416 ECHO. 



sequence of which, their strength is 

 increased. 



Q. Why are MOUNTAINS so NOISELESS and 

 quiet ? 



A. Because the air of mountains is 

 very rarefied; and as the air becomes 

 rarefied, sound becomes less intense. 



Q. How do you know that the RARETY of air 

 DIMINISHES the intensity of SOUND ? 



A. If a bell be rung in the receiver 

 of an air-pump, the sound becomes 

 fainter and fainter as the air is exhausted, 

 till at last it is quite inaudible. 



Q. What is the cause of ECHO ? 



A. Whenever a sound-wave strikes 

 against any obstacle (such as a wall or hill), 

 it is reflected (or thrown back) j and this 

 reflected sound is called an ECHO. 



The same laws govern echo as light. (See p. 370.) 

 Q. What places are most FAMOUS for ECHOES ? 

 A. Caverns, grottoes, and ruined 

 abbeys; the areas of antique halls; the 

 windings of long passages ; the aisles of 

 cathedral churches; mountains, and ice- 

 bergs. 



Q. Why are caverns, grottoes, and ruined 

 abbeys FAMOus/or ECHOES? 



