83 



therefore, concerning the effect of a sounding body upon 

 the air surrounding it ? 



EXPERIMENT 81. THE REFLECTION OF SOUND 



Place two wide glass tubes at any convenient angle, and 

 between them insert a piece of thick cardboard, as shown 

 in the diagram. Put a watch at the end A of one of the 

 tubes, and the ear at 

 the end of the other 

 tube. No sound can 

 be heard at B, 

 because the sound 

 waves which travel 

 directly from A to B 

 are cut off by the 

 cardboard. 



Put a second piece 

 of cardboard in posi- 

 tion C, SO that it FIG. 60. The ticks of the watch are reflected 



makes equal angles 



with the two tubes, and again place the ear at B. The 



ticks of the watch become audible. Why ? 



Change the position of the cardboard C so that it makes 

 unequal angles with the two tubes. Is the intensity of the 

 sound altered, or does it remain the same ? Why ? 



EXPERIMENT 82. RESONANCE 



Set up the apparatus as shown in the diagram. By rais- 

 ing and lowering B the height of the water in A can be 

 changed. 



