ATMOSPHERE AND THE ACOUSTIC EFFICIENCY OF FOG-SIGNAL MACHINERY. 237 
a large expanse of water.*' When, in addition, the direction of the wind is in the 
direction of sound propagation, we have the most favourable conditions of audibility. 
Unfortunately, such conditions are, as a rule, not available to ships at sea. Herein 
lies the probable explanation of the statements made by inhabitants of the country 
surrounding a fog-signal station, that the range of audibility is, on the average, 
considerably greater over the land than it is over the sea, in spite of the directive 
effect of the diaphone trumpet and of the obstructing effect of the fog-alarm buildings 
on the landward side. 
(iii.) Note on Atmospheric Losses. 
In order to obtain a rough estimate of energy losses in the atmosphere, phono¬ 
meter observations were taken over a series of circular courses having the fog-signai 
station as centre. The results are shown graphically in Charts 6, 9, 12, 13, and 14, 
and are analysed in detail in Appendix II., Table I. The flux of energy in the 
sound-waves across portions of spheres of different radii subtending the same solid 
angle at the diaphone is calculated in C.G.S. units. From a knowledge of the 
acoustic output as determined in Appendix III., we may estimate the energy flux 
which should be contained in the same solid angle under ideal conditions of propaga¬ 
tion. The results seem to indicate that a large proportion of the atmospheric losses 
occur at no very great distance from the diaphone trumpet. Once the sound has 
penetrated beyond a radius of about 3000 feet, it will continue to travel on a fairly 
calm day (wind not greater than 3 miles per hour) with comparatively little loss. In 
some cases the observations show that at greater distances the sound may be actually 
reinforced by contributions arising from reflections or refractions from the upper 
regions of the atmosphere, and to some extent, possibly, by the sound scattered by 
atmospheric eddies reaching the observer from all directions. 
In future studies of losses occurring in the immediate neighbourhood of a fog-signal 
station, the influence of the buildings in setting up eddy-motion near the siren 
trumpet with the wind in certain quarters should be kept in mind. A typical case 
of the atmospheric disturbance to leeward of a building is described by Dines! in a 
paper on “ cliff eddies.” 
(iv.) Note on the Effect of Fog. 
According to the verdict of the 1873 South Foreland tests, it would appear that 
the passage of sound through the atmosphere is not impeded by fog, or by falling 
snow, hail, or rain. No fog occurred during the Trinity House experiments conducted 
at St. Catherine’s Point in 1901. It is easily understood that the passage of sound 
may be favoured by the comparatively calm and homogeneous condition of the 
* The Gulf of St. Lawrence is about 25 miles wide in the neighbourhood of Father Point, 
t Dines, J. S., “ Fourth Report on Wind Structures,” ‘ Technical Report of the Advisory Committee 
' for Aeronautics,’ 1912-13, p. 325. 
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