326 NATURE OF SOUND. 



ACOUSTICS. 



32. Nature and Propagation of Sound. The phe 

 nomena of Sound, which form the subject of study ii 

 the science of Acoustics, are classed together under on 

 name, in the first instance because they are perceive* 

 by us through one particular organ of sense the Eai 

 The primary meaning of the term sound may accord 

 ingly be defined as any external action capable of ex 

 citing in us the sensation of hearing. When, howevei 

 those actions, which we perceive as sound, are examine 

 as to their physical nature, it is found that they all COT 

 sist essentially in motion. In many cases this is easil 

 recognisable by the touch ; thus, for example, whe 

 sound is produced by a piano, or a violin, or a tuning 

 fork, a vibratory motion may be felt in some parts of tt, 

 sounding bodies. These vibrations are not accidenta 

 if they are prevented by mechanical means, the soun 

 ceases. If the vibrating strings of the piano, or those < 

 the violin, or the prongs of the tuning-fork, be touche 

 with the fingers, the sound is immediately stopped. 



In order that a sounding body may be heard it is n< 

 sufficient for it to perform appropriate movements ; it 

 necessary that these movements should be imparted ' 

 the ear. The movements of sounding bodies are pr 

 pagated in most cases by the air, sometimes also, bii 

 much less frequently, by liquid or solid bodies. Tl 



