132 P.ITCH HEARING. SECT. XVI. 



acute. The lowest man's voice makes 396 vibrations in a second, 

 whilst the highest woman's voice makes 2112. Very deep tones 

 are not heard by all alike, and Dr. Wollaston, who made a variety 

 of experiments on the sense of hearing, found that many people, 

 though not at all deaf, are quite insensible to the cry of the bat 

 or the cricket, while to others it is painfully shrill. From his 

 experiments he concluded that human hearing is limited to about 

 nine octaves, extending from the lowest note of the organ to the 

 highest known cry of insects ; and he observes with his usual 

 originality that, " as there is nothing in the nature of the atmos- 

 phere to prevent the existence of vibrations incomparably more 

 frequent than any of which we are conscious, we may imagine 

 that animals like the Grylli, whose powers appear to commence 

 nearly where ours terminate, may have the faculty of hearing 

 still sharper sounds which we do not know to exist, and that 

 there may be other insects hearing nothing in common with 

 us, but endowed with a power of exciting, and a sense which 

 perceives vibrations, of the same nature indeed as those which 

 constitute our ordinary sounds, but so remote that the animals 

 which 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 experiments, 

 and those more recently made by Professor Wheatstone, 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 another. 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 fa 24,000 vibrations of a musical 



