ATMOSPHERE AND THE ACOUSTIC EFFICIENCY OF FOG-SIGNAL MACHINERY. 
235 
head of an observer at a distance. On the contrary, when travelling with the wind, 
the wave-front is continually tilted downwards, thus explaining the familiar fact that 
sound may be more easily heard, and at a greater distance, when the observer is to 
leeward of the source. This explanation was given independently by Reynolds* * * § 
and confirmed by a number of observations. These conclusions have been supported, 
on the whole, by the results of the various fog-signal tests already referred to 
During the last few years, through the development of aviation, a great deal of 
attention has been directed to the investigation of the structure of the atmosphere. 
It has been known for a long time that the wind near the earth’s surface is the seat 
of innumerable eddies, t Quite recently the subject of eddy motion in the atmosphere 
has been definitely formulated in mathematical terms by Taylor.| Observations by 
Dines§ and others on the gustiness of the wind are interpreted by Taylor as 
indicating that the eddy-motion has a tolerably regular structure, and that in a wind 
of given velocity and vertical gradient, a fairly definite diameter may be assigned to 
the eddies. In atmospheric movements these eddies, which have a certain degree of 
permanence, play the part of molecules in the kinetic theory of gases. Thus the 
conception of “ eddy-viscosity ” and “ eddy-conductivity ” as regards the transfer of 
momentum and heat in the atmosphere is employed with success in the interpretation 
of many meteorological phenomena. 
The conception of eddy-motion has recently been employed by Taylor|| to 
account for the attenuation of sound in a windy atmosphere. It is shown that eddies 
scatter a certain portion of the energy of the sound-waves in all directions at the 
expense of that in the wave-train advancing in any particular direction. The 
mechanism of attenuation of sound by eddies is thus somewhat analogous to that of 
the extinction of light in the atmosphere by the scattering effect of dust or fog 
particles. In the opinion of the writer many of the results observed in connection 
with fog-signal tests may be interpreted in terms of the eddy-motion theory. For 
instance, the remarkably loud and well-sustained echoes often observed after a loud 
blast has been sounded are probably nothing more than the scattered and re-scattered 
sound-waves reaching the observer from all directions. It thus appears that in the 
eddy-structure of the atmosphere we must look for the interpretation of Tyndall’s 
state of “ acoustic flocculence,” while the eddies themselves are the realization of his 
“ acoustic clouds.” Further evidence in favour of the “ eddy-motion ’ theory is 
* Reynolds, ‘Royal Soc. Proc.,’ 22, 1874, p. 531; or ‘ Scientific Papers,’ vol. i., p. 89. 
t Mallock, A., “ Note on Wind Velocity and Gusts,” ‘ Technical Report of the Advisory Committee 
for Aeronautics,’ 1912-13, Report No. 93, p. 329. 
t Taylor, G. I., “Eddy Motion in the Atmosphere,” ‘Phil. Trans.,’ 215 A., pp. 1-26. 
§ Dines, J. S., ‘Technical Report of the Advisory Committee for Aeronautics,’ 1911-12, pp. 213-230 ; 
1912-13, pp. 310, et seq. 
|| The writer is indebted to Mr. Taylor for communicating these results by letter and for permission 
to mention them here prior to the publication of his paper on the subject. 
VOL. CCXVIII.—A. 2 I 
