THE general public, and especially those who live or have their business in the metropolis, have recently experienced the bewildering effects of fog on land, and will, perhaps, on that account have a sympathetic interest in what is being done to mitigate the perils and in- conveniences to shipping traffic when fog settles down over the" navigable waters adjacent to our coasts, when all warning sea-marks are hidden from view, when the beams of the most powerful electric light are quenched, and the seaman has only his chart, compass and lead to guide him through the devious channels leading to our ports and harbours. Owing to the enormous number of vessels continually passing and repassing our shores, the danger of collision is always present, even when it is fine and clear; but in thick weather this danger is largely increased, and the invisibility of light * From The Times, 29th November, 1901.

and beacons renders the perils of rocks and shoals much more threatening. The seaman's worst enemy is fog, and its mischievous effects can only partially be guarded against. For many years the Trinity House and other lighthouse authorities have endeavoured to. solve the problem of conveying information to the mariner when fog shrouds all things from sight. The only other human sense which—so far as is at present known—can be appealed to is that of hearing, and efforts have accordingly been directed to the production of effective and distinctive sounds which may be sent out from points of land or lightships moored some distance from the coast, to indicate the locality to passing ships. About twenty-seven years ago the Trinity House, with the powerful aid of Professor Tyndall, made some extensive trials with sound signals at South Foreland. It was then demonstrated, inter alia, that fog offered no obstruction to the passage of soundthrough the atmosphere, and at the same time it was shown that the siren principle of producing powerful blasts of sound was more effective than any other method. Since that time siren sound signals have been largely intro- duced on the coasts of the United Kingdom, and without a doubt have been of the greatest service to mariners.

During the past summer the Trinity House, with the co-operation of the Scottish and Irish Lighthouse Boards, have carried out some further trials with sound signals, this time at St.

Catherine's Point, in the Isle of Wight, observations on the sounds produced being made at sea. The objects of these trials were apparently to test practically certain theoretic improvements which had from time to time been introduced, to make trial of some new forms of sounding apparatus and trumpets, to determine the relative efficiency of notes of high or low pitch, to ascertain the air-pressure at which the siren yields the most effective results, to settle a question which had been raised as to the relative merits of sirens or reed horns as sound producers, to find out whether the sound was better distri- buted as required over the sea by one large trumpet with large-sized siren, or two smaller trumpets fixed at an angle embracing the arc required to be guarded. The special committee ap- pointed by the Trinity House to carry out these trials consisted of Captain G. R. WVYAN, Deputy Master (Chair- man), Admiral H. B. STEWART, Captains A. E. BELL, A. W. CLARKE, and H. ACTON BLAKE, Elder Brethren, Mr. CHARLES A. KENT, Secretary to the Corporation, with Mr. E. PRINCE EDWARDS as clerk to the Committee.

Lord RAYLEIGH, Scientific Adviser, and Mr. THOMAS MATTHEWS, M.Inst.C.E., Engineer-in-Chief to the Trinity House, were also associated with the Committee, and rendered valuable advice and assist- ance in carrying out the investigation.

This committee have now made their report, the details of which, though, perhaps, caviare to the multitude, are of considerable interest to those con- cerned with coast-marking affairs and to those who go down to the sea in ships and have their business in the great waters. Some of the experiences recorded by the committee are striking. In one trial, a new form of disc-siren giving a very low note, 98 vibrations per second, out-distanced every other instrument; the sound was carried for 20J miles as a good sound and might have been taken further, all other sounds, of higher pitch, being left behind at ten miles and less.

This was on a fine, calm day; but on another occasion, with a very little force of wind blowing against the sound, the same instrument, sounding the same note was not heard beyond two miles, and, curiously enough, on that occasion the higher pitched note of another instru- ment was heard at a slightly increased distance. This, and other similar ex- periences, have caused the committee to conclude that " for calm weather a low- pitched note is more suitable than a high- pitched one; but when the wind is opposed to the course of the sound waves, or the sea is rough and noisy, a high- pitched note penetrates further than a low-pitched one." As fog is seldom accompanied by wind or noisy sea, it •would seem that a low-pitched note is most suitable for a fog-signal. Another remarkable experience to which the committee draw particular attention is the occurrence of what is called & soundless zone or silent area. Up to a mile from the signal station the sound of the instrument would be plainly heard ; as the listener proceeded further it would sink to extreme faintness and at times to inaudibility ; then at about three miles' distance the sounds would be recovered and carried in full hearing for a considerable distance. This phenomenon was experienced many times during the trials at St. Catherine's during fine, calm weather with smooth sea, but with any wind or disturbed sea surface it was not observed. Professor Tyndall's attention had been drawn to this matter by General Duane, of the United States Lighthouse Board, and he attributed the occurrence to the sound waves striking the smooth surface of the near sea and being reflected upwards,' the reflected waves thus interfering for a certain distance with the direct waves proceeding from the sound producer.

The committee consider that this liability to interruption, of the sound at short distances should always be borne in mind by mariners, and that, if a sound signal is not heard when expected, the ship's position should be verified by the use of the lead. Another remarkable phenomenon manifested during the trials was the length and strength of the echoes from seaward breaking in upon the sounds from the station, and lasting, in the case of some of the siren blasts, ten times longer than the original sounds. These echoes were mostly noticed on shore at a little distance from the instruments, but they were also observed on board the " Irene," starting apparently from a point oppo- site to that from which the sound proceeded, and rapidly spreading out all over the horizon. The echoes were heard mostly on days when the sky was cloudless, the sea smooth, and the air tranquil ; with disturbances in the atmosphere or sea surface the echoes were very short or not heard at all.

Professor Tyndall attributed such echoes to reflections of the sound waves from invisible acoustic clouds in the atmos- phere, such clouds being formed by air currents differently heated or differently saturated with vapour. When fog prevails, areas of different temperature or saturation are not so likely to exist, the atmosphere is more homogeneous and the passage of sound waves is not obstructed. In the committee's report a suggestion is made that the echoed sound might possibly, in some circum- stances, be liable to be mistaken for the actual signal. This, of course, is not at all probable ; but, if it should so happen, it would be most bewildering, as the echoed sound would appear to come from a direction opposite to that of the signal station.

Coming now to the instruments tried, the committee report that the siren has continued to hold its position as the most effective sound producer for fog- signal purposes under all the conditions of wind and weather experienced ; that reed sounding instruments as at present developed are much inferior and only suitable for guarding positions of second- ary importance. It is possible that, if there were no limits as regards power expended, volume and pressure of air employed, and size of reed and its trumpet, a reed instrument might be brought up to an equality with the siren as now used, but this is taking no account of the possibility of further developing the capabilities of the siren in a similar manner. The new form of disc-siren rotated by a special motor appears to promise very well, its per- formance during the trials having been exceptionally good. A full description of the various instruments tested is given in the report. One point seems to have been brought into prominence in connection with the use of trumpets —namely, that the musical note pro- duced by the speed of rotation of a siren or by the vibrations of a reed should, to get the best effect, be in unison with the fundamental note of the associated trumpet. A new form of trumpet with a mouth of elliptical section has been designed by Lord Rayleigh, and the trials made with it were so encouraging that the committee consider the prin- ciple may with advantage be further developed. A note by Lord Rayleigh giving the theoretical grounds on which he advocates this particular form of construction is embodied in the text of the report. Another form of trumpet introduced of late years by the Trinity House for lightships and positions where the sound is required to be distributed all round is that of the mushroom head.

The round waves issuing from a vertical trumpet are intercepted a little above the aperture by an inverted cone with concave sides, the circumference at the top of the cone being larger than that of the aperture of the trumpet. By this so-called mushroom head the sound waves are deflected and caused to issue hori- zontally with equal force all round. The comparative trials made at St. Catherine's proved this form of trumpet to be quite as effective as had been anticipated.

The experiments appear to have been conducted with great care and com- pleteness, and the results obtained cannot fail to be of great service in all future installations of coast fog-signals.

The fact that no fewer than 4,600 observations were made, which have been classified and tabulated and their results shown in the appendix, is evi- dence that the investigation has been thoroughly carried out.Some reference is made in the re.port to the possibility of employing electrical signals, setheric or submarine, for con- veying warnings to mariners in foggy weather. The question was considered by the committee, but it was thought that experimental trials of that nature were beyond the scope of the inquiry and the means at the committee's disposal, although they quite recognize that, if such a method could be brought into practical service, it would afford the great advantage of independence of atmospheric conditions. The variability of these conditions and their uncertain and unforeseen influence on the passage of sound through the air are dwelt upon by the committee, who, after all said and done, think that a sound fog-signal must be regarded as " only an auxiliary aid to navigation, which cannot at all times be relied on."