LIGHTHOUSES. These important build ings are erected along the sea-shore, or upon rocks, from which lights are exhibited at night for the direction of mariners. Floating lights perform a similar office, being shown from the masts of vessels moored in certain positions, generally as beacons to enable ships to avoid shoals or sunken rocks in the testtuaries of great rivers.
The erection of lighthouses in this country has not proceeded upon any systematic plan, but in every instance they have been con structed simply because of the disastrous losses that had occurred for want of them. From this cause it arises that our lighthouse esta blishments in the several parts of the United Kingdom have until recently been conducted under entirely different systems, different as regards the constitution of the management, the rates or amount of the light-dues, and the principle on which they are levied. Under the operation of recent acts of parliament, all the public or general lighthouses around the coast of England are under the management of the Trinity House ; those around Scotland under the Commissioners of Northern Lights; and those around Ireland under the Ballast Board of Dublin. There is a second class of lighthouses, consisting of local or harbour lights, which are managed by corporations and local trustees, under powers given for that purpose; but even these local lights are being brought more and more under general central authority. There are at the present time about 320 lighthouses round the several coasts of the United Kingdom. These lighthouses are maintained by dues levied on all vessels which leave the harbours of the United Kingdom, at so much per ton per vessel, according to the number of lighthouses which the vessels pass. These dues, which have amounted in some years to nearly 400,0001., are in some instances very oppressive ; but gradual improvements are being introduced in the whole system.
A principal object in the establishment of these buildings is to give intimation to vessels approaching the coast during the night as to the place in which they are. It is therefore of importance that the lights exhibited on the same line of coast should have some differences, so as to be readily distinguished by mariners. The different appearances thus
required are given by having two lights placed either vertically or horizontally with respect to each other, or three lights, as at the Casket rocks, or by causing the lights to revolve or to appear only at certain intervals, and to remain in sight only for a given number of seconds at each appearance ; or by the employment of lamps of different colours, as in some of the harbour-lights, which do not require to be seen at a great distance.
The mode of lighting now generally used in this country is that of placing an argand burner in the focus of a parabolic reflector.' This instrument is made of silver strengthened with copper, and is about 3 or 4 inches infooal length, and 21 inches in diameter. The num ber and the arrangement of reflectors in each lighthouse depend upon the light being fixed or revolving, and upon other circumstances connected with the situation and importance of the lighthouse. The mode in use in the lighthouses of France consists in placing a large argand lamp, having four concentric wicks, and giving a very powerful light, in the centre of the upper part of the building, and placing around the lamp a series of glass lenses of a peculiar construction; thus using a refracting instead of a reflecting instrument to collect the light, and only one lamp instead of a greater number. The lens employed is about 30 inches square, plano-convex, and formed of separate rings or zones,whose com mon surfaces preserve nearly the came curva ture as if they constituted portions of one complete lens, the interior and useless part of the glass being removed.
Of the fixed lights of England and Scotland, in 1814,76 were catoptric, or reflecting lights ; while 18 were dioptric, or lens lights. The re flecting lights had, on an average, about 15 burners each; and the one burner of each lens light was about equal in efficiency to 14 burners on the reflecting principle. The Eng lish floating lights had an average power of about 12 burners each. The Irish lights are powerful ; they presented an average power of 21 burners to each fixed light. The average annual cost for keeping up the fixed public lights is about 450/. per lighthouse.