Lightships and Lighthouses. Frederick Arthur Ambrose Talbot
iron staging was erected, and from this point to the top of the cliffs behind a Bullivant cableway was stretched, up and down which the various requirements were carried, together with the workmen. This cableway, designed by Mr. W. T. H. Carrington, M.I.C.E., consulting engineer to Messrs. Bullivant and Co., Ltd., facilitated rapid and economical construction very appreciably. The span was about 600 feet between the erecting stage and the cliff summit, and there were two fixed ropes stretched parallel from point to point. One rope, 6 inches in diameter, had a breaking strain of 120 tons; the second, 5½ inches thick, had a breaking strain of 100 tons. At the seaward end the cables were anchored into the solid chalk. Everything required for the constructional operations was handled by this carrying system, and when it is recalled that some of the blocks for the lower courses weighed from 4½ to 5 tons, it will be recognized that such a method of handling these ungainly loads, with the care that was demanded to preserve the edges and faces from injury, solved an abstruse problem completely.
The base of the tower, the diameter of which is 47 feet, is solid to a height of 48 feet, except for a central circular space for storing drinking water. It was designed by Sir Thomas Matthews, M.I.C.E., the Engineer-in-Chief to the Trinity Brethren, and is a graceful building, the tower rising in a curve which is described as a “concave elliptic frustum.” From the base to the lantern gallery is 123½ feet, and 3,660 tons of Cornish granite were used in its construction. The over-all height to the top of the lantern is 153 feet. The building is provided with eight floors, comprising the living and sleeping quarters for the keepers, storage of oil, and other necessaries. The light, of the dioptric order, is of 83,000 candle-power, and the two white flashes given every fifteen seconds are distinguishable for a distance of seventeen miles, which is the average range of modern British lighthouses.
Although the constructional work was frequently interrupted by rough weather, every advantage was taken of calm periods. While from the point of daring engineering it does not compare with many of the other great lights of the world, yet it certainly ranks as a fine example of the lighthouse builder’s skill. Owing to the elaborate precautions observed, the achievement was not marred by a single fatality, although there were many thrilling moments, the sole result of which, however, was the loss of tools and sections of the plant, which in the majority of cases were recovered when the tide fell. The most serious accident was a crushed toe, which befell one of the masons when a stone was being bedded.
Although the lighthouse is subjected to the full fury of wind and wave, if skilfully erected it will withstand the ravages of both without creating the slightest apprehensions in the engineer’s mind. The stones are prepared so carefully that they fit one another like the proverbial glove, while the cement fills every nook and cranny. Occasionally, however, the cement will succumb to the natural disintegrating forces, and, becoming detached, reveal a point vulnerable to attack. The air within the interstice becomes compressed by the surging water, and thereby the fabric is liable to be shattered. Some years ago one or two of the lighthouses guarding the Great Lakes of North America were found to have become weakened from this cause. A novel remedy was evolved by an ingenious engineer. He provided each tottering lighthouse with an iron overcoat, enveloping it from top to bottom. The metal was not laid directly upon the masonry, but was so placed as to leave about a quarter of an inch between the inner face of the metal and the surface of the masonry. Liquid cement was then admitted under pressure—“grouting” it is called—into this annular space, and penetrating every crack and crevice in the masonry, and adhering both to the metal and the stonework, it practically formed another intermediate jacket, binding the two so firmly together as to make them virtually one. This novel procedure absolutely restored the menaced building to its original homogeneity and rigidity, so that it became as sound as the day on which it was built.
Nowadays, owing to the skill in designing and the workmanship displayed, one never hears of a modern lighthouse collapsing. Expense is no object; the engineer does not endeavour to thwart the elements, but follows a design wherein the minimum of resistance is offered to them.
CHAPTER III
THE LIGHT AND ILLUMINANTS
While it is the tower that probably creates the deepest impression upon the popular mind, owing to the round of difficulties overcome associated with its erection, yet, after all, it is the light which is the vital thing to the navigator. To him symmetry of outline in the tower, the searching problems that had to be solved before it was planted in a forbidding spot, the risks that were incurred in its erection—these are minor details. His one concern is the light thrown from the topmost height, warning him to keep off a dangerous spot and by its characteristic enabling him to determine his position.
I have described the earliest type of light, the open wood or coal fire blazing on an eminence. In due course the brazier gave way to tallow candles. This was an advance, certainly, but the range of the naked light was extremely limited. Consequently efforts were made to intensify it and to throw it in the desired direction. The first step was made with a reflector placed behind the illuminant, similar to that used with the cheap wall-lamp so common in village workshops. This, in its improved form, is known as the “catoptric system,” the reflector being of parabolic shape, with the light so disposed that all its rays (both horizontal and vertical) are reflected in one direction by the aid of a highly polished surface. While the catoptric system is still used on some light-vessels, its application to important lighthouses has fallen into desuetude, as it has been superseded by vastly improved methods. But the reflector, made either of silvered glass set in a plaster-of-Paris mould or of brightly polished metallic surfaces, held the field until the great invention of Augustin Fresnel, which completely revolutionized the science of lighthouse optics.
Fig. 2.—Fixed Apparatus of 360 Degrees.
Shows one ray throughout the complete circle.
(By permission of Messrs. Chance Bros. and Co., Ltd.)
Fresnel was appointed a member of the French Lighthouse Commission in 1811, and he realized the shortcomings of the existing catoptric method only too well. Everyone knows that when a lamp is lighted the luminous rays are diffused on every side, horizontally as well as vertically. In lighthouse operations the beam has to be thrown in a horizontal line only, while the light which is shed towards the top and bottom must be diverted, so that the proportion of waste luminosity may be reduced to the minimum. While the parabolic reflector achieved this end partially, it was far from being satisfactory, and Fresnel set to work to condense the whole of the rays into a horizontal beam. Buffon, a contemporary investigator, as well as Sir David Brewster, had suggested that the end might be met by building up a lens in separate concentric rings, but neither reduced his theories to practice.
Fresnel invented a very simple system. He took a central piece of glass, which may be described as a bull’s-eye, and around this disposed a number of concentric rings of glass. But these rings projected beyond one another. Each constituted the edge of a lens which, while its radius differed from that of its neighbour, owing to its position, yet was of the same focus in regard to the source of illumination. The parts were shaped with extreme care and were united in position by the aid of fish glue, the whole being mounted in a metal frame. The advantage of the system was apparent in the first demonstrations. The lenses being comparatively thin, only one-tenth of the light passing through was absorbed, whereas in the old parabolic reflectors one-half of the light was lost.
Fig. 3.—Single Flashing Apparatus (One Panel and Mirror).
(By permission of Messrs. Chance Bros. and Co., Ltd.)
This revolutionary development was perfected in 1822, and in the following year it was submitted to its first practical application on the tower of Cordouan in the Gironde. Several modifications were made by the inventor for the purpose of