While the greater number of the most famous sea-lights have been erected upon the solid foundation offered by rock, in one or two instances notable works have been consummated upon sand. The two most remarkable achievements in this particular field of enterprise are the Rothersand lighthouse, off the coast of Germany, in the North Sea, and the Fourteen Foot Bank, in Delaware Bay, U.S.A.

The Rothersand light became necessary owing to the expansion of the German mercantile marine and the development of the ports of the Weser and Elbe. The estuary of the Weser River is hemmed in by shoals and sandbanks, similar to those found at the entrance to Liverpool, London, and New York, rendering navigation extremely hazardous under the most favourable circumstances. Bremerhaven, on the Weser, had been selected as the home port for the North German Lloyd Atlantic liners, but it was threatened with abandonment unless the entrance to the waterway should undergo improvement. It was of no avail to dredge a deep channel through the treacherous ridges of sand, if the general proximity of the shoal were left unmarked. Consequently, in order to secure the interests of Bremerhaven, it was decided by the three border States—Prussia, Oldenburg, and Bremen—to provide a powerful light at this danger-point. The financial problem was solved by the agreement to levy a special tax upon all vessels entering the Weser, to defray the cost of providing the safeguard.

The undertaking was somewhat formidable. The shoal, being of soft sand, was liable to erosion and movement, owing to fluctuating and changing currents. Then, again, the proposed site, some thirty miles from Bremerhaven and about halfway between that port and the island of Heligoland, was exposed to the assaults of the North Sea, where even slight breezes ruffle the water considerably. From the soundings and observations that were made, it was evident that the foundations would have to be carried down to a great depth, and that ordinary systems of construction were quite impracticable. At this juncture the Society Harkort of Duisburg, which had accumulated great experience in subaqueous work, was approached and asked if it would undertake the enterprise at its own risk. This was tantamount to a “no cure, no pay” proposal. If they succeeded, they would be rewarded for their labours; if they failed, they would have to face a heavy loss.

This firm, after careful deliberation, allowed that the work could be accomplished, but in one way only. This was to construct a huge caisson—practically a gigantic barrel of steel—on shore, to launch and tow it to the site, and there to lower it until it rested on the bottom. Then, by a removal of the sand from beneath this caisson, it could be sunk to a great depth, and, the interior being filled with concrete, a huge artificial core of rock would be created, capable of supporting a tower. This system is employed extensively in connection with bridge-building operations, and the firm entertained no doubts concerning its feasibility at Rothersand. The society accordingly prepared its designs, and advanced an estimate for the cost of the work.

At this juncture an unexpected competitor appeared on the scene. One of the engineers engaged in the preparation of the Harkort designs severed his connection with that firm, and, securing the collaboration of two engineering colleagues, established a rival concern, which tendered for the contract. They would follow the same lines, but would complete it for £22,750, or $113,750, instead of £24,025, or $120,125, asked by the Duisburg firm. The lower price was accepted, the more readily since it included the foundations, whereas the Society Harkort set these down as an extra. Naturally, the society was somewhat chagrined at this turn of events, after all the trouble and care it had taken to discover the most satisfactory solution of the problem, but subsequently it had good reason not to regret its loss.

The new engineers set to work and during the winter of 1880–81 constructed a huge caisson, which was launched and on May 22 of the latter year started down the Weser in charge of tugs. Then came a whole string of accidents. One night the unwieldy fabric got adrift and drove its nose into a sandbank, where it settled down with the tide. The towing cables were attached once more, and after a great struggle the structure was extricated on the next high-tide, and resumed its journey. Reaching the site without further incident, it was lowered by admitting the water within the barrel. But this task being accomplished somewhat crudely, the water rushed in with such force that the caisson commenced to spin round like a top, as well as bobbing up and down like an angler’s float. It threatened to topple over and founder every moment, but, luckily keeping upright, finally touched bottom. Lowering was completed. Night having approached, workmen made themselves comfortable on the caisson, while the constructional steamer stood off and cast its anchor.

The men on the caisson, however, experienced one of the most sensational nights in their lives. As the tide rose, they found their novel home to be behaving somewhat curiously. It moved, and then heeled over. This was an alarming state of affairs, especially as the list gradually became worse and worse. They shouted frantically for help, but, a heavy fog having descended upon the shoal, their cries were absorbed by the white pall. At last the caisson careened over to such a degree that the men could not keep their feet, while the depressed edge was in danger of being submerged. The men crawled to the opposite or elevated side, and held on for their lives, expecting every moment that the structure would give a heave and roll over. It was a terribly anxious time for them, and at last, when the constructional steamer came alongside in the morning, they scuttled down the ropes from their perilous perch to the deck below, thankful for having escaped, as they thought, a certain watery grave.

The engineers spared no effort to save their work. They were harassed at every tide because the water rose above the depressed edge and flooded the interior. With all speed the wall at this point was increased in height, so as to prevent inundation. Then, stormy weather having cut away the sand under the elevated side, the structure gradually righted itself. When it had regained its vertical position, it was found that no serious damage had been done, but rather that the engineers had profited, inasmuch as the caisson had buried itself some 16 feet into the sand.

Winter was approaching, and so the engineers crowded on every man and effort possible, in order to get the structure sunk to the requisite level before work would have to be abandoned for the season. They departed from the engineer’s axiom, “Make haste slowly,” and paid the penalty. When the bad weather broke, compelling the return of all the workmen to shore, the fabric was left insecure. The lower part had been given its filling of concrete, but above a certain level the fabric depended only upon the iron shell of the cylinder. It was stiffened as much as possible with cross-timbers and bracing, but the elements soon made short work of this puny defence. The North Sea, in common with the other large stretches of water throughout the world, was swept by terrible storms that winter, and one morning, when the sea was scanned from shore through glasses, strange to say the caisson was nowhere to be seen. All sorts of rumours were circulated to account for its disappearance, among others being a sensational theory that the caisson, having reached swampy ground while being sunk, had simply dropped suddenly into the submarine quagmire, and had been swallowed up completely. But the divers, when they could get out to the site and could venture into the ocean depths, returned to the surface with a very different story. The waves had snapped off the top of the caisson at the upper level of the concrete within, and had carried it away. Thus ended summarily the first attempt to build a lighthouse upon the red sand at the entrance to the River Weser.

The project, however, was not abandoned. The Society Harkort was approached once more, and requested to undertake the work upon its own terms. The invitation was accepted, but the firm, realizing the abnormal risks incidental to the enterprise, revised their price, so as to provide for contingencies. It demanded a sum of £42,650, or $213,250, in return for which it undertook to supply a fully-equipped lighthouse less the illuminating apparatus. The terms were accepted, but the responsible authorities, having suffered a heavy loss from the first failure, decided to protect themselves against a similar disaster, so exacted a bond for £12,000, or $60,000, to be returned when the work should be completed and accepted by the Government. The Society Harkort, on its part, reserved the right to withdraw from the undertaking in the event of the caisson sharing the fate which overtook the first structure.

The contracts were signed in September, 1882, and the task was commenced. The first disaster was a blessing in disguise, for the new engineers were able to turn the mistakes of their predecessors to advantage. They designed a caisson of oval shape, with pointed ends, measuring 46 feet in length by 36 feet wide. It was an elaborate, staunch structure, towering to a height of 60¾ feet when launched. At a height of 8 feet from the bottom edge was a massive flooring built of iron. The space below constituted the area in which the men were to work upon the sea-bed, excavating the sand under compressed air, while the lower rim was a cutting edge, so as to facilitate the sinking of the mass as the sand was removed. The upper part of the caisson was divided into four floors, each of which was set aside for a specific purpose. The lowest was the concrete-mixing chamber; that above carried the machinery and boilers; the third floor formed the living-quarters for the men who worked and slept on the structure; while the top floor formed a deck, and carried two powerful cranes whereby the material was lifted from the boats which drew alongside. Of course, when the caisson had been lowered into the water and was eating its way deeper and deeper into the sand, these platforms had to be moved higher and higher from time to time, as the base of the tun became filled with concrete, the outer walls of the fabric being increased to keep the top well above high-water mark.

When the caisson was completed on shore and sent into the water, it was an impressive-looking monster. The shell itself weighed 245½ tons, and with the various accessories aboard the weight was brought up to some 335 tons. It then had to be loaded down to the required depth for towing, for which purpose ballast in the form of pig-iron, concrete, and bricks, to the extent of another 245 tons, was stowed aboard, while delicate precautions were taken to maintain stability. The combined efforts of 120 men, working day and night for 127 days, were required to erect this caisson, and on April 1, 1883, it was ready for its transportation to the site.

The towing operation was extremely difficult, and the voyage out was full of exciting incident. It was possible to advance only on the ebb-tide, and the towing cables, 5 inches in diameter, were specially manufactured for the operation. Two of the most powerful tugs owned by the North German Lloyd Steamship Company were requisitioned, three other steamers engaged in the conveyance of requirements between tower and shore accompanying the procession. Although the engineers were ready, the weather, with aggravating persistence, refused to clear sufficiently to produce the smooth sea and calm demanded for the safe journey of the ungainly craft. Day after day slipped by, with eighty men on the alert, and with fires banked and steam raised on the vessels, ready to weigh anchor at the first favourable moment. Fifty-five days passed before the weather bureau recommended that the conditions were suitable. Under the foregoing circumstances the expense of this delay may be realized.

Directly the intimation was conveyed that the tow could be attempted, there was a scene of indescribable activity and bustle in the Bremerhaven dock, where the caisson was moored. Full steam was raised on the tugs, and at half-past three in the morning of May 26 the mighty steel barrel moved out of the dock. The towing ropes were hitched on, and very slowly the “Colossus,” as the caisson was named, moved down the harbour, accompanied by the whole fleet of nine vessels engaged in construction work, so that the procession was imposing. It dropped down the river without incident, when, the tide turning, anchor was cast, and all was made fast until another advance could be made at four o’clock in the afternoon. But the rising tide was stronger than had been anticipated, and trouble was soon encountered. The caisson, pressed by the current, dragged and strained at the two tugs by which she was being towed, causing them to slip their anchors. It was an anxious moment. The two vessels could not hold the “Colossus”; in fact, they were being towed backwards by it. Hurriedly another tug was called up, and helped in the effort; but although the three steamers put on full steam ahead, they failed to keep the mass in check. Another tug was signalled, and then, under the combined effort of 350 horse-power, driving for all it was worth against the current, the four vessels mastered the swing of the scurrying water, and had the “Colossus” under control.

A little later the procession continued on its way to the North Sea, but when the boats came up with the Hoheweg lighthouse further disquieting news was received. The keepers signalled that the barometer was falling, and that a thunderstorm was hurrying across the North Sea from England. Anchors were thrown out hurriedly, and everything made snug and tight for the approaching storm. It burst with fearful severity. The waves got up, the wind blew with fiendish velocity in terrifying gusts, and the rain tumbled down in sheets. The engineers were on tenterhooks the whole hour and a half the storm raged, as they foresaw lively times if the unmanageable hulk broke loose. But the “Colossus” rode the gale as quietly as if moored to a wharf in dock. The storm, however, upset all calculations for the day. There was no possibility of getting the caisson out and sunk before nightfall, so the engineers prepared to pass the night at anchor, and to start off again with the dawn. The weather, ruffled by the thunderstorm, refused to settle down until a further day and night had been wasted. Then, at 7.30 in the morning, on a favourable tide, anchors were weighed, and, steaming hard through a broken sea, the tugs conveyed the caisson on its journey. At last the procession reached the buoy marking the site. The caisson was brought to rest, the water was admitted gently through the valves, and slowly, steadily, and vertically, the shell sank lower and lower, until a scarcely perceptible shock conveyed the intimation that it had touched bottom.

The most anxious part of the task was consummated with complete success: the caisson had been got to the site and sunk. Then the task of burying it deeply and irremovably in the sand was hurried forward. Workmen descended into the space beneath the bottom floor and the sea-bed. Under compressed air they excavated the sand within the area to permit the cutting edge to sink lower and lower. The sand, as removed, was lifted to the top of the “Colossus” and discharged overboard. Meanwhile the concrete-mixing machine got busy, and the stone heart of the tun was fashioned rapidly. Under this increasing weight the sinking operation was assisted very appreciably. By the middle of October the work had been advanced to such a stage that the total weight of the structure had been increased to over 3,350 tons, and the top deck of the caisson, which had grown in height by the attaching of successive rings of plates, was about 99 feet above the cutting edge, which had buried itself to a depth of 51 feet below low-water. Then work had to be abandoned, as the autumnal gales sprang up. The whole of the staff, with the exception of two men, who mounted guard over the work, were taken back to Bremerhaven. The gales increased in fury, culminating in a tempest similar to that which had destroyed the first caisson. Remembering the fate of that enterprise under such fearful pounding from wind and wave, the Harkort engineers naturally were somewhat anxious concerning the welfare of their handiwork under identical conditions. But the new creation was overwhelmingly strong where its predecessor was weak, although the seas, baffled in their efforts to upset the caisson, did not fail to leave their mark by knocking the superstructure and scaffolding about somewhat, as well as carrying away a few weighty pieces of the top hamper.

Work was resumed in February, 1884, and continued more or less regularly until November. Interruptions were of frequent occurrence, so that only about one-quarter of the time available could be turned to useful account. The structure which had been towed out of Bremerhaven a year previously had disappeared from sight, the rim of the barrel built on dry land being about 4 feet below water; but, of course, as the work proceeded and the caisson sank, its walls were extended upwards, as already explained. When the structure had been sunk to its designed depth, the steel shell was 107½ feet in height, from the cutting edge to the top projecting above the water, and nearly 40 feet of its height was buried in the Rothersand. To sink it to this level required the removal of 3,000 cubic yards of sand from beneath the bottom floor of the structure; while 49,100 tons of material were brought out from Bremerhaven and built into the steel shell to render it a solid elliptical mass, with the exception of a short central hollow space which has a narrow conduit connection with the outer sea, and which, fitted with a float, acts as a tide-gauge which may be read in the lighthouse. From this massive concrete pedestal rises the tower proper, which at the base is circular, with a diameter of 33¾ feet. This base rises in the form of a graceful concave curve to a height of 26 feet, and is solid except for two water-tanks. At the entrance level the tower is 23 feet in diameter. Above this are disposed four floors, comprising the cellar, storeroom, kitchen, and living-quarters for the men, crowned by the lantern, the gallery of which is 80½ feet above low-water. The external appearance of this interesting lighthouse is somewhat different from the general conception of such a building. Instead of being merely a circular top and lantern, there are three semicircular turret-like projections on the dwelling-room and lantern levels, which serve for directing and warning lights as well as for a lookout station.

The fickle character of the North Sea where it rolls over the Rothersand is reflected by an experience which befell the Harkort engineer and the superintendent of erection for the authorities, who wished to complete his duty of inspection. The finishing touches were being applied, a squad of twelve workmen being in the tower to continue the work during the winter. The early December day was fair and the sea smooth, as well as giving every indication of remaining quiescent for some hours. The superintendent had arranged to spend his Christmas holidays with some friends, and desired to complete his duty in good time, so that his sojourn might be free from care. The two started off in the steamer, and landed without effort. But while they were engaged in their work of inspection the wind and sea freshened, so that a boat could not be sent from the steamer to take them off. It was an amusing situation which was keenly enjoyed at Bremerhaven; but all would be right on the morrow, said everyone. But the next day the weather was worse, and continued so for day after day. When a fortnight had passed without it being possible to succour the weather-bound engineers, amusement gave way to anxiety, more especially as a signal was flying from the tower which conveyed the unwelcome intelligence that one of the workmen had fallen ill. The feelings of the superintendent may be imagined. He had visions of spending his Yuletide in a draughty, half-finished lighthouse tower, where comfort was conspicuous by its absence, and where seasonal fare such as he had been anticipating keenly was unknown. But on December 21 the constructional engineers, having grown impatient with the weather, sent out one of their boats, with instructions to bring everyone ashore at all hazards. The waves were running high and the wind was gusty, but the steamer anchored as near the lighthouse as she dared, and by means of her boats, which were in momentary danger of being swamped, brought off the two engineers as well as all the workmen except two. The latter remained behind as a guard, and, being given a good stock of seasonal provisions and other necessities, were left in their splendid isolation. The superintendent, after all, was able to enjoy his Christmas holidays.

The succeeding spring brought a resumption of toil, and by September the tower was completed except for the illuminating apparatus. One feature was observed during construction and had to receive attention. The free swing of the currents and tides, being obstructed by the tower, had commenced heavy erosion, big hollows being scooped out of the soft sea-bed around the caisson. As it was quite possible that in the course of time this scouring might imperil the safety of the building, protective works had to be undertaken. These were of an elaborate character, and comprised the sinking of mattresses, fashioned from brushwood, around the foundations, upon which were dumped boatloads of broken stone. This mattress had to be nearly 50 feet in width, and in some places about 15 feet in thickness. For this protective work alone some 176,550 cubic feet of brushwood, and 600 tons of block-stone to hold it down, were used. These measures, however, effectually overcame the danger of erosion.

On November 1, 1885, the light was shown for the first time, and the greatest peril at the entrance to the Weser was indicated far and wide by night and day. It was a magnificent achievement, carried through in the face of enormous difficulties, sensational incidents innumerable, and upon a foundation of disaster. The lighthouse is as firm as if it were anchored upon a solid granite rock, instead of having its roots thrust deep into treacherous shifting sand, and constitutes an imperishable monument to German engineering ability; while, all things considered, the cost was low, being only £43,400, or $217,000, in all. The light is electric, the power being supplied from a station on shore, and fed to the lighthouse through a submarine cable; the keepers are also in submarine telegraphic communication with the mainland.

When the United States set out to build a similar structure in the spacious Delaware Bay, they were confronted with a prospect just as forbidding, and a task in every way as difficult, as that offered in connection with the Rothersand. There is a dangerous shoal about twenty miles off the land, where the Atlantic beats with furious rage, and where vessels were apt to stick hard and fast. It was described as “Fourteen Foot Bank” by mariners, from the depth of the water flowing over the shoal, and this colloquialism has provided the name for the present guardian light. The open situation did not augur favourably for the completion of a lighthouse at this spot, but the American engineers were resolved to make the attempt. Accordingly, plans were prepared for a construction upon the caisson principle, which was the only method promising success.

The preliminary step was the fabrication of a caisson. The first part was more like a raft with sides. It was about 40 feet square, 5 feet thick, and with walls 7 feet deep. It was built of timber, the staves being 12 inches square, and upside down—that is, with the floor uppermost—on a building-slip, as if it were a ship, and was launched into the water upon similar lines. The sides and top were lined, so as to secure water-tightness. In the centre there was a circular space 5 feet in diameter to form the air-shaft.

As the structure was built upside down, the rim was brought to the lowermost position, and this formed the cutting edge, which was to be sunk into the sand. On this floating platform a circular iron cylinder was erected. This tube was 35 feet in diameter, and was built up in plates, 6 feet in width by 1½ inches thick. When three rings of iron were set up the cylinder was 18 feet in height. In order to sink it to a depth of 15½ feet into the water for towing purposes, it was charged with a layer of concrete, 9 inches in thickness, to serve as ballast, and in this condition the caisson weighed 400 tons. This huge barrel was built at Lewes, Delaware, and when it was launched two powerful steam-tugs set out to drag it to the shoal, twenty miles away. As the tide rises and falls a matter of 6 feet in these waters, and the currents are somewhat wicked, the engineers displayed no undue haste. They waited for the first favourable opportunity, and seized it. But it took the two tugs some six hours to reach the site; an average speed of about three and a half miles per hour cannot be construed into fast travelling.

When the mighty caisson had been warped and nudged dead into position over the desired spot, water was admitted. With a gurgling and hissing the hulk sank slowly into the sea. At last a slight jolt, which quivered through the mass, signified that the structure was resting on the bottom. The engineers gave a sigh of relief, but the next instant changed it to a cry of dismay. The caisson began to heel over to one side. Was it going to capsize? That was the absorbing fear. It canted more and more, until at last it had a list of 12 degrees. It had not sunk vertically! There was less than 16 inches of water between the sea-level and the rim when the caisson first jarred against the sand, and if it careened over too far the water certainly would rush in, roll the whole tub over, and tumble it hither and thither over the sea-bed. The engineers watched that caisson as closely as a cat watches a mouse-hole. Presently it eased up, and then, as the tide rose some six hours later, it began to right itself. The engineers were relieved once more. The danger was over. But their self-satisfaction was soon upset as the tide began to ebb, because again the cylinder gradually fell over on its side. The cause of this strange behaviour flashed upon them. The surface of the sandbank was not level! The mass in sinking had touched bottom on the highest point of the shoal, and was trying to find its own level.

Without any further delay, the engineers decided upon an ingenious means of correcting this erratic and dangerous action. The tugs were despatched hurriedly to Lewes to bring out cargoes of broken stone, which had been delivered for the preparation of the concrete. While the steamers pursued their errand, the engineers fashioned large pockets on the elevated section of the structure, into which the stone upon its arrival was placed. Gradually but surely the caisson not only was corrected, but the weighted end was induced to settle into the sand, until the opposite free edge in its turn was resting upon the shoal.

In this manner all danger of further canting now was removed. As the rim had been brought perilously near the water-level, and there was a possibility of flooding from a rough sea, the walls of the caisson were extended vertically with all haste; meanwhile two additional rings of iron were placed in position, and the top was brought about 20 feet above the water. While this work was in progress the structure gradually bit farther and farther into the sand, until at last it secured a firm hold.

At the earliest possible moment the air-compressors were set to work, and air was driven into the space between the cutting edge and the roof, in which the men were to work. This space was 40 feet square and 7 feet deep. The greater pressure of the air drove the water out from this space, and the men were able to enter through the air-lock and to work upon a dry surface, isolated from the surrounding sea by the fence formed by the cutting edge.

The men toiled in eight-hour shifts continuously, removing the sand within the space and sending it upwards to be discharged overboard. As the area was excavated, the cutting edge sank deeper and deeper, so that the structure became more and more firmly embedded. There was apprehension that the obstruction offered by the caisson to the movement of the currents might set up undermining around the cylinder, as in the case of the Rothersand; but the engineers arrested any tendency in this direction by dumping large pieces of stone overboard around the tub. Some 6,000 tons of stone were used for this purpose, so that the caisson has an impregnable protection.

As the structure sank lower and lower, owing to the excavation, concrete was dumped around the air-tube above the floor of the space in which the men were labouring, while successive rings of iron were added to the top of the cylinder. The men worked with great gusto in their novel situation, and, the task being prosecuted uninterruptedly throughout the day and night, the cylinder sank from 12 to 24 inches during the twenty-four hours. This labour was maintained until the cutting edge of the caisson was 33 feet below the surface of the shoal, when the engineers called halt. They considered that the task had been continued to a sufficient depth to secure the requisite rigidity for their lighthouse. The men left the working chamber, which was then tightly underrammed with sand, so as to form a solid foundation, while the air-shaft was filled up with rammed sand and sealed with a thick plug of concrete. The wall of the iron cylinder had been intermittently increased in height by the addition of successive rings of plates, until the rim was 70 feet above the cutting edge and projected about 30 feet above the water at low-tide. From the bottom to a height of 40 feet it is virtually a solid mass of concrete, protected by a skin of iron 1½ inches thick. Further concrete was added, bringing the solid section to within 10 feet of the rim, so that the concrete heart is about 53 feet in height and 35 feet in diameter. It is a solid circular rock sunk into the sand, and as firm and free from vibration as a granite core.

Upon this foundation a house for the light-keepers, crowned by a tower, was erected, the focal plane being 59 feet above mean high-water. It is fitted with a light of the fourth order, visible for thirteen miles.

One of the most important features in connection with the Fourteen Foot Bank light was its small cost, which was below the estimate, especially when it is compared with the German work. The United States Government appropriated a sum of £35,000, or $175,000, for the undertaking, but the total expenditure was less than £25,000, or $125,000, so that a sum of £10,000, or $50,000, was handed back to the Treasury—a most unusual event in connection with Government contracts. The lighthouse was finished and brought into service in 1886. The success of this novel enterprise prompted the authorities to essay a more daring project—the erection of a lighthouse upon the caisson principle on the Outer Diamond Shoal, off Cape Hatteras, North Carolina. But the storms encountered off this inhospitable coast have proved too overpowering for the engineer. Numerous attempts have been made, but disaster has been their invariable fate. The Diamond Shoal refuses to be indicated by anything except a lightship.