CHAPTER XII 

 SOUND 



What Causes Sound. When the string of a guitar 

 is plucked, it gives forth a continuous sound. If a light 

 piece of metal is held near the string, it will be struck a 

 number of tiny blows by the string in its movements. 



Experiment 59. Attach a pith ball or a light 

 glass ball to a string and hold it near a tuning fork 

 that has just been struck (Figure 188). The ball is 

 set in motion by the vibration of the fork. 



If we hold a piece of metal near a large bell 

 that has just been struck, we find that the 

 metal in the bell is in violent vibration. In 

 short we find that where a sound is pro- 

 duced, matter has been set in motion. It 

 FIG. 188. ma y b e the falling of a tree, the collision of 

 two bodies, the firing of a shot, or the blowing of a whistle ; 

 but an examination will show in every case that vibrat- 

 ing matter of some sort has caused the 

 sound. ., 



Experiment 60. Sound Waves. Suspend 

 a small electric bell in the receiver of a vacuum 

 pump (Figure 189). If we set the bell to ring- 

 ing and pump out the air, we find that the 

 sound becomes fainter as the vacuum becomes 

 greater. Now admit the air slowly and note the 

 increase in sound. FIG. 189. 



From this experiment it will seem a logical conclusion 

 that sound will not pass through a vacuum and that air 



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