SOUND. 595 



elastic membrane. These vibrations, being communicated to the 

 air, are conveyed by it to our nerve-endings, where they set up a 

 nerve-impulse. The impulse is transmitted along the nerve to 

 the brain, and there gives rise to the sensation with which we are 

 familiar as sound. 



The vibrations of the air are wave-like movements depending 

 upon a series of changes of density in the gases, the particles of 

 which move towards or from one another, and transmit the mo- 

 tion to their neighbors, so as to propagate the sound wave. To 

 demonstrate these vibrations a special apparatus must be used. 



When a tuning fork is struck it is thrown into vibration, and 

 a sound is given forth. But the vibrations are often so rapid and 

 so small that the motion of the tuning fork cannot be appreciated 

 by the eye. But if a fine point be attached to one prong of the 

 tuning fork or, indeed, any elastic body, such as a bar of metal 

 and this point be brought into contact with a moving smoked sur- 

 face, such as has been already described for similar records, a little 

 wavy line is drawn, showing that the vibrating fork moves up and 

 down at an even and regular rate. Each up and down stroke 

 indicates a vibration. The length of the wave, as drawn on the 

 evenly-moving surface of the recorder, shows the amount of time 

 occupied by each vibration. This is always found to be the same, 

 for a tuning fork of a given pitch, and thus the recording fork is 

 in constant use by the physiologist as an exact measure of small 

 intervals of time. The pitch of the note, then, depends upon the 

 rate or period of vibration, a note or tone of a certain pitch being 

 simply a sound caused by so many vibrations per second. The 

 quicker the vibration the higher the note, and the slower the 

 deeper, until, at the rate of about thirty per second, no sound is 

 any longer audible. Whether a note be produced by a metal 

 fork, a tense string, or any other vibrating body, if the number 

 of vibrations per second be the same, the note must have the same 

 pitch. 



The elevation of each vibration as seen in the tracing made by 

 a recording fork is different at different times. When the fork 

 is first struck the waves are high and well marked, and the ex- 

 cursions of the recording prong can be seen to become less and 



