Acetylene in the dissolved form is at present the best and most economical illuminant for entirely unwatched lights when electric current is not available. Its use has enabled many un attended lights to be established and maintained at a compara tively small cost in positions where a light attended by keepers would be impracticable as, for example, on some parts of the Australian coast.
The earliest light-vessel established in English waters was that placed at the Nore in 1732. The early lightships were of small size and carried lanterns of primitive construction sus pended from the yard-arms. Modern light-vessels are usually of steel construction and are of various dimensions. The following may be taken as the limits in the Trinity House service : Length ...... . . . 80 to 114 ft.
Until about 1895 the illuminating apparatus used in light vessels was almost exclusively of catoptric form, consisting for the most part of 21 in. silvered paraboloidal reflectors, having mineral-oil burners in focus, hung in gimbals to preserve the horizontal direction of the beam. In a few cases incandescent mantle burners or electric-filament lamps have been substituted for the old wick-burners in catoptric apparatus. Dioptric apparatus is now usually provided in new lightships, not only in Britain but in other countries also. The French lighthouse service in 1896 devised the first dioptric revolving light for a light-vessel. This ship, the was lit by an incandescent oil-gas burner. A much larger vessel, the Sandettie, of 342 tons displacement, was completed in 1902. The new type of floating light was afterwards adopted by other lighthouse authorities, and many vessels constructed on the lines of the Sandettie were built during the first decade of the loth century. In England the Talais and another similar vessel constructed in 1899 were later converted into unattended light-vessels.
first of the class was stationed at the Swin Middle in 1905 (now—I929—at the Mid-Barrow Station). Most of these ves sels were fitted with revolving dioptric lenses suspended in gim bals below the lens table and counterbalanced by a heavy pen dulum weight. The apparatus was mounted on ball bearings in some cases and on a mercury float in others, the lenses being revolved by clockwork or gas. Another method of suspending the
dioptric apparatus has recently been developed, the design being a Swedish invention. This device, known as the constant-level table, has been fitted in several vessels, including that at the Barrow Deep in the approaches to the Thames. In this vessel the illuminant is acetylene employed with a mantle. The lens is mounted on a table made to revolve on ball bearings by the gas on its way to the burner. The lens table is balanced, nears the centre of gravity, on a pivot in the lantern, and connected by three vertical pull-wires to a pivoted counter-balance weight, placed in the hull of the ship at the rolling centre of the vessel, which controls the movement of the upper table (Plate II., fig. 5). As the motion imparted to the lower balance weight is small, the swinging of the lens table is less than with the former arrange ment of pendulum and gimbal suspension. In the older vessels oil-gas illumination was employed but this has been replaced in the later ships by incandescent-acetylene or incandescent-oil burners, and high-powered gas-filled electric lamps. Four light vessels fitted with such electric lights and dioptric apparatus were in service on English stations in 1928.
An experimental electric-light installation on board a Mersey light-vessel in 1886 proved unsuccessful.
Fog signals, where provided on modern light-vessels, are gen erally in the form of sirens or diaphones worked by compressed air. The compressors are driven by steam or oil engines in the older installations and by semi-diesel engines in the more modern vessels.
The improvements made in recent years in the design and construction of unwatched lights and their proved reliability have made it possible for many attended light-ships in positions of secondary importance to be replaced by unwatched vessels. Large economies, both in the cost of construction and of maintenance, have thereby been effected. These unattended vessels range in size from small boats to large vessels, and are fitted with dioptric apparatus and acetylene or Blau-gas lighting.
Communication Between Light-vessels and the Shore.— As far back as 1886 experiments were instituted at the Sunk light-vessel, off the coast of Essex, with the object of providing telephonic communication with the shore by means of a sub marine cable. In spite of great difficulties experienced in main taining the cables several light-vessels were ultimately equipped with this means of communication, and cables were also laid to many pile lighthouses and isolated rock and island stations. Wire less telephone installations have now (1929) superseded all the cable communications with light-vessels.