282 SOUND 



water. An Indian puts his ear to the ground in order to de- 

 tect distant footsteps, because sounds too faint to be heard 

 through the air are comparatively clear when transmitted 

 through the earth. A gentle tapping at one end of a long 

 table can be distinctly heard at the opposite end if the ear is 

 pressed against the table ; if the ear is removed from the wood, 

 the sound of tapping is much fainter, showing that wood trans- 

 mits sound more readily than air. 



The velocity of sound. A sounding body always disturbs 

 and throws into vibration the air around it, and the air particles 

 which receive motion from a sounding body transmit their 

 motion to neighboring particles, these in turn to the next adja- 

 cent particles, and so on until the motion has traveled to very 

 great distances. But the transmission of motion from particle 

 to particle requires time. If the distance is short, so that few 

 air particles are involved, the time required for transmission is 

 very brief and the sound is heard at practically the instant it 

 is made. Ordinarily we are not conscious that it requires time 

 for sound to travel from its source to our ears, because the dis- 

 tance covered is too short. At other times we recognize that 

 there is a delay; for example, thunder reaches our ears after 

 the lightning which caused the thunder has completely dis- 

 appeared. If the storm is near, the interval of time between 

 the lightning and the thunder is brief, because the sound does 

 not have far to travel ; if the storm is distant, the interval is 

 mudrt longer, corresponding to the greater distance through 

 which the sound travels. 



Echo. If one shouts out of doors, the sound is sometimes 

 heard a second time a moment or two later. This is because 

 sound is reflected when it strikes a large obstructing surface. 

 If the sound waves from the shout meet a cliff or a mountain, 

 they are reflected back, and on reaching the ear produce a later 

 sensation of sound. 



