ATMOSPHERE AND THE ACOUSTIC EFFICIENCY OF FOG-SIGNAL MACHINERY. 231 
by the writer this characteristic termination was sometimes heard when the sustained 
note was completely inaudible. 
The air consumption is regulated by a set screw on the main valve ; by adjusting 
this screw so as to allow the valve more or less clearance, the air consumption is 
regulated until the amount consumed per blast is equal to the pumping capacity 
of the compressors between blasts. The pressure then attains a stationary state and 
the signals are issued automatically at the proper intervals without further attention 
on the part of the station operator. 
In fig. 3 is drawn to scale the diaphone tested by the writer at Father Point, 
showing schematically the arrangement of the differential thermometers employed 
in the acoustic output determinations. The short conical trumpet (semi-vertical 
angle 6°'5, over all length = 0’66 x wave-length) has been found to be the most 
efficient as a result of numerous tests carried out by the makers and the Canadian 
Government engineers.* This conclusion agrees with the practice of the French 
Lighthouse Service.! From the evidence discussed in the present paper as to the 
propagation of fog-signal sound-waves as waves of finite amplitude, it would appear 
that the function of the trumpet as a resonator is limited by serious propagation 
losses occurring before spherical divergence reduces the amplitude to such small 
values that these losses become negligible. It is hoped to study this aspect of the 
subject experimentally by the method of the oscillating valve mentioned in the next 
section ; data thus obtained may be useful in deciding the most efficient form of 
trumpet for different purposes.^ 
§ 10. Note on the Practical Measurement of the Characteristics of 
Sound-Waves. 
Although several laboratory methods have been successfully employed in the 
measurement of sound intensity, none of these seem to have been suitable for service 
under the exacting conditions arising in carrying out tests on fog-signal generators. 
It is sufficient to mention two of these methods, both of which enable the compression 
in a sound-wave to be measured in absolute units, and at the same time do not 
depend on the theory of propagation of sound-waves of small amplitude. Both of 
these methods would seem capable of employment under special conditions in the 
* The original diaphone was invented by Dr. Owen Hope-Jones, U.S. Patent, No. 702,557, 
June 17, 1903. Its utilisation for fog-alarm piu-poses was first suggested by Lieut.-Col. Anderson, 
Chief Engineer of the Department of Marine and Fisheries of Canada, who, with his chief assistant, 
Mr. B. H. Fraser, determined from numerous tests the lines along which the diaphone evolved into 
full-sized fog-signal apparatus in the hands of Mr. J. P. Northey, of Toronto. The present diaphones 
are the results of several improvements and modifications due to Mr. Northey which have been made 
the subject of a number of patents dating from 1903. (See under description of fig. 1.) 
t ‘ Trinity House Report,’ p. 32 (see footnote, p. 227, above). 
1 [Added February 14, 1919.—For conclusions on this point resulting from more recent work, see 
footnote, p. 247.] 
