RECENT EXPERIMENTS ON FOG SIGNALS. 213 
In a paper published in the " Philosophical Transac- 
tions" for 1876, Professor Osborne Reynolds refers to 
these echoes in the following terms: " Without attempt- 
ing to explain the reverberations and echoes which have 
been observed, Twill merely call attention to the fact that 
in no case have I heard any attending the reports of the 
rockets,* although they seem to have been invariable 
with the guns and pistols. These facts suggest that the 
echoes are in some way connected with the direction given 
to the sound. They are caused by the voice, trumpets, 
and the syren, all of which give direction to the sound; 
but I am not aware that they have ever been observed in 
the case of a sound which has no direction of greatest in- 
tensity." The reference to the voice, and other references 
in his paper, cause me to think that, in speaking of echoes, 
Professor Osborne Reynolds and myself are dealing with 
different phenomena. Be that as it may, the foregoing 
observations render it perfectly certain that the condition 
as to direction here laid down is not necessary to the pro- 
duction of the echoes. 
There is not a feature connected with the aerial echoes 
which cannot be brought out by experiments in the air of 
the laboratory. I have recently made the following experi- 
ment: A rectangle, x Y (p. 214), 22 inches by 12, was 
crossed by twenty-three brass tubes (half the number 
would suffice and only eleven are shown in the figure), 
each having a slit along it from which gas can issue. In 
this way twenty-three low flat flames were obtained. A 
sounding reed a fixed in a short tube was placed at one 
end of the rectangle, and a " sensitive flame,"f / t some 
distance beyond the other end. When the reed sounded, 
the flame in front of it was violently agitated, and roared 
boisterously. Turning on the gas, and lighting it as it 
issued from the slits, the air above the flames became so 
heterogeneous that the sensitive flame was instantly stilled, 
rising from a height of 6 inches to a height of 18 inches. 
Here we had the acoustic opacity of the air in front of the 
South Foreland strikingly imitated. J Turning off the gas, 
* These carried 12 oz. of gunpowder, which has been found by 
Colonel Fraser to require an iron case to produce an effective ex- 
plosion. 
f Fully described in my " Lectures on Sound," 3d edition, p. 227, 
\ " Lectures on Sound," 84 ed., p. 208. 
