LIGHTHOUSES ON SAND AND CAST-IRON LIGHTHOUSES.
Floating Lights—Objections to—Mitchell’s Screw-moorings—Experiments on the Maplin Sand—Foundation—Erection of Screw-pile Lighthouse—Details of the Wyre Lighthouse—Proposed Lighthouse on the Goodwin Sands—Metallic Lighthouses—Advantages of Metal over Stone—Details of Cast-iron Lighthouse at Morant Point, Jamaica.
Those dangerous approaches to a coast which, from the nature of the soil, have not till very lately admitted of the erection of a permanent lighthouse, are usually indicated to the navigator by floating lights; but these being nothing more than large lanterns suspended in the rigging of a vessel, necessarily possess but feeble illuminating power. This power is still further diminished in a gale of wind, when it is most wanted, by the pitching and floundering about of the vessel: every now and then she is submerged in the trough of the sea, covered with spray and drift, or, what is most to be dreaded, she is liable to be blown away from her moorings; an accident which has been productive of the most disastrous consequences to life and property.
The details already given will convey some notion of the difficulty and danger of planting a lighthouse on the solid rock in a stormy sea; we may naturally suppose that this difficulty and danger must be enormously increased in erecting a permanent residence on the shifting sands. Such, however, is by no means the case; one of the recent triumphs of engineering has proved that it is not always folly to build a house upon the sand.
This remarkable result has been accomplished chiefly by means of Mitchell’s screw-mooring, an instrument which consists essentially of an enormous cast-iron screw of about one turn and a half, having a hollow cylindrical centre; a wrought-iron spindle passes through the cylindrical socket; it is somewhat tapering in form, and when driven up tight is fixed thereto by a forelock passing through both; it is formed with a square head to receive the key for screwing it into the ground. It is also furnished with a collar of wrought iron fitted so as to turn freely on the upper part of the shaft of the spindle below the collar.
The attention of the Trinity House having been called to this instrument, it was considered applicable to the establishment of lighthouses on sands; and accordingly a series of experiments was undertaken at the cost of that honourable body. The spot selected for the purpose was on the verge of the Maplin Sand lying at the mouth of the Thames, about twenty miles below the Nore, forming the north side of the Swin or King’s Channel, which, on account of its depth, is much frequented by large ships, as also by colliers and other vessels from the north sea. The sand is shifting, and is dry at low water spring-tides, and hitherto a floating light has been maintained upon it. On this spot it was proposed to erect a fixed lighthouse of timber framing, with a lantern and residence for the attendants.
In the month of August 1838, operations were commenced by inserting nine of Mitchell’s patent mooring-screws, each four feet and a half in diameter, and furnished with shafts of wrought iron about twenty-five feet in length and five inches thick. One of these screws served as a centre to the remainder, which occupied the angles of an octagon forty-two feet in diameter. The screws were turned into the sands to the depth of twenty-one feet and a half, the upper extremities being left standing about five feet above the surface of the sands. For the purpose of fixing the screws, a stage or raft of timber, thirty feet square, was floated over the spot, with a capstan in the centre, which was made to fit on the top of the iron shaft, and firmly keyed to it. A power of about thirty men was employed for driving the screws, and their labours were continued until their united force could scarcely turn the capstan. This stage or raft, which had been formed in two thicknesses crossing each other at right angles, and bolted at their intersection, was, as a precautionary measure, allowed to remain. It covered the whole site within the piles, and also extended some distance beyond them. A curb about eighteen inches high was raised round this stage; on its surface was arranged a quantity of brushwood, and then about two hundred tons of rough stone, which sunk the stage into the sand and prevented it from being displaced. Between the spaces of the stage and the brushwood the sand was allowed to wash its way, and it soon filled the interstices of the stone. The whole mass soon became embedded below the surface of the sand, and gave considerable lateral support to the piles, and formed a solid body for the water to wash upon.
In this state the whole was allowed to remain for about two years, during which time every change in the surface of the sand was observed, and although early in the year 1839 violent storms occurred, yet the screw-piles stood firmly, and the sand at no time was lowered more than three feet. In August 1840 the raft was found to have completely settled down, the piles were as firm as if they had been screwed into clay; a lighthouse was therefore erected within the short space of three months; and on the 16th February, 1841, a dioptric fixed light was exhibited off this dangerous spot, and was visible ten miles off in all directions.
But while the preparatory steps for this lighthouse were being taken, a screw-pile lighthouse was begun and completed at Port Fleetwood on the Wyre, near Lancaster; which being the first of the kind ever constructed, deserves particular notice.
The preparatory stages were of a similar nature to those already described. The foundation was formed of seven screw-piles, six occupying the angles of a hexagon forty-six feet in diameter, and the seventh being placed in the centre. From each screw proceeded a pile fifteen feet in length, at the upper end of which was another screw for securing a wooden column. These columns were prepared of Baltic timber; the one in the centre was fifty-six feet, and each of the remainder forty-six feet in length, bound firmly round with iron hoops and coated with pitch.
The framing upon which the house stands is firmly secured round the centre column and to the heads of the outer columns by means of hollow cast-iron capitals let down over the heads of the columns and secured with screw-bolts. To give lateral strength to the building, round iron angle-braces were applied, by which means a resisting power equal to at least three hundred and fifty tons is presented in every direction.
The platform upon which the house stands is twenty-seven feet in diameter. The dwelling-house is twenty feet in diameter, and nine feet high: it has an outside door, and three windows, and is divided into two apartments, one having a fire-place; the floor is tiled, and the walls and ceiling lathed and stuccoed. Access to the platform is secured by means of a Jacob’s ladder of wrought iron secured to one of the columns: access to the lantern is by a winding stair within the house.
From the summit of the house rises the lantern; it is twelve-sided, ten feet in diameter, and eight feet high. The light is thus elevated about forty-six feet above low-water level. It is of the dioptric kind, and is bright, steady, and uniform, ranging over an horizon of eight miles, and visible at the distance of ten miles from a coaster’s deck. During foggy weather a bell is tolled by machinery. Tide-time for vessels of twelve feet draft is also denoted by signals. Signals put out by vessels requiring a Wyre pilot will also be understood at this lighthouse, where corresponding signals are hoisted until the pilot is provided.
This admirable and useful structure was erected in two of the shortest day months of the year, in which time day-light did not occur at any low-water period; the workmen therefore had to depend on torches and moonlight. Nor is the portability of this form of building its least advantage: should there occur any local changes which might threaten the safety of the house, it can be taken down, and erected in another site within a month.
Perhaps one of the boldest schemes ever devised for lighthouses was the structure proposed to be erected by Mr. Bush, on a plan patented by him, on the Goodwin Sands, or on the Varne in the channel between Folkestone and Cape Grisnez, in four fathoms water. This plan, was recommended to the consideration of parliament by several merchants, ship-owners, and other influential persons. The building proposed to be called ‘The Light of all Nations,’ was to consist of a Doric column one hundred and twenty-five feet high supporting a lantern twelve feet in diameter, surmounted by a colossal statue of the Queen, her sceptre being the point of a lightning-conductor. This column was to rise from a base one hundred feet in height, and fifty in diameter, to be formed by a caisson composed of cast-iron plates bolted together: the part under water was to be divided into four pyramidal chambers, opening into and supporting one another; the lower one resting on the rock beneath the sands, and the whole forming a conical core to the cylindrical base. The only part of this plan that was executed was the cast-iron caisson, which was deposited in its place among the sands. In this situation, during one dark and stormy night, it was struck by a ship and shivered to a thousand fragments. This untoward accident has led to the abandonment of the design.
One of the characteristics of this country is the mode in which we lay out the mineral wealth which nature has bestowed upon us so liberally in the shape of coal and iron. With the assistance of the former we mould the latter into a thousand shapes of usefulness, neatness, and durability, and so much attached are we to this material, that it is daily superseding the use of the more cumbrous wood and stone, and other substances which were once in great demand. Iron furnishes most of the multifarious instruments required in the mechanical and agricultural arts—it ministers alike to war and to peace, by furnishing the sword and the ploughshare. It supplies some of the most useful domestic apparatus for the kitchen, the parlour, and the bed-room, and now even the bedstead itself may be formed of iron. It has been long used in some of our great public works: we have iron roads—iron bridges—iron statues—steam-boats of iron—houses of iron, and lastly, iron lighthouses.
The suggestion of metallic lighthouses originated a few years ago with Captain Sir Samuel Brown, when it was proposed to place a lighthouse on the Wolf Rock near Land’s End, a position where it would be exposed to the most violent storms of the Atlantic. A plan for the erection of a stone lighthouse on this point had already been drawn up by Mr. Stevenson, which plan, Captain Brown thinks would require fifteen years for its execution, and cost one hundred and fifty thousand pounds. Captain Brown undertook to erect one of bronze ninety feet high for fifteen thousand pounds, and to complete it in four months. This plan, from whatever cause, was not entertained, and with the exception of a small lighthouse erected on the Gravesend pier, metallic lighthouses excited no attention until the year 1840, when application was made to Mr. Alexander Gordon, the eminent engineer, by the commissioners appointed by the House of Assembly, in the island of Jamaica, to light a dangerous point in that island, called Morant Point, for the erection of a suitable lighthouse at the smallest possible cost. On this occasion Mr. Gordon proposed the erection of a cast-iron structure, resembling in outline that of the Celtic towers of Ireland. His plans and estimates having been accepted, they were executed with remarkable celerity; and from an account furnished by Mr. A. R. Renton, (the manager of the factory at which the work for the lighthouse was done,) we derive most of the following particulars.
The advantage which iron, when not in contact with sea-water, possesses over stone or other materials, is that upon a given base a much larger internal capacity for dwellings and stories can be obtained, with equal stability. With this material plates can be cast in large surfaces, and with but few joints. A system of bonding the plates may also be adopted, which will ensure the perfect combination of every part, so as to form an entire mass, and thus the best form for strength and stability can easily be obtained. From the comparatively small bulk and weight of the component parts of the structure great facilities are afforded for transporting and erecting it. Thus, in less than three months from the date of the contract, the lighthouse about to be described was cast and erected on the contractor’s premises, and it was expected to have the light exhibited in Jamaica in three months more. The whole expense was said not to exceed one-third the cost of a similar building in stone.
The structure was to be founded on a coral rock a little above the level of the sea; the face of the rock is about ten feet below the surface of the sand, and was to be excavated to receive the base of the tower, resting on and cased with granite, to prevent the natural filtration of the sea-water from acting upon the iron. This course of granite is grooved to receive the flange of the lower plates of the tower, from which lightning-conductors are to be continued to the sea. The tower is of course itself a lightning-conductor of the best kind. The diameter of the tower-shaft is eighteen feet six inches at its base, diminishing to eleven feet under the cap; it is formed of nine tiers of plates each ten feet in height, varying from one to three quarters of an inch thick. The circumference is formed of eleven plates at the base, and nine at the top: they are cast with a flange all round the 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 cap consists of ten radiating plates, which form the floor of the light-room; they are screwed to the tower upon twenty pierced brackets, and are finished by an iron railing. The lower portion, namely, twenty-seven feet, is filled up with masonry and concrete, weighing about three hundred tons, and so connected with the rock itself as to form a solid core of resistance. The remaining portion of the building is divided into store-rooms and berths for the attendants in the lighthouse.
The light-room consists of cast-iron plates five feet high, on which are fixed metal sash-bars filled with plate-glass; these, terminating with a point, are covered with a copper roof, whence rises a short lightning-rod, trebly gilt at the point. 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 constitute a continuous light, but with periodical flashes. The Admiralty notice which announced the light for exhibition on the 1st November, 1842, states that the centre of the light is ninety-six feet above the level of the sea, and in clear weather the light can be seen from a distance of twenty-one miles.
To preserve as low a temperature as the circumstances and climate will permit, the iron shell was lined with a non-conducting material, as slate or wood, leaving an annular interstice, through which a constant ventilation is effected, so as to carry off the excessive heat.
To preserve the two lower tiers from rusting, they are coated with coal-tar. The tower itself is painted white. The only brasing which has been thought necessary is a few cross tiers at each horizontal joint, over which the iron-tongued wood-floors are laid.
The several rooms are provided with fire apertures, fitted with oak sashes filled with plate-glass. The approach to the doorway, which is about ten feet above the level of the sand, is by means of stone-steps; ladder-irons are also provided in the event of the stone-steps being carried away by a hurricane.
Over the entrance is a large tablet of iron supported by two smaller ones; and on them, on bas-relief, are inscribed the date of erection (1842), the names of the commissioners, of the engineer, founder, &c.
The whole of the castings were executed at the foundry (late Bramah and Robinson’s) at Pimlico, and put together in the yard of the manufactory, prior to their removal to Jamaica, where the work was re-erected by a derrick and crab from the inside, without the aid of any external scaffolding.
It is said that the whole expense of the lighthouse, including the passage over the Atlantic, did not exceed seven thousand pounds, and that the entire weight of the iron-work is about one hundred tons. The masonry was also prepared in this country, which (from the absence of building-stone in Jamaica) was found to be more economical than if the work had been done on the spot. Mr. Grove, as clerk of the works, and two labouring engineers, who had attended to the execution of the work in England, were sent out for the purpose of erecting the lighthouse, and the necessary apparatus upon the site which had been selected. The elevation of the lighthouse above the level of the sea is one hundred and three feet.
Since the completion of this lighthouse, Mr. Gordon has been employed by the Ordnance Office to furnish designs and specifications for a tower on the same principle, but of larger dimensions and improved details, which is to be erected on Gibbs’ Hill, in the island of Bermuda.