SOUND. 



This arrangement being made, and the alarum being placed 



within the receiver, upon a 

 soft cushion of wool e, so as 

 to prevent the vibration from 

 being communicated to the 

 pump-plate, let the receiver 

 be exhausted in the usual 

 way. When the air has been 

 withdrawn, let the bell be 

 made to ring by means of the 

 sliding- rod. No sound will 

 be heard, although the per- 

 cussion of the tongue upon 

 the bell, and the vibration of 

 the bell itself, are visible. 

 Now if a little air be ad- 

 mitted into the receiver, a 

 faint sound will begin to be 

 heard, and this sound will 

 become gradually louder in 

 proportion as the air is gra- 

 dually readmitted. 



In this case the vibrations 

 which directly act upon the 

 ear are not those of the air 

 contained in the receiver. 

 These latter act upon the re- 

 ceiver itself and the pump- 

 plate, producing in them sympathetic vibration; and those 

 vibrations impart vibrations to the external air which are trans- 

 mitted to the ear. 



If in the preceding experiment a cushion had not been inter- 

 posed between the alarum and the pump-plate, the sound of the 

 bell would have been audible, notwithstanding the absence of air 

 from the receiver. The vibration in this case would have been 

 propagated, first from the bell to the pump-plate and to the bodies 

 in contact with it, and thence to the external air. 



3. Since the propagation of undulations through the atmosphere 

 is progressive, an interval of time, more or less, must elapse 

 between the vibration of the sounding body and the perception of 

 the sound by a hearer, and such interval will be proportionate to 

 the distance of the hearer from the sounding body, and to the 

 velocity with which sound is propagated through the intervening 

 medium. This progressive propagation of sound can be directly 

 proved by experiment. 

 186 



