[MILLER] PHOTOGRAPHS OF FOG SIGNALS 163 



has been verified by the synthesis of the curve from the numerical 

 results of the analysis. The complete processes of, and apparatus 

 for, the analysis, synthesis, and correction of such curves have been 

 fully described and illustrated by the writer in the reference already 

 given- and in the Journal of the Franklin Institute.^ ^ 



The type of fog-alarm in use at the Father Point Station is that 

 known as the Northey diaphone. As described b}- Professor King, 

 the essential feature of this apparatus is a hollow plug-like piston in 

 which are cut a number of equidistant circumferential slits lying in 

 planes perpendicular to the axis of the piston. The piston fits closely 

 in a stationary cylinder in which are cut corresponding slits. Under 

 the influence of compressed air, by means of a special driving-head 

 and a valve system somewhat resembling that of a compressed air 

 riveter, the piston receives a rapid and regular oscillatory movement 

 which alternately opens and closes the slits in the cylinder. This 

 forms the "throat" of the fog-horn. Air at the pressure of about 25 

 pounds per square inch is supplied from compressors and storage 

 tanks to a chamber surrounding the exterior of the cylinder. As the 

 hollow piston oscillates the compressed air passes through the slits 

 into the hollow throat in intermittent pufts, giving rise to intense 

 sound waves which are propagated to the external atmosphere through 

 a conical trumpet tuned to resonance with the pitch of the signal tone. 



In the diaphone under observation the piston makes 87 to-and- 

 fro movements, or complete cycles, per second. The character of 

 the puffs of air emitted in the forward movement of the piston is 

 nearly the same as that produced by the backward movement, so 

 that practically, the period of the sound wave is that of the half cycle, 

 and its frequency is 174. The harmonic analyses of sounds described 

 in this paper are all based upon a fundamental frequency of 87, but 

 nearly all of the energy of the sound is confined to the even-numbered 

 terms of the harmonic components, so that the effective frequency 

 of the fog-alarm is 174. and this frequency is used in interpreting the 

 results. 



Table I gives the results of the analyses of all the photographs 

 taken on the acoustic axis of the horn, at distances ranging from 760 

 feet to 13,500 feet. The photographs represented b\' b, r, e, and / 

 were taken at about six o'clock, p.m., on September 24. 1917, while 

 all the others were taken between 10 o'clock a.m., and 12.30 o'clock, 



^The Henrici Harmonic Analyzer and Devices for Extending and Facilitating 

 Its Use, Dayton C. Miller, Journal of the Franklin Institute, September, 1916, pp. 

 285-322. 



^A 32-Element Harmonic Synthesizer, Dayton C. Miller, Joitrnd of the Franklin 

 Institute, January, 1916, pp. 51-81. 



