millions of miles. And yet it is influencing, in a measure, the chemical changes which are perpetually occurring upon the earth's surface, and by suitable means the changes may be recorded, estimated, and measured the force which was registered by the photograph having emanated from Sirius twenty-one years before!

Capella, which Admiral Smyth estimates at more than three times the distance of Sirius, also gave a photograph, when its spectrum was thrown upon a collodion plate, the effect being produced by rays, which left the star probably when the oldest person in the room was yet a boy.

[W. A. M.]

WEEKLY EVENING MEETING,

Friday, March 13, 1863.

THE REV. JOHN BARLOW, M.A. F.R.S. Vice-President,
in the Chair.

JOHN HALL GLADSTONE, Esq. Ph.D. F.R.S.
On Fogs and Fog Signals.

DURING the course of the inquiry made by the Royal Commission on Lights, Buoys, and Beacons, the attention of my colleagues and myself was called to the fog signals which form part of the apparatus of many lighthouses, and of all British light-ships. In the report we expressed our conviction, "that they are not sufficiently powerful, and recommend the provision of a more efficient warning in fog as subject of investigation and experiment." About the same time, some scientific men in Ireland stirred in the matter, and induced the British Association to appoint a committee, at the head of which is the Rev. Dr. Robinson, of Armagh, to bring the importance of the subject more directly under the notice of the legislature. These circumstances led me to turn my attention to fog; and I propose now to lay before you some of the results arrived at, with reference both to the meteorological phenomenon itself, and to the means adopted for preventing its disastrous consequences among the vessels that sail along our shores.

I have received voluminous returns of the occurrence of fog at about 250 stations, for which I am indebted to the kindness of the three general Lighthouse Boards,-the Trinity House, the Northern Commissioners of Lighthouses, and the Ballast Board of Dublin; also to the Board of Trade, through Admiral Fitz Roy; and to Mr. Vol. IV. (No. 37.)

E

Glaisher. I wish here also to express my thanks to several gentlemen who have aided me in the preparation of this discourse, especially Mr. Alexander Cuningham, who has just read a paper on the subject at the Royal Scottish Society of Arts, and to our friend Professor Wheatstone.

FOGS.

A fog is simply a cloud resting on the earth. In the first discourse of the present season, Professor Tyndall explained the formation of clouds from the aqueous vapour in the atmosphere; and defined a cumulus as "the visible capital of an invisible column of saturated air." A fog is the capital without the column. It is the moisture evaporating from the warm earth, or river, or sea, condensed at once by the colder air. Mr. Glaisher told us here how from his lofty position in the balloon, he saw a fog following all the windings of the Thames. This is a frequent observation, and it reminded me of a scene from the summit of the Righi one morning last summer. There lay in the valley of the Reuss a mist like a white sheet on the ground, but as the sun began to exert his power, and a light breeze to spring up, the uniform layer began to break into regular masses, and soon far beneath us there stretched a cirrus cloud, identical in aspect with those we so often see in the highest regions of the atmosphere.

Fog, then, is composed of minute particles of water, most likely in a globular form, for there seems to be no ground for the popular notion of vesicles of vapour. Smoke enters largely into the composition of that peculiar yellow fog which visits London a few times each year, a fog of wonderful darkness and quietness, and strangely bewildering.

This condensed vapour has a great effect in obstructing the passage of light; the sun himself cannot look through it. A slight mist seems to attack principally the more refrangible rays of the spectrum, so that the light appears redder than usual. I once analysed with a spectroscope the rays which reached Worthing from the great revolving light on Beachy Head, twenty-eight miles distant, and found that those only situated between Fraunhofer's lines C and F were transmitted. This was on what would be called a clear summer night. An objection has been raised against the orange-red glass used in many of the French lighthouses, that in misty weather all bright lights are reduced to very nearly that colour, and thus the distinction is lost; a misfortune that could hardly happen with the deep-red glass employed for the red lights of the British Isles. When the sun shines through a cloud or mist, we do not detect those atmospheric lines which make their appearance when his disk is near the horizon. Yet I have observed in London, when the sun at a considerable altitude loomed red through a slight fog, that the characteristic C 6, 8, and were visible.

There is, of course, every conceivable gradation between the lightest haze and the densest fog, and it is a difficult matter to draw a line of distinction between fog and mist. The value of the meteorological

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 high 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.