ECHOES 169 



movement of a body of air; for we do not feel any motion 

 of the air from a sounding body. To understand the for- 

 mation of sound waves we must remember that air is elastic; 

 that is, it can be compressed, but will expand again when 

 released. The sudden jar given the air by a bell (Fig. 164) 

 pushes the air particles near the bell forward, producing a 

 layer of compressed air. This layer then expands, and 

 in doing so gives a rapid push to the next layer of air. 

 The sound wave thus proceeds outward as a series of 

 short, rapid compressions and ex- 

 pansions of air. 



With a row of elastic balls (Fig. 165) we 

 can illustrate the way in which air particles 

 can give up their motion without going 

 forward. If we draw aside one of the balls, 

 and let it fall against the row, it gives its 

 motion to its neighbor. This gives its row of bails; only the last 



ball actually moves forward 



motion to the next ball, and so on down the 



line. Only the last ball, which has no ball to which it can give its 



motion, actually moves forward. 



191. Echoes. Sound waves are reflected as light waves 

 are (cf. 174). If, on a quiet day, we shout toward some 

 wall, or barn, or cliff, our shout may be returned to us as 

 an echo. To succeed we must be far enough away to per- 

 mit the shout to cease before the reflected sound can come 

 back. In a small room the echo blends with the original 

 sound, and we do not notice it; but in a large room or 

 hall the echo of one word or sentence may come back just 

 as another is being spoken. The result is a confused mix- 

 ture of sounds. Echoes are often prevented by the hang- 

 ing of curtains or wires across the room. These break up 

 the disturbing sound waves. 



