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THE POPULAR SCIENCE MONTHLY. 



much louder than any you have ever before heard from the fork alone. 

 This is because the air in the tumbler is set in vibration, and adds the 

 vibrations of its mass to those of the fork. That this is so you may 

 prove for yourself by the following experiment : 



Experiment ^Jf. — Being careful not to move the glass plate from 

 its present position (Experiment 43), stick it with wax to the tumbler. 

 Pour a little silica into the tumbler, and then hold it horizontally, and 

 vibrate the fork near its opening, observing attentively how the silica- 

 powder is acted on by the inclosed vibrating air. 



Fig. 27. 



Experiment J/.5. — Take a piece of thin linen paper about four and 

 a half inches square, and having wetted it paste it over the mouth of 

 the tumbler. When the paper has dried it will be stretched tightly. 

 Take a sharp penknife and carefully cut away the paper so as to make 

 an opening as shown at J5 in Fig. 27. Make this opening small at 

 first, and very gradually make it larger and larger. Hold the fork 

 over the opening after each increase in its size, and you will soon dis- 

 cover the size of the opening which causes the air inclosed in the tum- 

 bler to vibrate with the fork, and thus greatly to strengthen its sound. 

 You have now a mass of air in tune with the fork, and inclosed in a 

 vessel which has one of its walls formed of a piece of elastic paper. 

 With this instrument, which I have invented for you, you must make 

 some charming experiments. 



Experiment Jf6. — If the air in the tumbler vibrates to the A-fork, 

 it will, of course, vibrate to the A-pipe, which gives the same note as 

 the fork. Scatter some sand on the paper, and then sound the A-pipe 

 a foot or two from it. The sand dances vigorously about, and ends by 

 arranging itself in a nodal line parallel to the edges of the paper, in 



