Smeaton and Lighthouses Part 6

The framing upon which the house stands is firmly secured round the centre column and to the heads of the outer columns by means of hollow cast-iron capitals let down over the heads of the columns and secured with screw-bolts. To give lateral strength to the building, round iron angle-braces were applied, by which means a resisting power equal to at least three hundred and fifty tons is presented in every direction.

The platform upon which the house stands is twenty-seven feet in diameter. The dwelling-house is twenty feet in diameter, and nine feet high: it has an outside door, and three windows, and is divided into two apartments, one having a fire-place; the floor is tiled, and the walls and ceiling lathed and stuccoed. Access to the platform is secured by means of a Jacob's ladder of wrought iron secured to one of the columns: access to the lantern is by a winding stair within the house.

From the summit of the house rises the lantern; it is twelve-sided, ten feet in diameter, and eight feet high. The light is thus elevated about forty-six feet above low-water level. It is of the dioptric kind, and is bright, steady, and uniform, ranging over an horizon of eight miles, and visible at the distance of ten miles from a coaster's deck. During foggy weather a bell is tolled by machinery. Tide-time for vessels of twelve feet draft is also denoted by signals. Signals put out by vessels requiring a Wyre pilot will also be understood at this lighthouse, where corresponding signals are hoisted until the pilot is provided.

This admirable and useful structure was erected in two of the shortest day months of the year, in which time day-light did not occur at any low-water period; the workmen therefore had to depend on torches and moonlight. Nor is the portability of this form of building its least advantage: should there occur any local changes which might threaten the safety of the house, it can be taken down, and erected in another site within a month.

Perhaps one of the boldest schemes ever devised for lighthouses was the structure proposed to be erected by Mr. Bush, on a plan patented by him, on the Goodwin Sands, or on the Varne in the channel between Folkestone and Cape Grisnez, in four fathoms water. This plan, was recommended to the consideration of parliament by several merchants, s.h.i.+p-owners, and other influential persons. The building proposed to be called 'The Light of all Nations,' was to consist of a Doric column one hundred and twenty-five feet high supporting a lantern twelve feet in diameter, surmounted by a colossal statue of the Queen, her sceptre being the point of a lightning-conductor. This column was to rise from a base one hundred feet in height, and fifty in diameter, to be formed by a caisson composed of cast-iron plates bolted together: the part under water was to be divided into four pyramidal chambers, opening into and supporting one another; the lower one resting on the rock beneath the sands, and the whole forming a conical core to the cylindrical base. The only part of this plan that was executed was the cast-iron caisson, which was deposited in its place among the sands.

In this situation, during one dark and stormy night, it was struck by a s.h.i.+p and s.h.i.+vered to a thousand fragments. This untoward accident has led to the abandonment of the design.

One of the characteristics of this country is the mode in which we lay out the mineral wealth which nature has bestowed upon us so liberally in the shape of coal and iron. With the a.s.sistance of the former we mould the latter into a thousand shapes of usefulness, neatness, and durability, and so much attached are we to this material, that it is daily superseding the use of the more c.u.mbrous wood and stone, and other substances which were once in great demand. Iron furnishes most of the multifarious instruments required in the mechanical and agricultural arts--it ministers alike to war and to peace, by furnis.h.i.+ng the sword and the ploughshare. It supplies some of the most useful domestic apparatus for the kitchen, the parlour, and the bed-room, and now even the bedstead itself may be formed of iron. It has been long used in some of our great public works: we have iron roads--iron bridges--iron statues--steam-boats of iron--houses of iron, and lastly, iron lighthouses.

The suggestion of metallic lighthouses originated a few years ago with Captain Sir Samuel Brown, when it was proposed to place a lighthouse on the Wolf Rock near Land's End, a position where it would be exposed to the most violent storms of the Atlantic. A plan for the erection of a stone lighthouse on this point had already been drawn up by Mr.

Stevenson, which plan, Captain Brown thinks would require fifteen years for its execution, and cost one hundred and fifty thousand pounds. Captain Brown undertook to erect one of bronze ninety feet high for fifteen thousand pounds, and to complete it in four months.

This plan, from whatever cause, was not entertained, and with the exception of a small lighthouse erected on the Gravesend pier, metallic lighthouses excited no attention until the year 1840, when application was made to Mr. Alexander Gordon, the eminent engineer, by the commissioners appointed by the House of a.s.sembly, in the island of Jamaica, to light a dangerous point in that island, called Morant Point, for the erection of a suitable lighthouse at the smallest possible cost. On this occasion Mr. Gordon proposed the erection of a cast-iron structure, resembling in outline that of the Celtic towers of Ireland. His plans and estimates having been accepted, they were executed with remarkable celerity; and from an account furnished by Mr. A. R. Renton, (the manager of the factory at which the work for the lighthouse was done,) we derive most of the following particulars.

The advantage which iron, when not in contact with sea-water, possesses over stone or other materials, is that upon a given base a much larger internal capacity for dwellings and stories can be obtained, with equal stability. With this material plates can be cast in large surfaces, and with but few joints. A system of bonding the plates may also be adopted, which will ensure the perfect combination of every part, so as to form an entire ma.s.s, and thus the best form for strength and stability can easily be obtained. From the comparatively small bulk and weight of the component parts of the structure great facilities are afforded for transporting and erecting it. Thus, in less than three months from the date of the contract, the lighthouse about to be described was cast and erected on the contractor's premises, and it was expected to have the light exhibited in Jamaica in three months more. The whole expense was said not to exceed one-third the cost of a similar building in stone.

The structure was to be founded on a coral rock a little above the level of the sea; the face of the rock is about ten feet below the surface of the sand, and was to be excavated to receive the base of the tower, resting on and cased with granite, to prevent the natural filtration of the sea-water from acting upon the iron. This course of granite is grooved to receive the f.l.a.n.g.e of the lower plates of the tower, from which lightning-conductors are to be continued to the sea.

The tower is of course itself a lightning-conductor of the best kind.

The diameter of the tower-shaft is eighteen feet six inches at its base, diminis.h.i.+ng to eleven feet under the cap; it is formed of nine tiers of plates each ten feet in height, varying from one to three quarters of an inch thick. The circ.u.mference is formed of eleven plates at the base, and nine at the top: they are cast with a f.l.a.n.g.e all round the inner edges; and when put together these f.l.a.n.g.es form the joints, which are fastened together with nut and screw-bolts, and caulked with iron cement. The cap consists of ten radiating plates, which form the floor of the light-room; they are screwed to the tower upon twenty pierced brackets, and are finished by an iron railing.

The lower portion, namely, twenty-seven feet, is filled up with masonry and concrete, weighing about three hundred tons, and so connected with the rock itself as to form a solid core of resistance.

The remaining portion of the building is divided into store-rooms and berths for the attendants in the lighthouse.

The light-room consists of cast-iron plates five feet high, on which are fixed metal sash-bars filled with plate-gla.s.s; these, terminating with a point, are covered with a copper roof, whence rises a short lightning-rod, trebly gilt at the point. The light is of the revolving kind, consisting of fifteen Argand lamps and reflectors, five in each side of an equilateral triangle, and so placed as to const.i.tute a continuous light, but with periodical flashes. The Admiralty notice which announced the light for exhibition on the 1st November, 1842, states that the centre of the light is ninety-six feet above the level of the sea, and in clear weather the light can be seen from a distance of twenty-one miles.

To preserve as low a temperature as the circ.u.mstances and climate will permit, the iron sh.e.l.l was lined with a non-conducting material, as slate or wood, leaving an annular interstice, through which a constant ventilation is effected, so as to carry off the excessive heat.

To preserve the two lower tiers from rusting, they are coated with coal-tar. The tower itself is painted white. The only brasing which has been thought necessary is a few cross tiers at each horizontal joint, over which the iron-tongued wood-floors are laid.

The several rooms are provided with fire apertures, fitted with oak sashes filled with plate-gla.s.s. The approach to the doorway, which is about ten feet above the level of the sand, is by means of stone-steps; ladder-irons are also provided in the event of the stone-steps being carried away by a hurricane.

Over the entrance is a large tablet of iron supported by two smaller ones; and on them, on bas-relief, are inscribed the date of erection (1842), the names of the commissioners, of the engineer, founder, &c.

The whole of the castings were executed at the foundry (late Bramah and Robinson's) at Pimlico, and put together in the yard of the manufactory, prior to their removal to Jamaica, where the work was re-erected by a derrick and crab from the inside, without the aid of any external scaffolding.

It is said that the whole expense of the lighthouse, including the pa.s.sage over the Atlantic, did not exceed seven thousand pounds, and that the entire weight of the iron-work is about one hundred tons. The masonry was also prepared in this country, which (from the absence of building-stone in Jamaica) was found to be more economical than if the work had been done on the spot. Mr. Grove, as clerk of the works, and two labouring engineers, who had attended to the execution of the work in England, were sent out for the purpose of erecting the lighthouse, and the necessary apparatus upon the site which had been selected. The elevation of the lighthouse above the level of the sea is one hundred and three feet.

Since the completion of this lighthouse, Mr. Gordon has been employed by the Ordnance Office to furnish designs and specifications for a tower on the same principle, but of larger dimensions and improved details, which is to be erected on Gibbs' Hill, in the island of Bermuda.

CHAPTER IX.

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 n.o.ble 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 s.p.a.ce.

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 s.p.a.ce, 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 s.p.a.ces 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-gla.s.s. 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 a.s.sistance 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 polis.h.i.+ng, 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 astonis.h.i.+ng; and by comparing its highly-polished and regularly-curved surface with the previous gla.s.s 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 subst.i.tute 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 cla.s.sed as _stationary_, _revolving_, _flas.h.i.+ng_, 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 _flas.h.i.+ng_ light is a modification of the revolving light, and is practically a beautiful example of the infinite celerity of the pa.s.sage 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 gla.s.s, 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 s.h.i.+pping.

When the French were recovering from the long night of terror, during which their commerce had been ruined and their s.h.i.+ps 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 subst.i.tution of gla.s.s 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 l.u.s.tre 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-gla.s.s, 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 gla.s.s 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 ma.s.s 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 quant.i.ty 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.