Engd. by J. Moffat Edinr.
PLATE XXIII.
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.