434 Lord Bayleigh on Shadows. [Jan. 15, 



by the small scale arrangement represented diagrammatically in 

 5^ig. 4. A strip of zinc, about 2 feet wide and 12 feet long, is 

 bent into the form of a semicircle ; this forms the model of the 

 whispering gallery. The bird-call B is adjusted so that it throws 

 the sound tangentially against the inner surface of the zinc : it 

 thus takes the place of the whisperer. The sensitive flame F takes 

 the place of the listener. A flame is always more sensitive to sound 

 reaching it in one direction than in others ; the flame F is therefore 

 adjusted so that it is sensitive to sounds leaving the gallery tan- 

 gentially. The flaring of the flame shows that sound is reaching it : 

 if an obstacle is interposed in the straight line F B the flame flares 

 as before ; but if a lath of wood W, which need not be more than 

 2 inches wide, is placed against the inner surface of the zinc, the 

 flame recovers, showing that the sound has been intercepted. Thus 

 the sound creeps round the inside surface of the zinc, and there is no 

 disturbance except at points within a limited distance from that 

 surface. 



[E.] 



