184 
PROFESSOR TYNDALL ON THE ATMOSPHERE 
light began to dawn upon the subject, revealing many old errors and some novel truths. 
As the results had a scientific as well as a practical bearing, I requested permission to 
lay them before the Royal Society, the prompt and cordial consent of the Elder Brethren 
being the response. 
§ 2. Condition of the Question. 
A few extracts and references will suffice to show the state of the question when this 
inquiry began. “ Derham,” says Sir John Herschel, “ found that fogs and falling rain, 
but more especially snow, tend powerfully to obstruct the propagation of sound, and 
that the same effect was produced by a coating of fresh-fallen snow on the ground, though 
when glazed and hardened at the surface by freezing it had no such influence”* * * § . 
In a letter addressed to the President of the Board of Trade in 1863f, Dr. Robinson, 
of Armagh, thus summarizes our knowledge of fog-signals : — " Nearly all that is known 
about fog-signals is to be found in the Report on Lights and Beacons ; and of it much is 
little better than conjecture. Its substance is as follows: — 
“ Light is scarcely available for this purpose. Blue lights are used in the Hooghly ; 
but it is not stated at what distance they are visible in fog : their glare may be seen 
further than their flamej. It might, however, be desirable to ascertain how far the 
electric light or its flash can be traced §. 
“ Sound is the only known means really effective ; but about it testimonies are con- 
flicting, and there is scarcely one fact relating to its use as a signal which can be consi- 
dered as established. Even the most important of all, the distance at which it ceases 
to be heard, is undecided. 
“Up to the present time all signal-sounds have been made in air, though this medium 
has grave disadvantages : its own currents interfere with the sound-waves, so that a gun 
or bell which is heard several miles down the wind is inaudible more than a few furlongs 
up it. A still greater evil is that it is least effective when most needed ; for fog is 
a powerful damper of sound.” 
Dr. Robinson here expresses the universally prevalent opinion, and he then assigns 
the theoretic cause. Eog, he says, “ is a mixture of air and globules of water, and at each 
of the innumerable surfaces where these two touch, a portion of the vibration is reflected 
and lost || Snow produces a similar effect, and one still more injurious.” 
Reflection being thus considered to take place at the surfaces of the suspended par- 
ticles, it followed that the greater the number of particles, or, in other words, the denser 
the fog, the more injurious would be its action upon sound. Hence optical transparency 
came to be considered as a measure of acoustic transparency. On this point Dr. Robinson, 
in the letter referred to, expresses himself thus: — “At the outset, it is obvious that, to 
* Essay on Sound, par. 21. 
f Report of the British Association for 1863, p. 105. 
+ A very sagacious remark : see letters in the Appendix. 
§ Powerful electric lights have been since established and found ineffectual. 
|| This is also Sir John Herschel’ s way of regarding the subject. Essay on Sound, par. 38. 
