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Proceedings of Indiana Academy of Science 



lens is less than it would be were the stream of hot gases replaced by 

 a solid cylinder of the same size. 



Figure 2 is a picture of a sound wave generated by an electric 

 spark E between two opaque circular disks. W is a portion of the 

 original wave, N of the wave reflected from the near surface of the 

 lens, and T the wave transmitted by the lens L. D and D are portions 

 of the wave diffracted about the edges of the lens into the geometrical 

 shadow. Note that the conditions are entirely different in the region V, 

 apparently almost a sound void. The hot gases due to the passage of 

 the spark absorb the energy in the region V and refraction by the outer 



FiR. 3. Sound wave absorption and refraction by a stream of hot gases. 



portions of the hot gas region causes the wave to move in the direc- 

 tions R and S, a dispersing effect. Thus normal diffraction into the 

 region V is, to a certain extent, prevented. 



In figure 3, G is a stream of hot gases issuing from a vertical cylin- 

 drical pipe P, about an inch in diameter and two feet long. The hot 

 gases came from the flame of a Bunsen burner just below the lower 

 end of the pipe. An electric spark E behind the disk produced the 

 sound wave W. R is the wave reflected from the surface of the pipe, 

 D the wave diffracted about the pipe, and S the refracted wave (R and 

 S in figure 2) in advance of the others and therefore undergoing disper- 

 sion. When the locomotive is moving rapidly the hot gases form a sort 



