AZIMUTHAL CONDENSING LIGHT. The above is description of the general principles on which lighthouses are illuminated. In placing a light in some situation, regard, however, must he had to the physical peculiarities of the localities. The following plans of Thomas Stevenson may he cited as examples. Where it is only necessary to illuminate a narrow sound, it is obvious that the requirements are very different than where the light is distributed equally all around the hori zon. On the side next the shore, no light is re quired at all; across the sound, a feeble light is all that is necessary, because the distance at which it has to be seen is small, owing to the narrowness of the channel; while up the sound and down the sound the sea to be illuminated is to he of greater or lesser extent, and requires a corresponding intensity. If the light were made sufficiently powerful to answer for the greater distance, it would he much too powerful for the shorter distance across the sound. Such an ar rangement would occasion an unnecessary waste of oil, while the light that was cast on the land ward side would be altogether useless. Fig. 5 represents (in plan) the condensing light, by Adel' the light proceeding froth the flame is allocated in the different azimuths in proportion to the distances at which the light requires to be seen by the mariner in those azimuths. Let us suppose that the rays marked a require to be seen at the greatest distance down the kound, and those marked b to a somewhat smaller distance up the sound. In order to strengthen those arcs, the spare light proceeding landward, which would otherwise be lost, is intercepted by por tions of holophotes, B and C, subtending spheri cal angles proportioned to the relative ranges and angular spaces of the arcs a and b. The p.rtions of light thus intercepted are made parallel by the holophotes, and fall upon straight prisms as and bb respectively, which again re fract them in the horizontal plane only: and, after passing through focal points (independent for each prism), they emerge in separate equal beanie. and diverge through the same angles as
a b respectively. In this way. the light pro ceeding up and down the sound is strengthened in the required ratio by utilizing, in the manner we have described, the light which would other wise have been lost on the land, and a consider able saving is effected.
are vessels fitted out with lights moored at sea in the vicinity of reefs. Prior to 1807 the lantern was hung at the yard-aria. Robert Stevenson then introduced a system of lanterns, grouped around a copper tube capable of receiving the vessel's mast. which passed through the tube. in this way proper optical appliances could lie employed. and the lantern lowered on the mast so as to pass through the roof of a house on the deck. where the lamps are filled or trimmed. in 1S64 six floatina lights were constructed for the Huedi under the direc tion: of the Messrs. Stevenson, in which the (Hop true principle was applied. Eight half-fixed light apparatus of glass with spherical mirrors be hind were placed in the lantern round the mast so as to show in every azimuth rays from three of them at once. In 1000 the United States had 44 lightships in position, each ship showing either one or two lights. Each light is corn posed of eight reflectors 12 inches in diameter set upon a ring which encircles the mast and can be lighted and hoisted to the masthead at night and and housed by day. Certain of the British lightships show revolving light-, some of which are red. Lightships are employed in those positions where lighthouses are necessary but where they have not been becau-e of the great difficulty of erecting them.