c 



98 SOUND 



that at which it struck. Now roll it so as to strike 

 exactly at right angles (as cd in Fig. 73 strikes ab), and 

 the ball will be reflected so as to come straight back to 

 your hand. 



In just the same way sound waves are reflected from 

 any building, hill, rock, or bank of woods which they 



strike. Usually these reflected 

 & waves pass off in a different di- 

 rection; but when they strike 

 squarely at right angles, they 

 come back to their starting 

 point and there produce a faint 

 sound. This sound is called 



an echo. Because the waves have lost some energy in 

 traveling, the echo is generally weak; if the air is too 

 full of other sound waves, these weak echoes may not 

 be heard. 



110. Reverberation. In a large empty hall sound 

 waves may be reflected from wall to wall in many 

 directions at the same time. The effect of this confu- 

 sion of waves upon the ear is not a distinct echo, but 

 rather a roar. Such an effect is called reverberation. It 

 may be noticed in caves, wells, and other inclosed 

 empty spaces. 



111. Forced and Sympathetic Vibrations. When a 

 vibrating body touches another, its motion may start 

 vibration in that other. In some cases this may be done 

 also ly sound waves, the waves in the air having enough 

 energy to arouse vibration in certain bodies that they 

 strike. This sort of vibration is of two kinds, forced 



