THE LIGHTHOUSE SYSTEM.
Imperfect Illumination of the old Lighthouses—First Improvements—The Argand Lamp and Reflecting Mirrors—Revolving Lights—The Catoptric System—Varieties of Lights—The Dioptric System—Its Details—Introduction of this Method into Great Britain—Comparison of the two Methods—The Drummond and Voltaic Lights—Gurney’s Lamp—Captain Basil Hall’s Experiments—Ventilation of Lighthouses.
Since there is something more or less common in the modes of lighting and in the general economy of all lighthouses, a general view of the subject is likely to prove of more interest than particular details.
In consequence of the rotundity of the earth, the distance at which a beacon light ceases to be visible depends upon its elevation. The height to which a lighthouse may be carried is a simple question of expense. The greater part of the pharos of the Romans were much higher than the most celebrated modern towers. Yet, as it respects optical effect, the feeble rays which were diffused from the wood or coal-fires at their summits, could never have traversed the thick fogs which in all climates occasionally overspread the lower regions of the atmosphere.
Nevertheless, as to the strength of the light, the modern lighthouses were, until lately, little superior to the ancient. At the time of the erection of the Eddystone lighthouse civil engineering was greatly in advance of practical optics. That noble structure was lighted by tallow candles, without reflectors or the aid of any kind of apparatus for concentrating the light. ‘For more than half a century this feeble light was all that directed the mariner in the very high-road of commerce.’ So late as the year 1811 it was lighted with twenty-four wax candles. In 1812 the Lizard lighthouse, certainly one of the most important in the kingdom, was maintained with coal-fires. The Bidstone, a leading light to the port of Liverpool, was furnished with an enormous spout lamp, with a wick twelve inches in width, the smoke from which was so great as to completely darken the upper surface of its reflector. The first important improvement was the introduction of that admirable invention the Argand Lamp, with a double stream of air. Four or five of these lamps would doubtless give as much light as the large fires kept by the Romans; but if those lamps are furnished with reflecting mirrors, the luminous effect is prodigiously increased.
The light of inflamed bodies spreads itself equally in all directions. One portion is absorbed by the ground, another is dissipated in space. The navigator, whose route we are anxious to enlighten, profits only by the rays that proceed in a horizontal direction, or nearly so, from the lamp to the sea. But such of the horizontal rays as are directed towards the land are of course entirely lost to the purposes of the lighthouse. This zone of horizontal rays forms not only a very small portion of the total light, but has also the serious inconvenience of becoming much weaker by divergence, so as to convey to a distance but a very feeble light. To destroy this divergence, and to profit by all the light of the lamp, was the task to be accomplished, before lighthouses could be rendered useful to the full extent.
The application to this purpose of deep metallic mirrors, known under the name of parabolic mirrors, has been found effectual to the purposes required. When a lamp is placed in the focus of such a mirror, all the rays which emanate from it are reflected from the polished surface, and converge in one direction: their original divergence is destroyed, and they form, as they issue from the apparatus, a cylinder of light, parallel with the axis of the mirror. This light would be transmitted with undiminished brilliancy to a great distance, did not the atmosphere absorb a portion of it.
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.
By this arrangement each lens transmits to all the points of the horizon in succession a light equivalent to that of from three to four thousand lamps with double currents, and eight times greater than the light produced by the silver parabolic reflectors; it is, according to Arago, the same amount of light as would be obtained if it were possible to bring together the third of the whole number of gas-lights which illumine the streets, the shops, and the theatres of Paris; and this wonderful result is obtained from a single lamp.
This lamp has four concentric burners, which are defended from the action of the excessive heat produced by their united flames, by means of a superabundant supply of oil, which is thrown up from the cistern below by a clock-work movement, and constantly overflows the wicks. A very tall chimney is necessary in order to supply fresh currents of air to each wick with sufficient rapidity to support the combustion. The carbonization of the wicks is not very rapid; and after they have been burning a long time, the flame is not sensibly diminished, as the great heat evolved from the mass of flame promotes the rising of the oil in the cotton.
In the year 1820, in the course of some investigations connected with the Trigonometrical Survey of Great Britain, and conducted by a deputation of scientific persons from London and Paris, M. Fresnel exhibited from the French side of the channel, by means of his lens and a large lamp, a powerful light which was observed by the English across the channel. The brilliancy of this light so struck Lieut.-Col. Colby, of the Royal Engineers, who was engaged in these observations, that he immediately corresponded with Mr. Stevenson as to its probable use upon the Scottish coast. A considerable time was occupied in inquiry and negotiation, when at length, on the 26th October, 1836, the light at the Isle of May was changed from the catoptric to the dioptric system, and a committee of the Royal Society of Edinburgh met at Dunbar, a distance of thirteen miles from the lighthouse, to make observations on the two lights, which were exhibited in contrast. In their report they conclude:—
‘1. That at a distance of thirteen miles the mean effect of the new light is very much superior to the mean effect of the old light (perhaps in the ratio of two to one). 2. That at all distances the new light has a prodigious superiority to the old, from the equality of its effects in all azimuths. 3. That the new light fulfils rigorously the conditions required for the distribution of light to the greatest advantage. 4. That at distances much exceeding thirteen miles, the new light must still be a very effective one, though to what extent the committee have not observed. The light is understood to be still a good one, when seen from Edinburgh at a distance of about thirty miles.’
On a further comparison of results, it was found that the light of one of the great annular lenses, used in the revolving lights of the first order, was equal to the united effect of about eight of the large reflectors employed in the revolving lights on the Scottish coast. At the Isle of May and Inchkeith the quantity of sperm-oil consumed by the great lamp is equal to that burned by fourteen of the Argand lamps used in the Scotch lights. Hence by dioptric means the consumption of oil necessary for the fourteen reflectors will produce almost as powerful a light as that which would require the oil of twenty-four reflectors in the catoptric system, and consequently there is an excess of oil equal to that consumed by ten reflectors, or four hundred gallons in the year against the Scotch system.
The Dutch were the first to adopt Fresnel’s system. In the year 1834 the Commissioners of Northern Lighthouses sent Mr. Alan Stevenson to Paris to inspect the system, and his report was so favourable, that the reflecting apparatus of the revolving light at Inchkeith was removed, and the dioptric instruments substituted. The new light was exhibited on the evening of the 1st of October, 1835, and so great was the satisfaction afforded, that a similar change was made at the fixed light of the Isle of May. The Trinity-House of London followed next in adopting the improved system, and a revolving dioptric light of the first order was erected at the Star Point in Devonshire.
In the lighthouses of this country sperm-oil is the most usual fuel. In France[6] an oil is burned called Colza oil, expressed from the seeds of a species of wild cabbage. In the lighthouses on the Mediterranean olive-oil is used. In a few lighthouses near large towns coal-gas has been advantageously adopted. Much also has been said in favour of the Drummond and Voltaic lights, which, on account of their prodigious intensity would appear to be most desirable; but the uncertainty which attends their exhibition renders it at present impossible to adopt them: but there is a yet more fatal objection—the smallness of the flame renders them wholly inapplicable to dioptric instruments, which require a great body of flame in order to produce a degree of divergency sufficient to render the duration of the flash in revolving lights long enough to answer the purpose of the mariner.
In the year 1835, Mr. Gurney proposed a lamp of great power in which the flame of oil or wax was sustained by streams of oxygen gas, a method said to be more economical than the combustion of oil in atmospheric air. The Trinity House entertained the proposal, and instituted a number of experiments. In applying this light to reflectors it is intended to use three small flames, each about three-eighths of an inch in diameter, productive, it is said, of an effect equal to that of ten Argand lamps. But for lenses the burner has seventeen films of flame, and is said to possess six times the power of the Fresnel lamp.
In the year 1840, Captain Basil Hall instituted a series of experiments to ascertain whether the well-known superior brilliancy of a revolving light could not be obtained for a fixed or continuous light, that is, for one equally visible in all directions at the same moment. His idea was, that by giving a certain velocity of revolution to a series of lenses round a fixed light, as in Fresnel’s arrangement, a continuity of illuminative power, equal almost in brilliancy to that of a slowly revolving light, might be produced. The apparatus was arranged so as to cause a series of eight lenses one foot in diameter and three feet focal distance to revolve with any velocity up to sixty revolutions per minute round a central lamp. The light from this lamp being concentrated by refraction through the eight lenses into eight pencils, having a divergence of about eight degrees each, illuminated when at rest not quite fifty degrees of the horizon; but when this system of lenses was put into rapid motion, every degree of the three hundred and sixty degrees of the horizon became illuminated, so that to spectators placed all round the horizon the light would appear continuous and equally brilliant in every direction. The only question would be, whether or not this continuous light is essentially less intense than the light seen through the lenses at intervals when in slow motion; and this is a point which further inquiry must decide.
One of the causes which has tended to improve the brilliancy of lighthouses, has produced inconveniences, which long existed without remedy. During the combustion of a pound of oil, the union of its hydrogen with the oxygen of the air produces more than a pound of water in the state of vapour. When a cold wind is blowing upon the lantern of the lighthouse from without, this vapour is condensed into water upon the inner surface of the glass, and in very severe weather forms a crust of ice, in some cases, as much as four inches thick in the course of one night. This not only very much dims the brilliancy of the light to the sailor, but also entails a great amount of labour on the light-keepers, and injury to the lantern. The combustion of the oil also produces a large quantity of carbonic acid gas, which is of a very deleterious nature, and in many cases rendered the light-keepers’ rooms almost uninhabitable. Under these circumstances, the Trinity House made application to Dr. Faraday to investigate the subject, with a view to the discovery of some remedy. With his usual skill and sagacity, Dr. Faraday instituted a number of inquiries and experiments, and visited some of the principal lighthouses. The result was the contrivance of a complete method of ventilating lighthouses. On the dioptric system, the remedy was simple: it was merely to erect a tall chimney over the central lamp, and lead it out at the roof; by which means, the draught of the lamp was improved, and all the products of combustion carried off. On the catoptric system, with revolving lights, each lamp was furnished with a chimney, which passed out at its upper extremity, through a small hole in the reflector into a fixed central hollow shaft, which served the purpose of a ventilating chimney to all the lamps. These plans are said to have been eminently successful in removing the inconveniences, which rendered the light less efficient, and the lighthouse an unwholesome and even dangerouse place of abode.
[6] In the year 1836 the coast of France were provided with no less than ninety-six lighthouses.
THE END.
Parker’s
Collections in Popular
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Collections in Popular Literature,
publishing by
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It has frequently been suggested to the Publisher, that he might render an acceptable service to the friends of Education, and greatly assist those who desire to promote the intellectual amusement of the people, by producing a series of Popular Books, at low prices, calculated, by their unexceptionable tendency, for general use in families; from which School Libraries might be formed, Reward Books selected, and Lending Libraries supplied; which, on account of their convenient form and size, would be welcome as Fireside and Travelling Companions; books, in short, which might be found instructive and entertaining wherever introduced.
These suggestions he is now carrying out, in compliance with certain conditions, namely, that the works produced shall be unexceptionable in subject and in treatment; that the series be sufficiently varied to meet the requirements of all classes of readers; and that each book shall be complete in itself, and procurable for a very small sum.
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The Plan will embrace new and improved Editions of certain Standard English books, but the majority of the works will be newly written, translated, compiled, or abridged, for the present purpose; and the volumes will appear from time to time in sufficient variety to extend simultaneously, and in due proportion, the various branches of Popular Literature. The whole will be prepared with an especial view to the diffusion of sound opinions—to the promulgation of valuable facts and correct principles—and to the due indulgence of general literary taste.
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