AS A VEHICLE OE SOUND. 
199 
colleagues is as follows: — “Avant de terminer cette note, nous ajouterons seulement 
que tous les coups tires a Montlhery y etaient accompagnes d’un roulement semblable a 
celui du tonnerre, et qui durait de 20" a 25". Rien de pared n’avait lieu a Yillejuif; 
il nous est arrive seulement d’entendre, a moins d’une seconde d’intervalle, deux coups 
distincts du canon de Montlhery. Dans deux autres circonstances le bruit de ce canon 
a ete accompagne d’un roulement prolonge. Ces phenomenes n’ont jamais eu lieu qu’au 
moment d’apparition de quelques nuages ; par un ciel completement serein le bruit etait 
unique et instantane. Ne serait-il pas permis de conclure de la qu a Yillejuif les coups 
multiples du canon de Montlhery resultaient d’echos formes dans les nuages, et de tirer 
de ce fait un argument favorable a l’explication qu’ont donnee quelques physiciens du 
roulement du tonnerre V’ * * 
It is not here stated that at Montlhery the clouds were seen when the echoes were 
heard. The explanation of the Montlhery echoes is, if I understand right, an inference 
from observations made at Villejuif. The inference I think requires qualification. 
Some hundreds of cannon-shots have been fired at the South Foreland, many of them 
when the heavens were completely free from clouds, and never in a single case has a 
“ roulement ” similar to that noticed at Montlhery been absent. It follows, moreover, 
so hot upon the direct sound as to present hardly a sensible breach of continuity between 
the sound and the echo. This could not be the case if the clouds were its origin. A 
reflecting cloud, even at the short distance of 1000 yards, would leave a silent interval 
of 5 seconds between sound and echo ; and had such an interval been observed at 
Montlhery, it could hardly have escaped record by the philosophers stationed there. 
However this may be, the foregoing observations prove that air of perfect visual 
transparency is competent to produce echoes of great intensity and long duration. The 
subject is worthy of additional illustration. On the 8th of October, as already stated, 
the syren was established at the South Foreland. I visited the station on that day, and 
listened to the syren-echoes. They were far more powerful than those of the horn. 
Like the others they were perfectly continuous, and faded, as if into distance, gradually 
away. The direct sound seemed rendered complex and multitudinous by its echoes, 
which resembled a band of trumpeters first responding close at hand, and then retreating 
rapidly towards the coast of France. The syren-echoes had 11 seconds, those of the 
horn 8 seconds duration. 
I moved away from the station so as to lower the power of the direct without at the 
same time weakening the reflected sound. This was done by dropping into the sound- 
shadow behind an adjacent eminence. The echoes thus heard were still more wonderful 
in sight over any considerable part of the horizon, they were frequently accompanied by a long continued roll 
like thunder .” — Essay on Sound, par. 38. 
Observations and experiments recorded further on show the conclusion that clouds, as such, have any sensible 
power at all of reflecting sound, to be problematical, if not erroneous. 
* Connaiss. des Temps, 1825, p. 361. 
