SECT, xvi.] THEORY OF SOUND. 143 



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 Reflection of Sound Echoes 

 Thunder Refraction of Sound Interference of Sounds. 



ONE of the most important uses of the atmosphere is the 

 conveyance of sound. Without the air, deathlike silence 

 would prevail through nature, for in common with all sub- 

 stances 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 inaudible, 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 trans- 

 mission 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 propagation 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. 



