THE ATMOSPHERE AND FOG-SIGNALING. 545 



Comparative experiments made at the outset gave a slight advan- 

 tage to the upper instruments. They, therefore, were for the most 

 part employed throughout the subsequent inquiry. 



Oitr 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 3^ miles. 1 The wind, however, was 

 hisfh and the sea rou^h, 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 = calm to 12 = a hurricane, a little practice in 

 common producing a remarkable unanimity between different observ- 

 ers as regards the force of the wind. Its force on May 19th was 6, 

 and it blew at ri^ht 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 ab- 

 sent, 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 consid- 

 ered a decisive comparative experiment between horns and guns as 

 instruments for fog-signaling. The distinct report of the tAvelve 

 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 pufi" 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, demon- 

 strated. 



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, 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 Fore- 

 land 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 superi- 

 ority of the gun as a fog-signal. No single experiment is, to my 



1 In all cases nautical miles are meant. 

 vol. vi. 35 



