S O U N D 



affect the distance to which a sound can be 

 heard. Anything which tends to stir up the air 

 and make it less uniform, such as little currents 

 of warm rising air, help to prevent the passage of 

 the sound waves ; just as glass, which is usually 

 transparent, will not let light through as easily 

 when it is scratched or powdered up. Again, 

 a very violent wind is full of eddies and irregular 

 movements, and soon makes a sound die away 

 even in the direction of the wind. In a fog the 

 air is usually still, and as the sunlight does not 

 reach the ground there is no local heating. This 

 is a state of the air in which it is generally uni- 

 form, and it is well known to sailors that sounds 

 can often be heard distinctly for long distances 

 in foggy weather ; indeed, it is not difficult to 

 observe this for ourselves. 



We have seen that sounds are carried by 

 waves or vibrations moving through the air. If 

 the source vibrates very quickly, the sound it 

 gives out is high and shrill ; if it vibrates more 

 slowly, the sound is deeper and lower in tone. 

 There are many things in Nature which create 

 these vibrations. Our voices are made for this 

 purpose ; they set up vibrations of various times 

 and amplitudes, and so produce the words and 

 notes of our speech and song. A blow will 

 cause the particles of the objects that strike, or 

 are struck, to vibrate or move very rapidly, 

 and these movements are passed on as sound- 

 waves to the air through which they travel till 



(47) 



