Smeaton and Lighthouses / A Popular Biography, with an Historical Introduction and Sequel

by Anonymous

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.