SOUND. 121 



a quantity of hydrogen gas, and then speak, his 

 voice is scarcely audible. 



The loudness of sounds becomes smaller in pro- 

 portion as they come from a greater distance. 

 This enables us to judge of the distance of objects, 

 in some degree at least, by the sounds which 

 proceed from them. Moreover it is found that 

 we can judge of the position of objects by the 

 ear: and this judgment seems to be formed by 

 comparing the loudness of the impression of the 

 same sound on the two ears and two sides of the 

 head.* 



The loudness of sounds appears to depend on 

 the extent of vibration of the particles of air, and 

 this is determined by the vibrations of the sound- 

 ing body. 



3. The pitch, or the differences of acute and 

 grave, in sounds, form another important pro- 

 perty, and one which fits them for a great part 

 of their purposes. By the association of different 

 notes, we have all the results of melody and har- 

 mony in musical sound ; and of intonation and 

 modulation of the voice, of accent, cadence, em- 

 phasis, expression, passion, in speech. The song 

 of birds, which is one of their principal modes of 

 communication, depends chiefly for its distinc- 

 tions and its significance upon the combinations 

 of acute and grave. 



* Mr. Gough in Manch. Mem. vol. 



