DESIGN for the BELL ROCK LIGHT HOUSE by Mr. RENNIE.
Drawn by J. Slight
Engraved by R. Scott
PLATE VII.
Pl. VII.
Fig. 1. shews the sites of the principal and diagonal supports of cast-iron of the Pillar-formed Light-house, modelled by the writer prior to his landing on the Rock in the year 1800, as alluded to in his report, included in the Appendix, No. IV. at page [440]. This Figure is a Plan in which letters a a a a a a a a point out the feet of the eight principal columns; e the central shoe or socket, intended to receive the feet of the diagonal supports or braces, which correspond in number with those of the main columns. This fabric, at the base, was to form a common diameter of about 35 feet, diminishing to 12 feet at the top, or immediately under the Light-room.
Fig. 2. The great columns in this design, which are sunk into the Rock, and the diagonal braces d d d d, which butt against, or step into the shoe c, are also strongly connected by the braces b b b b b b b b, and cross ties marked c c c c c c. The habitable part of this building is formed by two concentric rings of copper, marked a f, a f, the one upon the outside, and the other upon the inside, of the main columns. The space between these rings was to be filled with plaster-of-Paris, or some light substance, to render the apartments air-tight. Letter g is the position of the entrance-door, and the space below the first floor, marked h, is a coal magazine. The better to throw off the sprays of the sea, the lower part, as will be seen from the diagram, is of a conical form inverted.
Fig. 3. Shews the mode of fixing the principal columns, to the Rock, and of building or connecting them together, being an enlarged section of the foot of one of the main columns. The hollow cylindrical tube marked b b, is sunk 24 inches, and acts as a steady pin or joggle to the column which is wedged into the Rock, as shewn at c c c c, by driving oaken and iron wedges alternately, instead of running them up with melted lead. The next length of the column, marked a a a a, is slipped over this tube, and has a seat cut in the rock at right angles to the oblique direction of the column, to which it is connected by the cutter or spear-bolts marked d d d, whose directions respectively cross each other, as shewn in the section.
Fig. 4. represents one of the joints of the main columns, which occur at every 7 feet of their length, from the rock to the habitable part of the Light-house. This description of joint is what is technically termed spigot and faucet; a a is the upper half, b b b the lower, c c the joint, d d d d the collar-piece clasping the joint, and connecting it with the horizontal arms or braces e e, which, in Fig. 2, are marked b, and in connection with the cross ties c, and diagonal-supports d, bind the whole firmly together.