Sound 



181 



FIG. 101. When the tuning fork vibrates, the glass needle makes a wavy line 

 on the smoked paper on the drum. 



under water,, and fishes hear without any air. So, to 

 be accurate, we should say that sound is vibrations of 

 any kind of matter. And the vibrations travel better 

 in most other kinds of matter than they do in air. Vi- 

 brations move rather slowly in air, compared with the 

 speed at which they travel in other substances. It 

 takes sound about 5 seconds to go a mile in air ; in other 

 words, it would go 12 miles while an express train went 

 one. But it travels faster in water and still faster in 

 anything hard like steel. That is why you can hear 

 .the noise of an approaching train better if you put your 

 ear to the rail. 



Why we see steam rise before we hear a whistle blow. 

 But even through steel, sound does not travel with 



