MISCELLANEOUS LIGHTHOUSE EQUIPMENT Modern lighthouse lanterns usually consist of a cast-iron or steel pedestal, cylindrical in plan, on which is erected the lantern glazing, surmounted by a domed roof and ventilator (fig. 12). Adequate ventilation is of great importance, and is provided by means of ventilators in the pedestal and a large ventilating dome or cowl in the roof. The astragals carrying the glazing are of wrought steel or gun-metal. They are frequently arranged heli cally or diagonally, thus causing a minimum of obstruction to the light rays in any vertical section and affording greater rigidity to the structure. The glazing is usually in. thick plate-glass curved to the radius of the lantern. In situations of great exposure the thickness is increased. Lantern roofs are of sheet steel or copper secured to steel, gunmetal or cast-iron rafter frames. At certain lighthouses it is found necessary to erect a grille or network out side the lantern to prevent the numerous sea birds, attracted by the light, from breaking the glazing by impact. Lanterns vary in diameter from 5 to 16 ft. or more, according to the size of the optical apparatus. For first-order apparatus a diameter of 24 ft.
is usual and 12 ft. for second-order.
The lantern, gallery handrails and principal metallic structures in a lighthouse should be connected to a copper lightning con ductor (q.v.) carried to a point below low water or terminating in an earth plate embedded in wet ground.
Revolving-light apparatus are rotated by clockwork mechanism actuated by weights or spring driven. The clocks are fitted with speed governors and also warning apparatus to indicate when re-winding is required. Where current is available small electric motors are often employed either for automatic re-winding of the clockwork or for driving the rotating mechanism direct. In modern gas-lit apparatus the pressure of the gas is sometimes made use of to revolve the lens table.
At rock and other isolated stations, accommodation for the keepers is usually contained in the towers. In the case of land lighthouses, dwellings are provided in close proximity to the tower. The watch room should be situated immediately under the lantern floor. Oil is generally stored in galvanized steel tanks.