THE EEASON WHY. 165 



' My heart maketh a noise in me : I cannot hold my peace, because thou hast 

 heard, O my soul, the sound of the trumpet, the alarm of war." JEB. iv. 



192,000 miles in a second; while sound,}*? which we hear, travels 

 only at the rate of a quarter of a mile in a second. 



726. Why does the tread of soldiers, when marching in 

 long ranJcs, appear to be irregular ? 



Because the sounds proceeding from different distances, reach 

 our ears in varying periods of time. 



727. What are the numbers of vibrations in a second 

 that produce the various musical sounds ? 



C or Do, 480 vibrations in a second ; B or Si, 450 vibrations ; A 

 or La, 400 vibrations ; G or Sol, 360 vibrations ; F or Fa, 320 

 vibrations ; E or Mi, 300 vibrations ; D or Re, 270 vibrations ; C or 

 Do, 240 vibrations. It is thus seen that the more rapid the 

 vibrations, the Uglier the note, and vice versa. 



728. Why does the length of a wire or string determine 

 the sound that it produces ? 



Because the shorter the string the more rapid are its vibrations 

 when struck. 



729. Why does the tension of a wire or string affect its 

 vibrations ? 



Because when the string or wire is tight, a touch communicates 

 vibrations to all its particles ; but when it is loose the vibrations 

 are imperfectly communicated. 



730. Why are some notes low and solemn, and others high 

 and quick ? 



Because the vibrations of musical strings vary from 32 vibrations 

 in a second, which produces a soft and deep bass, to 15,000 

 vibrations in a second, which produces the sharpest treble note. 



731. Why can our voices be heard at a greater distance 

 when we speak through tubes ? 



Because the vibrations are confined to the air within the tube, 

 and are not interfered with by other vibrations or movements in 

 the air ; the tube itself is also a good conductor of sound. 



