Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/636

616 LIGHTHOUSE in 1706 and completed in 1709, in the form of a frustum of a cone 92 feet high. The work consisted principally of timber, the lower part being oak carefully bolted together, aad also to the rock. Above the lower structure of oak courses of stone, cramped together and fixed to the timber work and to the rock, were added in order to give weight to the structure. This lighthouse stood for forty-six years, and was destroyed by fire in 1755. In every respect the simplicity of the structure and the judicious character of the details of the design may be regarded as models of engineering. First, it rested upon a stepped level base, was circular in plan, did not wholly depend upon fixtures but upon weight, preserved a uniform external surface devoid of outside projections and ornamentation ; and, above all, the engineer did not by splaying out the base needlessly throw away the small diameter which the rock afforded, but with much judgment adopted the conical form. Smeaton s Eddystone Tower. This justly celebrated work, which consisted entirely of stone, was commenced in 1756, and the masonry was finished in 1759. Smeaton was the first engineer who adopted a structure of masonry for a sea tower and dovetailed joints for the stones, which averaged a ton in weight. This work cannot be regarded as a safe model for general imitation in exposed situations, and RudyeVd s earlier tower was certainly as successful in resisting the forces to which it was exposed. Rudyerd unquestionably selected for so small a rock as the Eddystone a preferable form to that adopted by Smeaton. The sharply curved profile in Smeaton s design greatly reduced the diameter of the building at a small height above the rock, and so re duced its strength. Smeaton s reasoning about the similarity of a tower exposed to the surf and an oak tree resisting the wind was very conclusively shown to be fallacious by the late Mr Alan Stevenson. The arching of the floors, as shown in fig. 1, was also a source of weakness which the introduction of the iron chains, shown black in the diagram, was intended to counteract. Mr Douglass in 1878 stated that "for several years the safety of the Eddystone had been a matter of anxiety and watchful care to the corporation of the Trinity House, owing to the great tremor of the building with each wave stroke." He also stated that the projecting cornice at the top had been lifted, and that the rock itself had been considerably undermined. A new tower has now (1882) been erected in place of Smeaton s by Mr Douglass. Bell Rock Lighthouse Tower. The Bell Rock, which lies 12 miles off the coast of Forfarshire, is fully exposed to the assaults of the German Ocean. The rock is of considerable extent but of a low level, the tower being covered about 16 feet at high water of spring tides. Mr R. Stevenson, of Edinburgh, when he first landed on the rock, decided to adopt a stone tower as Smeaton had done at the Eddy- stone, but he deviated largely from that design in the thickness of the walls, in raising the tower to 100 feet instead of 68 feet, and the level of the solid to 21 feet Fig. 2. above high water instead of 11 feet. Instead of employ ing arched floors as at the Eddystone, he adopted lintel stones for the floors which formed part of the outward walls, and were feathered and grooved as in carpentry, besides having dovetailed joggles across the joints where they formed part of the walls. It will be seen on fig. 2 that the floors instead of being sources of weakness, as in Smeaton s tower, were converted into effective bonds tying the walls together. He also used a temporary beacon or barrack on the rock for the engineer and his workmen to live in while the tower was in progress. The bill intro- dueed into parliament for this work in 1802 was not passed in consequence of financial difficulties. As the Bell Rock was scarcely dry at low water, while the Eddystone was scarcely covered at high water, the commissioners, in order to fortify Mr Stevenson s views, consulted Mr Telford, and before going to parliament for the second time they also, on Mr Stevenson s suggestion, obtained for the scheme the support of Mr Rennie, with whom he could afterwards advise in case of emergency during the progress of the work. The second bill was passed in 1806, and the works, which were begun in 1807, were finished in 1810, and the light was exhibited in 1811. The total weight of the tower is 2076 tons. Skerryvore Lighthouse. The Skerry vore Rocks, 12 miles off the island of Tyree in Argyllshire, which is the nearest land, are wholly open to the Atlantic. The works, designed and carried out by the late Mr Alan Stevenson, were com menced in 1838 and finished in 1843. The first tem porary barrack was destroyed in 1838, and another erected on a more sheltered part of the rock. The tower, which is of a hyperbolic curve, is 138 feet high, 42 diameter at the base, and 16 at the top. Its weight is 4308 tons. Bishop Rock. The Bishop Rock, lying off the Scilly Islands, is fully exposed to the Atlantic. It was designed by the late Mr James Walker, and carried out by Mr J. N. Douglass. It is 100 feet above high water, 34 feet in diameter at the base, and 17 at top. The lowest part of the foundation of tower is covered about 19 feet at high water springs ; the solid is 20 feet above high water, where the walls are 9 feet thick, and decrease to 2 feet at the top. Owing to the great tremor in this building, it has lately been found necessary to strengthen it by an internal structure of ironwork. Wolf Rock. This much exposed rock is about midway between Scilly and the Lizard Point, and is submerged to the depth of about 2 feet at high water. The first design for a lighthouse was in 1823, by Mr R. Stevenson, but it was not till 1862 that a lighthouse was commenced under the superintendence of Mr Douglass, from a design by the late Mr Walker. It is 116 feet high, 41 feet 8 inches diameter at the base, decreasing to 17 at the top, and the walls are 7 feet 9 inches thick, decreasing to 2 feet 3 inches. The shaft is a concave elliptic frustum, and contains 3290 tons. The lower part of the tower has projecting scarcements in order to break up the sea, but, as has been already stated in the case of the Eddystone, such projections are not in accordance with the principle of uniformity of external surface, and are, therefore, more likely to produce disturbance of the masonry than to add to its stability. Dhu Heartach Rock Lighthouse. The Dhu Heartach Rock, which is 35 feet above high water, is 14 miles from the island of Mull, which is the nearest shore. The maximum diameter of the tower, which is of parabolic outline, is 36 feet, decreasing to 16 feet; the shaft is solid for 32 feet above the rock; the masonry weighs 3115 tons, of which 1810 are contained in the solid part. The temporary barrack for the workmen was made of malleable iron bars with an iron drum on the top in which the workmen lived. This tower was designed by Messrs D. & T. Stevenson, and occupied six years in erection, the length of the working season being only about two and a half months in each year. Chickens Rock Lighthouse. The Chickens Rock lies 1 mile off the Calf of Man. The curve of the tower, which is 123 feet 4 inches high, is hyperbolic, the diameter varying from 42 feet to 16 feet. The tower is submerged 5 feet at