i8o 



Common Science 



as it turns is like the soft wax cylinder. The glass needle 

 is like the recording needle of a phonograph. The 

 chief difference is that you have struck the tuning fork 

 to make it and the needle vibrate, instead of making 

 it vibrate by air waves set in motion by your talking. 

 It is because these vibrations of the tuning fork are 

 more powerful and larger than are those of the recording 

 needle of a phonograph that you can see the record on 

 the recording drum, while you cannot see it clearly on 

 the phonograph cylinder. 



In all ordinary circumstances, sound is the vibration 

 of air. But in swimming we can hear with our ears 



FIG. ioo. How the apparatus is set up. 



