190 WELLSS NATURAL PHILOSOPHY. 



,^ . . .,. 407. Air is the usual medium tliroush ■which 



What IS the . °. 



usual medium sound is coDveved to the ear. The vibrating' 



through which i i . i ... 



Bound is propa- bodv impErts to the au- m contact with it an 



gated f >/ ± 



undulatory, or wave-like movement, which, 

 propagating itself in every direction, reaches the ear, and 

 produces the sensation of sound. 



What are eon- 408. Vibrating bodies which are capable of 

 •reus bodies? ^^^^ imparting undulations to the air, are 

 termed sounding, or sonorous bodies. 



The aerial vibrations, or undulations thus caused, propagate themselves 

 from the center of disturbance in concentric circles, in the same way that 

 waves spread out upon the smooth surface of water. If such waves of water, 

 propagated from a center, encounter any obstruction, as a floating body, they 

 will bend their course round the sides of the obstacle, and spread out obUquely 

 beyond it. So the undulations of air, if interrupted in their progress by a 

 high wall or other similar impediment, will be continued over its summit and 

 propagated on the opposite side of it. 



In a sound-wave or imdulation of the air, as in a wave of water, there is 

 no permanent change of place among the particles, but simply an agitation, 

 or tremor, communicating from one particle to another, so that each particle, 

 like a pendulum which has been made to oscillate, recovers at length its 

 original position. 



This motion may be best illustrated by comparing it to the motion pro- 

 duced by the wind in a field of grain. The grassy waves travel visibly over 

 the field in the direction in which the wind blows ; but this appearance of 

 an object moving is only delusive. The only real motion is that of the heads 

 of the grain, each of which goes and returns as the stalk stoops or recovers 

 itself This motion affects successively a line of ears in the dfrection of the 

 wind, and affects simultaneously all the ears of which the elevation or de- 

 pression forms one visible wave. The elevations and depressions are propa- 

 gated in a constant direction, while the parts with which the space is filled 

 only vibrate to and fro. Of exactly such a nature is the propagation of sound 

 through air. 



Under what ^^^- ^^ °^ substaucc intervenes between the 

 sho'^uid^wrbe "^i^i'3'ting body and the organs of hearing, no 

 ssoundV ^^''^ sensation of sound can be produced. 



This is readily proved by placing a bell, rung by the action 

 of clock-work, beneath the receiver of an air-pump, and exhausting the air. 

 No sound will then be heard, although the striking of the tongue upon the 

 bell, and the vibration of the bell itself, are visible. Now, if a little air be 

 admitted into the receiver, a faint sound will begin to be heard, and this 

 sound will become gradually louder in proportion as the air is gradually read- 

 mitted, imtil the air within the receiver is in the same condition as that without. 



