330 PROPAGATION OF SOUND. 



is, in the direction towards ft-- -because the particle: 

 at b are under the ordinary pressure, while thosi 



, situated close to the vibratin< 

 a o c ..".... 



prong are in a state of condensa 

 ll tion. Compression now takes plac 



II ; at b ( B, fig. 209), the particles a 



a moving in the direction of th 



"* . . .7 arrow. By their inertia the pai 



c II tides maintain this direction c 



'. their motion even for an instan 



*J I after the prong has already con 



menced its backward motion th* 



FIG. 209. . r f ~ c\f\t\ /-A 



is, away from us (tig. 209, 6). / 

 that moment the particles at b are already moving t< 

 wards c, and cause a compression of the air at c, whi 

 the particles between a and the prong suffer rarefactio 

 because the prong moves to the left while the particl 

 a move to the right. But as soon as this takes plac 

 and the air close to the prong becomes rarefied, and 

 considerably less pressure than the still somewhat cor 

 pressed air at b, then the particles a reverse the di 

 tiori of their motion, and move towards the prong (fi 

 209, D) ; the rarefaction is now between a and ft, wh 

 the compression proceeds beyond c towards d. Al 

 the air has now again acquired the original densii 

 In this manner the compression continues to approa 

 us, and is followed by the rarefaction, which ad vane 

 in a similar manner, because the particles now at 

 flow towards the rarefied part, causing a rarefaction at 

 and so on. 



The tuning-fork does not perform merely a sin; 

 vibration, but the vibrations are uniformly repeated, 



