124: SCIENCE OF COMMON THINGS. 



Bounds of Insects. Echoes. "Where echoes occur. 



fluttering of their wings or other membranous parts of 

 their structure. 



7 84 How do locusts produce sound f 



They are furnished with an air-bladder, or a species 

 of bagpipe, placed under and rather behind their wings. 



CHAPTEE III. 



KEFLECnON OF SOUNDS. 



785 What is an echo f 



An echo is a reflection of sound. 



786 Will you explain the manner in which an echo is produced f 



When a wave or undulation of water strikes against 

 a smooth surface, it is reflected, or turned back, and 

 waves moving in an opposite direction are produced. 

 The same thing takes place with a sound-wave of air : we 

 hear first the sound proceeding directly from the sono- 

 rous body ; then, if the sound-wave strikes against a 

 proper surface, at a suitable distance, it is turned back, 

 and we hear a repetition of the sound. This repetition 

 we call an echo. 



787* Are echoes often Tieard at sea or on extensive plains f 



Yery rarely ; at sea or on an extensive plain there 

 are no surfaces to reflect sound. It sometimes happens, 

 however, that in these situations the clouds reflect sound. 



788 In what places do echoes most frequently occur f 



In caverns, large halls, valleys and mountainous 

 passes, the windings of long passages, etc. 



789 Why are these places famous for echoes ? 



Because the sound-waves cannot flow freely forward, 

 but continually strike against opposing surfaces, and 

 are turned back. 



