234 
PEOEESSOE TYNDALL ON THE ATMOSPHEEE 
The point referred to in these letters is, I think, one of practical importance. The intensity of -the light is 
•due to the concentration of the combustion of a considerable amount of matter into the fraction of a second of 
time. It is a question well worthy of consideration whether on board light-ships and elsewhere the combustion 
of a definite amount of gunpowder, or of gun-cotton, at definite intervals during fog may not turn out to be a 
simple and useful form of signal. The combustion, of course, would in this case be effected with a view to the 
production of the maximum light instead of the maximum sound. 
In dealing with this subject the gas-gun proposed by Mr. Wigham would naturally be considered. 
Remark added, May 27. — The more I think of it, and the more I experiment upon it, the more important 
does this question of flashes appear to me. In one of the sections of the foregoing paper experiments on 
artificial fogs are described. The densest of these were suddenly and strikingly illuminated throughout by the 
combustion of half a grain of gunpowder, and of a still smaller quantity of gun-cotton. The cutting off and 
restoration of the candle-light, or the electric light, used to test the density of the fog, produced a similar effect. 
It is its suddenness that renders the lightning-flash so startlingly vivid through a cloud. A revolving light like 
the South Stack does not fulfil the necessary conditions. Its revolution is slow, and the angular spaces between 
the beams being filled by laterally scattered light, the differential action is practically abolished. At a 
distance the luminosity, when uniform, may be so feeble as to be unseen, while its sudden extinction and 
revival would render it sensible. 
Remarkable Instances of Acoustic Opacity. 
In his excellent lecture entitled “ Wirkungen aus der Feme,” Dove has collected some striking cases of the 
interception of sound. During the battle of Cassano on the Adda, between the Due de Vendome and the 
Prince Eugene, an army corps stationed under the Duke’s brother five miles up the river failed to join the battle 
through not hearing the cannonade. In a river-valley, particularly on a warm day, it would, in my opinion, 
be perilous to place much dependence upon sound. Near Montereau on the Seine, during the battle between 
Napoleon I. and the King of Wurtemberg, which lasted seven hours, no sound of the conflict was heard by 
Prince Schwartzenberg 13 miles up the river. A Prussian officer sent thither at noon first heard the cannonade 
at a distance of 4f miles from the field of battle. This happened on a day apparently resembling in point of 
mildness and serenity our 3rd of July. In the battle of Liegnitz, where Frederick the Great overthrew Laudon, 
the sound of the battle was unheard by Field-Marshal Daun, who was posted on a height A\ miles from the 
battle-field. Dove himself recounts the fact of his having failed to catch a single shot of the battle of Katzbach 
at 41 miles distance, while he plainly heard the cannonade of Bautzen 80 miles away. “ 
The stoppage of the sound in the foregoing cases Dove referred, and doubtless correctly, to the nun-homo- 
geneous character of the air. He also notes the exceedingly interesting observation that in certain clear winter 
days, when the sun has already attained some power, the semaphore is difficult to decipher, the reason being 
that by the solar warmth upward currents of warm and downward currents of cold air (similar to those of 
Humboldt on the plain of Antures) are established, and that such days are also unfavourable to the transmission 
of sound. In another passage, however, he seems to indorse the prevalent notion that the optical transparency 
of the air and its power to transmit sound go hand in hand ; whereas in our experiments days of the highest 
optical transparency proved themselves acoustically most opaque. 
But nothing of this description that I have read equals in point of interest the following account of the battle 
of Gain’s Farm, for which I am indebted to the Eector of the University of Virginia. 
“ Lynchburgh, Virginia, 
March 19th,, 1874. 
“Sir, — I have just read with great interest your lecture of January 16th, copied by Littell’s ‘Living Age’ 
from ‘ Nature,’ on the acoustic transparency and opacity of the atmosphere. The remarkable facts you mention 
induce me to state to you a fact which I have occasionally mentioned, but always where I am not well known, 
with the apprehension that my veracity would be questioned. It made a strong impression on me at the time, 
but was an insoluble mystery until your discourse gave a possible solution. 
