* 



') i' 

 The Senses of Animals. 261 



Hearing is a telassthetic sense. Through it we become 

 aware of certain vibratory states of more or less distant 

 objects. The vibrations of these bodies are transferred to 

 the air or other medium surrounding the body, and are 

 transmitted through the air or other medium to the ear. 

 The sound-waves traverse the air at a rate of 337 metres 

 (1106 feet) in a second ; but they travel about four times 

 as fast in water. If the vibration is periodic or regular, 

 the sound is called a tone ; non-periodic or irregular sounds 

 are noises. The pitch of a tone is determined by the 

 number of vibrations in a second. The lowest or gravest 

 tone most of us can hear is that where there are about 30 

 vibrations in a second ; twice this number give us a tone 

 of an octave higher ; twice this again, another octave ; and 

 so on. In musical composition, tones from about 40 to 

 about 4000 vibrations per second are employed. This is 

 a range of somewhat over six octaves. But many of us 

 are capable of hearing sounds over a range of about ten 

 octaves, that is to say, from 30 to 30,000 vibrations per 

 second. The upper limit of hearing is, however, very 

 variable. Some people are deaf to tones of more than 

 15,000 or 20,000 vibrations per second.* Others may hear 

 shrill tones of 40,000, or even in rare cases 50,000. I 

 could as a boy hear the shrill squeak of a bat ; now I am 

 quite deaf to it. A friend of mine in South Africa was 

 unable to hear the piping of the frogs in the pond, which 

 was to me so loud as almost to drown the tones of his 

 voice. 



Apart from the pitch of a note is its quality. The 

 same note struck on different instruments or sung by 

 different persons has a different ring. This is determined 

 by the number and intensity of overtones, or partials, which 

 are associated with the fundamental tone. Suppose the 

 deep fundamental tone of 33 vibrations be sounded ; with it 

 there may be associated overtones, eight or nine in number, 

 all of which are simple multiples (twice, thrice, four times, 



* A friend of mine informs me that his limit is about 17,500 per second, 

 20,000 being quite inaudible. 



