210 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
heard the bell ofW estminster with great distinctness to-day, but that it had been heard still 
more loudly last night through exceedingly dense fog. On many clear days, they informed 
me, they fail to hear it at all. To-day the clangour at 12 o’clock was very loud. 
At 3 p.m. I went again to the Serpentine ; stationed my assistant, Mr. Cottrell, with 
a whistle and organ-pipe on the walk below the south-west end of the bridge dividing 
Hyde Park from Kensington Gardens, while I went to the eastern end of the Serpentine. 
There I heard distinctly both the whistle and the pipe sounding Mi 3 , which corresponds 
to 380 waves a second. The whistle was best heard. I then changed places with my 
assistant, and listening attentively at the bridge heard for a time the distinct blast of 
the whistle only. The organ-pipe at length sent its deeper note to me across the water ; 
it sometimes rose to great distinctness, and sometimes fell to inaudibility. The whistle 
showed the same intermittence as to period, but in the 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-sound proved itself 
the most efficient in piercing the fog. 
There seemed to me to be an extraordinary amount of sound in the air on Dec. 10 ; 
it was filled with a resonant roar from the Bayswater and Knightsbridge roads. The 
railway-whistles, which were frequently blown, were extremely distinct, while the 
fog-signals exploded at the various metropolitan stations kept up a loud and almost 
.constant cannonade. I could by no means reconcile this state of things with the state- 
ments so categorically made regarding the influence of fog on the sound of carriage- 
wheels and guns. 
The Serpentine presented the instructive appearance to which I have already referred. 
The water was warmer than the air, and the ascending vapour was instantly in great part 
condensed, thus revealing its distribution. Instead of being uniformly diffused, it formed 
wreaths and strige. I am pretty confident that had the vapour been able to maintain 
itself as such, the air to-day would have been more opaque to sound. In other words, 
I believe that the very cause which diminished the optical transparency of the atmo- 
sphere augmented its acoustic transparency. 
On the 11th of December, the fog being denser than before, at 2.50 I stationed 
Mr. Cottrell near the bridge, and took up a position myself near the end of the Serpen- 
tine. The whistle and the Mi 3 pipe were sounded in succession. I heard every blast 
of the whistle, and occasional blasts of the pipe. We reversed our positions, and I 
heard substantially the same, perhaps a little fainter. On joining my assistant at the 
bridge I heard the loud concussion of a gun, and was informed by a police-inspector 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 in relation to the present question ; so 
I immediately telegraphed to my friend Professor Abel, asking him for information. He 
was absent in Portsmouth when the telegram arrived, but on his return he found that 
guns had been fired at the proof-butts about the time mentioned in my telegram. On 
the following day he kindly furnished me with the following particulars : — 
