The preparatory steps were similar to those already described. The foundation of the lighthouse was formed of seven screw-piles, six of them occupying the angles of a hexagon 46 feet in diameter, the seventh being in the centre. From each screw proceeds a pile 15 feet in length, having at the upper end another screw for securing a wooden column. These columns are of Baltic timber, the one in the centre being 56 feet, the others 46 feet in length, firmly secured with iron hoops and coated with pitch. The platform, upon which the house stands, is 27 feet in diameter, the house itself being 20 feet in diameter and 9 feet high. From the summit of the house rises a twelve-sided lantern, 10 feet in diameter and 8 feet high. Altogether the light is elevated about 46 feet above low-water level, and ranges over an horizon of eight miles. The light is of the dioptric kind—bright, steady, and uniform, and when the weather is too foggy to allow it to be seen, a bell is tolled by machinery, to give the needful warning.
At the period when screw-pile lighthouses were being thus successfully erected, other and most valuable suggestions were being made for the building of bronze and cast-iron lighthouses. The great advantage of iron over stone and other materials in those portions of the building not actually in contact with sea-water soon became apparent. Upon a [pg 183]given base a much larger internal capacity could be obtained; plates could be cast in large surfaces and with few joints, and a system of binding adopted which should ensure the perfect combination of every part. The comparatively small bulk and weight also of the component parts gave great facilities for the transport and rapid construction of such structures. The initial cast-iron lighthouse was designed by Mr. Gordon in 1840, and was cast and put together within three months from the date of the contract. It was then taken to pieces and shipped for Jamaica, on which island it now lights up Morant Point, a point of great danger. The Commissioners of the House of Assembly had applied to Mr. Gordon to supply a suitable lighthouse at the smallest possible cost, and in furnishing them with the structure of cast-iron he fulfilled their wishes admirably, the expense not exceeding one-third of the cost of a similar building in stone. This elegant lighthouse, the outline of which resembles that of the Celtic towers of Ireland, was exhibited to visitors while it stood complete in the contractor’s premises. The diameter of the tower is 18 feet 6 inches at the base, diminishing to 11 feet under the cap. The tower is formed of nine tiers of iron plates, each tier being 10 feet high and about three-quarters of an inch thick. At the base of the structure eleven plates are required to form the circumference, at the top nine plates; they are cast with a flange around their inner edges, and when put together these flanges form the joints, which are fastened together with nut-and-screw bolts and caulked with iron cement. The interior of the tower, to the height of 27 feet, was to be filled up with masonry and concrete of the weight of 300 tons; the remainder is divided into store-rooms and berths for the attendants. The tower is finished by an iron railing, within which rises the light-room, also of cast-iron, with windows of plate-glass. A copper roof and a short lightning-rod complete the whole. The Admiralty notice announced the exhibition of this light on Morant Point November 1st, 1842, and stated that the elevation of the light is 97 feet above the level of the sea, and that in clear weather it is visible at a distance of twenty-one miles. The light is of the revolving kind, consisting of fifteen Argand lamps and reflectors, five in each side of an equilateral triangle, and so placed as to produce a continuous light, but with periodical flashes. The tower is painted white, and the lower portion is coated with coal-tar to preserve it from rust. It rests on a granite base, and is also cased with granite near the foundation, the more certainly to prevent the action of the sea-water on the metal.
While the engineer had attained some of his greatest triumphs in the construction of lighthouses, the optician had not once directed his attention to the invention of a brilliant light, worthy to be placed upon the structure which proudly rose high above the fierce waves with the strength and solidity of a rock. During a period of forty years after the completion of the Eddystone tower by Smeaton, the lantern was illuminated by tallow candles stuck in hoops, just as a stand or booth is lighted at a country fair, and so lately as the year 1811 it was lighted with twenty-four wax candles. In 1812 the Lizard Light was maintained with coal fires; and in 1816, when the Isle of May Light, in the Firth of Forth, was taken possession of by the Commissioners of the Northern Lighthouses, a coal fire was exhibited in a chauffer—a description of light which had been exhibited for 181 years. In 1801 the light at Harwich, in addition to the coal fire, had a flat plate of [pg 184]rough brass on the landward side, to serve as a reflector. Such methods of lighting were of course very deficient in power, and did not enable the mariner to distinguish one light from another—a point which is often of as much importance as the brilliancy of the light itself. Prior to the invention of the Argand lamp (about 1784) the production of a strong and brilliant light from a single source was scarcely possible, and even such a lamp, by its unassisted powers, would not be of very great value in giving early notice to the mariner of his approach to the coast, which ought to be the primary object of a lighthouse. As the rays of a luminous body proceed in all directions in straight lines, it is obvious that in the case of a single lamp the mariner would derive benefit only from that small portion of light which proceeded from the centre of the flame to his eye. The other rays would proceed to other parts of the horizon, or escape upwards to the sky, or downwards to the earth, and thus be of no value to him. By increasing the number of burners a small portion of light from each burner would slightly increase the effective action, but by far the greater portion of the light produced would escape uselessly above and below the horizon and also at the back of each flame. Next, these defects were remedied, and the efficiency of the light greatly increased, by placing behind each lamp a reflector of such a form as to collect the rays that would otherwise be lost, and throw them forward to the horizon. The adoption of such a method has led to what is called the catoptric system of lights.
REVOLVING LIGHT APPARATUS.
(From Drawings supplied by Messrs. W. Wilkins & Co.)
Alan Stevenson states that the earliest notice he has been able to find of the application of paraboloidal mirrors to lighthouses is in a work on “Practical Seamanship” (Liverpool, 1791), by Mr. William Hutchinson, who notices the erection of the four lights at Bidstone and Hoylake for the entrance of the Mersey, in 1763, and describes large paraboloidal moulds of wood lined with mirror glass and smaller ones of polished tin-plate, as in use in those lighthouses. In France M. Téulère, a Member of the Royal Corps of Engineers of Bridges and Roads, is regarded as the inventor of the catoptric system of lights. In a memoir dated 26th June, 1783, he is said to have proposed for the Cordouan Lighthouse a combination of paraboloidal reflectors with Argand lamps, arranged on a revolving frame, a plan which was actually carried into execution, under the direction of the Chevalier Borda.[60] The plan was so successful that it was soon adopted in England by the Trinity House of London; and in Scotland the first work of the Northern Lights Board, in 1787, was to light a lantern on the Old Castle of Kinnaird Head, in Aberdeenshire, by means of parabolic reflectors and lamps. These reflectors were formed of facets of mirror-glass placed in hollow paraboloidal moulds of plaster. The more complicated arrangement of lenses placed round a centre in concentric circles is due to the great Fresnel, a practical man of science, whose abilities are acknowledged as fully in England as in France.
The oil used in the lighthouses of the United Kingdom has generally been sperm. Colza, the expressed oil of the wild cabbage (Brassica oleracea), was very generally used in France, and occasionally in Great Britain. Gas is used in a few places, where its application is easy. There can hardly be any doubt now, however, that the coming light will be the electric, since its steady production is becoming a matter of scientific certainty. As early as 1857 Professor Holmes submitted to the Trinity House a method of employing this light, which was submitted to Faraday, and approved. The Board then allowed a trial at the South Foreland Lighthouse. The light was first displayed on the 8th of December, 1858. In June, 1862, it was permanently fixed at Dungeness. In Faraday’s Report to the Trinity House, published in 1862, he says: “Arrangements were made on shore by which observations could be made at sea, about five miles off, on the relative light of the electric lamp and the metallic reflectors with their Argand oil-lamps, for either could be shown alone, or both together. At the given distance the eye could not separate the two lights, but by the telescope they were distinguishable. The combined effect was a glorious light up to five miles; then, if the electric light was extinguished, there was a great falling off in the effect, though, after a few moments’ rest to the eye, it was seen that the oil-lamps and reflectors were in their good and proper state. On the other hand, when the electric light was restored, the glory rose to its first high condition.... During the day-time I compared the intensity of the light with that of the sun, and both looked at through dark glasses. Its light was as bright as that of the sun, but the sun was not at its brightest.”
The number of lights on a well-frequented coast being considerable, it is of the utmost importance to arrange them so as to enable the mariner easily to distinguish [pg 186]them from each other. Catoptric lights admit of nine separate distinctions:—1, fixed; 2, revolving white; 3, revolving red and white; 4, revolving red with two whites; 5, revolving white with two reds; 6, flashing; 7, intermittent; 8, double fixed lights; 9, double revolving white lights. Mr. Stevenson thus defines their distinctive features:—“The first exhibits a steady and uniform appearance which is not subject to any change, and the reflectors used for it are of smaller dimensions than those employed in revolving lights. This is necessary in order to permit them to be ranged round the circular frame, with their axes inclined at such an angle as shall enable them to illuminate every point of the horizon. The revolving light is produced by the revolution of a frame with three or four sides, having reflectors of a larger size grouped on each side with their axes parallel, and as the revolution exhibits once in two minutes or once in a minute, as may be required, a light gradually increasing to full strength and in the same gradual manner decreasing to total darkness, its appearance is extremely well marked. The succession of red and white lights is produced by the revolution of a frame whose different sides present red and white lights, and these afford three separate distinctions, namely, alternate red and white, the succession of two white lights after one red, and the succession of two red lights after one white light. The flashing light is produced in the same manner as the revolving light; but, owing to a different construction of the frame, the reflectors on each of eight sides are arranged with their rims or faces in one vertical plane, and their axes in a line inclined to the perpendicular. A disposition of the mirrors, which, together with the greater quickness of the revolutions, which shows a flash once in five seconds of time, produces a very striking effect, totally different from that of a revolving light, and presenting the appearance of the flash alternately rising and sinking, the brightest and darkest periods being but momentary; this light is further characterised by a rapid succession of bright flashes, from which it gets its name. The intermittent light is distinguished by bursting suddenly into view and continuing steady for a short time, after which it is suddenly eclipsed for half a minute. Its striking appearance is produced by the perpendicular motion of circular shades in front of the reflectors, by which the light is alternately hid and displayed. This distinction, as well as that called the flashing light, is peculiar to the Scotch coast. The double lights (which are seldom used except where there is a necessity for a leading line, as a guide for taking some channel or avoiding some danger) are generally exhibited from two towers, one of which is higher than the other. At the Gulf of Man a striking variety has been introduced into the character of leading lights, by substituting for two fixed lights two lights which revolve in the same periods and exhibit their flashes at the same instant; and these lights are of course susceptible of the other variety enumerated above, that of two revolving red and white lights, or flashing lights, coming into view at equal intervals of time. The utility of all these distinctions is to be valued with reference to their property of at once striking the eye of an observer and being instantaneously obvious to strangers. The introduction of colour as a source of distinction is necessary in order to obtain a sufficient number of distinctions; but it is in itself an evil of no small magnitude, as the effect is produced by interposing coloured media between the burner and the observer’s eye, and much light is thus lost by the absorption of those rays which are held back in order to cause [pg 187]the appearance which is desired. Trial has been made of various colours, but red, blue, and green alone have been found useful, and the two latter only at distances so short as to render them altogether unfit for sea-lights. Owing to the depth of tint which is required to produce a marked effect, the red shades generally used absorb from four-sevenths to five-sixths of the whole light—an enormous loss, and sufficient to discourage the adoption of that mode of distinction in every situation where it can possibly be avoided. The red glass used in France absorbs only four-sevenths of the light, but its colour produces, as might be expected, a much less marked distinction to the seaman’s eye. In the lighthouses of Scotland a simple and convenient arrangement exists for colouring the lights, which consists in using chimneys of red glass, instead of placing large discs in front of the reflectors.”
The construction of the lantern is a point of importance; and one of the first order will cost about £1,260. On the level of the top of the lower glass a narrow gangway is usually built for the keeper to stand upon in order to clean the panes, an operation which in snowy weather may have to be frequently repeated during the night. At some of the lighthouses on the Mediterranean the lantern is at certain seasons so completely covered with moths as to obscure the light and to require the attendance of men with brooms. Mr. Tomlinson was informed by the keepers at the Eddystone that bees and other insects were much attracted by the light, and collected round the lantern in great numbers. Larks and other birds flew against it, and, becoming stunned with the blow, were picked up on the balcony and were cooked by the men for breakfast. The lantern is very liable to injury in high winds, or the glass may be broken by large sea-birds coming against it on a stormy night, or by small stones violently driven against it by the wind. Extra plates of glass are always kept to take the place of broken panes. The number of light-keepers employed varies, ranging from two to four, and in the latter case one is usually allowed to remain on shore, the men taking the privilege in turns. When the situation admits, it is usual to have the keeper’s rooms in a building outside the lighthouse to avoid dust, which is most injurious to the delicate apparatus of the light-room. Great cleanliness is enforced in all that belongs to a lighthouse, the reflectors and lenses being constantly burnished, polished, and cleansed.
And so we have traced the history and progress of lighthouses, and it is hard to believe that any great change can be advantageously made in their construction, though their mode of illumination will doubtless be greatly improved. As we have seen, the electric light was used practically in a lighthouse long before it was in the streets of the great metropolis, and not in a merely experimental way, but with the most successful results.