SECT, xvi.] EXTENT OF HEAKING. 147 



ceives vibrations of the same nature indeed as those which 

 constitute our ordinary sounds, but so remote that the 

 animals who perceive them may be said to possess another 

 sense, agreeing with our own solely in the medium by which 

 it is excited." 



M. Savart, so well known for the number and beauty of his 

 researches in acoustics, has proved that a high note of a given 

 intensity, being heard by some ears and not by others, must 

 not be attributed to its pitch, but to its feebleness. His expe- 

 riments, and those more recently made by Professor Wheat- 

 stone, show that, if the pulses could be rendered sufficiently 

 powerful, it would be difficult to fix a limit to human hearing 

 at either end of the scale. M. Savart had a wheel made 

 about nine inches in diameter with 360 teeth set at equal 

 distances round its rim, so that while in motion each tooth 

 successively hit on a piece of card. The tone increased in 

 pitch with the rapidity of the rotation, and was very pure 

 when the number of strokes did not exceed three or four 

 thousand in a second, but beyond that it became feeble and 

 indistinct. With a wheel of a larger size a much higher tone 

 could be obtained., because, the teeth being wider apart, the 

 blows were more intense and more separated from one an- 

 other. With 720 teeth on a wheel thirty-two inches in 

 diameter, the sound produced by 12,000 strokes in a second 

 was audible, which corresponds to 24,000 vibrations of a 

 musical chord. So that the human ear can appreciate a sound 

 which only lasts the 24,000th part of a second. This note 

 was distinctly heard by M. Savart and by several people 

 who were present, which convinced him that with another 

 apparatus still more acute sounds might be rendered audible. 



For the deep tones M. Savart employed a bar of iron, two 

 feet eight inches long, about two inches broad, and half an 

 inch in thickness, which revolved about its centre as if its 

 arms were the spokes of a wheel. When such a machine ro- 

 tates, it impresses a motion on the air similar to its own, and, 

 when a thin board or card is brought close to its extremities, 



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