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some body which is capable of vibrating; for instance, in 

 an absolute void there could be no sound, and a very 

 slight investigation shows that unless some body, be it solid, 

 fluid, or gaseous, be present, sound, if even it might be 

 produced, cannot propagate itself. 



As it appears of much theoretical importance to ascertain 

 whether sound can be produced without primary vibration, 

 that is, vibration generating the sound, disregarding for 

 the time the question whether sound is transmitted by 

 vibrations, it is important to notice an instrument of French 

 invention, called the Syrene, in which streams of air acting 

 upon a disc, pierced with twenty-five small holes, arranged 

 in a circle and drilled obliquely, cause the disc to revolve, 

 so that in revolving the disc acts as a valve, alternately 

 closing and opening the apertures twenty-five times in each 

 revolution, and the air passing through the disc when 

 revolving, rapidly produces a musical tone, which appears 

 to be produced by the interrupted impulses of the air, and 

 not by vibration. 



As nearly all other phenomena of sound are the effects 

 of vibration of elastic bodies, it will be advantaofeous to 

 pay close attention to some of the properties of elasticity. 

 All known bodies yield or are compressed in a slight 

 degree, when they are struck by or impinge against 

 another hard body ; and the bodies which are termed 

 elastic, after such compression, return to their original 

 shape ; but other bodies have the property of elasticity 

 only in a very low degree. 



Elasticity may be defined to be the property which enables 

 the particles of a body which have been slightly disturbed 

 to return to their original positions. But we are ignorant 

 of even the more approximate causes of elasticity, and we 

 shall remain in ignorance of many similar and apparently 

 very simple properties of bodies, until the intimate con- 



s 



