SOUND AND HEARING 417 



bottle. Shake the bottle so that the bell can be seen to ring but, 

 does not strike the sides of the bottle. Can the sound be heard 

 distinctly ? 



Experiment 137. Suspend a pith ball by a light thread so that 

 it may swing freely. Strike a tuning fork and quickly place it in 

 very light contact with the pith ball. The ball will be set in mo- 

 tion by the vibrations of the tuning fork. 



In Experiment 136 it was found that if the air was ex- 

 hausted and the bell did not touch the sides of the bottle, 

 almost no sound was heard when the clapper of the bell 

 showed that the bell was ringing. This shows that the sounds 

 we usually hear are transmitted in some way by the aid of 

 the air. In Experiment 137 the sounding 

 body was seen to be vibrating. Since 

 these vibrations set the pith ball moving, 

 we may understand that the air surround- 

 ing the tuning fork must also have been 

 set in motion. 



Sound has been found to be a wave 

 motion in a material medium. If a 

 scratch is made on the end of a long log, 

 it can be heard if the ear is placed at the other end of the 

 log, when it cannot be heard if the ear is away from the log. 

 In this case the medium is the wood. 



If a stone is dropped into a quiet pond, the rippling waves 

 developed will extend often to the farthest shore of the pond, 

 but a chip floating near where the stone fell will not be moved 

 from its position except up and down. Thus the waves 

 traveled outward from the point of origin, but there was no 

 outward movement of the water. If a long rope, attached 

 at one end and held in a horizontal position, is suddenly 

 struck with a stick, a wave motion will travel along the 



