ON THE MOTIONS OF SOUND, 413 







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

 bility in the larger experiments of 1822. Happily an incidental ob- 

 servation of great significance comes here to our aitl. It was observed 

 and recorded at the time that while the reports of the guns at Ville- 

 juif were without echoes, a roll of echoes, lasting from twenty to 

 twenty-five seconds, accompanied every shot at Montlhery, beino- 

 heard by the observers there. Arago, the writer of the report, 

 referred these echoes to reflection from the clouds, an explanation 

 whicli I think we are entitled to regard as problematical. The report 

 says that " all the shots fired at Montlheiy were there accompanied 



Fig. 2. 



by a rolling sound like that of thunder." ' I have italicized a very 

 significant word a word which fairly applies to our exijeriments on 

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

 tion of continuity between explosion and echo, but which could hardly 

 apply to echoes coming from the clouds. For, supposing the clouds 

 to be only a mile distant, the sound and its echo would have been 

 separated by an interval of nearly ten seconds. But there is no men- 

 tion of any interval; and, had such existed, surely the word "fol- 

 lowed," instead of" accompanied," would have been the one employed. 

 The echoes, moreover, appear to have been continuous, while the 

 clouds observed seem to have been separate. " These phenomena," 

 says Arago, "never took jjlace except at the moment when some 

 clouds appeared." ^ But from separate clouds a continuous roll of 

 echoes could hardly come. "When to this is added the experimental 

 fact that clouds far denser than any ever formed in the atmosj^here 

 are demonstrably incapable of sensibly reflecting sound, while cloud- 

 less ail', which Arago pronounced echoless, has been proved capable 

 of powerfully reflecting it, I think we have strong reason to question 

 the hypothesis of the illustrious French philosopher. 



And considering the hundreds of shots fired at the South Fore- 

 land, with the attention specially directed to the aerial echoes, when 

 no single case occurred in which echoes of measurable duration did 

 not accompany the report of the gun, I think Arago's statement, that 

 at Villejuif no echoes were heard when the sky was clear, must simply 

 mean that they vanished with great rapidity. Unless the attention 



' Tyndall quotes the French. Ed. ^ The French quoted by Tyndall. Ed. 



