148 Royal Society : — 



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 neighbourhood 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 6 feet from it. The sonorous vibrations were without sen- 

 sible action upon the flame. 



The experiment was repeated and varied in many wa} r s. Screens 

 of various sizes were employed ; and instead of reversing the posi- 

 tions 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 expe- 

 rimenter, 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 these 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 



Pig. 2. 



B 



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 C, where its action on 

 the screen was greatly augmented, it ceased to have any action on 

 the flame at A. 



We are now, I think, prepared to consider the failure of rever- 

 sibility in the larger experiments of 1822. Happily an incidental 

 observation of great significance comes here to our aid. It was 

 observed and recorded at the time that while the reports of the 

 guns at Villejuif were without echoes, a roll of echoes, lasting from 

 20 to 25 seconds, accompanied every shot at Montlhery, being heard 

 by the observers there. Arago, the writer of the Report, referred 

 these echoes to reflection from the clouds, an explanation which I 

 think we are entitled to regard as problematical. The report says 

 that " tous les coups tires a Montlhery y etaient accompagnes d'un 

 roulement semblable a celui du tonnerre." I have italicized a very 

 significant word — a word which fairly applies to our experiments 

 on gun-sounds at the South Foreland, where there was no sensible 



