THE REASON WHY. 167 



The morniug is come unto thee, O thou that dwellest in the land : the time is 

 come, the day of trouble is near, and not the sounding again of 

 the mountains." EZEKIEL vil. 



a long pole be heard by applying the ear to the opposite 

 extremity ? 



Because wood is a good conductor of sound, and its atoms are 

 susceptible of considerable vibration. It is, therefore, chosen in 

 numerous instances for the construction of musical instruments. 



Deaf persons have been known to derive pleasure from music by placing their 

 hands upon the wood- work of musical instruments while being played upon. 



739. Why is the hearing of deaf persons assisted by ear- 

 trumpets ? 



Because ear-trumpets collect the vibrations of the air into a focus, 

 and make the sounds produced thereby more intense. 



740. Why are sounding-boards used to improve the hearing 

 of congregations ? 



Because, being suspended over, and a little behind, the speaker, 

 they collect the vibrations of the air, and reflect them towards the 

 congregation. 



741. What are echoes ? 



Echoes are sounds reflected by the objects on which they strike. 



742. Why do some echoes occur immediately after a 

 sound ? 



Because the reflecting surface is very near ; therefore the sound 

 returns immediately. 



743. Why do some echoes occur a considerable time after 

 a sound ? 



Because they are at a considerable distance, and the sound takes 

 time to travel to it, and an equal time to return. 



744. Why do some echoes change the tone and quality of 

 sound? 



Because the reflecting surface, having vibratory qualities of its 

 own, mingles Us own vibrations with that of the. sound. 



745. Wliy are there sometimes several echoes to one 

 sound ? 



