Comparative experiments made at the outset gave a slight advantage to the upper instruments. They, therefore, were for the most part employed throughout the subsequent inquiry.
Our first experiments were a preliminary discipline rather than an organized effort at discovery. On May 19th the maximum distance reached by the sound was about 312 miles.[1] 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 19th was 6, and it blew at right angles to the direction of the sound.
The same instruments on May 20th covered a greater range of sound; but not much greater, though the disturbance due to local noises was absent. At four 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 favorable to the transmission of sound. We crept a little farther away, and by stretched attention managed to hear at intervals, at a distance of six miles, the faintest hum of the horns. A little farther on we again halted; but though local noises were absent, and though we listened intently, we 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 experiment between horns and guns as instruments for fog-signaling. The distinct report of the twelve 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 half-past twelve precisely the puff of an 18-pounder, with a three-lb. charge, was seen at Dover Castle, which was about a mile farther off than the South Foreland. Thirty-six seconds afterward 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 1⁄2miles, 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, and at 9.7 miles the report of a fourth gun was heard by 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
- ↑ In all cases nautical miles are meant.
