214 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
During the exceedingly dense and “ dripping ” fog of January 22 I placed myself near 
the same railings and heard every stroke of the bell. On the same day an assistant at 
the end of the Serpentine, when the fog was densest, heard the Westminster bell 
striking loudly eleven. Towards evening this fog began to melt away, and at 6 o’clock 
I went to the end of the Serpentine to observe the effect of the optical clearing of the 
atmosphere upon the sound. Not one of the strokes reached me. At 9 o’clock and at 
10 o’clock my able assistant Mr. Cottrell was in the same position, and on both occa- 
sions failed to hear a single stroke of the bell. It was a case precisely similar to that 
of December 13, when the dissolution of the fog was accompanied by a decided acoustic 
thickening of the air. 
On the morning of the 5th of February a dense fog filled London. At 10.45 I went 
to the Green Park and heard the chimes of the great bell in a position where on many 
clear calm clays no sound had been audible. I walked thence to the eastern end of the 
Serpentine and heard the chimes at 11 a.m. Eight of the subsequent strokes of the bell 
reached me with marked power, the ninth and tenth (doubtless through the passage of 
an acoustic cloud) sank almost to inaudibility, while in the eleventh the sonorous power 
was restored. The air seemed dead calm at the time ; no observable motion could be 
seen among the lightest twigs of the adjacent trees, but my breath was wafted in a 
direction opposed to the sound. Such observations show what instructive results may 
he obtained from a mode of observation accessible to all. 
§ 13. Experiments on Artificial Fogs. 
The smoke from smouldering brown paper was allowed to stream upwards into the 
tunnel X Y, Plate XVIII. ; the action upon the sound-waves was strong, rendering the 
short and agitated sensitive flame F tall and quiescent. 
Instead of the smoke and heated air, the heated air alone from four red-hot pokers 
was permitted to stream upwards into the tunnel ; the action on the sound-waves was 
very decided, though the tunnel was optically empty. 
A thick fog of chloride of ammonium was sent into the tunnel ; a candle placed at 
one end could not he seen at the other, still there was no appreciable action upon the 
waves of sound. 
Air first passed through ammonia, and then through hydrochloric acid, was sent into 
the tunnel ; the agitated flame was rendered immediately quiescent, indicating a very 
decided action on the sound-waves. The flask containing the hydrochloric acid was, 
however, very warm, suggesting that differences of air-temperature might have come 
into play. 
Air passed through perchloride of tin and sent into the tunnel produced exceedingly 
dense fumes. The action on the sound-waves was very strong. 
The dense smoke of resin, burnt before the open end of the tunnel and blown into it 
with a pair of bellows, had the effect of stopping the sound-waves, so as to still the 
agitated flame. 
