STUDIES IN NATURE 



makes this trembling or vibration cease, and 

 the sound then stops. Now let us think what 

 effect this trembling of the object would have 

 on the air around it. If we put our hand or a 

 stick into water and move it rapidly to and fro, 

 we see that a series of waves spreads out over 

 the surface of the water on all sides. Here the 

 waves are visible, and the effect of a vibrating 

 body on the air which surrounds it is just the 

 same, though we cannot see the waves which 

 spread out through the transparent and invisible 

 air. We cannot see the waves, but we can hear 

 them, for these waves are sound. 



Now that we have concluded that sounds are 

 waves which usually travel through the air, there 

 are many other things about them we want to 

 know. For instance, how fast do the waves 

 move? We may make experiments on this 

 question in one of several ways. If a gun is 

 fired late in the afternoon of a still day at a 

 distance from us which we know, we can observe 

 the time between seeing the flash and hearing 

 the noise by looking at the seconds-hand of a 

 watch. If the distance is half-a-mile, we shall 

 find that the time is about two-and-a-half 

 seconds. Thus sound travels through still air 

 at the rate of about noo feet in one second, 

 about twelve times as fast as the quickest 

 express train. In this way, too, we can gain 

 some idea of the distance of a thunderstorm, 

 for there is always an interval of a few moments 



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