LIGHTHOUSE

An establishment for the exhibition of a light or landmark to direct the mariner. The use of lights for such a purpose is of very high antiquity; but their early history is involved in much obscurity. In the ancient world there were lighthouses at Ostea, Kavenna, Puteoli, Caprea, Rhodes, on the Thracian Bosphorus, . (Suetonii Opera, tom. i. p. 755.); but by far the most celebrated lighthouse in antiquity was "that erected by Ptolemy Soter on the small island of Pharos, opposite to Alexandria, — " nocturn is ignibus cursum navium regens." (Pliny, lib. v. c. 31.) Its extraordinary height, which some authors have estimated at 500 feet and upwards, procured for it a place among the wonders of the world; and, according to Joseph us, its "beaming summit " could be seen at a distance of 300 stadia, — about 42 British miles. It is said to have cost 800 talents; and its celebrity was such that Pharos rapidly became, and still continues to be in many countries, a generic term equivalent to lighthouse. The most celebrated lighthouses of modern times are, the Tour de Carduan, at the entrance of the Gironde, in France; the Eddy stone lighthouse, opposite to Plymouth Sound; and that more recently constructed on the Bell Rock, opposite to the Frith of Tay. The first of these was begun in 1584, and finished in 1611. It is 18GJ feet (English) in height; and besides being of the highest importance to the sailor on so dangerous and frequented a coast, it forms at the same time a splendid specimen of architectural beauty^ The Eddy stone lighthouse, constructed by the celebrated engineer Smeaton, was completed in 1759;. is regarded as a masterpiece of its kind; and, as has well been observed, bids fair to be little le lasting than the rocks upon which it stands. The Bell Rock lighthouse was built by Mr. Stevens on on the model of the Eddy stone. Numerous lighthouses marking the most dangerous points, and the entrance to the principal harbours, are now erected in most civilized maritime countries. In the Baltic and the Sound they are particularly abundant; the Dutch have 20 lights on their coast, and in the Zuyder Zee; and on the northern and western coasts of France there are no fewer than 89 excellent lights. (The Admiralty has lately published lists of all these lights.) But the coasts of no country are so well provided with lighthouses as those of the United Kingdom. For England they are under the management of the Brethren of the Trinity House; for Ireland, under that of the Board for the Improvement of the Port of Dublin; and for Scotland, under that of the Commi ioners of Northern Lighthouses meeting at Edinburgh. It would swell our pages to too great an extent were we to attempt to give a complete list of the various lighthouses of this country; we must therefore beg to refer the reader to the Coimnercial Dictionary for full particulars upon this subject, and shall devote what remains of our limits to a view of the different principles adopted in the construction of their important machinery. 1 he ancient mode of exhibiting lights as beacons to the mariner consistcnl in burning wood or coal in a chauffer on the top of a tower; and till the year 1807, the Eddy stone light was nothing better than the feeble blaze of a few tallow candles, without any apparatus for concentrating the light or giving it any particular direction. But as rays of light proceeding from a luminous focus are equally dispersed over the surface of the sphere which has the focus for its centre, it is evident that with LIGHTHOUSE. out some means of giving the light a horizontal direction, the greater part of it must be wholly lost; for only those rays which are directed in the plane of the horizon, or at least which are depre ed only a few degrees below it, can be seen by a ship at a distance. Hence the first object to be attained is to prevent the lo of light by throwing the whole of it forward in the plane of the horizon, in order that its intensity may be increased in the greatest po ible degree. Now there are two principles on which this may be accomplished, — reflexion and refraction. The object is accordingly carried into effect by a catoptric or dioptric apparatus. Sometimes both principles are combined in the same apparatus. Catoptric System — The usual mode of applying the catoptric principle is by burning an Argand lamp in the focus of a parabolic mirror. This mode of illumination appears to have been first carried into effect at the Carduan lighthouse above mentioned, under the direction of Borda, about the year 1780. A few years later reflecting mirrors were placed in some of the English lighthouses, under the direction of the Trinity House; and in 1786 the principle was adopted in the only two beacons then existing on the coast of Scotland,— viz. the Isle of May in the Firth of Forth, and the Cumbrae Isle in the Clyde. Soon afterwards it was adopted generally in this country. Borda's reflector was formed of a sheet of copper plated with silver; those applied in Scotland were formed of small facets of mirror gla , placed in hollow parabolic moulds of plaster. The mirrors in general use in the British lighthouses at the present time are of copper lined with silver; the focal length is about 3 or 4 inches, and the diameter at the outer edge about 21 inches. If the curvature of a reflector could be m.ade truly parabolic, and if the light i ued from a mathematical pomt, the rays would be reflected from the mirror exactly parallel to the axis of the generating curve, and the beam of projected light would be a cylinder having a diameter equal to that of the mirror. Such a form of the beam would render the light nearly usele , on account of the small portion of the horizon which would be illuminated; and it is accordingly nece ary to give the rays a certain degree of divergence: this is practically effected by the size of the flame. The burner of the lamp is usually an inch in diameter; and the origin of the luminous rays being thus at a small distance from the focus, instead of being reflected parallel they are projected in a cone having a divergence of about 14°. The curvature of the, also vergence. mirror may also be so adjusted as to increase the diIn order to produce a light of sufficient intensity, several parabolic mirrors, sometimes as many as eight, are placed on a frame, with their axes all parallel to each other, so that the light reflected by all of them is blended together in the same beam. To form a revolving light, the frame is attached to a horizontal axis, which is turned by means of clock machinery; and in this manner the different quarters of the horizon are succe ively illuminated. But as a rapid motion would be inconvenient, the frame has usually three or four sides, on each of which the same number of mirrors and lights is placed; so that the illumination is repeated three or four times in one revolution. To form a stationary light, a number of reflectors are placed round a circular frame, having their axes on the radii of the circle. This arrangement has one obvious defect; namely, that the illumination will not be equally intense at all azimuths, but strongest in the direction of the several axes, and feeblest in the direction of lines bisecting the several angles formed by each pair of contiguous axes. The defect is one which cannot be entirely remedied in a stationary light on the catoptric principle. Dioptric System. — The introduction of lenses for the purpose of giving the light a horizontal direction is of recent date. A project for this purpose is indeed mentioned by Smeaton in his account of the Eddy stone lighthouse, and about tlie end of the last century the method was tried at some lighthouses in the south of England; but from the imperfect figure of the lenses, and the absorption of the light caused by the great thickne.-a of the gla , it did not succeed. But the art of grinding spherical lenses having been since brought to greater perfection, and a means of greatly diminishing the absorption, and also of constructing lenses of a much greater size, having been found in the use of polyzonal lenses (that is to say lenses formed of several pieces separately prepared and afterwards united, — S6'e Polyzonal Lens), the system has been revived of late years, and in many instances carried succe fully into execution. The merit of first applving such lenses to lighthouses belongs to the late Auguste'Fresnel, of the Academy of Sciences of Paris. The annular lenses, constructed under the direction of Fresnel for the principal lighthouses in France, are plano-convex, havmg a focal distance of about 3 feet; and they are formed of crown gla , as being le liable to striee than flint gla . The effective divergence of the cone of light projected by a lens is generally le than in [s. 677]