Wolf Trap Lighthouse.—This building was erected during the years 1893 and 1894 on Wolf Trap Spit in Chesapeake Bay, near the site of the old openwork structure which was swept away by ice early in 1893. The new tower is formed upon a cast iron caisson 30 ft. in diameter sunk 18 ft. into the sandy bottom. The depth of water on the shoal is 16 ft. at low water. The caisson was filled with concrete, and is surmounted by a brick superstructure 52 ft. in height from low water to the focal plane of the light. A somewhat similar structure was erected in 1885-1887 on the Fourteen Foot Bank in Delaware Bay, at a cost of £24,700. The foundation in this case was, however, shifting sand, and the caisson was carried to a greater depth.
Rothersand Lighthouse.—This lighthouse, off the entrance to the river Weser (Germany), is a structure of great interest on account of the difficulties met with in its construction. The tower had to be founded on a bottom of shifting sand 20 ft. below low water and in a very exposed situation. Work was begun in May 1881, when attempts were made to sink an iron caisson under pneumatic pressure. Owing to the enormous scour removing the sand from one side of the caisson it tilted to an alarming angle, but eventually it was sunk to a level of 70 ft. below low-water mark. In October of the same year the whole structure collapsed. Another attempt, made in May 1883, to sink a caisson of bi-convex shape in plan 47 ft. long, 37 ft. wide and 62 ft. in height, met with success, and after many difficulties the structure was sunk to a depth of 73 ft. below low water, the sides being raised by the addition of iron plating as the caisson sank. The sand was removed from the interior by suction. Around the caisson foundation were placed 74,000 cub. yds. of mattress work and stones, the interior being filled with concrete. Towards the end of 1885 the lighthouse was completed, at a total cost, including the first attempt, of over £65,000. The tower is an iron structure in the shape of a concave elliptic frustum, its base being founded upon the caisson foundation at about half-tide level (fig. 21). The light is electric, the current being supplied by cable from the shore. The focal plane is 78 ft. above high water or 109 ft. from the sand level. The total height from the foundation of the caisson to the top of the vane is 185 ft.
Other famous wave-swept towers are those at Haulbowline Rock (Carlingford Lough, Ireland, 1823); Horsburgh (Singapore, 1851); Bayes d’Olonne (Bay of Biscay, 1861); Hanois (Alderney, 1862); Daedalus Reef, iron tower (Red Sea, 1863); Alguada Reef (Bay of Bengal, 1865); Longships (Land’s End, 1872); the Prongs (Bombay, 1874); Little Basses (Ceylon, 1878); the Graves (Boston, U.S.A., 1905); Jument d’Ouessant (France, 1907); and Roche Bonne (France, building 1910).
Jointing of Stones in Rock Towers.—Various methods of jointing the stones in rock towers are shown in figs. 6 and 22. The great distinction between the towers built by successive engineers to the Trinity House and other rock lighthouses is that, in the former the stones of each course are dovetailed together both laterally and vertically and are not connected by metal or wooden pins and wedges and dowled as in most other cases. This dovetail method was first adopted at the Hanois Rock at the suggestion of Nicholas Douglass. On the upper face, one side and at one end of each block is a dovetailed projection. On the under face and the other side and end, corresponding dovetailed recesses are formed with just sufficient clearance for the raised bands to enter in setting (fig. 23). The cement mortar in the joint formed between the faces so locks the dovetails that the stones cannot be separated without breaking (fig. 24).
Table I.—Comparative Cost of Exposed Rock Towers.
| Name of Structure. | Total Cost. | Cub. ft. | Cost per cub. ft. of Masonry. | ||||
| Eddystone, Smeaton (1759) | £40,000 | 0 | 0 | 13,343 | £2 | 9 | 11½ |
| Bell Rock, Firth of Forth (1811) | 55,619 | 12 | 1 | 28,530 | 1 | 19 | 0 |
| Skerryvore, west coast of Scotland (1844) | 72,200 | 11 | 6 | 58,580 | 1 | 4 | 7¾ |
| Bishop Rock, first granite tower (1858) | 34,559 | 18 | 9 | 35,209 | 0 | 19 | 7½ |
| Smalls, Bristol Channel (1861) | 50,124 | 11 | 8 | 46,386 | 1 | 1 | 7¼ |
| Hanois, Alderney (1862) | 25,296 | 0 | 0 | 24,542 | 1 | 0 | 7¼ |
| Wolf Rock, Land’s End (1869) | 62,726 | 0 | 0 | 59,070 | 1 | 1 | 3 |
| Dhu Heartach, west coast of Scotland (1872) | 72,584 | 9 | 7 | 42,050 | 1 | 14 | 6 |
| Longships, Land’s End (1872) | 43,869 | 8 | 11 | 47,610 | 0 | 18 | 5 |
| Eddystone, Douglass (1882) | 59,255 | 0 | 0 | 65,198 | 0 | 18 | 2 |
| Bishop Rock, strengthening and part reconstruction (1887) | 64,889 | 0 | 0 | 45,080 | 1 | 8 | 9 |
| Great Basses, Ceylon (1873) | 63,560 | 0 | 0 | 47,819 | 1 | 6 | 7 |
| Minot’s Ledge, Boston, Mass. (1860) | 62,500 | 0 | 0 | 36,322 | 1 | 17 | 2 |
| Spectacle Reef, Lake Huron (1874) | 78,125 | 0 | 0 | 42,742 | 1 | 16 | 2 |
| Ar’men, France (1881) | 37,692 | 0 | 0 | 32,400 | 1 | 3 | 3 |
| Fastnet, Ireland (1904) | 79,000 | 0 | 0 | 62,600 | 1 | 5 | 5½ |
Effect of Waves.—The wave stroke to which rock lighthouse towers are exposed is often considerable. At the Dhu Heartach, during the erection of the tower, 14 joggled stones, each of 2 tons weight, were washed away after having been set in cement at a height of 37 ft. above high water, and similar damage was done during the construction of the Bell Rock tower. The effect of waves on the Bishop Rock and Eddystone towers has been noted above.
Land Structures for Lighthouses.—The erection of lighthouse towers and other buildings on land presents no difficulties of construction, and such buildings are of ordinary architectural character. It will therefore be unnecessary to refer to them in detail. Attention is directed to the Phare d’Eckmühl at Penmarc’h (Finistère), completed in 1897. The cost of this magnificent structure, 207 ft. in height from the ground, was largely defrayed by a bequest of £12,000 left by the marquis de Blocqueville. It is constructed entirely of granite, and is octagonal in plan. The total cost of the tower and other lighthouse buildings amounted to £16,000.