184 SENSE OF HEARING. 



afforded to the mind by this sense, it is necessary to enter into a brief 

 physical disquisition on sound. 



2. SOUND. 



If a body, by percussion or otherwise, be thrown into vibration, 

 every vibration excites a corresponding wave in the air ; and these 

 oscillations are propagated in all directions, until gradually lost in dis- 

 tance; but if they strike on the organ of hearing with the necessary 

 force, a sensation is produced, which is called sound or noise. The 

 term, however, is frequently used to signify, not only the sensation, 

 but the affection of the air, or of the sonorous body by which the 

 sensation is effected. 



That bodies move or oscillate when they produce sound admits of 

 easy detection. We can see it in drums, bells, musical strings, &c., 

 whose vibrations are extensive ; and can arrest them, and with them 

 the sound, by putting the hand upon the body, or muffling it. When- 

 ever a sonorous body is struck, a change in the relative position of its 

 molecules is produced. These, by virtue of their elasticity, tend to 

 return to their former place. This is done by a series of oscillations, 

 which are, at first, more extensive; but become gradually less, until 

 they finally cease. The rapidity of these oscillations is greater in bodies 

 that are hard and elastic ; and hence it has been concluded, that these 

 two qualities render a body sonorous. It is not, however, a matter of 

 facility to say, what is the precise cause of the difference of sound in 

 analogous bodies. It must be dependent upon intimate composition, but 

 of what nature is not intelligible to us. There are but one or two 

 individuals in Great Britain, who have been celebrated for the fabrica- 

 tion of the larger order of bells for churches, colleges, &c., and in 

 certain countries the art is comparatively unknown. Resonance is 

 entirely owing to the intimate composition of the body, and is beauti- 

 fully and singularly exhibited in the Chinese gong, the sound of which 

 continues to rise for some time after a succession of rapid and forcible 

 blows has been inflicted. 



But, in order that sonorous oscillations may affect the organ of sense, 

 an intermediate body is necessary to repeat and transmit them. This 

 body is called the vehicle of sound, and it is usually air. M. De Lamarck 

 supposes the existence, in the atmosphere, of a vibrative fluid of great 

 subtilty, which pervades the globe as well as the bodies on its surface ; 

 and Geoffroy St. Hilaire affirms, that sound " is a matter resulting from 

 the combination of the external air, with the polarized air of the sono- 

 rous body!" but these are topics that belong to works on higher 

 physics-. 



Air, by virtue of its elasticity, is admirably adapted as a vehicle for 

 sound ; and the loudness of the sound conveyed by it is greatly de- 

 pendent upon its density. If we put a bell under the receiver of an 

 air-pump, and exhaust the air, the sound becomes gradually more and 

 more faint, and when the air is exhausted is not heard at all. For the 

 same reason a pistol fired on the top of the Himala Mountains gives a 

 much feebler report than in the valleys beneath. 



Sympathetic sounds afford additional evidences of the carrying power 



