a shipwreck. The difficulty of landing was great, owing to the immense force (three tons to the superficial foot) with which the Atlantic broke upon it, and caused the delay of the scheme till 1838, when it was undertaken by Mr. Alan Stevenson, who followed generally the plan of the Bell Rock, and, in spite of disasters and tempests, completed his work in 1844. Its cost was nearly £87,000; its diameter at the base, forty-two feet, and at the top, sixteen feet; and its light is a guide to mariners for eighteen miles.
The total number of lights in Great Britain in January, 1871, afloat and ashore, was five hundred and fourteen. Lighthouses in England are usually painted white or red, while those in Scotland are left their natural color. The number of lights of all classes on the English coast averages one to nearly every eleven miles; or, if the floating lights are excluded, one to every fourteen miles; as a general rule, the horizon to be lighted is limited to fifteen or twenty miles, and the height above the sea level varies from ninety to five hundred and forty feet; depending, of course, on the situation of the foundation.
The French lighthouse system is very perfect and comprehensive; the authorities class their lights in four divisions, according to their power and range of visibility. The phares of the first class are visible for thirty, those of the second for twenty-five, those of the third for fifteen miles; while the fourth class, or harbor lights, are seen only for six miles. Of thirteen of the principal French lighthouses, the height varies from one hundred and fifty-seven to three hundred and ninety-seven feet, and the cost from £4,000 to £38,000; while the range of visibility is from eighteen to twenty-seven miles. The greatest recorded distance at which an oil lamp has been seen is that of the holophotal light of Allepey, in Travancore, which has been visible from an elevated position forty-five miles away. This seems very wonderful, but is almost equalled by the revolving light of Buccalieu, in Newfoundland, which throws its beneficent beams for forty nautical miles.
Something must now be said on the subject of the lights themselves, which, beginning with the old-fashioned beacon fires, have not yet reached perfection. The earliest system was merely that of blazing fires on the open ground; then a candle was tried placed in a lantern, and this was adopted at Eddystone, which was first lighted by twenty-four candles in a sort of chandelier. The use of oil lamps instead of candles is said to have been introduced by the celebrated engineer Borda, about 1780 or 1790. Various kinds of oil have been tried; for fifty years spermaceti was used, but the preference is now given to rape-seed, or colza, as the most economical and reliable.
The next step in advance was the adoption of reflectors, which gave rise to the catoptric system, which was universal for half a century. The light thrown from a parabolic reflector is sent out in parallel rays, and can be seen for a great distance. Then came the dioptric system, which consisted in magnifying one large flame through a lens. It had been suggested to Smeaton, 1759, and had been actually employed in the Portland lighthouse in 1789, but, through some mismanagement, it had fallen into disfavor for many years till revived by Augustin Fresnel in a paper read before the French Academy of Sciences, in 1822. The French government at once adopted the dioptric system, which has been improved and perfected by Leopold Fresnel, Alan Stevenson, Arago, and Faraday; it is to this last-named distinguished philosopher that we are indebted for the present efficient mode of ventilating lighthouse lamps.
The light now generally adopted in British lighthouses is a mixture of the two systems; inside the great glass lantern, usually about twelve feet high, is placed another framework of glass, corresponding to the shape of the lantern, and which, enclosing the lamp, is composed of, firstly, a band of glass round the middle, called the lenticular belt, which considerably magnifies the flame; the top consists of a number of prisms, which intercept the light which would otherwise be lost on the roof; and in the lower part of the apparatus is another set of prisms, which, in a similar manner, prevent the light being wasted below.
A curious light is one shown at Stornoway Bay, where the position of a rock is indicated by means of a beam of parallel rays thrown from the shore upon an apparatus fixed in the top of a beacon erected upon the rock itself; this is called an apparent light, from its appearing to rise from a flame on the rock, while in reality it proceeds from the shore six hundred and fifty feet distant, and is refracted by glass prisms placed on the beacon.
Many other means of lighting, besides that of oil lamps, have been tried, though none of them are yet extensively adopted;
