460 LIGHTHOUSE ing, cramping, dowelling, and by the use of hydraulic mortar, have never been surpassed. The experiments made by Smeaton on hydrau- lic cements in connection with the construc- tion of this work were particularly valuable, and are still quoted. The erection of the light- house was, on account of its position, the diffi- culty of access to its site, and the fact that Smeaton had determined to build it of stone, attended with the greatest difficulties. The genius and energy of the engineer triumphed over all obstacles, and the work was finished in 1759. It has stood for more than 100 years, a monument of the skill of its designer and builder, and an example to all engineers. An- other noted lighthouse structure is the Bell Eock lighthouse off the E. coast of Scotland. This rock is situated in the German ocean, llm. from the Scot- tish coast, on the N. side of the frith of Forth, and nearly op- posite that of Tay. It is about 427 ft. long and 230 ft. brcrad, but the vicinity is dan- gerous over an area of about 1,400 by 300 ft. The rock is a red- dish sandstone, and the part upon which the lighthouse is built is 12 ft. below high water of springtides, the rise of these tides being 16 ft. The lighthouse is built principally of sand- stone found on the mainland in the vi- cinity, the outer ca- sing of the lowest 30 ft. being of granite. It was commenced in 1807, and was fin- ished late in 1810. The designers were Messrs. Rennie and Robert Stevenson, and it was constructed by the latter. The difficulties of the erection of this lighthouse were nearly as great as those encountered by Smeaton in his work, but the large size of the rock gave it an advantage, and Smeaton's expe- rience was made useful by Stevenson in its con- struction. The form is similar to that of the Eddystone. The diameter of the bottom course is 42 ft., and that of the course just below the cornice 15 ft. The stone work is 102 ft. high, in which height is included that of a parapet wall, octagonal in plan, which surmounts the tower. This wall is 6 ft. high, and its sides are 5 ft. long ; upon it the lantern is placed. The account of the erection of this lighthouse, written by Mr. Stevenson and published in 1824, contains an accurate history of the Scot- tish lighthouses. The Skerryvore lighthouse, FIG. 9. Bell Eock Lighthouse. off the W. coast of Scotland, is also notable on account of the difficulties of its construction. The Skerryvore rocks are situated about llm. S. W. of the island of Tyree, and 50 m. from the mainland. They are in the track of large vessels bound from the Clyde and Mersey (Glasgow and Liverpool) around the north of Ireland, and many wrecks have taken place upon them. The erection of a lighthouse was authorized in 1814, but it was not till 1834 that a survey was made, the result of which was the discovery of a solid gneiss rock 160 FIG. 10. Skerryvore Lighthouse. ft. long and 70 ft. wide, upon which it was determined to erect the lighthouse. Alan Stevenson, a son of the constructor of the Bell Rock lighthouse, succeeded his father a,s engineer of the commissioners of northern lights, and to him was committed the difficult task of the erection of the Skerryvore light- house. The form chosen for the tower is a shaft surmounted by a belt and capital, upon which is the parapet wall. The shaft is a solid of revolution formed by revolving a rec- tangular hyperbola about its asymptote. The diameter of the lowest course is 42 ft., that
