Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/639

 LIGHTHOUSE 617 high-water springs. The solid is 32 feet in height, weighing 2050 tons, the whole weight of the tower being 3557 tons. The walls decrease from 9 feet 3 inches to 2 feet 3 inches in thickness. The work was designed by Messrs D. & T. Stevenson, and was begun iu 1869 and completed in 1874:. Great Basses Lighthouse near Ceylon. Great Basses lighthouse lies 6 miles from the nearest land, and was designed by Mr Douglass. The tower has a cylindrical base 32 feet in diameter, above which is a tower 67 feet 5 inches high and 23 feet in diameter. The walls vary in thickness from 5 feet to 2 feet. The tower, including the base, contains about 2768 tons, and the work was finished in three years. There are several other lighthouses in Ireland, India, and America which merit more attention than our space admits of, and we shall therefore conclude with directing the attention of the engineer to the important influence of the configuration of rocks in modifying the breaking of waves. It cannot but excite surprise that some of the structures which were erected on the Eddystone should have withstood the waves so long as they did. This fact seems to lead to the conclusion that the Eddystone Rock, at one time at least, acted to some extent as a shelter to the structures which were built on it. During a summer gale when Dhu Heartach lighthouse was being erected fourteen stones each of 2 tons weight, which had been fixed on the tower by joggles and Portland cement at the level of 37 feet above high water, were torn out and swept off into deep water, as shown on Plate VII. At the Bell Hock stones of 2 tons weight were several times swept away during the construction of the tower, while it is a remarkable fact that no stones were ever moved at the Eddystone. But what is more striking, the thin glass panes of Winstanley s first tower stood successfully through a whole winter s storms at the same level above the water as that at which the fourteen heavy blocks were swept away at Dhu Heartach, where it was found necessary from the experience acquired when constructing the light house to raise the solid base of the tower to nearly the same height above the water as the glass panes in Smeaton s tower, which have hardly ever been broken during the storms of more than a hundred years. The conclusion then which seems fairly deducible from these facts is that the level of the plane of dangerous impact of the waves above high water depends upon the relation subsisting between their height and the configuration of the rocks above and below high water, as well as perhaps on the configuration of the bottom of the sea near the lighthouse. Thus, while the rock at Dhu Heartach, from its height above high water, forms a great protection against the smaller class of waves, it operates as a dangerous conductor to the largest waves, enabling them to exert a powerful horizontal force at a much higher level than they would had the rock been lower. The lighthouse engineer must therefore beware of taking it for granted that Smeaton s Eddystone tower is a model for general imitation, and must carefully consider as best he can in what way the configur ation of the rock may affect the stability of the tower which he has to design. Unfortunately in the present state of our information no specific directions can be laid down for his guidance in this matter, but the following general rules of construction may be given : (1) The tower should have a low centre of gravity, and sufficient mass to prevent its being upset by the waves. (2) It should be throughout circular in the horizontal plane, and either straight or continuously curved in the vertical plane, so as to present no abrupt change of outline which would check the free ascent of the rising waves, or the free descent of the falling waves, or the free vent of those passing round the tower. All external scarcements in the vertical plane, or polygonal outline in the horizontal plane, are therefore objectionable. (3) Its height, c&tcris paribus, should be determined by the distance at which the light requires to be seen by the sailor. The rule for determining tin s height will be afterwards given. (4) Where the rock is soft, or consists of ledges which are easily torn up, the tower should spring from the foun dation-course at a low angle with the surface of the rock, so as to prevent its being broken up by reaction of the waves from the building ; or, in other words, the tower must have a curved profile. But special care should be taken to sink the foundation-courses below the surface of the rock, as the superincumbent weight decreases with the sine of the angle of inclination of the wall. If the rock overhangs, owing to the wearing action of the waves, the tower should, if possible, be built at a distance from the place where this dangerous action is in progress. (5) Where the rock is hard and of ample area, the tower may be of such a curved form as will best suit the economic arrangement of the materials, so as to avoid an unnecessary thickness of the upper walls. (6) When the rock is hard, but of small dimensions, the diameter above the base should not be suddenly reduced by adopting a curved profile, but a conical outline should be adopted ; and if the rock be hard, but of yet smaller dimensions, a cylindric form of greater height should be adopted so as to thicken the walls, and to increase the weight and therefore increase the friction, which is directly proportional to the weight of the blocks of masonry. In all cases where the rock is small the thickness of the walls should be decreased by steps or scarcements internally, but nver externally. (7) The level of the top of the solid part of the tower, and the thickness of the walls above it, should, in different towers having the same exposure, be determined^in each case by the level and configuration of the rock and of the bottom of the sea. (8) The best position for the tower is not necessarily the highest part of the rock. It should, in each case, be selected so as to secure the greatest protection in the direc tion of the maximum fetch and deepest water near the reef. (9) The tower should not, it possible, be erected across any chasm which divides the rock, or in the direction of any gully which projects into it, especially if it be of converging form, which would concentrate wave action. (10) No permanent fixture of the tower to the rock is required for increasing the.stability of the structure. The foun dation-course (unless where a curved profile is adopted) becomes, indeed, in the end the most stable of all, because it has the greatest weight above it, to keep it in its place. (11) The stones should, however, be sufficiently connected together, and fixed to the rock, in order to prevent their bein washed away during the construc tion of the work, when they have no superincumbent weight to keep them in their beds. (12) The tower should rest on a truly level base, or on level steps cut in the rock. (13) The pressure of all the materials within the tower should act vertically, so as not to produce a resolved force acting laterally as an outward thrust. (14) The tower should be of such height and diameter, with walls of such thickness, as to prevent the masonry being disturbed by the impact of the waves. (15) The entrance door should be placed on that side of the tower where the length of fetch is shortest, or where from the configuration of the reef and the depth of water the force of the waves is least. This was determined at the Bell Rock by the distribution of the fiicus which grew on the lower parts of the tower dur ing the first winter, the vege tation being least where the waves were heaviest. (16) The materials should be of the highest specific gravity that can be readily obtained, and, in some special cases, lead, or dove-tailed blocks of cast iron set in cement, might perhaps be employed. Plate VII. shows sec tions on the same scale of a number of the ^p more remarkable light- -~ house towers. Fig. 3 shows an iron _- pile light erected at - Haneda, in the Bay of --;__. Yedo, Japan. Fig. 3. Modes of Uniting the Stones and Courses of Jfasonry. Fig. 4 (p. 618) shows the mode of combining the stones during construction at different lighthouses iu the United Kingdom and in America. XIV. 78