547 



CHAP. XXV. 



HEARING. 



BY Hearing we are able to appreciate the vibratory motions 

 of elastic bodies, when their frequency is within certain limits. 

 Some experiments by Dr. Wollaston prove that these limits 

 vary in different individuals ; but the average extent of the scale 

 of sounds perceptible to the human ear has been estimated to 

 be between 30 and 12,000 vibrations of the sonorous body per 

 second. 



The undulations to which these vibrations give rise may be 

 transmitted through any substance, either aeriform, liquid, or 

 solid : but the air is the ordinary medium by which they reach 

 the ear. The velocity of transmission depends on the specific 

 elasticity of the substance ; according to the latest experiments, 

 sound travels through air at the rate of about 1142 feet per 

 second. 



With regard to the sensation of sound, three independent qua- 

 lities must be distinguished : a 



1st. The tune, or pitch ; which depends on the frequency with 

 which the vibrations succeed each other. 



2d. The loudness, or intensity; which is determined by the 

 amplitudes of the vibrations. 



3d. The timbre: For this word, adopted in France to express 

 the specific differences of sound which are not comprehended in 

 any of the preceding definitions, there is no analogous term in 

 our language ; nor have we at present the least idea of the true 

 causes of these modifications of sound. In some cases the inde- 

 finite expression quality of tone is employed. 



When two or more sounds are heard simultaneously, or succes* 

 sively, the mind by a peculiar faculty perceives the relative fre- 



8 C. Wheatstone, Experiments on Sound. Annals of Philosophy. New Series, 

 vol. vi. p. 81. 



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