COMPOSITION OF SOUND WAVES. 291 



of waves will interfere, the one with the other. If the 

 waves be of equal intensity, the algebraic sm of these 

 component forces will be zero. The air particles, thus 

 acted upon, will remain at rest ; this means silence. In 

 Fig. 229, an attempt is made to represent this effect to the 

 eye, the uniformity of tint indicating the absence of con- 

 densations and rarefactions. Tfius, by adding sound 

 to sound, both may be destroyed. This is the lead- 

 ing characteristic property of wave motion. The 

 phenomenon here described is called interference 

 of sound. 



(a.) The sound of a vibrating tuning-fork held in the hand is 

 almost inaudible. The feebleness results largely from interference. 

 As the prongs always vibrate in opposite directions at the same 

 time, one demands a rarefaction where the other demands a con- 

 densation. By covering one vibrating prong with a pasteboard 

 tube the sound is more easily heard. (Fig. 230.) 



(&.) Hold a vibrating tuning-fork near the ear, and slowly turn 

 it between the fingers. During a single complete rotation, four 



