Sound 171 



jar and notice that the sound gradually dies away. Let the air 

 into the jar again and notice that the sound is again heard and 

 continues to grow louder. 



A similar experiment may be performed by using an alarm 

 clock set to ring while under the bell jar. 



These experiments show that sound is transmitted by the air. 

 Air, however, is not the only substance that will transmit sound. 

 Have someone stand at the farther end of a long table and scratch 

 it lightly with a pin. You cannot hear it. But if you place your 

 ear against the table you can hear it readily because the wood 

 carries the sound to you. This is an example of sound being 

 carried by another substance than the air. Sound may be trans- 

 mitted by any gas, liquid, or solid. Remember that both matter 

 and energy are needed to produce sound and to carry it to us. 



Speed of Sound. We all know that sound requires time to 

 travel to our ears. Perhaps you have watched a distant engine 

 when its whistle was blown. If so, you noticed that the steam 

 rushed from the whistle an instant before you heard the sound, 

 and you continued to hear the sound for an instant after the 

 steam stopped rising from the whistle. Perhaps, also, you have 

 watched a carpenter in the distance driving a nail, and have noticed 

 that his hammer was raised for another blow before you heard 

 the sound of the preceding one. 



Experiments to Show the Speed of Sound. To show the 

 speed of sound, have one person pound something uniformly. 

 As you watch him, move away as rapidly as possible. Notice that 

 the interval that elapses between the time when you see the 

 hammer strike and the time when you hear the sound of the 

 stroke increases with the distance. If you continue moving away, 

 you will finally reach a point where the hammer will appear 

 to strike at exactly the same time you hear a report. This is 

 because time enough elapses between the blow and the report 

 for the hammer to be raised and brought down a second time. 

 If the person stops pounding, notice that you receive one report 

 after the pounding has ceased. If you time the blows struck by 

 the hammer, and then measure your distance from this point to 

 the hammerer, you have the time needed for the sound to 



