Such signals have been established to some extent upon our own coasts, and to a still greater extent along the coasts of Canada and the United States. But the evidence as to their value and performance is of the most conflicting character, and no investigation sufficiently thorough to clear up the uncertainty has hitherto been made. In fact, while the _velocity_ of sound has formed the subject of refined and repeated experiment by the ablest philosophers, the publication of Dr. Derham’s celebrated paper in the “Philosophical Transactions” for 1708 marks the latest systematic inquiry into the causes which affect the _intensity_ of sound in the atmosphere. Jointly with the Elder Brethren of the Trinity House, and as their scientific adviser, I have recently had the honor of conducting an inquiry designed to fill the blank here indicated. One or two brief references will suffice to show the state of the question when this investigation 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.”[59] 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.”