323 SOUND. 



If a person should have a bar of steel vibrating at the 

 rate of twelve and a half times a second, a very low hum 

 would be heard, so low that it would scarcely be perceiv- 

 ed that it was a sound. Then if the experimenter should 

 have another bar of steel, so adjusted that it would 

 vibrate, twice as fast, that is, at the rate of tioentyjive 

 times per second, it would evidently produce two vibra- 

 tions to every one of the first bar. Thus the bars would 

 correspond at every other vibration, and no two bars can 

 correspond more closely. 



A person might for a moment think that if one vibrates 

 thirteen times, when the other vibrates twelve and a half, 

 they will coincide more nearly. But in this case, they 

 would only coincide at the thirteenth vibration, whereas 

 if one vibrates twentyjive, while the other does twelve and 

 a half, they correspond every second beat. Now it is 

 found that the two sounds produced by two bars, vibra- 

 ting one twice as fast, as the other, unite more completely 

 and more pleasantly, than any other sound ; and one of 

 these is called the octave of the other. 



Fifty vibrations in a second would produce a sound 

 an octavs above the one vibrating twentyfive times, and 

 one hundred the octave above that. The next would be 

 200 in a second , the next 400 ; then 800 ; then 1600; 

 then 3200; and last 6400. This makes in all nine 

 octaves, in which is included the whole compass of 

 musical sound. Beyond that number the sound becomes 

 inaudible. 



These octave distances are subdivided ; that is, be- 

 tween every two of these sounds a multitude of others 

 may be made. The whole number of possible sounds, 

 therefore, is immensely great, but all must be comprised 

 between the first and last of the sounds above described. 

 The first, produced by twelve and a half vibrations being 

 a low hum scarcely distinguishable as a sound, and the 

 others increasing in shrillness until the last, produced 

 by 6400 vibrations in a second is almost lost to the ear. 

 The human voice produces only a small portion of these 

 possible sounds, perhaps those from one hundred to eight 

 vibrations in a second. 



