ON THE MOTIONS OF SOUND. 411 



W^ith the caution which characterized him on other occasions, and 

 which has been referred to admiringly by Faraday,' Arago made no 

 attempt to explain this anomaly. His words are: "As for the very 

 notable differences of intensity constantly observed in the sound of 

 the cannon accordingly as it was propagated from north to south be- 

 tween Villejuif and Montlhery, or from south to north between the 

 latter station and the former, we will not at present try to explain it, 

 as we could present to the reader only conjectures unsupported by 

 evidence." ^ 



I have tried, after much perplexity of thought, to bring this sub- 

 ject within the range of exjDeriment, and have now to submit to the 

 Royal Society a possible solution of this enigma. The first step was 

 to ascertain whether the sensitive flame referred to in my recent 

 paper in the " Philosophical Transactions " could be safely employed 

 in experiments on the mutual reversibility of a source of sound and an 

 object on which the sound impinges. Now, the sensitive flame usually 

 employed by me measures from eighteen to twenty-four inches in 

 height, while the reed employed as a source of sound is less than a 

 square quarter of an inch in area. If, therefore, the whole flame or 

 the pipe which fed it were sensitive to sonorous vibrations, strict ex- 

 periments on reversibility with the reed and flame might be difficult, 

 if not impossible. Hence my desire to learn whether the seat of sensi- 

 tiveness 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 upon 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 three 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 violently agi- 

 tated by the sounding-reed, i?. On shifting the tube, or the burner, 

 so as to concentrate the sound on a portion of 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 



' " Researches in Chemistry and Physics," p. 484. 



^ Tyndall gives the passage in French, as found in the " Connaissance des Temps," 

 1825, p. 370. Ed, 



