188 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
to sea in the axes of the horns. The maximum distance reached by the sound was about 
three and a half miles*. The wind, however, was high and the sea rough, so that local 
noises interfered to some extent with our appreciation of the sound. 
Mariners express the strength of the wind by a series of numbers extending from 
0 = calm to 12 = a hurricane, a little practice in common producing a remarkable 
unanimity between different observers as regards the force of the wind. Its force on 
May 19 was 6, and midway between the axes of the two trumpets it blew at right angles 
to the direction of the sound. 
The same instruments on the 20th of May covered a greater range of sound ; but not 
much greater, though the disturbance due to local noises was absent. At 4 miles 
distance in the axes of the horns they were barely heard, the air at the time being calm, 
the sea smooth, and all other circumstances exactly those which have been hitherto 
regarded as most favourable to the transmission of sound. We crept a little further 
away, and by stretched attention managed to hear at intervals, at a distance of 6 miles, 
the faintest hum of the horns. A little further out we again halted ; but though local 
noises were absent, and though we listened intently, w T e heard nothing. 
This position, clearly beyond the range of whistles and trumpets, was expressly 
chosen with the view of making what might be considered a decisive comparative expe- 
riment between horns and guns as instruments for fog-signalling. The distinct report 
of the 12 o’clock gun fired at Dover on the 19th suggested this comparison, and through 
the prompt courtesy of General Sir A. Horsford we were enabled to carry it out. At 
12.30 precisely the puff of an 18-pounder, with a 3-lb. charge, was seen at Dover Castle, 
which was about a mile further off than the South Foreland. Thirty-six seconds after- 
wards the loud report of the gun was heard, its complete superiority over the trumpets 
being thus, to all appearance, demonstrated. 
We clinched this observation by steaming out to a distance of 8^ miles, where the 
report of a second gun was well heard by all of us. At a distance of 10 miles the 
report of a third gun was heard by some of us, and at 9 *7 miles the report of a fourth 
gun was heard by us all. 
The result seemed perfectly decisive. Applying the law of inverse squares, the sound 
of the gun at a distance of 6 miles from the Foreland must have had more than two 
and a half times the intensity of the sound of the trumpets. It would hardly have been 
rash under the circumstances to have reported without qualification the superiority of 
the gun as a fog-signal. No single experiment is, to my knowledge, on record to prove 
that a sound once predominant would not be always predominant, or that the atmosphere 
on different days would show preferences to different sounds. On many subsequent 
occasions, however, the sound of the horn proved distinctly superior to that of the gun. 
This selective power of the atmosphere revealed itself more strikingly in our autumn 
experiments than in our summer ones ; and it was sometimes illustrated within a few 
hours of the same day : of two sounds A and B for example, A would have the greatest 
range at 10 a.m. and B the greatest range at 2 p.m. 
* In all cases nautical miles are meant. 
