TRANSMISSION OF SOUND BY THE ATMOSPHERE. 
185 
mony. Mr. Douglass stated that he had found in his experience 
but little difference in the travelling of sound in foggy or in 
clear weather. He had distinctly heard in a fog, at the Smalls 
rock in the Bristol Channel, guns fired at Milford Haven, 
twenty-five miles away. Mr. Beazeley, moreover, has heard 
the Lundy-Island gun “ at Hartland Point,” a distance of ten 
miles, during dense fog. Mr. Beazeley ; s conclusion, indeed, 
accurately expresses the state of our knowledge when he wrote. 
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 evi- 
dence as to the effect of fog upon sound , is extremely conflict- 
ing.” When, therefore, it is alleged, as it is so often alleged, 
that the power of fogs to deaden sound is well known, the 
disjunctive not is to be inserted before the predicate. 
The real enemy to the transmission of sound through the 
atmosphere has, I think, been clearly revealed by the foregoing 
inquiry. That enemy has been proved to be not rain, nor hail, 
nor haze, nor fog, nor snow — not water, in fact, in either a 
liquid or a solid form — but water in a vaporous form mingled 
with air so as to render it acoustically turbid and flocculent. 
This acoustic turbidity often occurs on days of surprising op- 
tical transparency. Any system of measures, therefore, founded 
on the assumption that the optical and acoustic transparency 
of the atmosphere go hand in hand must prove delusive. 
There is but one solution to this difficulty ; it is to make 
the source of sound so powerful as to be able to endure loss by 
partial reflection, and still retain a sufficient residue for trans- 
mission. Of all the instruments hitherto examined by us, the 
syren comes nearest to the fulfilment of this condition ; and 
its establishment upon our coasts will, in my opinion, prove 
an incalculable boon to the mariner. 
An account of the observations made during the recent fog will 
be included in the paper shortly to be presented to the Royal 
Society. These observations add the force of demonstration to 
others recorded in the paper, that fogs possess no such power 
of stifling sound as that hitherto ascribed to them. Indeed, 
the melting away of fog on December 13 was accompanied by 
an acoustic darkening of the atmosphere so great that, at a 
point midway between the eastern end of the Serpentine, 
where a whistle was sounded, and the bridge, the sound pos- 
sessed less than one-fourth of the intensity which it possessed on 
the day of densest fog. 
Thus, I think, has been removed the last of a congeries of 
errors which for more than a century and a half have been 
associated with the transmission of sound by the atmosphere. 
(This Paper was read before the Royal Society, Jan. 15, 1874.) 
