SOUND 



between seeing the flash of the lightning and 

 hearing the noise of the thunder that accom- 

 panies it. 



When the wind is blowing, we all know, or 

 may easily observe, that sounds are heard much 

 further in the direction of the wind than against 

 it. The church bells or the railway train, which 

 we mentioned before, are clearly heard when 

 the wind blows from them to us, and not when 

 it blows the other way. In order to explain 

 this, people often say that sound is carried by 

 the wind, and cannot travel against it. This 

 shows the danger of hastily concluding that we 

 understand things before we have considered 

 them in the light of all the information we can 

 get. We know that sound travels twelve times 

 faster than an express train, and if we measure 

 the speed of a strong wind we shall find that, in 

 England at all events, it is rarely more than 

 about half the speed of a train. Thus a fairly 

 strong wind will only increase the speed of a 

 sound that is travelling with it by a small part, 

 and, if the sound is moving against it, it can 

 only diminish it by the same amount. It fol- 

 lows that the effect of wind on the speed of 

 sound is very slight, and we must look further 

 for an explanation of the fact that sounds are 

 heard much further with the wind than against 

 it. When the wind blows it is clear that the 

 lower parts of the great current of air, which 

 strike the trees, houses, and other things on the 



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