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measure of the fog’s power ” to obstruct the sound. This is 
quite in accordance with prevalent notions ; and granting that 
the sound is dissipated, as assumed, by reflection from the 
particles of fog, the conclusion follows that the greater the 
number of the reflecting particles, the greater will be the waste 
of sound. But the number of particles, or, in other words, the 
density of the fog, is declared by its action upon light ; hence 
the optical opacity will be a measure of the acoustic opacity. 
This, I say, expresses the opinion generally entertained, 
u clear still air ” being regarded as the best vehicle for sound. 
We have not, as stated above, experimented in really dense 
fogs ; but the experiments actually made entirely destroy the 
notion that clear weather is necessarily better for the trans- 
mission of sound than thick weather. Some of our days of 
densest acoustic opacity have been marvellously clear optically, 
while some of our days of thick haze have shown themselves 
highly favourable to the transmission of sound. Were the 
physical cause of the sound-waste that above assigned, did that 
waste arise in any material degree from reflection at the limiting 
surfaces of the particles of haze, this result would be inex- 
plicable. 
Again, Derham, as quoted by Sir John Herschel, says that 
“ falling rain tends powerfully to obstruct sound.” We have 
had repeated reversals of this conclusion. Some of our ob- 
servations have been made on days when rain and hail de- 
scended with a perfectly tropical fury ; and in no single case did 
the rain deaden the sound ; in every case, indeed, it had pre- 
cisely the opposite effect. 
But falling snow, according to Derham, offers a more serious 
obstacle than any other meteorological agent to the trans- 
mission of sound. We have not extended our observations at 
the South Foreland into snowy weather; but an observation of 
my own made on December 29, in the Alps, during a heavy 
snow-storm, distinctly negatives the statement of Derham. 
Beverting to the case of fog, I am unable in modern ob- 
servations to discover anything conclusive as to its alleged 
power of deadening sound. I had the pleasure of listening 
to a very interesting lecture on fog-signals, delivered by 
Mr. Beazeley before the United-Service Institution ; and I have 
carefully perused the printed report of that lecture, and of a 
paper previously communicated by Mr. Beazeley to the Insti- 
tution of Civil Engineers. But in neither of these painstaking 
compilations can I find any adequate evidence of the alleged 
power of fogs to deaden sound. 
Indeed, during the discussion which followed the reading of 
Mr. Beazeley’s paper, an important observation in an opposite 
sense was mentioned by Mr. Douglass, to whose ability and 
accuracy as an observer I am able to bear the strongest testi- 
