118 SCIENCE OF COMMON THINGS. 



Sound vibrations may be rendered visible. Telocity of sound. 



lations of air, if interrupted in their progress by a high wall or other simi- 

 lar impediment, will be continued over its summit and propagated on the 

 opposite side of it. 



739 When a sonorous body is struck, do all the particles of which it it 

 composed really move or vibrate ? 



They do ; and the body itself, no matter how com- 

 pact and solid it may be, really changes its form with 

 each vibration. 



740 How may the sound vibrations in a solid body be rendered visible f 



By many simple contrivances as by a ball hung by 

 a string to a bell, by pieces of paper placed on the 

 strings of a violin, or by sand placed upon the sound- 

 ing-board of a piano or any other stringed instrument. 



741 How fast does sound travel f 



About 13 miles in a minute, or 1142 feet in a second 

 of time. 



Why is the flash of a gun fired at a distance seen long before th 

 report is heard ? 



Because light travels much faster than sound. 



Light would go 480 times round the whole earth while sound is going 

 its 13 miles. 



743 How is a knowledge of the velocity of sound made applicable to the 

 measurement of distances f 



Suppose a flash of lightning to be perceived, and on 

 counting the seconds that elapse before the thunder is 

 heard, we find them to amount to 3 ; then as sound 

 moves 1142 feet in a second, it will follow that the 

 thunder-cloud must be distant 1142 x 3 = 3997 feet. 



744 Why do windows rattle when carts pass by a house ? 



1. Because glass is sonorous and the air communi- 

 cates its vibrations to the glass, which echoes the same 

 sound ; and 



2. The window-frame being shaken, contributes to 

 the noise. 



"Window-frames are shaken, 1. By sound-waves striking against them. 

 2. By a vibratory motion communicated to them by the walls of the 

 house. 



745 Why. is the sound of a bell stopped by touching the bell with our 

 finger f 



Because the weight of our finger stops the vibratiima 



