Plate XXIII
DESIGN FOR A LIGHT-HOUSE, SUGGESTED UPON VISITING THE WOLF ROCK SITUATE EIGHT MILES S. S. W. FROM THE LAND’S-END.
Pl. XXIII.
In the range of aquatic buildings applicable to the purposes of a Light-house, which the writer has examined, he was forcibly struck with the magnitude of the Tour de Corduan, on the French coast. This magnificent edifice measures about 145 feet in diameter at the base, and 150 feet in height. Its cubical contents may perhaps be stated at the immense quantity of 339,432 feet; of which the basement alone forms about 200,000 feet. This building has undergone considerable alterations since its completion in the year 1609, as appears from Belidor’s Architecture Hydraulique, tom. ii. At the time of its alteration from a coal fire to an oil light with reflectors, the upper parts in particular seem to have been greatly simplified, by the removal of several of its exterior ornamental appendages.
The Edystone Light-house, owing to the smallness of the rock, as appears from Mr Smeaton’s Narrative of this celebrated building, measures only 26 feet in diameter, at the level of the first entire course; but if there had been space on the Rock for extending it equally on all sides, the ground-course, according to the curve of the outward walls, and the position of the foundation-stone, would have measured 32 feet in diameter. The height of the cupola of the Edystone Light-house is 90 feet, and the cubical contents of the masonry is about 13,147 feet. The Bell Rock Light-house, measures 42 feet in diameter at the base; its height, from the foundation to the cupola, is 118 feet; and the cubical contents of the masonry, as appears from the Table in Appendix, No. VI. is 28,530 feet.
Though the design represented in Plate XXIII., is more or less applicable to several situations upon the coast, yet the writer, in making this Sketch, had special reference to the Wolf Rock, which, as noticed at page 423, he visited in the Orestes sloop of war, commanded by Captain Smith. The extreme dimensions of the upper surface of this rock are about 115 feet, by 90 feet. It is not liable to be covered by the ordinary rise of the tide, though little of it appears above water in spring-tides. The Rock consists of grey porphyry, and is extremely hard. Its outline is somewhat uniform, and the depth of water in its vicinity is from 20 to 40 fathoms. The dangerous position of this reef, in reference to the navigation of the British Channel, led to the proposition of having a Light-house upon it many years since. The erection, however, was ultimately made upon the Long-Ships Rocks, lying about one mile off the Land’s-End.
With the construction and dimensions, therefore, of the Light-houses above alluded to in view, the design delineated in this Plate is given, as the result of the writer’s knowledge and experience on subjects of this kind. Without, however, entering into particulars as to the mode in which such an operation should be conducted, he merely notices, in reference to the various curves delineated in Figs. 1, 2, 3, and 4. as applicable to Light-houses upon sunken rocks, that he prefers the curve of the diagram represented in Fig. 3., as the outline of a building for a situation like the Wolf Rock.
Fig. 1. is formed by the supposed revolution of the cycloidal curve round the axis of a building, whose base is 50 feet, and which, at the entrance-door or top of the solid, measures 25 feet in diameter; the lines produced beyond these dimensions, which would form the habitable part of the Light-house, being tangents to the curves below. Between the base a b, and its parallel c d, this Figure contains 29,635 cubic feet.
Fig. 2. is in like manner formed by the revolution of the logarithmic curve round the axis of a building of similar dimensions at the base and top of the solid with Fig. 1., and contains 31,867 cubic feet.
Fig. 3. is obtained by the revolution of a parabola round the axis of the supposed building. The contents of the solid part, ascertained as in the two former Figures, is 34,006 cubic feet, being 4,371 cubic feet more than that of the cycloidal curve, and 2,139 cubic feet more than in the logarithmic curve.
Fig. 4. is formed by the revolution of the hyperbola between the two parallels a b, and c d, and contains 39,655 cubic feet, or 5,649 cubic feet more than the parabola. But, from the divergent nature of this curve, it is not applicable to the purposes of a Light-house tower, in its pure or simple form.
Fig. 5. is the section of a design formed by the revolution of the parabola round the axis of a building, as its asymptote, whose base a b, measures 56 feet in diameter, and parallel c d, at the top of the solid, is 36 feet; and height to the entrance-door, 35 feet. The contents of this Figure between these parallels is calculated at 45,000 cubic feet; but the whole of the masonry of the design is estimated at 70,624 cubit feet. Its general features may be stated as similar to those of the Edystone and Bell Rock Light-houses, the parts being only enlarged and the parabolic instead of the logarithmic curve, adopted for its outline. In this design, the parabolic curve is continued from the basement to the cope-stone of the Light-room, exclusively of the projection for the cornice and balcony. The masonry is intended to be 120 feet in height, estimating from the medium level of the sea, of which the solid, or from the foundation to the entrance-door, forms 35 feet, the stair-case 25 feet; and the remaining 60 feet of its height is occupied with six apartments, and the walls of the Light-room. In the stair-case a recess is formed for containing the machinery, for raising the stores to the height of the entrance-door; here a small hole, marked g g, is perforated through the building, for the admission of the purchase-chain. The thickness of the walls immediately above the solid, marked c c, is 12 feet; at the top of the stone staircase or level e f, they are 8 feet, and where the walls are thinnest, as at h i, immediately under the cornice, they measure 2 feet. k k represents a drop-hole formed in the courses of the stair-case and solid, for the range of the weight of the machinery of a revolving Light. The ascent to this building, as at the Bell Rock, is intended to be by an exterior stair or ladder of brass, and the interior communication between the several apartments by means of flights of circular oaken-steps.
Fig. 6. represents the first entire course of this design, drawn to a scale double the size of the former Figure, and shows the manner in which the courses are proposed to be built or connected with each other. In every building of this kind, it is proper that two or more of the lower courses, according to the situation and circumstances of the rock, should be sunk or imbedded in it. Since the erection of the present Light-house on the Edystone, the practice of dove-tailing and trenailing stones in water-buildings has been occasionally followed; and there can be no doubt that in some instances it is attended with advantage. But it is also true, that, independently of the extra quantity of rock and workmanship required, the stones are thereby greatly weakened, and rendered much more difficult to be landed in safety in such situations, and that there is often more apparent than real utility in cutting them agreeably to this system of building, as the great bond of the fabric still resolves itself into the gravity of the materials. It may likewise be noticed, that, unless in very peculiar circumstances, the process of trenailing stones newly laid in mortar ought to be avoided, as it becomes necessary to bore holes into the course immediately below, which is apt to break the bond of the mortar by the tremulous motion of the jumper or chisel and hammer.
To avoid these disadvantages, and more effectually to preserve the square or simplest form of the stones, it is proposed, in this new design, to adopt the same mode throughout, that was followed with the courses of the void or habitable part of the building at the Bell Rock, viz. to have zones or joggle-belts worked on the upper beds, and corresponding grooves cut into the under beds, of all the stones. In the plan represented in this Figure these zones are intended to be 1 foot in breadth, and 1½ inch in depth or thickness, forming the concentric rings, marked c c c c c c, which become so many girths to the course, superseding the necessity both of the dove-tailing system, and of the cubic joggles described in Plate XVI.
In tracing the general aspect of Fig. 6., it may be observed that the outer circle of stones is more lightly shaded, being intended for granite, while the hearting or interior part is of a darker shade, as descriptive of sandstone. The whole course of the building here represented, is uniformly simple, and when laid in its place, no mode of attachment can be conceived more effectually to add to the strength and connection of the fabric than these circular belts. Letter d refers to the recess worked in the outward wall of the solid, measuring 2 feet in breadth and 1 foot in depth, for the reception of a flight of brazen-steps for ascending to the entrance-door, instead of the greater projection of the spiral-formed stair delineated in Plate VII., Fig. 6., or the uncertain means of communication by a rope-ladder. The recess alluded to on the outward wall is worked with a small projection, which gradually falls into the circle of the building, without presenting any abrupt face to the waves. A stair of this description, upon a sloping wall, is ascended with great ease.
Fig. 7. is a section of part of the solid of the building, of which a b is the foundation course, and, in reference to Fig 6., letters c c, &c. correspond with the zones or joggle-belts; d d, shew a section of part of the brazen-ladder, answerable to the purposes of a stair, and a pair of skids, for preventing the joints of the building from being injured in taking up stores to the Light-house. This ladder may also be applied as a part of the thunder-rod, as is the case at the Bell Rock.
Fig. 8. is a plan of an entire course of the void or habitable part of the Light-house, showing one of the joggle-belts of the bed-joints. The form of joggle for the end-joints here shewn is also simple, and suited for preserving the strength of the materials more entire than the method of dove-tailing.
Fig. 9. is a section of three courses of the void of the building, showing the connection of the bed-joints.
FINIS
P. Neill, Printer.