SECT, xvi.] INTERFEKENCE OF SOUNDS. J55 



will resemble a rattle ; and, if the strings be in perfect unison, 

 there will be no beats, since there will be no interference. 

 Thus by interference is meant the co-existence of two undu- 

 lations in which the lengths of the waves are the same. And, 

 as the magnitude of an undulation may be diminished by the 

 addition of another transmitted in the same direction, it 

 follows that one undulation may be absolutely destroyed by 

 another when waves of the same length are transmitted in 

 the same direction, provided that the maxima of the undula- 

 tions are equal, and that one follows the other by half the 

 length of a wave. A tuning-fork affords a good example of 

 interference. When that instrument vibrates, its two branches 

 alternately recede from and approach one another; each 

 communicates its vibrations to the air, and a musical note is 

 the consequence. If the fork be held upright about a foot 

 from the ear, and turned round its axis while vibrating, at 

 every quarter revolution the sound will scarcely be hearcf, 

 while at the intermediate points it will be strong and clear. 

 This phenomenon arises from the interference of the undula- 

 tions of air coming from the two branches of the fork. When 

 the two branches coincide, or when they are at equal dis- 

 tances from the ear, the waves of air combine to reinforce each 

 other ; but at the quadrants, where the two branches are at 

 unequal distances from the ear, the lengths of the waves 

 differ by half an undulation, and consequently destroy one 

 another. 



