Sounds we cannot Hear. 415 



quite forty-eight thousand vibrations in the same time.* As 

 there is some reason to suppose that those persons who can 

 best distinguish very acute, are least able to hear very grave 

 notes, we may probably assume that a range of about twelve 

 octaves, as expressed in musical notation, includes the hearing 

 capacity of human ears ; to all vibrations of greater or less 

 rapidity, such ears are absolutely deaf. To produce this 

 range of sound on an organ would require pipes varying in 

 length from 140 feet to about the third of an inch. 



No reason whatever can be assigned why vibrations not 

 included in such a scale should not, though quite inaudible to 

 us, be distinguished as sounds by ears adapted for the purpose, 

 and whose range of hearing, though perhaps not more exten- 

 sive than our own, embraces a different series of notes. This 

 is rendered more probable by the fact that the ordinary voices 

 of many small animals do but just come within the range of 

 the human ear. 



To many persons the voice of the common shrew-mouse is 

 inaudible ; some cannot hear the sound of the cricket ; and a 

 few are deaf to the chirp of the' house-sparrow. Very few 

 persons are conscious of the scream of the common bat. 

 Though well enough acquainted with that animal, I can hear 

 no sound uttered by it. I have seen a bat scream when I have 

 held it in my hand — that is, I judged from its struggles, and 

 the action of the jaws and tongue that it was shrieking — but 

 I could hear no sound whatever. Yet a friend, who hears 

 more acute notes than I can, tells me that bats are very noisy 

 little creatures. 



The average tone of the human voice is not far from holding 

 the middle place in the range of sound audible to the human 

 ear — being about six octaves above the lowest, and six octaves 

 below the most acute sounds distinguishable. Should the 

 range of hearing of the bat be as extensive as our own, and, as 

 is likely, hold about the same relative position with regard to 

 its voice as ours does to the human voice, it would follow that 

 sounds would be audible to the bat which were six octaves 

 higher in tone than the most acute audible to us. Such 

 sounds would require two and half millions of vibrations in the 

 second to produce them. 



The voice of the bat is probably the shrillest sound audible 

 to human ears, consequently all animals whose voices are still 

 more acute are, as far as our senses are concerned, completely 

 mute. It does not, however, at all follow that their muteness 



* M. Despretz gives thirty-two vibrations in a second as the limit of deep 

 sounds ; and 73,700 for acute sounds. — See Ganot's Physics, by Atkinson. 

 Savart gives seven to eight vibrations in the second as the lowest limit of 

 audibility when the sound is very loud. — Vide Lardners Handbook, "Acoustics.'* 



