I 7 6 



Common Science 



FIG. 96. An interesting experiment in sound. 



them vibrate? Tap it again, and hold the fork close to 

 your ear. Can you hear the sound ? 



The experiment which follows will show that we usually 

 must have air to do the vibrating to carry the sound. 



Experiment 56. Make a pad of not less than a dozen 

 thicknesses of soft cloth so that you can stand an alarm 

 clock on it on the plate of the air pump. The pad is to 

 keep the vibrations of the alarm from making the plate 

 vibrate. A still better way would be to set a tripod on the 

 plate of the air pump and to suspend the alarm clock from 

 the tripod by a rubber band. Set the alarm so that it 

 will ring in 3 or 4 minutes, put it under the bell jar, and 

 pump out the air. Before the alarm goes off, be sure that 

 the air is almost completely pumped out of the jar. Can 

 you hear the bell ring? Distinguish between a dull trilling 



