$8 THE ART OF PROJECTING. 



ACOUSTICS. 



THE TUNING-FORK. 



The vibrations of an ordinary tuning-fork may be ex- 

 hibited in the following way. Having made the fork to 

 vibrate, hold it at a in the divergent beam (Fig. 21), 

 and swing it in its plane of vibration at right angles to 

 the beam of light. Its shadow will present a curious, 

 fan-like appearance. If the fork is polished it will re- 

 flect enough light to exhibit the same appearance when 

 looked at while vibrating and swinging. 



Another way is to hang light pith or cork balls so 

 they just touch the fork, or other sounding body, and 

 project the ball in any convenient way. As soon as the 

 body begins to vibrate it will drive the ball away from 

 it. Two forks in unison may be used in this way, to 

 show sympathetic vibration. Hang a cork ball half an 

 inch in diameter so it will just touch the side of one of 

 the forks near the end, and project the ball and fork. 

 At some distance set the other fork to vibrating, and 

 put it upon its resonant case, or place the stem upon 

 the floor or some resonant surface. The ball will be at 

 once thrown off from the first fork, showing that it has 

 been set vibrating. 



Professor Mayer has described a number of interest- 

 ing experiments to illustrate the change of wave-lengths 

 by the motion of translation of the sounding body, in 

 the American journal of Science, April, 1872. 



THE KALEIDOPHONE. 



To the end of a piece of steel wire, two or three feet 

 long, and an eighth of an inch in diameter, <? /(Fig. 37), 



