THE ATMOSPHERE AND FOG-SIGNALING. 691 



The whistle showed the same intermittence as to period, but in an 

 opposite sense ; for when the whistle was faint the pipe was strong, 

 and vice versa. To obtain the fundamental note of the pipe it had to 

 be blown gently, and on the whole the whistle proved the most efficient 

 in piercing the fog. 



An extraordinary amount of sound filled the air during these ex- 

 periments. The resonant roar of the Bayswater and Knightsbridge 

 roads ; the clangor of the great bell of Westminster ; the railway- 

 whistles, which were frequently blown, and the fog-signals exploded 

 at the various metropolitan stations, were all heard with extraordinary 

 intensity. This could by no means be reconciled with the state- 

 ments so categorically made regarding the acoustic impenetrability 

 of a London fog. 



On the 1 1 tli of December, the fog being denser than before, I heard 

 every blast of the whistle, and occasional blasts of the pipe, over the 

 distance between the bridge and the eastern end of the Serpentine. 

 On joining my assistant at the bridge, the loud concussion of a gun 

 was heard by both of us. A police-inspector affirmed that it came 

 from Woolwich, and that he had heard several shots about 2 p. m. 

 and previously. The fact, if a fact, was of the highest importance ; 

 so I immediately telegraphed to Woolwich for information. Prof. 

 Abel kindly furnished me with the following particulars : 



" The firing took place at 1.40 p. m. The guns proved were of comparatively 

 small size 64-pounders, with 10-lb. charges of powder. 



" The concussion experienced at my house and office, about three-quarters 

 of a mile from the butt, was decidedly more severe than that experienced when 

 the heaviest guns are proved with charges of 110 to 120 lbs. of powder. There 

 was a dense fog here at the time of firing." 



These were the reports heard by the police-inspector ; on subse- 

 quent inquiry it was ascertained that two guns were fired at about 

 3 p. m. These were the guns heard by myself. 



Prof. Abel also communicated to me the following fact : 



" Our workman's bell at the arsenal-gate, which is of moderate size and any 

 thing but clear in tone, is pretty distinctly heard by Prof. Bloxam only when 

 the wind is northeast. During the whole of last week the bell was heard with 

 great distinctness, the wind being southwesterly (opposed to the sound). The 

 distance of the bell from Bloxam's house is about three-quarters of a mile as the 

 crow flies." 



Assuredly no question of science ever stood so much in need of 

 revision as this of the transmission of sound through the atmosphere. 

 Slowly but surely we mastered the question ; and the further we ad- 

 vanced the more plainly it appeared that our reputed knowledge re- 

 garding it was erroneous from beginning to end. 



On the morning of the 12th the fog attained its maximum density. 

 It was not possible to read at my window, which fronted the open 



