272 



NATURE OF SOUND. 



the nearest layer of air particles follows it ; a rarefaction 

 of the air is thus produced, A sound wave, therefore, 

 consists of two parts, a condensation and a rarefac- 

 tion. The motion of any air particle is backward and 

 forward in the line of propagation, and not " up and down " 

 across that line, as in the case of water waves. A series of 

 complete sound waves consists of alternate condensations 

 and rarefactions in the form of continually increasing 

 spherical shells, at the common centre of which is the 

 sounding body. Any line of propagation of the sound 

 would be a radius of the sphere. 



424. Sound Media. The air particles impart their 

 motion to other particles because of their elasticity. Any 

 elastic substance may become the medium for the 

 transmission of sound, but such a medium is neces- 

 sary. The elasticity of a body 

 may be measured by the re- 

 sistance it opposes to compres- 

 sion. The less the compres- 

 sibility, the 

 greater the 

 elasticity. 



FIG. 215. 



(a.) That sound 

 is not transmit- 

 ted in a vacuum 

 is shown as fol- 



lows: A lar e 



glass globe, pro- 

 vided with a stop-cock, contains a 

 small bell suspended by a thread. 

 When the air is pumped from the 

 globe and the globe shaken, no 

 sound is heard, although the clap- 

 per of the bell is seen to strike 



FIG. 216. 



