THE ATMOSPHERE AND FOG-SIGNALING. 693 



Extraordinary fluctuations were also observed in the case of the 

 church-bells heard in the morning ; in a few seconds they would sink 

 from a loudly-ringing peal into utter silence, from which they would 

 rapidly return to loud-tongued audibility. The intermittent drifting 

 of fog over the sun's disk, by which his light is at times obscured, 

 at times revealed, is the optical analogue of these effects. As regards 

 such changes, the acoustic deportment of the atmosphere is a true 

 transcript of its optical deportment. 



At 9 p. m. three strokes only of the Westminster clock were heard ; 

 the others were inaudible. The air had relapsed in part into its 

 condition at 7 P. m., when all the strokes were heard. 



The quiet of the park this evening, as contrasted with the resonant 

 roar which filled the air on the two preceding days, was very re- 

 markable. The sound, in fact, was stifled in the optically clear but 

 acoustically flocculent atmosphere. 



On the 13th the fog being displaced by thin haze, I went again to 

 the Serpentine. The carriage-sounds were damped to an extraor- 

 dinary degree. The roar of the Knightsbridge and Bayswater roads 

 had subsided, the tread of troops which passed us a little way off was 

 unheard, while at 11 a.m. both the chimes and the hour-bell of the 

 Westminster clock were stifled. Subjectively considered, all was 

 favorable to auditory impressions ; but the very cause that damped 

 the local noises extinguished our experimental sounds. The voice 

 across the Serpentine to-day, with my assistant plainly visible in front 

 of me, was distinctly feebler than it had been when each of us was 

 hidden from the other in the densest fog. 



Placing the source of sound at the eastern end of the Serpentine, 

 I walked along its edge from the bridge toward the end. The distance 

 between these two points is about 1,000 paces. After five hundred 

 of them had been stepped, the sound was not so distinct as it had been 

 at the bridge on the day of densest fog ; hence, by the law of inverse 

 squai-es, the optical cleansing of the air through the melting away of 

 the fog had so darkened it acoustically, that a sound generated at the 

 eastern end of the Serpentine was lowered to one-fourth of its intensity 

 at a point midway between the end and the bridge. 



To these demonstrative observations one or two subsequent ones 

 may be added. On several of the moist and warm days at the begin- 

 ning of this year I stood at noon beside the railing of St. James's 

 Park, near Buckingham Palace, three-quarters of a mile from the clock- 

 tower, which was clearly visible. Not a single stroke of " Big Ben " 

 was heard. On January 19th, fog and drizzling rain obscured the 

 tower ; still from the same position I not only heard the strokes of the 

 gi-eat bell, but also the chimes of the quarter-bells. 



During the exceedingly dense and " dripping" fog of January 22d, 

 from the same railings, I heard every stroke of the bell. At the end 

 of the Serpentine, when the fog was densest, the Westminster bell 



