 

 it, extends to the tympanum or drum of the ear, 

 and affects the nerves with the sensation called 

 sound. 



Air is the usual vehicle of sound, but it is not 

 absolutely essential; the vibration of a solid body 

 being capable of affecting the ear without the in- 

 tervention of air, as may be observed by striking a 

 poker which is held by a string and pressed against 

 the ear. It is necessary, however, that there be 

 some substance extending from the sounding body 

 to the ear, as sound cannot be conveyed through a 

 vacuum. Saussure observed, that the noise of a 

 pistol fired on the top of Mont Blanc, where the 

 air is highly rarefied, was not greater than the 

 report of a small cannon used as a toy. On the 

 contrary, the noise of a bell, inclosed in a receiver 

 of condensed air, is louder than before the air was 

 condensed. 



As sound is owing to the elasticity of bodies, 

 the most elastic are the most sonorous: for example, 



q 2 



