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 | 1112 |
| 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 | 734 |
| Bishop Rock, first granite tower (1858) | 34,559 | 18 | 9 | 35,209 | 0 | 19 | 712 |
| Smalls, Bristol Channel (1861) | 50,124 | 11 | 8 | 46,386 | 1 | 1 | 714 |
| Hanois, Alderney (1862) | 25,296 | 0 | 0 | 24,542 | 1 | 0 | 714 |
| 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 | 512 |
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.
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| Fig. 19.—Maplin Pile Lighthouse. |
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.
The tower at Île Vierge (Finistère), completed in 1902, has an elevation of 247 ft. from the ground level to the focal plane, and is probably the highest structure of its kind in the world.
The brick tower, constructed at Spurn Point, at the entrance to the Humber and completed in 1895, replaced an earlier structure erected by Smeaton at the end of the 18th century. The existing tower is constructed on a foundation consisting of concrete cylinders sunk in the shingle beach. The focal plane of the light is elevated 120 ft. above high water.
Besides being built of stone or brick, land towers are frequently
