Lightships and Lighthouses. Frederick Arthur Ambrose Talbot
more baffling, as he is compelled to carry his underwater work down to a point where a stable foundation may be secured. When the Leasowe lighthouse was built on the sandy Wirral shore, the builders were puzzled by the lack of a suitable foundation for the masonry tower. An ingenious way out of the difficulty was effected. In the vicinity an incoming ship, laden with a cargo of cotton, had gone ashore and had become a total wreck. The cotton was useless for its intended purpose, so the bales were salvaged and dumped into the sand at the point where the lighthouse was to be erected. The fleecy mass settled into the sand, and under compression became as solid as a rock, while its permanency was assured by its complete submersion. The stability of this strange foundation may be gathered from the fact that the tower erected thereon stood, and shed its welcome light regularly every night, for about a century and a half, only being extinguished two or three years ago as it was no longer required.
In the Old World, and, indeed, in the great majority of instances, the lighthouse is what is described as a “monolithic structure,” being built of courses of masonry, the blocks of which are dovetailed together not only laterally, but also perpendicularly, so that, when completed, the tower comprises a solid mass with each stone jointed to its fellow on four or five of its six sides. This method was first tried in connection with the Hanois lighthouse, off the Guernsey coast, and was found so successful that it has been adopted universally in all lighthouses which are exposed to the action of the waves.
The upper face and one end of each block are provided with projections, while the lower face and the other end are given indentations. Thus, when the block is set in position, the projections fit into corresponding indentations in the adjacent blocks, while the indentations receive the projections from two other neighbouring pieces. The whole is locked together by the aid of hydraulic cement. Consequently the waves, or any other agency, cannot possibly dislodge a stone without breaking the dovetails or smashing the stone itself. For the bottom layer, of course, the surface of the rock is pared away sufficiently to receive the stone, which is bedded in cement adhering to both the rock and the superimposed block. A hole is then drilled through the latter deep into the rock beneath, into which a steel rod or bolt is driven well home, and the hole is sealed up with cement forced in under such pressure as to penetrate every interstice and crevice.
The iron supports constitute the roots, as it were, of the tower, penetrating deep into the heart of the rock to secure a firm grip, while the tower itself resembles, in its general appearance, a symmetrical tree trunk, this form offering the minimum of resistance to the waves. The lower part of the tower is made completely solid by the dovetailing of the integral blocks, and is cylindrical in shape up to a certain predetermined level which varies according to the surrounding conditions and the situation of the light. Some years ago the lighthouse assumed its trunk-like shape at the bottom course, rising in a graceful concave curve to the lantern; but this method has been abandoned, inasmuch as, owing to the decreasing diameter of the tower as it rose course by course above its foundations, the lowest outer rings of masonry did not have to withstand any of the superimposed weight, which naturally bears in a vertical line. By carrying the lower part to a certain height in the form of a cylinder, and then commencing the concave curve of the tower, the pressure of the latter is imposed equally upon the whole of its foundations. The latter may be stepped—i.e., one tier of stones may project a little beyond that of the one immediately above—but this arrangement is adopted in order to break the smashing force of the waves.
The conditions attending the actual building operations upon the rock, which may be accessible only for an hour or two per day in calm weather, prevent the blocks of granite being shaped and trimmed upon the site. Accordingly, the lighthouse in the first place is erected piecemeal on shore. A horizontal course of stones is laid to see that each dovetail fits tightly and dead true. The next course is laid upon this, and so on for perhaps eight or ten courses, the trimming and finicking being accomplished as the work proceeds. Each projection has to be only just big enough to enter its relative indentation, while the latter must be exactly of the requisite dimensions to receive the projection, and no more. Each stone is then given an identification mark, so that the masons on the rock may perceive at a glance its precise position in a course, and to what ring of stones it belongs. Therefore the mason at the site has no anxiety about a stone fitting accurately; he has merely to set it in position upon its bed of cement.
On shore—generally in the quarry yard—when a series of courses have been temporarily built up in this manner and have received the critical approbation of the resident engineer, the topmost course is removed and retained, while the other blocks are despatched to the site. This topmost course forms the bottom ring in the next section of the lighthouse which is built up in the yard, and the topmost course of this section in turn is held to form the bottom course of the succeeding part of the tower, and so on from foundation to lantern parapet.
During the past two or three years reinforced concrete has been employed to a certain extent for lighthouse construction, but granite of the finest and hardest quality still remains the material par excellence for towers erected in exposed, sea-swept positions. The Russian lighthouse authorities have adopted the ferro-concrete system in regard to one or two shore lights, especially on the Black Sea, while another fine structure upon this principle was built by the French Service des Phares in 1905 at the entrance to the River Gironde. The system has also been adopted by the Canadian lighthouse authorities; one or two recent notable lights under their jurisdiction have been constructed in this material, although on somewhat different lines from those almost invariably followed, so far as the general design is concerned.
While the masonry or monolithic structure is the most durable and substantial structure, it is also the most expensive. In many parts of the world, notably along the Atlantic coastline of the United States, what are known as “screw-pile lighthouses” are used. These buildings vary in form, some resembling a huge beacon, such as indicates the entrance to a river, while others convey the impression of being bungalows or pavilions on stilts. The legs are stout, cylindrical, iron members, the lower ends of which are shaped somewhat after the manner of an auger, whereby they may be screwed into the sea-bed—hence the name. This system has been employed for beacons over dangerous shoals; and while they are somewhat squat, low-lying lights, they have proved to be highly serviceable.
Iron has been employed also for lighthouse constructional work, the system in this case being a combination of the screw pile and the tower, the latter, extending from a platform whereon the living-quarters are placed and mounted clear of the water, on piles, being a huge cylindrical pipe crowned by the lantern. One of the most interesting and novel of these iron lighthouses is the Hunting Island tower off the coast of South Carolina. In general design it resembles the ordinary lighthouse wrought in masonry, and it is 121½ feet in height from the ground to the focal plane. It is built of iron throughout, the shell being in the form of panels, each of which weighs 1,200 pounds.
This type of tower was selected owing to the severe erosion of the sea at the point where it is placed. When it was erected in 1875, at a cost of £20,400, or $102,000, it was planted a quarter of a mile back from the sea. This action was severely criticized at the time, it being maintained that the light was set too far from the water’s edge to be of practical value; but the hungry ocean disappointed the critics, because in the course of a few years the intervening strip of shore disappeared, and the necessity of demolishing the light and re-erecting it farther inland arose. On this occasion the engineers determined to postpone a second removal for some time. The tower was re-erected at a point one and a quarter miles inland, and the sum of £10,200, or $51,000, was expended upon the undertaking. The iron system, which was adopted, proved its value in this work of removal piece by piece, because, had the tower been carried out in masonry, it would have been cheaper to set up a new light, as was done at Cape Henry.
Fig. 1.—Sectional Diagram of the Ar-men Lighthouse, showing Yearly Progress in Construction.
It guards the “Bay of the Dead,” off Cape Finisterre. Commenced in 1867, it was not finished until 1881.
Some of the American coast lights are of the most primitive and odd-looking character, comprising merely a lofty skeleton of ironwork. The lamp is a head-light, such as is carried by railway