Plate VII. — Original Designs for the Bell Rock Light-House
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.
Fig. 5. represents the lower floor of the five apartments, in which a a a a a a a a, as in the foregoing Figs. 1, 2, and 3. shew the main columns passing between the outer covering and inner lining of copper. Letter c marks the entrance door, shewn at g, in Fig. 2., in which d is the plat at the top of the ladder, formed on one of the columns, which may also be conceived as passing up through the habitable part of the building; the floors of which are formed with plates of cast-iron.
Fig. 6. represents a Light-house of masonry, modelled by the writer immediately after his first landing upon the Rock. The better to illustrate this design, the solid or lower part is given in elevation, and the upper or habitable part in section. Compared with the Edystone Light-house, upon the principles of which it was designed, the chief differences consist in the exterior spiral-formed stair, intended to afford permanent access to the Light-house, and also to serve as a landing quay or wharf at high-water. The steps of this stair were to be 3 feet in length, forming a kind of scarsement upon the outer-wall, there being nothing to obstruct the force of the sea under the stair; as it formed part of the main-wall, it may be conceived calculated to have become a kind of spiral buttress to the building. In the upper or habitable part, the interior walls diminish gradually, instead of forming abrupt scarsements, and the floors were laid horizontally, instead of being arched.
Fig. 7. Shews one of the floors, each stone of which forms part of the outward walls, extending inwards to a centre stone, independently of which they were to be connected, by means of copper-batts, with a view to preserve their square form at the extremity, instead of dove-tailing. These stones were also modelled with joggles sidewise, upon the principles of the common-floor, termed Feathering in carpentry; and also with dove-tailed joggles across the joints, where they formed part of the outward wall, as shewn in this figure.
Fig. 8. is a plan of one of the courses of the void or habitable part of this design, shewing both the joggles of the end joint and the girth, raised upon the upper beds of the courses, and sunk into the groove cut in the lower beds of the respective covering courses.
Figs. 9. and 10. exhibit different modes of connecting the stones and courses of the solid of the building, both perpendicularly and horizontally, by means of dove-tailing. But as a certain degree of weakness is unavoidably incident to every system of dove-tailing, and, as the method of forming the bed-joints of the void above alluded to is equally effective, in its application to the solid or lower part of the building, as represented in Plate XXIII., this is considered preferable to the dove-tailing system.
Fig. 11. is part of a course, shewing a simple mode of connecting the stones in water-buildings, so as to avoid dove-tailing, and preserve, as much as possible, the entire figure of the stones. This is effected by inserting square joggles into the joints, and allowing these joggles to project 6 inches into the course immediately above, thereby forming a connection horizontally between the stones of a course, and vertically with the several courses.
Fig. 12. is also a course of masonry, in which the square or entire form of the materials is preserved, while the stones are bound both horizontally and vertically, by means of metallic-batts and plates, as represented in the enlarged diagram described in the following Figure.
Fig. 13., a is the dove-tailed cross-head or plate; b is a section both of the batt and plate, in their places, shewing their connection with two courses of the building marked A B; c shews the mode in which the batt is fixed into the lower course, by means of wedges; d the upper end of the batt and plate, which is also wedged, and appears, when in its place, as in Fig. 11., marked also a a a. But these several modes of connecting the materials of masonry are considered more or less objectionable, when compared with the girths delineated in the design represented in Plate XXIII.
Figs. 14. and 15. are a reduced plan and elevation of the Bell Rock Light-house by Mr Rennie, in the year 1807. They will be found so obvious upon inspection, as not to require explanation, and are here preserved as the only plans or drawings furnished for this work by that eminent engineer.