238 DR. LOUIS VESSOT KING ON THE PROPAGATION OF SOUND IN THE FREE 
atmosphere usually met with in foggy weather. When the fog lies over the sea in 
the form of low-lying, distinctly separated banks, as is often the case in the Gulf of 
St. Lawrence, conditions are very different. In these circumstances it has been 
pointed out by Catford # as a result of five years’ observations that “ when both fog- 
signal and observer are immersed in the same bank of fog, little interference may be 
expected : if the fog-signal is in fog, and the observer in clear atmosphere, or vice 
versa,, great interference may be expected, still more so if the signal is in one fog- 
bank with the observer in another bank ; a bank of fog will often reflect sound very 
strongly and definitely.” 
The influence of fog particles on the passage of sound has been studied theoreti¬ 
cally by Sewell,! who found that a fog containing 10 6 particles per cubic centimetre 
having diameters as small as 0'002 mm. would not interfere appreciably with the 
propagation of waves of small amplitude over the ordinary range of frequencies. 
Although preparations were made to navigate during a fog, no opportunity 
occurred of carrying out an acoustic survey under these conditions during the entire 
series of Father Point tests. It has been repeatedly asserted to the writer by 
lighthouse-keepers and others that the sound of a diaphone as heard at a moderate 
distance (within 1000 feet) seems to be very appreciably stifled. On the only 
occasion that fog occurred this conclusion was confirmed by the writer as judged by 
ear. Before the phonometer could be set up to test the point objectively, the fog 
lifted, and no occasion of carrying out further investigations presented itself subse¬ 
quently. It is not impossible, in the case of very intense waves in the immediate 
vicinity of the diaphone trumpet, that a heavy fog may give rise to a marked 
extinction through its effect on the viscosity, thermal conductivity, and specific heat 
of the atmosphere. From what has already been said on the subject of the propaga¬ 
tion of waves of finite amplitude, it is easily seen how a change in the physical 
properties of air occasioned by the presence of fog| may be the cause of increased 
energy losses associated with the- propagation of waves of large amplitude. The 
existence of such losses may not be inconsistent with better audibility at a great 
distance, resulting from propagation under conditions of improved atmospheric 
homogeneity generally prevailing in foggy weather. 
(v.) Note on the Determination of the Direction of Sound. 
It is important that the navigator be able to ascertain as accurately as possible 
the bearing of a fog-alarm from the signals which he hears. From what lias already 
* Catford, E. 0. (Engineer-in-charge of the Platte Foug&re Fog-signal Station, Guernsey), “ Fog 
Signals,” ‘Engineer,’ 119, Feb. 5, 1915, pp. 129-130. 
t Sewell, C. J. T., “ The Extinction of Sound in a Viscous Atmosphere,” &c., ‘ Phil. Trans., 5 
210, A, 1910, pp. 239-270. 
j A study of the physical characteristics of fog has recently been commenced by V ELLS, P. V., and 
Thomas, A. L., ‘ Bulletin No. 5 of the International Ice Observation and Ice Patrol Service, 5 U.S. 
Treasury Department, Washington, 1916. 
