PROPAGATION OF SOUND. 329 



The motion of the particles of air during the propa- 

 gation of sound resembles to some extent that of the 

 particles of water during the propagation of a wave ; 

 hence sound is said to be propagated by an undulatory 

 or wave-like motion of particles of air. 



The resemblance is, however, confined to the fact 

 that each particle performs the same definite move- 

 ment but commences its motion somewhat later than 

 he preceding one. The path described by each particle 

 s essentially different in the two cases. When a wave 

 propagated through water, each particle describes a 

 drcle ; when sound is propagated through air, each 

 article of air moves in a straight line, backwards and 

 brwards, in the direction in which the sound is pro- 

 >agated. A wave of water is formed by a series of crests 

 and hollows, (or elevations and depressions) ; a wave 

 f sound, by a series of alternating compressions and 

 rarefactions of air. Such compressions and rarefactions 

 f the air must take place whenever a body, surrounded 

 >y air, is set into rapid vibrations that is, when its 

 parts are made to move rapidly to and fro through a 

 short space. Suppose a tuning-fork to be struck, and, for 

 :the sake of simplicity, that one of its prongs vibrates 

 i3xactly in the direction towards and away from us. 

 AS soon as the prong commences to move towards 

 ! is, the nearest particles of air (a in fig. 209, A) are 

 Compressed, while the particles farther away at b are 

 is yet unaffected by the impulse in consequence of 

 heir inertia. Before the prong commences its back- 

 ward motion, the compressed particles of air expand 

 'igain, and the expansion takes place in that direction, 

 n which they meet with the least resistance that 



