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CHAPTER lY. 



THE SENSE OF HEABING. 



The sensation of sound is due to vibrations of the air 

 striking on the drum of our ear. The intensity of the 

 sound depends on the extent or amplitude of the sound- 

 wave ; while the pitch of the tone depends on the fre- 

 quence of vibration, and consequently on the number of 

 waves which strike the ear during a given interval. 

 The fewer the number of vibrations in a second, the 

 deeper the sound ; the more numerous, the shriller it 

 becomes. Our pianos generally begin with the of 

 32 vibrations in a second, and extend to A'"' of 3520 

 vibrations. The number of vibrations for the tone 

 A', which is that of the hum of a bee, is about 440 

 in a second. If the vibrations are fewer than 30 in a 

 second, they produce only a buzzing and groaning 

 sound, while the shrillest sound we can hear is produced 

 by about 35,000 vibrations in a second. 



It may seem curious that there should be any dif- 

 ficulty in ascertaining whether an animal can hear. 

 But, in the first place, in order to experiment on 

 them, we are often obliged to place them in situa- 

 tions very unlike those to which they are accustomed ; 

 and, secondly, it is by no means always easy to say 



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