THE REASON WHY. 167 



" The morning is cornc unto thee, O thou that dwellest in the land : the time ii 



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



the mountains." EZEKIEL viz. 



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

 extremity ? 



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

 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- 



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 its own vibrations with that of the sound. 



745. Why are there sometimes several echoe* to one 

 sound f 



