AS A VEHICLE OF SOUND. 
219 
with its transmission on October 14, when the wind had the same strength and the 
same direction, namely N.N.W. with a force of 2. My notes of the observations show 
that the latter was throughout a day of extreme optical clearness. The range was 10 
miles. On February 7 the ‘Argus’ heard the sound at 11 miles; and it was also 
heard through the densest fog at the Varne light-vessel, which is 12f miles from the 
Foreland. 
But this important circumstance is also to be borne in mind : on February 7 the syren 
happened to be pointed not towards the ‘ Argus,’ but towards Dover. Had the yacht 
been in the axis of the syren, it is highly probable that the sound might have been heard 
all the way across to the coast of France. 
The .fact that Mr. Troughton was able through the fog to “ locate ” the Foreland, and 
that his fixing of the direction should be subsequently found correct, is also one of 
great importance. 
Early in the morning of the 20th of February there was also a fog in the neighbour- 
hood of the South Foreland. It was a patchy fog, at intervals very thick and then 
comparatively clear. The ‘Argus’ steamed out at 8 a.m., and carried the sounds to a 
distance of 10 miles along the axial line of the instruments. At that distance the 
horn-sounds were better than those of the syren, although at shorter distances the syren 
had greatly excelled the horn. At the Varne light-ship (12f miles) the horn-sounds 
only had been heard during the fog. At the 10-miles position the atmosphere was 
beautifully clear, the sun shining strongly, and the fog-bank lying over the land clearly 
defined, extending seaward about 5 miles. 
Mr. Troughton states that the sounds of this day were very inferior to those of the 
much thicker and more homogeneous fog of February 7. 
Since the publication of the first notices of this investigation various communications 
have reached me, to two of which I should like to refer. The Rev. George H. Hetling, 
of Fulham, has written to me with a circumstantiality which leaves no room for doubt 
that he has heard the Portland guns at a distance of 44 miles through a dense fog. 
The Duke of Argyll has also favoured me with the following very interesting account 
of his own experience. Coming as it does from a disciplined scientific observer, it is 
particularly valuable. “This fact” (the permeability of fog by sound) “I have long 
known, from having lived a great part of my life within four miles of the town of 
Greenock, across the Frith. Ship-building goes on there to a great extent, and the 
hammering of the caulkers and builders is a sound which I have been in the habit of 
hearing with every variety of distinctness, or of not hearing at all, according to the state 
of the atmosphere ; and I have always observed on days when the air was very clear, 
and every mast and spar was distinctly seen, hardly any sound was heard ; whereas on 
thick and foggy days, sometimes so thick that nothing could be seen, every clink of every 
hammer was audible, and appeared sometimes as close at hand.” 
It is hardly necessary for me to say a word to guard myself against the misconception 
that I consider sound to be assisted by the fog itself. Fog I regard as the visible result 
2 f 2 
