LEVERS 57 



hear it as well when your ear is away from the pipe? What 

 conclusion can you draw from this fact? 



Try the same experiment with a long, wooden board. 



EXERCISE 55 

 HOW SOUNDS ARE STRENGTHENED 



Apparatus and Materials. Wide-mouth bottle, tuning fork or 

 table fork, metric rule, water. 



a. Into a wide-mouth bottle, such as a milk bottle or fruit 

 jar, pour a little water, and hold over the mouth of the bottle 

 (near it, but not touching) the prongs of a vibrating tuning fork 

 or table fork. If the sound produced by the fork does not 

 increase in loudness, add more water, so that the depth of the 

 water will be increased about 1 cm., and set the fork in vibra- 

 tion once more. Do this again and again until the sound is 

 reinforced, or strengthened, so that you can hear it very plainly. 

 For a common, Rogers Brothers', plated table fork the rein- 

 forcement was found to be loudest when the water was 9 cm. 

 from the top of the bottle. That is to say, the vibrating air 

 column was 9 cm. high. What is the length for your fork? 



6. Try another tuning fork or table fork. Strike it, and note 

 whether its pitch is higher or lower than that of the first fork. 

 If it is lower, make the air column longer; that is, take out some 

 of the water. If the pitch is higher, make the air column 

 shorter. Get the length of the air column in this case also. 



The reinforcement of sound by means of a second vibrating 

 material (here it is air) is called resonance. 



EXERCISE 56 

 LEVERS 



Apparatus and Materials. Strip of wood for lever, fulcrum, books, 

 a weight, such as a flat stone. 



