HEARING. 375 



chord or wire of a stringed musical instrument, as 

 well as its different lengths, determine the fre- 

 quency of its vibrations ; a greater tension, or a 

 shorter length, rendering them more frequent, and 

 consequently producing a higher note ; and on the 

 contrary, the note is rendered more grave by either 

 lessening the tension, or lengthening the chord or 

 wire. In a wind instrument, the pitch depends 

 chiefly on the length of the tube producing the 

 sound. 



There are, therefore, two principal qualities in 

 sound recognisable by the ear, namely, its loudness, 

 or intensity, and its pitch, or musical note ; the 

 former depending on the force of the vibrations ; 

 the latter, on their frequency. To these a third 

 may be added, namely, the tone, or the quality 

 called by the French " le timbre :" and which ap- 

 pears to depend on the greater or less abruptness 

 of the individual impulses. 



These acoustic principles are to be borne in mind 

 in studying the comparative physiology of hearing; 

 and since the functions of the different parts of the 

 organ of this sense are, as yet, but imperfectly 

 understood, I shall, in treating of this subject, de- 

 viate from the plan I have hitherto followed, and 

 premise an account of the structure of the ear in its 

 most highly developed state, as it appears to be in 

 Man. 



