ACOUSTICS. 69 



breadth of the fork, a quarter of an inch thick, and half 

 an inch broad, this to be cemented to the end of the 

 fork. The fork will not vibrate at all with this attach- 

 ment at first ; but if a thin wedge is cut out from each 

 side of the rubber, until it moves very freely, the vibra- 

 tions of the fork will not be much interfered with ; at 

 the same time the amplitude of the vibrations will be 

 much increased. 



When the mirror is fastened to each fork, it will be 

 advisable to determine their pitch, which may be done 

 by comparing them with a properly-tuned piano, organ, 

 or another tuning-fork with known pitch. 



EXPERIMENTS WITH THE FORKS. 



/ The Sinuous Line. Cut off most of the light from 

 the lantern or porte lumiere with a diaphragm, so that 

 the beam is not more than an inch in diameter and 

 consists of parallel rays. Adjust the fork so that it 



J?ig. 45. 



will vibrate perpendicularly, and place it so that the 

 beam of light will fall upon the mirror at its end. 

 This should be again reflected to the screen by a mir- 

 ror m held in the hands, to swing the beam around the 

 room. When the fork is made to vibrate by striking it 

 with a small billet of wood, if the mirror m is held still, 



