Smeaton and Lighthouses / A Popular Biography, with an Historical Introduction and Sequel

by Anonymous

It must, however, be admitted, that this method is not free from defect. It is true, we direct towards the horizon of the sea, a vast number of rays, which would have been lost upon the ground, in space, or landward; we also destroy the primitive divergence of those rays which fall within the range of the seaman; but the cylinder of reflected light is of no greater size than that of the mirror; the zone which it illuminates has precisely the same dimensions, at whatever distance, and, unless we employ a number of similar reflectors differently inclined, there will be a number of large spaces in the horizon completely obscure, from which the pilot will never see any signal whatever. This serious objection has been removed by imparting, by means of clock-work, a uniform rotatory motion to the reflector. The collection of rays proceeding from the mirror is thus directed to all the points of the horizon in succession. Every vessel perceives the signal-light during one instant, and immediately after it is seen to disappear; and if, in a great extent of coast, the different lights revolve in different times, the various signals become thus individualized. According to the interval of time, which elapses between two successive appearances or eclipses of the light, does the sailor recognize the part of the coast which is in view: he is thus no longer liable to mistake a planet, or a star of the first magnitude, at its rising or setting, or a fire lighted on the coast by fishermen, charcoal-burners, &c. for the light of the lighthouse; mistakes, which have often led to the most deplorable wrecks.

The reflectors originally employed were casts in plaster of Paris, from a mould formed to the parabolic curve, and lined with facets of mirror-glass. The power of these reflectors, however, was comparatively small, from the reflecting surface being composed of numerous pieces, in each of which only one point coincided with the curve of the parabola.

The Trinity House having been at great pains to improve the reflecting apparatus on the coast of England, with the advice and assistance of eminent scientific men, adopted parabolic reflectors made of silvered copper; and these, from their superior effects, have ultimately been introduced into all the lighthouses of the united kingdom. In the northern lighthouses, the reflectors consist of copper coated with silver, in the proportion of six ounces of silver to one pound avoirdupois of copper, which are rolled together, and then, with much labour and great nicety, by a process of hammering and polishing, formed to the parabolic curve of a mould made with mathematical precision. The focal distance of the curve is four inches. The diagram for the Bell-Rock reflectors was drawn by Professor Leslie, and the mould was made by Mr. Adie the optician. The powers of this elegant production of the mechanical art are said to be quite astonishing; and by comparing its highly-polished and regularly-curved surface with the previous glass reflector, the superiority of the former seems to be immense: indeed, its influence extends to the horizon formed by the height of the lighthouse-tower and the earth’s curvature. The reflectors in general use measure over the tips twenty-one inches as applicable to stationary, and twenty-live inches for revolving lights.

The Catoptric or reflecting system was first adopted under the direction of Borda, at the Corduan Lighthouse, probably about the year 1780. The system was soon introduced into England; and one of the first acts of the Northern Lights’ Board, so early as 1786, was to substitute reflectors in place of coast-lights, which till then had been the only beacons on the Scotch coast.

In the improved lights the best spermaceti oil and the Argand lamp have been introduced. The keepers are professionally adepts in the management of lamps; and should a drop of oil be spilt, the floor is covered with painted floorcloth to receive it. The Argand lamp-burners are tipped with silver, to prevent the waste and imperfection to which copper is subject, from the excessive heat of the burner.

In appearance the lights may be classed as stationary, revolving, flashing, and intermittent. In the first, as its name implies, the light has a steady and uniform appearance, and the reflectors, which are smaller than those used for revolving lights, are ranged in circular zones upon a chandelier or piece of iron frame-work, with their axes inclined at such an angle as shall enable them to illuminate every part of the horizon. The revolving light consists of a frame built upon a perpendicular shaft, and the reflectors, which are of large size, are ranged on perpendicular planes or faces, which are made to revolve in periodic times, by means of a train of machinery kept in motion by a weight. When one of those illuminated planes or faces is brought towards the eye of the observer, the light gradually increases to full strength: when, on the contrary, the angle between two of these faces comes round, the observer is in darkness. By these alternate changes, the characteristic of the lighthouse is as distinctly marked to the eye of the mariner as the opposite extremes of light and darkness can make it. The flashing light is a modification of the revolving light, and is practically a beautiful example of the infinite celerity of the passage of light. The reflectors are here also ranged upon a frame, with faces which are made to revolve with considerable rapidity; and the light thus emerging from a partial state of darkness exhibits a momentary flash, resembling a star of the first magnitude, and thereby produces a very striking effect. The intermittent light bursts suddenly into view, like a star of the first magnitude, and continues a stationary light a minute and a half, when it is as suddenly eclipsed for half a minute; and by this simple arrangement a strongly marked distinction in the lights of the coast is introduced. This is accomplished by the perpendicular motion of shades before the lights. A variety of all these lights is introduced by interposing before the reflectors plates of red glass, which produce the beautiful red light alluded to in the lines of Sir Walter Scott, when he notices the ‘ruddy gem of changeful light.’ The red and white light is caused by the revolution of a frame on the sides of which the lights are placed alternately, with and without coloured media. There are varieties in this kind of light, some being so arranged that two white lights should be seen in succession, and then one red; and others, that two red should be seen, and then one white. When there is a necessity for what is called a leading-line, as a guide for taking some channel, or avoiding some danger, double lights are exhibited from two towers, one of which is higher than the other; and when seen in one line, these form a direction for the course of the shipping.

When the French were recovering from the long night of terror, during which their commerce had been ruined and their ships disabled, they directed attention to lighthouses, and resolved to discard the very imperfect and insignificant reflectors then in use. They investigated the subject with their usual scientific skill, and the result was the invention and adoption of the system of lenses instead of reflectors, known as the Dioptric system.

A transparent lens reduces to parallelism all the luminous rays which traverse it, whatever be their original amount of divergence, provided these rays proceed from a point or focus suitably situated. The substitution of glass lenses for reflectors is not a new idea, since we find that a proposal to that effect was made by a London optician to Mr. Smeaton, in 1759, for illuminating the Eddystone lighthouse, but was not adopted by him. M. Fresnel mentions that lenses had been used in England so far back as 1789, in the tower light-room at Portland Island, but from some cause or other were discontinued.

On account of the great loss of light by reflexion at the surface of mirrors, the French adopted the lenses, and they soon discovered the source of failure in our use of them; they saw that, in order to render lenses superior to reflectors, the intensity of the illuminating flame must be considerably increased, as well as the size of the lenses; also, that these lenses must have a very short focus; and that, if constructed by the ordinary rules, their thickness would be great, their transparency diminished, and their weight far too great for the safety of the machinery whereby the lights were revolved. Fresnel therefore adopted the ingenious device proposed by Condorcet, that of constructing a lens of a number of distinct pieces. This method was also proposed by Dr. Brewster, in 1811. Fresnel also invented a lamp, with a number of concentric wicks, the lustre of which was twenty-five times greater than the best lamps then existing.

In a lighthouse on the dioptric system, the lantern is constructed with eight sides, which form an octagonal prism around the lamp in the centre. The centre of each side is occupied by a plano-convex lens, something similar to a burning-glass, having a diameter of about fifteen inches. This central lens is not sufficient to cover the entire side. Indeed, a lens of sufficient size for the purpose would be very costly and bulky, even supposing it could be manufactured. To remedy this defect, the central lens is surrounded by a series of glass rings, the external surface of which is so formed as to have precisely the same optical effect as the great central lens. A transverse section of one of these zones or rings presents the form of a wedge, one side of which is slightly curved.