ATMOSPHERE AND THE ACOUSTIC EFFICIENCY OF FOG-SIGNAL MACHINERY. 259 
of incipient “ silent zones,” the amplitude diminishing with distance and increasing again as the distance 
is further increased. The comparatively small amplitudes along line VII. are due in part to the wind 
blowing against the direction of sound propagation and in part to the obstructing effect of the wharf. 
Nos. 3 to 14. Long Range Acoustic Surveys. (Tables 3 to 14, Charts 3 to 141) 
These observations were taken from the C.G.S. “ Lady Evelyn ” (Plate (iii).). The phonometer was set 
up on the main deck in such a position that an unobstructed view of the fog-signal station could be 
obtained. When the phonometer reading fell to 0 - l mm. the signal was still sufficiently loud for practical 
purposes. Signals which could still be heard, although incapable of affecting the phonometer, are entered 
in the tables as O’O. In the remarks, the intensity of the signal as estimated by ear is entered according 
to the following scale —distinct (between O’O and 0 - 05 on the phonometer), audible, just audible, barely audible, 
inaudible. When a signal was missed or not heard owing to disturbing ship noises a — is entered in the 
table. On landing at Father Point wharf after a series of observations, a number of phonometer readings 
were taken at this position (W, Chart 1), and a note made of the duration of the echoes. A number of 
readings were also taken at a control station behind the fog-signal station (S, Chart 1), and observations 
were made of the air pressure operating the diaphone. It would have been desirable to have had a third 
observer available to regulate the compressors and keep the pressure constant; it was fortunate, as pointed 
out in Appendix I., that the pitch of the diaphone varies over a very small range for considerable 
variations of operating pressure. Also, as may be seen from an inspection of tests 1 and 3 of 
Appendix III., the acoustic output does not vary between very wide limits over a considerable range of 
pressures. Thus, although the pressures at the fog-signal station could not be maintained as constant as 
could be desired, it is concluded that the results of the acoustic survey as shown graphically on Charts 3 to 14 
are comparable as regards conditions at the diaphone, and that the extraordinary variations of sound 
distribution observed from day to day are due for the most part to changes in atmospheric conditions. 
We proceed to direct attention to some of the outstanding features of the results obtained on these long- 
range surveys. 
No. 3. August 26, 1913. Table 3 and Chart 3. 
Wind .(as recorded at Father Point meteorological station) very variable. Note effect on sound and the 
tendency to form regions of inaudibility. Close to the fog-signal station (observations 84 to 89) note the 
deadening effect of a wind blowing from the direction of the shore as compared with the influence of a 
steadier and more homogeneous wind blowing from the sea (observations 25 to 30). Unfortunately, no 
anemometer was available for shipboard use at the time these observations were taken. 
VOL. CCXVIII.-A. 2 M 
