SECT. XVI. PROPAGATION OF SOUND. 129 





Library. 



SECTION XVI. 



Sound Propagation of Sound illustrated by a Field of Standing Corn 

 Nature of Waves Propagation of Sound through the Atmosphere 

 Intensity Noises A Musical Sound Quality Pitch Extent of 

 Human Hearing Velocity of Sound in Air, Water, and Solids 

 Causes of the Obstruction of Sound Law of its Intensity -r- Reflection 

 of Sound Echoes - Thunder Refraction of Sound Interference 

 of Sounds. 



ONE of the most important uses of the atmosphere is the con- 

 veyance of sound. Without the air, deathlike silence would 

 prevail through nature, for in common with all substances it has 

 a tendency to impart vibrations to bodies in contact with it. 

 Therefore undulations received by the air, whether it be from 

 a sudden impulse, such as an explosion or the vibrations of a 

 musical chord, are propagated in every direction, and produce 

 the sensation of sound upon the auditory nerves. A bell rung 

 under the exhausted receiver of an air-pump is inaiidible, which 

 shows that the atmosphere is really the medium of sound. In 

 the small undulations of deep water in a calm, the vibrations of 

 the liquid particles are made in the vertical plane, that is, up and 

 down, or at right angles to the direction of the transmission of 

 the waves. But the vibrations of the particles of air which 

 produce sound differ from these, being performed in the same 

 direction in which the waves of sound travel. The propagatio 

 of sound has been illustrated by a field of corn agitated by the 

 wind. However irregular the motion of the corn may seem on a 

 superficial view, it will be found, if the velocity of the wind be 

 constant, that the waves are all precisely similar and equal, 

 and that all are separated by equal intervals and move in equal 

 times. 



A sudden blast depresses each ear equally and successively in 

 the direction of the wind, but, in consequence of the elasticity of 

 the stalks and the force of the impulse, each ear not only rises 

 again as soon as the pressure is removed, but bends back nearly 

 as much in the contrary direction, and then continues to oscillate 



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