ACOUSTICS. 49 



ACOUSTICS. 



Sound defined and explained Air chief conductor of Sound Intensity of Sound 

 depending on the density of the media Cause of the vibrations of a sounding 

 body Velocity of Sound Conductors of Sound Echo Speaking and Hear- 

 ing TVumpets Invisible Girl, &c. 



THIS science treats of the nature, laws, and pheno- 

 mena of sound. Sound has been denned as a sensation 

 of the mind communicated by the ear, or it is the effect 

 of some external collision of bodies which produces a tre- 

 mulous motion or vibration, and which is communicated 

 to the mind by means of the ear. It is generally under- 

 stood that the surrounding air or atmosphere is the me- 

 dium of sound : but air is not the sole conductor of sound ; 

 fluids in general, and solids of all kinds, will transmit it, 

 though not to a like extent.* 



That air is the chief conductor cannot but be acknow- 

 ledged ; and that without that or some other fluid, as a 

 medium, no sound would be heard, is evident from the 

 fact of a bell rung under an exhausted receiver being 

 inaudible, and becoming gradually audible as the air is 

 admitted. 



The intensity of sound is found to be in proportion to 

 the density of the air; so that it has been ascertained by 

 experiment that sound can be heard half as far again in 

 carbonic acid gas as in common atmospheric air, while 

 in hydrogen gas it can scarcely be heard at all. On the 

 tops of high mountains the voice is considerably less 

 audible than in valleys; as also the report of a gun is 

 much less in strength and more acute in tone. 



Sound appears to be communicated to the air in cir- 

 cular undulations, similar to the small waves produced 

 on the surface of water when a stone is thrown into it ; 

 and this is evident to the ear in the tones of a church- 

 bell while its sounds are dying away. 



When a sonorous body is struck it becomes in a state 



* It has been supposed that there is some subtle fluid, probably of 

 n electrical nature, in the composition of bodies whose office is to 

 transmit sound ; and that bodies transmit sound in proportion to the 

 quantity of this fluid contained in them. 



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