THE SIX GATEWA YS OF KNOWLEDGE. 279 



a low note. If the period is gradually accelerated 

 you hear the low note gradually rising, becoming 

 higher and higher, more and more acute, and 

 if it gets up to 256 periods per second, we have 

 a certain note called C in the ordinary musical 

 notation. I believe I describe it correctly as the 

 low note C, of the tenor voice the gravest C that 

 can be made by a flute. The note of a two-foot 

 organ pipe open at both ends has 256 periods 

 per second. Go on higher and higher to 5 12 

 periods per second, and you have the C above 

 that the chief C of the soprano voice. Go 

 above that to 1,024, you get an octave higher. 

 You get an octave higher always by doubling 

 the number of vibrations per second, and if you 

 go on till you get up to about 5,000 or 6,000 or 

 10,000 periods per second, the note becomes so 

 shrill that it ceases to excite the human ear, and 

 you do not hear it any longer. The highest note 

 that can be perceived by the human ear seems 

 to be something like 10,000 periods per second. 

 I say " something like," because there is no very 

 definite limit. Some ears cease to hear a note 



