The entrance and lowest apartment is at the top of this solid masonry, and there is an external stair and platform for landing when the tide suits. The walls of this apartment are feet thick ; it is occupied by the water-tanks, fuel, and other heavy necessaries. The second, which is much more ample, in consequence of the reduced thickness of the walls, contains the oil and other stores necessary for the lights. The third floor is the kitchen, and the fourth the bedroom for the keepers ; and the fifth room is the library, and place for the reception of sueh strangers as have hardihood to visit this sea-girt pillar. Over all these is the light-room, with double glazed windows, and wholly fire-proof, except the external dead-lights, which are put on as occasion requires. The balcony around the light-room is well secured by a cast-iron railing, supported with brass, and having a strong top-tail of that metal. The parapet of the light-rootn is six feet high, and from it a door opens to the balcony. The sashes of the windows arc of cast-iron, the glazing strong plate double, as we have said, and the dome is of copper. The lights are revol ving ones, and show alternately a white and red light, produced by stained glass. They are very powerfid, and can readily be seen at a distance of 20 miles or more, unless Ns-hen the atmosphere is ; and unfortunately, no light has yet been discovered which can so far penetrate a thick fig as to warn a ship of danger in time for even a chance of escaping.
As the best substitute that circumstances admit of, turn hells of 12 cwt. each are tolled constantly day and night, when the atmosphere is foggy, by means of the same machinery which moves the lights. In calm weather. during which fl)p-s are most frequent, the sound of these bells can be heard all over the surface of the rock which is absolutely dangerous ; and thus they justify the appellation of Bell-rock lighthouse, and remind one of the Abbot of St. Thotnas.
The rise of the tide over the foundations of the lighthouse is about 16 feet at ordinary spring floods ; and when the sea is perfectly tranquil, the structure seems resting on the waters. Altogether, indeed, the Bell-rock lighthouse is a structure of great interest, and one which has been the means of saving many lives and much property. Consider ing that the workmen had to contend with the violence of the sea, upon a rock 12 miles from land, and with its highest point 12 feet below the surface of high water; it will be readily understood that the commencement of the work, and all the early stages of it. must have been attended with great difficulty and no small danger. A faithful history of its construction, drawn up with even the tithe of the talent which Smeaton displays in his report on Eddystone, would be an interesting and instructive work ; unfortunately, there was no Smeaton conversant with all the details of the Bell rock ; and the result is, that all the published accounts of it are meagre. and some of them perhaps not true.
Another noble erection of this kind is that on the Skerry more rock, ofT' the west coast of Scotland. This building was constructed from the designs of Mr. Alvan Stevenson, the talented engineer to the Scottish Lighthouse Board, and cost in its erection, with the harbour for the tender and other necessaries, £S7.000 ; it was first illuminated in 1844. The light is 150 feet above the sea, and the structure and its appliances exhibit every refinement and improvement hitherto etlected by modern science in the varied particulars of the system.
Although, however, the talent and practical ability of such men *as Smeatnn and Rennie enabled them to overcome all the difficulties of constructing such buildings as we have described, other situations, where also it was desirable to erect them, presented obstacles of another character, perhaps even still more troublesome to deal with ; we allude to the erecting lighthouses on such shifting and dangerous sands as the Goodwin, &c. Here the engineer has no solid rock to build on ; instead of a substantial foundation on which to base his work, he has to work on a treacherous material, which slides from under him, and engulfs all that is placed on it.
Of the means for meeting such difficulties, the first to be noticed is the of Mr. Alexander :1Iitehell, C. a of Belfast. This principle was first employed in the con struction of the foundation of the Maplin Lighthouse, on the north side of the mouth of the Thames, on which Dow exhibited a red light. This was commenced in 1S3S, and is as firm now as when first erected ; it stands on the outer edge of the Maplin Sand. This dangerous shoal is composed of sand at the surfilee, and afterwards of sand and mud ; it is exceedingly soft and penetrable. and therefore the creed m of a lighthouse upon such a foundation must be considered as a great achievement.
The principle, of this serew-pile lighthouse, is having a series of piles nine in number—eight in the angles of an octagon, and one in the centre. These piles consist of a shaft of hammered iron, five or six inches in diameter. having a single turn of the flange of a screw four feet in diameter. This pile is screwed with great facility into the sand, to the depth of 22 feet ; and each, it was ealoulated, would bear a weight of 64 tons. Nine piles were fixed in nine con secutive days in the summer of 1b3S, and upon this foundation of Mr. Mitchell's, the light-room was erected under the direction of Mr. Walker, the engineer to the Trinity Board.