ATMOSPHERE AND THE ACOUSTIC EFFICIENCY OF FOG-SIGNAL MACHINERY. 229 
devised by the writer. The excellent pitch regulation and relatively pure quality 
of note emitted by the diaphone made the use of the Webster phonometer in the 
measurement of sound intensity at a distance especially reliable. 
§ 9. Description of the Diaphone. 
The essential features of this modification of the siren are shown in fig. 1, where 
^ the principal dimensions of the diaphone actually tested at Father Point are also 
indicated. The apparatus consists essentially of a brass cylinder into which fits 
accurately a hollow piston. The head of this piston is of somewhat larger size and 
fits into a corresponding cylinder fitted with valve-ports so arranged that the 
admission of compressed air into this driving cylinder (referred to as “ driving air ”) 
causes the piston to oscillate through a small amplitude (0'15 inch) at half the 
frequency of the note which it is desired to produce. Both the cylinder and piston 
are cast of brass and carry on the outer surface of the former and the inner surface 
of the latter, six light longitudinal ribs; in this way the cylinder and piston may 
be cut transversely by a series of narrow ports ( T V inch wide) at equal distances 
apart. As the piston vibrates the main air supply (referred to as the “ sounding 
air”) is admitted intermittently from a cast-iron chamber completely surrounding 
the cylinder and connected through a large relay valve to the compressed-air tanks 
and compressors. 
The variation of pitch with pressure (referred to as “pitch regulation”) was 
measured by means of the phonometer and was found to be relatively small; it is 
seen from Table III. of Appendix I. that the pitch only increases from 169 to 182 
complete vibrations per second for an increase of operating pressure from 12 to 
2 7 lbs./sq. in. above atmospheric. This characteristic, which contributes greatly 
to the satisfactory performance of the instrument, is probably due to the fact that 
the frequency is governed largely by the mass of the piston and the elastic reaction 
of the air in the trumpet (toned to resonance at pitch 180) and in the space behind 
the piston-head (“ cushioning effect ”). 
The operation of the valves so as to give the blasts at the required intervals is 
automatic. By means of a suitably adjusted cam mechanism connected to the main 
shaft of the engine driving the compressors, a small valve is first of all opened 
causing a second valve to admit the “driving air” to the driving cylinder, thus 
setting the piston into vibration ; the same valve also admits air into a large relay 
valve, allowing the main “ sounding air ” to pass into the diaphone a fraction of 
a second later, so that the note begins to sound only when the piston has attained 
its normal frequency. In fact, the sharp commencement of the note and its sudden 
termination by a rapid fall in pitch during a small fraction of a second is characteristic 
of the diaphone signal and is very favourably commented on by navigators as entirely 
distinct from any siren note which might be sounded from a passing ship and ot 
great value in determining the direction of the sound. During the tests carried out 
2 h 2 
