216 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
ascended through it, many minutes elapsed before the candle-flame could be seen through 
the attenuated fog. 
Steam from a copper boiler was so copiously admitted into the cupboard as to fill it 
with a dense cloud. No real cloud was ever so dense, still the sound passed through it 
without the least sensible diminution. This being the case, cloud-echoes, I think, are 
not a likely phenomenon. 
In any and all of these cases, when a couple of Bunsen’s burners were ignited within ‘ 
the cupboard containing the fumes, less than a minute’s action rendered the air so 
heterogeneous that the sensitive flame was completely stilled. 
The same occurred when the air within the cupboard was optically clear. 
The foregoing experiments were repeated, the density of the fumes being tested by 
their action on the electric light. The density of these acoustically inactive fogs was 
such as to cut off the light totally, even when concentrated by a lens. The fumes 
themselves were illuminated by diffused light, but the carbon-points could not be seen. 
The action of rain, hail, and snow was imitated by dense showers of water, sand, 
seeds, and bran : their action, within the limits employed, was nil. 
§ 14. Observations at the South Foreland. 
Satisfactory and, indeed, conclusive as these results were, I desired exceedingly to test 
in fog the instruments actually employed at the South Foreland. On the 10th of 
February it gave me great pleasure to receive the following note and enclosures from 
the Deputy Master of the Trinity House : — 
“ My deae Tyndall, — The enclosed will show how accurately your views have been 
verified, and I send them on at once without waiting for the details. I think you will 
be glad to have them, and as soon as I get the report it shall be sent to you. I made 
up my mind ten days ago that there would be a chance in the light foggy-disposed 
weather at home, and therefore sent the ‘ Argus ’ off at an hour’s notice, and requested 
the Fog Committee to keep one member on board. On Friday I was so satisfied that 
the fog would occur that I sent Edwaeds down to record the observations 
“Very truly yours, 
“ Feed.' Aeeow.” 
The enclosures referred to were notes from Capt. Atkins and Mr. Edwaeds. Capt. 
Atkins writes thus to the Chairman of the Fog-Signal Committee : — 
“ As arranged I came down here by the mail express, meeting Mr. Edwaeds at Cannon 
Street. We put up at the { Dover Castle,’ and next morning at 7 I was awoke by the 
sounds of the syren. On jumping up I discovered that the long-looked-for fog had 
arrived, and that the ‘ Argus ’ had left her moorings. 
“ However, had I been on board, the instructions I left with Teoughton [the Master 
of the ‘ Argus ’] could not have been better carried out. About noon the fog cleared 
up and the ‘ Argus ’ returned to her moorings, when I learned that they had taken both 
syren- and horn-sounds to a distance of 11 miles from the station, where they dropt a 
