COMPOSITION OF SOUND WAVES. 283 



1128 feet, find the corresponding numbers of vibrations of vocal 

 chords. 



11. A person stands before a cliff and claps his hands. In f of a 

 second he hears the echo. How far distant was the cliff ? 



Recapitulation. In this section we have considered 

 the Definition of sound; Undulations; the Pe- 

 riod, Length and Amplitude of waves ; the 

 Cause and Propagation of sound ; sound Waves 

 and Media ; the Velocity in air and in other media, 

 and the effect of Temperature; the difference between 

 Noise and Music ; the Three Elements of 

 musical sounds ; the relation of Loudness to ampli- 

 tude and distance; Acoustic Tubes; the cause of 

 differences in Pitch ; the relation that exists between 

 Pitch and Period, or Wave Length ; Re- 

 fraction and Foci of sound; Reflection of sound; 

 Echoes. 



./ 



COMPOSITION OF SOUND WAVES; MUSICAL 

 INSTRUMENTS. 



443. Sympathetic Vibrations. The string of a 

 violin may be made to vibrate audibly by sounding near 

 it a tuning-fork of the same tone. By prolonging a vocal 

 tone near a piano, one of the wires seems to take up the 

 note and give it back of its own accord. If the tone be 

 changed, another wire will give it back. In each case, 

 that wire is excited to audible action, which is able to 



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