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THE POPULAR SCIENCE MONTHLY 



Now, these appearances are as if there were an actual outpouring of 

 water from the cavity made by the stone ; but if we strew small pieces 

 of paper or other light material on the water-surface before we drop 

 the stone, we find that it is not the water which moves outward, but 

 only the state of things the wave. Each particle of water moves in a 

 circular or elliptic path in a vertical plane lying along the direction of 

 the wave, and so comes again to its original place. Hence it is that only 

 the phase goes on how it goes on will easily be gathered from Fig. 2. 



Let us now pass to a disturbance of another kind, from two dimen- 

 sions to three, from the surface of water to air. 



We hear the report of a gun or the screech of a railway-whistle, or 

 any other noise which strikes the ear. How comes it that the ear is 

 struck ? Certainly no one will imagine that the sound comes from the 

 cannon or from the railway-whistle like a mighty rush of air. If it 

 came like a wind we should feel it as a wind, but as a matter of fact 



Fig. 4. Shells of Compressed and Rarefied Air produced by a Source of Sound. 



no rush of this kind is felt. It is clear, therefore, that we do not get a 

 bodily transmission, so to speak, as we get it in the case of the ball 

 thrown from one boy to the other. We have a state of things passing 

 from the sender of the sound to the receiver ; the medium through 



