48 JORNAL AND PROCEEDINGS. 
It is a frequently expressed opinion that foggy atmosphere is 
adverse to the transmission of sound. Experiments have proved 
that the reverse is the case; sounds travelling not only further but 
with greater volume in misty air. It has been proved, however, 
that there are what are technically called acoustic fogs, viz. : strata 
or sections of the atmosphere which are almost impervious to sound 
though conveying to the eye no indication of any difference in 
quality from the surrounding air. Many experiments at the various 
gun and signal stations on the coast have given curious and interest 
ing results in this connection. 
A vessel approaching the shore heard a fog signal (a 1o-inch 
steam whistle) distinctly at six miles distance; when it reached a 
distance of three miles the sound vanished and was not heard again 
until within a quarter of a mile of the signal. 
Signals are often heard at a great distance in one direction, 
while in another are scarcely audible for a mile. This is not the 
effect of wind, as the signal is frequently heard further against the 
wind than with it. Difficulties with fog signals arise from the fact 
that they seem to be sometimes surrounded by a belt of varying 
thickness of non-homogeneous air from which the sound appears to 
be entirely absent. This action is common to all ear signals and 
has been observed at times at all signal stations, at one of which the 
signal is situated on a bare rock 20 miles from land and with no 
surrounding objects to affect the sound. 
Experiment and observation lead to the conclusion that these 
anomalies in the action of fog signals are to be attributed mainly to 
the want of uniformity in the surrounding air, and that snow, rain, © 
fog and wind have much less influence than has been generally 
supposed. It is on record that at the Battle of Gain’s Farm, in 
Virginia, two men watched the battle from an opposite hill about 
one and a half miles distant ; the day was a mid-summer day of 
perfect clearness ; they saw the musket-fire of both sides and the 
flash of guns ; they watched all the proceedings for two hours of a 
battle where 50,000 men were engaged and too pieces of field 
artillery zezthout hearing a single sound. In the intervening valley 
was a swamp and on each side of it a clearing, part cultivated and 
part not, giving conditions capable of providing several belts of air 
varying in the amount of watery vapour and arranged at right angles 
to the travel of the acoustic waves. 
