212 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
which I supposed to be exactly abreast of him, marked it, and on the following day, 
when the fog had cleared away, found that I was perfectly exact. If undisturbed by 
echoes, the ear, with a little practice, becomes capable of fixing with great sharpness 
the direction of a sound. 
On reaching the Serpentine this morning the clangour of a peal of bells, which then 
began to ring, seemed so close at hand that it required some reflection to convince me 
that they were ringing to the north of Hyde Park. The sounds fluctuated wonderfully 
in power, sometimes pouring forth a wealth of sound and then sinking to sudden penury. 
They disclosed to the mind’s eye the condition of the atmosphere through which these 
varying sounds were transmitted. Prior to the striking of eleven by the great bell of 
Westminster, a nearer bell struck with loud clangour. The first five strokes of the 
Westminster bell were afterwards heard, one of them being extremely loud ; the six last 
strokes were inaudible. I subsequently stationed my second assistant to attend to the 
12-o’clock bells. The clock which had struck so loudly at 11 was unheard at 12, while 
of the Westminster bell eight strokes out of twelve rendered themselves audible. To 
such astonishing changes is the atmosphere liable. 
Wishing to test still further the acoustic fluctuations of the day, I sent Mr. Cottrell 
and a younger assistant at 7 p.m. to the Serpentine. The Westminster bell striking 
seven was not at all heard, while the nearer bell already alluded to was heard distinctly. 
The fog had now cleared away, and the lamps on the bridge could be seen from the 
eastern end of the Serpentine burning brightly ; but instead of the sound sharing the 
improvement of the light, what might be properly called an acoustic fog took the place 
of its predecessor. The whistle and organ-pipe were sounded, three blasts of each in 
succession, several times ; one series only of the whistle was heard, all the other blasts 
being quite inaudible. Three series of the organ-pipe were heard, but exceedingly faintly. 
On reversing their positions and sounding as before, nothing whatever was heard. 
At 8 o’clock the chimes and hour-bell of the Westminster clock were very loud. The 
“ acoustic fog ” had shifted its position or temporarily melted away. 
An assistant placed at the end of the Serpentine sounded the whistle and pipe for 
fifteen minutes without interruption. An observer at the bridge noticed the fluctuations 
of the sound. Sometimes the whistle was heard alone, sometimes the organ-pipe. 
Sometimes both whistle and pipe began strongly and ended by sinking almost to inaudi- 
bility. Extraordinary fluctuations were also observed in the case of the bells to which 
reference has been already made : 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, as already stated, the optical analogue of these 
acoustical effects. In fact, 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 in part relapsed into its condition at 7 p.m., when all the strokes 
