Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/651

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LIGHTHOUSE 629

Floating Lights.

Prior to 1807 the only kind of floating light was a ship with small lanterns suspended from the yard-arms or frames. The late Mr Robert Stevenson introduced in that year a lantern which surrounded the mast of the vessel, and was capable of being lowered down to the deck to be trimmed. By his plan a lantern of much greater size could be used, and with this increase of size a larger and more perfect apparatus could be employed which admitted of gearing for working a revolving light, as adopted by the Trinity House. In fig. 68, aa is the mast, b tackle hook, c, c brass flanges for fixing parts of lantern together, e and g weather guards, h plate glass front of lantern, i shutter by which lamps are trimmed, k lamps, l silver reflector. Revolving catoptric apparatus was applied to floating lights in England, and M. Letourneau, in 1851, proposed to employ a number of sets of dioptric apparatus in one lantern; figs. 69 and 70 show elevation and plan of one of the Hooghly floating lights on the dioptric principle, designed for the Indian Government by Messrs Stevenson. It will be observed that not fewer than four of the separate lights are always in view from every part of the compass at once. The "Seven Stones" lightship, which has all the recent Trinity House improvements, is moored off the Lands End in 42 fathoms. Her moorings consist of a 40 cwt. mushroom anchor, and 315 fathoms of 1½ inch studded chain cable. The vessel is timber-built, copper-fastened throughout, sheathed with Muntz metal. Her length is 103 feet between perpendiculars; extreme breadth, 21 feet 3 inches; depth of hold from the strake next the timbers to the upper side of the upper deck beams, 10 feet 3 inches. In the event of the vessel breaking adrift, she is provided with sails, the mizzen being frequently used for steadying the vessel at her moorings. The cost of the vessel, fully equipped for sea, with illuminating and fog signal apparatus complete, was about £9500.

Fig. 68. Fig. 69. – Dioptric Floating Light. Elevation. Fig. 70. – Dioptric Floating Light. Horizontal Section.

Lanterns.

Lanterns are generally constructed of diagonally framed astragals (fig. 71) to avoid the obscuration of light in any one azimuth as would be the case with dioptric fixed light apparatus were vertical astragals used. The astragals are made of gun-metal, having a tensile strain of 33,000 lb to the square inch; the dome is made of copper plates rivetted together; and the glass is the best plate ¼ inch thick. Storm panes are kept at hand which can be applied in a few minutes in the event of a pane being broken, which sometimes occurs from large birds driving against the lantern and by stones thrown up from the face of cliffs by sea and wind during storms. Mr Douglass's cylindric or helical lanterns (fig. 72) have steel astragals, and the panes, which are also cylindrical, are ½ inch thick.

Fig. 71. – Diagonal Lantern. Fig. 72. – Helical Lantern.

Lightning Conductors.

Lightning Conductors. – Each lighthouse tower is furnished with a copper lightning conductor consisting of a ¾ inch copper rod or wire rope. These conductors terminate about 18 inches above the top of dome of lantern in a forked shape with two platinum points; the lower end bifurcated and furnished with large copper earth-plates is sunk into moist earth or into the sea. The conductor is put into metallic connexion with all large masses of metal outside or inside the lightroom. See LIGHTNING CONDUCTORS.

Machinery.

Machinery. – Revolving lights are profiled by clock-work machinery actuated by a falling weight. The machines are provided with a maintaining power which keeps the apparatus moving while the machine is being wound up, and a bell is attached which rings to warn the keepers when the machine requires winding up. A clock face is also attached which keeps time with the lightroom clock when the apparatus is revolving at the proper speed.

Masking Screens.

Masking Screens. – For the purpose of cutting off sharply the light proceeding from any fixed light apparatus, the late Mr J. M. Balfour devised what he called radial masking screens. These screens, which are in use at several lighthouses where the fairway for ships is narrow, consist of a series of thin plates placed with their edges towards the apparatus, and with such a distance between them, and of such a length as will secure that only rays having the desired divergence can pass through between them.

With revolving lights it is not possible to effect a "cut off" as in the case of fixed lights, on any particular bearing; for the direction of the axis of the luminous beam projected by the lens or reflector is being continually changed in the horizontal plane by the revolution of the frame on which it is fixed. Mr Stevenson's mode of overcoming this difficulty is by fixing a light metallic screen or coloured glass shade outside of the revolving apparatus and on the safety side of the danger arc. These screens are fitted with rollers for running on a slightly inclined plane or circular path. Small projecting rods or snugs fixed to the sides of each lens are, in revolving, brought against the edge of the screen, and gradually press the screen before it up the inclined plane. By the time the lens reaches the edge of the danger arc the screen has been pushed to the top of the inclined plane, and the full beam of light, coming from the now entirely uncovered lens, points in the required line of cut off, which is the border of the danger arc. But whenever the further revolution of the apparatus causes the snugs to pass clear of the edges of the screen so as to free it from their pressure it immediately runs back again to its original position in front of the lens, so as to prevent any of the light being now sent seawards. By this continued reciprocative movement of the screen, as lens after lens comes round, the same effect will be successively produced and the light will always be cut off on the lines of obscuration, so that the flashes can never be seen within the danger arc.

Mathematical formulæ.

Mathematical investigations and formulæ for constructing the optical instruments for lighthouses will be found in the books undernoted by Alan Stevenson, Swan, Chance, Nehls, Reynaud, Allard, &c., to which the reader is referred.

Literature.

Literature. – Smeaton, Narrative of the Eddystone Lighthouse, London, 1793; Stevenson, Account of the Bell Rock Lighthouse, Edinburgh, 1824; Fresnel, Mémoire sur un nouveau système d'éclairage des phares, Paris, 1822; Brewster, Treatise on Burning Instruments, 1812; Fanale di Salvore, nell' Istria, Illuminato a Gaz, Vienna, 1821; Account of a New System of Illumination for Lighthouses, 1827; Alan Stevenson, Account of Skerryvore Lighthouse, with Notes on Lighthouse Illumination, 1847; Stevenson, Treatise on the History, Construction, and Illumination of Lighthouses, London. 1850; Report of the Lighthouse Board of America, Washington, 1852; J. T. Chance, "On Optical Apparatus used in Lighthouses," in Proc. Inst. C. E., vol xxvi.; "Dioptric Apparatus in Lighthouses for the Electric Light," Proc. Inst. C. E., vol. lii; Douglass, "The Wolf-Rock Lighthouse," Proc. Inst. C E., vol. xxx; "Electric Light applied to Lighthouse Illumination," Proc Inst. C. E., vol. lvii.; Thos. Stevenson, Lighthouse Illumination, 2d ed., Edinburgh, 1871 (German translation with appendix, Ueber die Berechnung von Leuchtthurmhohen und Leuchtthurm-apparatus, by Chr. Nehls, Hanover, 1878); L. Reynaud, Mémoire sur l'éclairage et le balisage des côtes de France, Paris, 1804; Thos. Stevenson, Lighthouse Construction and Illumination, London, 1881; W. Swan, "New Forms of Lighthouse Apparatus," Trans. Roy. Scot. Soc. of Arts, vol. viii.; M. E. Allard, Mémoire sur l'intensité et la portée des phares, Paris, 1876; Id., Mémoire sur les phares électriques, Paris, 1881; Proc. Inst. C. E., passim. (T. ST.)