« VorigeDoorgaan »
returns in my possession depends greatly on this, and there is reason to fear, even from internal evidence, that one lighthouse keeper calls by the name of fog what another keeper thinks sufficiently described as mist or haze. Yet these keepers have a certain general sentiment and similarity in their way of thinking, and with many the practical definition of a fog is when it is necessary to sound the signal. Among amateur observers on land the greatest discrepancy prevails; but nautical men seem to have a general agreement as to what amount of thickness is to give a claim to the designation fog. For such observations Mr. Cuningham suggests that a pole, painted vermilion, should be set up at a hundred yards from the station, and that such an amount of mistiness as renders it invisible should alone be named fog. The colour of the pole should, of course, be in strong contrast to the objects behind it. This definition is somewhere about that practically adopted by seamen ; it has been accepted by Mr. Glaisher, and it is to be hoped that it will be generally adopted by all observers.
From the returns received, the following deductions may be drawn :
I. While many fogs are quite partial in their character, others cover a large extent of country. The irregular distribution of a London fog is a matter of frequent observation. Thus, last Tuesday, the day of the royal wedding, one of the densest yellow fogs obscured Westminster, whilst at Bayswater there was nothing more than a murky mist. The long-continued fog of November last in London, extending from the 19th to the 25th, was observed also at Berkhampstead, Oxford, and Banbury, but other parts of England seem to have been clear. The occurrence of fog at the lighthouse stations all round the coast during the first six months of the year 1861 has been especially studied, and the fogs of that April have been represented on a map. Some of them cover large portions of the British Isles. For instance, that of the 14th stretched all round Ireland, except the south-west corner, crossed the Irish sea to the headlands of Wales and the south-western isles and coasts of Scotland, and made its appearance again across the mainland in the Firth of Forth.
II. Some months are marked by fogs much more than others. For instance, along the south coast of England, February and September are comparatively free, while January and June are foggy months. November is notorious for fog in London, but does not seem to deserve that character elsewhere.
III. Some years are much more visited by fogs than others. For instance, 1861 was freer than 1858 along most parts of the coast.
IV. Different localities are very variously visited by fog. England does not deserve that pre-eminent character for mistiness which is attributed to it by the popular imagination of the Continent. The value of the returns in showing the relative distribution of fog in different places is seriously affected by the different standards in the minds of the observers, but the following points seem pretty clearly made out: A fog is more uniformly distributed over the surface of the sea than on the adjoining coasts. Fog is not particularly prevalent about sandbanks, or low headlands; but where cliffs or bigh hills catch the southwest wind just after it has swept the ocean, as at the Isle of Wight, the Start Point, Lundy Island, and the Rocks of Pembrokeshire, the numbers run very high. The highest return is from Barra Head, the southernmost point of the Hebrides, where winds surcharged with moisture from the Gulf Stream strike the cold northern rocks, and wrap them in cloud or fog.
Fog SIGNALS. As light only very imperfectly penetrates a fog, attempts have been made to warn vessels of their approach to danger, or to acquaint them with their position, by means of sound. The fog signals actually in use are as follows :-Bells are employed at many of the lighthouses, and in the Irish light-ships, the finest, perhaps, being two near Dublin, and that at the Copeland Island, in the Irish sea, which is rung by machinery, and is said to have been heard thirteen miles off. At the end of the pier at Boulogne there is a large bell, in the centre of a large parabolic reflector facing the sea. It is struck by three hammers alternately, the motive power being a falling weight. Gongs are made use of in all the light-vessels belonging to the Trinity House. Guns are fired on board the Kish light-ship, from the mountain above the South Stack Lighthouse, at Fleetwood, and elsewhere. A very powerful steam-whistle has for some time been in operation at Partridge Island, near St. John's, New Brunswick, a part of the world peculiarly affected by fog.
At the Skerries, near Holyhead, terns and other sea-birds are encouraged, as their cries serve as a warning to vessels during fog ; but unfortunately some rats escaped from the 'Regulus,' which was wrecked there about seven years ago, and they are destroying the birds. A cat has been tried, but she preferred birds to rats.
The comparative efficiency of these various methods is a very serious question ; indeed, there are grave objections to the use of sound at all as a fog signal. The difficulties are as follows:-A sound indicates the proximity, but not the exact direction, of a danger. In this respect it is totally different from a light. Yet the mere warning is something; and probably a suitable ear-trumpet would give a better idea of the direction than is obtained without it. There is evidence that vessels have sometimes steered by a sound; for instance, -" In the winter of 1860, the steamer 'Iron Duke' having been drawn by the flood tide to the northward past Howth, was attracted by the sound of the bell, and steered by it safely towards Kingston, until the bell on the east pier of that harbour told her of its proximity, and ultimately led her into safety.” The Royal Commissioners on Lights made special inquiry about the Boulogne bell, and found that some of the captains of steamers frequenting that port could find their way in by the sound of the bell, in thick weather; at least, in conjunction with the use of the lead.
An objection to the use of most of these fog signals is the fear that they may be mistaken for other sounds, or other sounds mistaken for them. Bells are frequently being rung on shipboard; the firing of guns is the well-known sign of distress; and steamers in a fog are in the habit of whistling as they go along. Gongs do not seem open to this objection ; and in the case of other sounds, it might be obviated by having a definite system of repetition, as is done with the flashing and revolving lights. My friend, the Rev. T. Pelham Dale, * has indeed suggested a means of signalling, in which musical notes are employed.
Another difficulty is, that even loud sounds cannot be heard far to windward if a breeze is blowing ; but this is of less importance, as a fog usually occurs in calm weather.
But the great objection to sound as a fog signal is, that a fog stops the waves of sound as it does those of light. It is well known that sound will not traverse a heterogeneous medium, such as air loaded with mist. As to the fact of such signals being rendered inefficient by the very thing which they are intended to penetrate, the testimony of mariners is somewhat conflicting. Mr. Alexander Cuningham, who is the secretary of the Northern Commissioners of Lighthouses, says: “ Many years ago, having landed from the lighthouse tender on the small skerry in the Portland Firth, a fog came on. We hurried off in the hope of reaching the vessel ; but before doing so, the fog shut her completely from our view. We pulled in the direction (having a compass) in which the tender was last seen ; but those who know the rapidity and variety of the tides in that dangerous locality, will easily be prepared to hear that our efforts were unsuccessful We lay about the spot for some time, firing our fowling-pieces, and at last pulled for the shore. Next morning the vessel came in sight; and on comparing notes, we found that we must have been within a very short distance of her; and they had been firing small six-pound carronades all night, and we never heard them, nor did those on board hear our guns.”
Yet, on the other hand, we have accounts of bells being heard at a distance of some miles during a fog, and the steam-whistle near St. John's is said by the captains and pilots of steamers frequenting that port to be most serviceable, and to be generally heard for four or six miles during strong breezes blowing on shore. It is, indeed, quite possible that fogs of the same intensity may still have a very different effect upon the same signal, and that for two reasons ; first, one fog may reach far up into the atmosphere, presenting a high wall to every vibration; while another may be a thick layer lying on the surface of the earth, with an open space above, through which the swelling waves of sound may freely pass. Secondly, air perfectly saturated with moisture is no bad conductor of sound ; a fog under such atmospheric
* Marryatt's Signals, ed. 1856.
conditions may therefore be far less obstructive than when the medium is more heterogeneous,
Considering these objections to the use of sound as a fog signal, must we abandon it altogether? We cannot do so, as we have nothing better to substitute. We fall back upon the recommendation of the Royal Commission, that further experiments should be made, which it is hoped will be gradually adopted by the authorities, as the majority of the other scientific recommendations have been.
Experiments should be performed on the manner and degree in which fog absorbs or destroys sounds of different pitch, or of different characters, for instance, a sharp sound or a prolonged sound; on the various means of producing loud sounds, as to their pitch, volume, convenience, costliness, &c. ; whether a repetition of the same sound, or some variation in note, octave, frequency, &c., be desirable ; on the influence exerted by the height above the sea at which the sound originates; on the influence of a background, such as a tower, cliff, or hill, in reflecting the sound, or of a concave mirror; on the best means of directing a sound to a particular azimuth, or of determining its direction when on board ship.
Captains Close and Nisbet, of the Trinity Board, have made some experiments on one of these points at Holyhead Mountain, where a gun was fired from near the surface of the sea, and another at a considerable height, and the respective reports were listened to from various distances at sea. They found the upper gun was heard best for six miles, after which it lost its superiority. But the most remarkable result was the irregularity of the noise from the lower gun, which, at certain points in fact was not heard at all, though the flash was distinctly seen.
Powerful means of producing sound, besides those already mentioned, have been suggested. Mr. Cowper has planned a large steamtrumpet for lighthouse stations, which may be made to revolve. There is something similar in America worked by Erichsen's engine, with which is associated the name of Mr. Daboll. And Professor Holmes, of the Magneto-Electric Light, has also a steam-trumpet, which can be adapted for different notes, and gives a buzzing sound of wonderful intensity. Suggestions on this point were also made to the Royal Commissioners by several scientific men. Thus, horns were strongly recommended by more than one; but Mr. Mallet prefers explosive sounds ; and Sir John Herschel says :-“ It would be worth trial, what would be the effect of a battery of whistles, blown by high-pressure steam, or by a combination of three, or several sets of three, pitched exactly to harmonic intervals (key-note, third, fifth, and octave), but all of a very high pitch, and with a rattle (analogous to the pea in a common whistle), which intensifies the action on the auditory nerve." Captain Ryder believes a gun might be constructed to produce the very distinctive sound of an explosion, followed instantaneously by a whistle.
There is another and very promising field for experiment, the
transmission of sound through the water itself. The experiments of M. Colladon on the Lake of Geneva proved the great distance to which sound is transmitted through water, and the velocity and directness of its course. *
In his observations he employed a bell, let down into the water ; but this is a bad instrument for signalling, as its vibrations are almost instantly stopped. Many arrangements would appear to be preferable. The Syren, which was so called by its inventor, M. Cagniard de la Tour, because it would sing under water, is well adapted to give any vote that is found desirable.f Long glass tubes, vibrating longitudinally, are said to produce immense volumes of sound in water: and other means might be devised. As the sound remains in the water, it would be necessary to make some communication between it and the ear of the listener. M. Colladon employed an apparatus like a spoon, with a tube for handle. By this means a mariner might listen for signals made at any important station, such as the Lizard Point, and might not only hear them at a great distance, but determine approximately their direction, unaffected by the state of the atmosphere above.
[J. H. G.]
WEEKLY EVENING MEETING,
Friday, March 20, 1863.
MAJOR-GENERAL EDWARD SABINE, R.A. President R.S. D.C.L.'
Vice-President, in the Chair.
BALFOUR STEWART, Esq. F.R.S. On the Forces concerned in producing Magnetic Disturbances.
When a bar of steel has been magnetized, it has acquired a tendency to assume a definite position with respect to the Earth. Nothing is more widely known than this important fact, but at the same time there is nothing in Science more mysterious than its cause. We may endeavour to explain it by asserting that the Earth acts as a magnet; but whence it has acquired this magnetism, how it is distributed, and what are the causes of its many changes, are amongst the most perplexing and the most important of those problems in physical science which are yet unsolved.