SOUND 



buildings ; and in the same way we can tell from 

 the noise of a railway train when it is going 

 through a deep cutting, into a tunnel, or out on 

 to an open embankment. Even if we did not 

 know the country at all, and were sitting there 

 for the first time on a dark evening, we should 

 learn a great deal about the character of the 

 neighbourhood by listening attentively to the 

 sounds, and thinking about their direction and 

 their meaning. 



We sometimes come across one curious 

 and rather misleading effect of sound, which 

 requires a little consideration, and that is an 

 echo. We make a sound, probably with our 

 faces turned to a wall or wood, and very shortly 

 the sound comes back to us ; it is, as it were, 

 reflected to us, just as a looking-glass will reflect 

 our faces. The sound-waves when they reach 

 the wall or clump of trees strike against them 

 and are turned back in an orderly fashion. We 

 can actually see this happen to waves of a 

 different sort. Let us drop a pebble in a basin 

 of water or a quiet pool ; it sets up waves in the 

 water which travel out on all sides to the edge, 

 against which they strike and rebound and be- 

 gin to travel back again. We do not often hear 

 an echo, because we have to choose the position 

 in which we stand very carefully ; as we know, 

 to see ourselves in the looking-glass we must 

 stand in front of it, and, moreover, not every- 

 thing in front of which we stand will act as a 



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