198 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
had failed to reach us by transmission 1 The case would then be strictly analogous to the 
reflection of light from an ordinary cloud to an observer placed between it and the sun. 
My first care in the early part of the day in question was to assure myself that our 
inability to hear the sound did not arise from any derangement of the instruments on 
shore. Accompanied by Mr. Edwakds, who was good enough on this and some other 
days to act as my amanuensis, at 1 p.m. I was rowed to the shore, and landed at the base 
of the South Foreland Cliff. The body of air which had already shown such extraor- 
dinary power to intercept the sound, and which manifested this power still more impres- 
sively later in the day, was now in front of us. On it the sonorous waves impinged, 
and from it they were sent back to us with astonishing intensity. The instruments, 
hidden from view, were on the summit of a cliff 235 feet above us, the sea was smooth 
and clear of ships, the atmosphere was without a cloud, and there was no object in 
sight which could possibly produce the observed effect. From the perfectly transparent 
air the echoes came, at first with a strength apparently but little less than that of the 
direct sound, and then dying gradually and continuously away. A remark made by my 
talented companion in his note-book at the time shows how the phenomenon affected 
him : — “ Beyond saying that the echoes seemed to come from the expanse of ocean, it 
did not appear possible to indicate any more definite point of reflection.” Indeed no 
such point was to be seen; the echoes reached us, as if by magic, from absolutely 
invisible walls. 
Here, in my opinion, we have the key to many of the mysteries and discrepancies of 
evidence which beset this question. The foregoing observations show that there is no 
need to doubt either the veracity or capability of the conflicting witnesses, for the varia- 
tions of the atmosphere are more than sufficient to account for theirs. The mistake 
indeed hitherto has been, not in reporting incorrectly, but in neglecting the monotonous 
operation of repeating the observations during a sufficient time. I shall have occasion 
to remark subsequently on the mischief likely to arise from giving instructions to 
mariners founded on observations of this incomplete character. 
The question of aerial echoes has an historic interest. While cloud-echoes have been 
accepted as demonstrated by observation, it has been hitherto assumed that audible 
echoes never occur in optically clear air. We owe this opinion to the admirable report 
of Arago on the experiments made to determine the velocity of sound at Montlhery and 
Villejuif in 1822*. Arago’s account of the phenomenon observed by him and his 
* Sir John Herschel gives the following account of Arago’s observation : — “ The rolling of thunder has been 
attributed to echoes among the clouds ; and if it is considered that a cloud is a collection of particles of water, 
however minute, in a liquid state, and therefore each individually capable of reflecting sound, there is no reason 
why very loud sounds should not he reverberated confusedly (like bright lights) from a cloud. And that such 
is the case has been ascertained by direct observation on the sound of cannon. Messrs. Arago, Matthietj, and 
Prony, in their experiments on the velocity of sound, observed that under a perfectly clear sky the explosions 
of their guns were always single and sharp ; whereas when the sky was overcast, and even when a cloud came 
