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THE POPULAR SCIENCE MONTHLY 



sensitiveness," and they prove the flame to be an appropriate instru- 

 ment for the contemplated experiments on reversibility. 



The experiments proceeded tlius : The sensitive flame being jjlaced 

 close behind a screen of cardboard eighteen inches high by twelve 

 inches wide, a vibrating reed, standing at the same height as the root 

 of the flame, was placed at a distance of six feet on the other side of 

 the screen. The sound of the reed, in this position, produced a strong 

 agitation of the flame. 



Fig. 1. 



The whole upper half of the flame was here visible from the reed ; 

 hence the necessity of the foregoing experiments to prove the action 

 of the sound on the upper portion of the flame to be nil, and that the 

 waves had really to bend round the edge of the screen so as to reach 

 the seat of sensitiveness in the neighborhood of the burner. 



The positions of the flame and reed were reversed, the latter being 

 now close behind the screen, and the former at a distance of six feet 

 from it. The sonorous vibrations were without sensible action upon 

 the flame. 



The experiment was repeated and varied in many ways. Screens 

 of various sizes were employed, and, instead of reversing the positions 

 of the flame and reed, the screen was moved so as to bring, in some 

 experiments the flame, and in other experiments the reed, close behind 

 it. Care was also taken that no reflected sound from the walls or 

 ceiling of the laboratory, or from the body of the experimenter, 

 should have any thing to do with the effect. In all cases it was shown 

 that the sound was effective when the reed was at a distance from the 

 screen, and the flame close behind it ; while the action was insensible 

 when the positions were reversed. 



Thus let s e. Fig. 2, be a vertical section of the screen. "When the 

 reed was at A, and the flame at B, there was no action ; when the 

 reed was at B, and the flame at A, the action was decided. It may be 

 added that the vibrations communicated to the screen itself, and from 

 it to the air beyond it, were without effect ; for when the reed, which 

 at B is effectual, was shifted to <7, where its action on the screen was 

 greatly augmented, it ceased to have any action on the flame at A. 



