SOUNDING BODIES 



275 



it to make a faint sound. When a piano is being played, we are 

 often startled to find that a window pane or an ornament re- 

 sponds to some note of the piano. If two tuning forks of ex- 

 actly identical periods (that is, of the same frequency) are 

 placed on a table as in 

 Figure 174, and one is 

 struck so as to give forth 

 a clear sound, the second 

 fork will likewise vibrate, 

 even though the two 

 forks may be separated 

 by several feet of air. 

 We can readily see that 

 the second fork is in 

 motion, although it has 

 not been struck, because 

 it will set in motion a pith ball suspended beside it; at first 

 the pith ball does not move, then it moves slightly, and 

 finally bounces rapidly back and forth. If the periods 

 of the two forks are not identical, but differ in the slight- 

 est degree, the second fork will not respond to the first 

 fork, no matter how long or how loud the sound of the first 

 fork. If we suppose that the fork vibrates 256 times each 

 second, then 256 gentle pulses of air are produced each 

 second, and these, traveling outward through the air, reach 

 the silent fork and tend to set it in motion. A single pulse 

 of air could not move the solid, heavy prongs, but the accu- 

 mulated action of 256 vibrations per second soon makes itself 

 felt, and the second fork begins to vibrate, at first gently, 

 then gradually stronger, and finally an audible tone is given 

 forth. 



The cumulative power of feeble forces acting frequently at 

 definite intervals is seen in many ways in everyday life. A 



