Prof. Tyndall on Acoustic Reversibility. 149 



solution 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 mention of any interval ; and had such existed, surely 

 the word " followed," 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. 

 " Ces phenomenes," says Arago, " nont jamais eu lieu qu'au mo- 

 ment de l'apparition de quelques nuages." 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 atmosphere are demonstrably incapable of sen- 

 sibly reflecting sound, while cloudless air, 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 dura- 

 tion 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 rapi- 

 dity. Unless the attention were specially directed to the point, a 

 slight prolongation of the cannon-sound might well escape obser- 

 vation ; and it would be all the more likely to do so if the echoes 

 were so loud and prompt as to form apparently part and parcel of 

 the direct sound. 



I should be very loth to transgress here the limits of fair criti- 

 cism, or to throw doubt, without good reason, on the recorded ob- 

 servations of an eminent man. Still, taking into account what has 

 been just stated, and remembering that the minds of Arago and 

 his colleagues were occupied by a totally different problem (that 

 the echoes were an incident rather than an object of observation), 

 I think we may justly consider the sound which he called "instan- 

 taneous " as one whose aerial echoes did not differentiate them- 

 selves from . the direct sound by any noticeable fall of intensity, 

 and which rapidly died into silence. 



Turning now to the observations at Montlhery, we are struck 

 by the extraordinary duration of the echoes heard at that station. 

 At the South Foreland the charge habitually fired was equal to 

 the largest of those employed by the French philosophers ; but on 

 no occasion did the gun sounds produce echoes approaching to 20 

 or 25 seconds' duration. It rarely reached half this amount. 

 Even the syren-echoes, which were more remarkable and more 

 long-continued than those of the gun, never reached the duration 

 of the Montlhery echoes. The nearest approach to it was on the 

 17th of October, 1873, when the syren-echoes required 15 seconds 

 to subside into silence. 



On this same day, moreover (and this is a point of marked 



