Prof. Tyndall on Acoustic Reversibility. 147 



Now the sensitive flame usually employed by me measures from 

 18 to 24: inches in height, while the reed employed as a source of 

 sound is less than a square quarter of an inch in area. If, there- 

 fore, the whole flame, or the pipe which fed it, were sensitive to 

 sonorous vibrations, strict experiments on reversibility with the 

 reed and flame might be difficult, if not impossible. Hence my 

 desire to learn whether the seat of sensitiveness was so localized in 

 the flame as to render the contemplated interchange of flame and 

 reed permissible. 



The flame being placed behind a cardboard screen, the shank 

 of a funnel passed through a hole in the cardboard was directed 

 upou the middle of the flame. The sound-waves issuing from the 

 vibrating reed placed within the funnel produced no sensible 

 effect upon the flame. Shifting the funnel so as to direct its shank 

 upon the root of the flame, the action was violent. 



To augment the precision of the experiment, the funnel was con- 

 nected with a glass tube 3 feet long and half an inch in diameter, 

 the object being to weaken by distance the effect of the waves dif- 

 fracted round the edge of the funnel, and to permit those only 

 which passed through the glass tube to act upon the flame. 



Presenting the end of the tube to the orifice of the burner (b, 

 fig. 1), or the orifice to the end of the tube, the flame was vio- 

 lently agitated by the sounding-reed, E. On shifting the tube, 

 or the burner, so as to concentrate the sound on a portion of 



Pis'. 1. 



the flame about half an inch above the orifice, the action was 

 nil. Concentrating the sound upon the burner itself about half 

 an inch below its orifice, there was no action. 



These experiments demonstrate the localization of " the seat of 

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

 strument for the contemplated experiments on reversibility. 



The experiments proceeded thus : — The sensitive flame being 

 placed close behind a screen of cardboard 18 inches high by 12 

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

 root of the flame, was placed at a distance of 6 feet on the other 

 side of the screen. The sound of the reed, in this position, pro- 

 duced a strong agitation of the flame. 



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



L2 



