EXPLANATION OF SOUND 95 



largely caused in the air itself, as when a gun is fired 

 or we clap our hands ; often the motion is so great that 

 we lose sight of the little vibrations, as when a load of 

 rocks is dumped. In some cases the motion may be 

 felt even if we cannot see it. 



Experiment 82. Hold a pan in the hand and strike it. Can 

 the vibration be felt ? Strike several sharp blows on an iron bar 

 held in the hand. Do you feel a tingling sensation ? Blow a horn 

 and touch it lightly with the finger. Sometimes the sound waves 

 from a distant blast or a heavy cannon will shake the windows 

 which they strike. 



At least we must understand that sound occurs only 

 in the ear. However a vibrating body may arouse sound 

 waves in the air, it is still nothing but a vibrating body. 

 There is no noise in a gun which is fired, and no sound 

 in a piano nothing but motion. To be sure, we are 

 made aware of the motion because of the sound that it 

 produces in the ear ; but the source of this sensation is 

 still only a body in vibrating motion. 



QUESTIONS 



1. What two sorts of motion are there ? What is a wave? 



2. What is a vibration? Explain how each particle moves 

 during one complete vibration. 



3. Define a medium. Define wave length and rate of vibration. 



4. Define sound. Of what use is this sensation to us? 



5. What are sound waves ? What is the first cause of waves 

 in the air? Are all of these waves alike? Are they all sound 

 waves ? 



6. Show how a vibrating fork may cause waves in the atmos- 

 phere. What property of air makes it a good medium to carry 

 sound waves? 



