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OUR PHYSICAL WORLD 



tubes of equal length each closed at one end, the one that has 

 the greater diameter will give out the lower note when one blows 

 across the open end. The pipes on the organ that produce the 

 bass notes are long and of large diameter, while those for the 

 high notes are short and have a small bore. So the wind blow- 

 ing over the opening at the top of the chimney produces a deep 

 note, and we say the chimney roars. This is due to the fact 

 that the pitch of the note emitted by a vibrating string or air 

 column depends on the rate of vibration. The shorter the 

 string or column of air, other things being equal, the more 

 rapidly it vibrates and the higher the note emitted; the greater 



FIG. 171. A fife, showing change of length of air column 



the caliber of the string or tube, the less rapid the vibration. A 

 long or thick string or column of air means a greater mass, and 

 the greater the mass, the less rapidly it swings into motion. 



More than that, a note of a given pitch is always produced 

 by exactly the same number of vibrations. Thus the piano is 

 tuned so that middle C is given off by a string vibrating at the 

 rate of 256 vibrations per second. The C note one octave higher 

 is produced by double the number of vibrations, and the one 

 an octave lower by half as many. Match on the piano the pitch 

 of a mosquito's or bee's hum and you can tell how many times 

 per second the insect's wings are beating the air, for you can calcu- 

 late the number of vibrations for any musical note. 



