In a very clear and able letter, addressed to the President of the Board of Trade in 1863,[60] 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 flame.[61] It might, however, be desirable to ascertain how far the electric light, or its flash, can be traced.[62]
“‘Sound is the only known means really effective; but about it testimonies are conflicting, and there is scarcely one fact relating to its use as a signal which can be considered 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. “Fog,” 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.[63] ... Snow produces a similar effect, and one still more injurious.”
Reflection being thus considered to take place at the surfaces of the suspended particles, 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 optic transparency came to be considered 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 make experiments comparable, we must have some measure of the fog’s power of stopping sound, without attending to which the most anomalous results may be expected. It seems probable that this will bear some simple relation to its opacity to light, and that the distance at which a given object, as a flag or pole, disappears may be taken as the measure.” “Still, clear air” was regarded in this letter as the best vehicle of sound, the alleged action of fogs, rain, and snow being ascribed to their rendering the atmosphere “a discontinuous medium.”
Prior to the investigation now to be described, the views here enunciated were those universally entertained. That sound is unable to penetrate fogs was taken to be “a matter of common observation.” The bells and horns of ships were affirmed “not to be heard so far in fogs as in clear weather.” In the fogs of London the noise of the carriage-wheels was reported to be so much diminished that “they seem to be at a distance where really close by.” My knowledge does not inform me of the existence of any other source for these opinions regarding the deadening power of fog than the paper of Derham, published one hundred and sixty-seven years ago. In consequence of their à priori probability, his conclusions seem to have been transmitted unquestioned from generation to generation of scientific men.
§ 2. Instruments and Observations
On the 19th of May, 1873, this inquiry began. The South Foreland, near Dover, was chosen as the signal-station, steam-power having been already established there to work two powerful magneto-electro lights. The observations for the most part were made afloat, one of the yachts of the Trinity Corporation being usually employed for this purpose. Two stations had been established, the one at the top, the other at the bottom, of the South Foreland Cliff; and at each of them trumpets, air-whistles, and steam-whistles of great size were mounted. The whistles first employed were of English manufacture. To these was afterward added a large United States whistle, and also a Canadian whistle, of great reputed power.