146 Royal Society :— 



brought to f bear upon the work), in the first instance, with more 

 especial reference to the positions, thicknesses, and individualities 

 of the lines ; the final revision will consist of an absolute inten- 

 sity reproduction of the photographs. 



" On Acoustic Reversibility." By J. Tyndall, D.C.L., LL.D., F.R.S. 



On the 21st and 22nd of June, 1822, a Commission appointed 

 by the Bureau of Longitudes of France executed a celebrated 

 series of experiments on the velocity of sound. Two stations had 

 been chosen, the one at Yillejuif , the other at Montlhery, both lying 

 south of Paris, and 11*6 miles distant from each other. Prony, 

 Mathieu, and Arago were the observers at Yillejuif, while Hum- 

 boldt, Bouvard, and Gay-Lussac were at Montlhery. Guns, charged 

 sometimes with 2 lbs. and sometimes with 3 lbs. of powder, were 

 fired at both stations, and the velocity was deduced from the interval 

 between the appearance of the flash and the arrival of the sound. 



On this memorable occasion an observation was made which, as 

 far as I know, has remained a scientific enigma to the present 

 hour. It was noticed that while every report of the cannon fired 

 at Montlhery was heard with the greatest distinctness at Yille- 

 juif, by far the greater number of the reports from Yillejuif failed 

 to reach Montlhery. Had wind existed, and had it blown from 

 Montlhery to Yillejuif, it would have been recognized as the cause 

 of the observed difference ; but the air at the time was calm, the 

 slight motion of translation actually existing being from Villejuif 

 towards Montlhery, or against the direction in which the sound was 

 best heard. 



So marked was the difference in transmissive power between the 

 two directions, that on the 22nd of June, while every shot fired at 

 Montlhery was heard " a merveille " at Yillejuif, but one shot out 

 of twelve fired at Yillejuif was heard, and that feebly, at the other 

 station. 



"With 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 : — " Quant 

 aux differences si remarquables d'intensite que le bruit du canon 

 a toujours presentes suivant qu'il se propageaient du nord au sud 

 entre Yillejuif et Montlhery, ou du sud au nord entre cette seconde 

 station et la premiere ; nous ne chercherons pas aujourd'hui a l'ex- 

 pliquer, parce que nous ne pourrions offrir au lecteur que des con- 

 jectures denuees de preuves"t. 



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

 subject within the range of experiment, and have now to sub- 

 mit to the Royal Society a possible solution of the 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. 



* Kesearckes in Chemistry and Physics, p. 484. 

 t Connaissance des Temps, 1825, p. 370. 



