AS A VEHICLE OF SOUND. 
185 
make experiments comparable, we must have some measure of the fog’s power of stopping 
sound, without attending to which the most anomalous results may be expected. It 
seems probable that this will bear some simple relation to its opacity to light, and that 
the distance at which a given object, as a flag or pole, disappears may be taken as the 
measure.” “ Still clear air” is regarded in this letter as the best vehicle of sound, the 
alleged action of fogs, rain, and snow being ascribed to their rendering the atmosphere 
“ a discontinuous medium.” 
To Mr. Alexander Beazeley we are indebted for an extremely useful summary of 
existing knowledge regarding fog-signals*. He classifies the various instruments 
hitherto employed, and gives some account of their performance. As regards the 
action of fog upon sound, the statements made in the body of his papers agree with 
those just quoted from Dr. Robinson. “ Fogs,” he says, “ have a remarkable power of 
deadening sound, and act in this respect so irregularly, that experiments made during 
clear weather have little or no practical value, except as mere competitive trials of 
different instruments.” 
In the discussion which followed the reading of Mr. Beazeley’s paper at the Institu- 
tion of Civil Engineers, Dr. Gladstone, who was a member of the Commission on Lights 
and Beacons, is reported to have said, “ A difficulty in the use of sound was this, that 
fogs deadened sound very materially ; but the evidence was very contradictory on that 
point. In a fog on land it was difficult to hear the passing of carriages or noises at a 
short distance ; and so in a fog at sea these signals found a difficulty in penetrating the 
fog against which they are intended to be a protection.” 
On the same occasion Mr. James N. Douglass, the Engineer of the Trinity House, to 
whose ability as an observer I am able to bear strong testimony, stated that in his 
experience “he had found but little difference in the travelling of sound in foggy or 
in clear weather. He had distinctly heard in a fog, at the Small’s Rock in the Bristol 
Channel, guns fired at Milford, twenty-five miles ^ff.” Mr. Beazeley had also heard 
the Lundy-Island gun “at Hartland Point, a distance of ten miles, during dense fog;” 
so that, in winding up his paper, he admitted “ that the subject appeared to be very little 
known, and that the more it was looked into the more apparent became the fact that 
the evidence as to the action of fog upon sound is extremely conflicting.” 
In a paper presented to the Literary and Philosophical Society of Manchester on the 
16th of December, 1873, Professor Osborne Reynolds affirms the prevalent doctrine with 
great distinctness, and makes a very ingenious attempt to explain it. “ That sound,” 
says Professor Reynolds, “ does not readily penetrate a fog is a matter of common 
observation. The bells and horns of ships are not heard so far during fogs as when the 
weather is clear. In a London fog the noise of the wheels is much diminished, so that 
they seem to be at a distance when really close by.” 
My knowledge does not inform me of the existence of any other source for these 
* Proceedings of tlie Institution of Civil Engineers, Marcli 14, 1871 ; and Lecture at the United- Service 
Institution, Hay 24, 1872. 
MDCCCLXXIV. 2 B 
