ATMOSPHERE AND THE ACOUSTIC EFFICIENCY OF FOG-SIGNAL MACHINERY. 227 
Part II.—ON THE MEASUEEMENT OF SOUND INTENSITY AND OF 
THE ACOUSTIC OUTPUT OF FOG-SIGNAL APPAEATUS. 
§ 8.—Note on Previous Fog-Signal Experiments. 
As the determination of the physical principles underlying the design of sound¬ 
generating machinery is of great importance with reference to the construction of 
these aids to navigation, the testing of fog-signal equipment has in the past been 
made the subject of several lengthy and detailed reports. The mode of propagation 
of sound-waves under the conditions which exist at sea has also at various times 
engaged the attention of a number of scientific men, Among the more important 
investigations we may mention Tyndall’s experiments,* carried out at the South 
Foreland in 1874 and Henry’s Eeportt published in the same year. In these 
experiments the decrease of intensity with distance and the effect of winds, fog and 
other atmospheric conditions on the audibility of the signals were investigated. 
Intensity was estimated in terms of audibility as judged by the unaided ear. The 
results are now well known and are described in considerable detail in text-books and 
treatises on sound.| 
An extensive series of tests, carried out at a large number of stations on the North 
Atlantic coast of the United States, are described in detail in Livermore’s Eeport§ 
published in 1894. Here an attempt was made to estimate the intensity of sound in 
terms of a physical scale. While the ear still remained the instrument of comparison, 
observations were controlled by means of standards consisting of small musical 
instruments whose notes imitated in pitch and quality that of the fog-signal as heard 
from a considerable distance. These were enclosed in small sound-proof boxes, and 
the intensity of the sound was varied by a sliding cover which could be adjusted to 
open to different sized apertures. It is stated that estimates of intensity obtained 
in terms of this arbitrary scale by different observers proved to be in satisfactory 
agreement. 
In 1901 a detailed report was issued by a Committee of the Trinity House|| on 
experiments carried out at St. Catherine’s Point, Isle of Wight, under the scientific 
direction of Lord Eayleigh and Sir Thomas Matthews. These tests were for the 
most part confined to an investigation of the relative merits of various forms of sound 
* Tyndall, “On the Atmosphere as a Vehicle of Sound,” ‘Phil. Trans.,’ vol. 164, pp. 183-244. An 
account of these experiments is given in Tyndall’s ‘ Sound.’ 
t Henry, ‘Report of the Lighthouse Board of the United States for the year 1874,’ Washington. 
+ Rayleigh, ‘Sound,’ 1896, vol. II., p. 129, et seq.; Lamb, ‘Dynamical Theory of Sound,’ 1910, 
pp. 216-222. 
§ Livermore, W. R., “Report upon Fog-Signal Experiments,” ‘Report of the U.S. Lighthouse Board,’ 
Appendix No. V., 1894, Washington. 
|| ‘ Report of the Trinity House Fog-Signal Committee on Experiments conducted at St. Catherine’s 
Point, Isle of Wight,’ 1901. (Published by Darling & Son, London, 1901.) 
VOL. CCXVIII.— A. 2 IT 
