Sound 177 



7. Do not neglect a gathering in the ear. The germs from 

 it may cause a dangerous disease requiring a surgical opera- 

 tion. 



To Test the Hearing. In a quiet room, a person with per- 

 fect hearing can hear a loud-ticking watch at a distance of 

 several feet. To determine whether a person has approximately 

 normal hearing, hold such a watch five feet from each ear in 

 turn, while covering the other ear. If the watch has to be brought 

 within three feet of the ear before the sound of the ticking is 

 heard, the hearing of that ear is about three-fifths normal; if 

 within two feet, two-fifths normal, and so forth. 



Reflection of Sound. Sound waves in the air are reflected 

 when they come against walls and other barriers. Since sound 

 sends out waves in all directions, a listener in a room hears, both 

 the original sound and its reflections. The reflections, if imme- 

 diate, may intensify the sound. Otherwise they may interfere 

 with its sharpness. Frequently the voice of a speaker is more 

 distinct in a crowded hall than in an empty hall, as the presence of 

 people helps to break up troublesome reflections of the speaker's 

 voice. Sound, like light, after it has been reflected from several 

 surfaces, may be focused, or collected into a single point, where 

 it will be heard better than at any other point. The famous whis- 

 pering gallery of St. Paul's church in London is constructed on 

 this principle. Persons at opposite sides of the huge gallery 



are able to hear each other . _ 



when they merely whisper. 

 The sound is carried from 

 one person to another by the 

 reflections along the curve of 

 the dome. Similar odd effects 

 occur in Statuary Hall of the 

 Capitol at Washington, D. C. 



Echoes. An echo is 



j i M Ernest F. Conwtiy. 



caused by ^sound vibrations A PLACE WHERE EcHOES MAY BE 

 coming against a large sur- HEARD 



face, such as a massive rock, the side of a large building, 

 a hill, or woods, at some distance from the source of the 



