The details already given will convey some notion of the difficulty and danger of planting a lighthouse on the solid rock in a stormy sea; we may naturally suppose that this difficulty and danger must be enormously increased in erecting a permanent residence on the shifting sands. Such, however, is by no means the case; one of the recent triumphs of engineering has proved that it is not always folly to build a house upon the sand.
This remarkable result has been accomplished chiefly by means of Mitchell's screw-mooring, an instrument which consists essentially of an enormous cast-iron screw of about one turn and a half, having a hollow cylindrical centre; a wrought-iron spindle passes through the cylindrical socket; it is somewhat tapering in form, and when driven up tight is fixed thereto by a forelock passing through both; it is formed with a square head to receive the key for screwing it into the ground. It is also furnished with a collar of wrought iron fitted so as to turn freely on the upper part of the shaft of the spindle below the collar.
The attention of the Trinity House having been called to this instrument, it was considered applicable to the establishment of lighthouses on sands; and accordingly a series of experiments was undertaken at the cost of that honourable body. The spot selected for the purpose was on the verge of the Maplin Sand lying at the mouth of the Thames, about twenty miles below the Nore, forming the north side of the Swin or King's Channel, which, on account of its depth, is much frequented by large ships, as also by colliers and other vessels from the north sea. The sand is shifting, and is dry at low water spring-tides, and hitherto a floating light has been maintained upon it. On this spot it was proposed to erect a fixed lighthouse of timber framing, with a lantern and residence for the attendants.
In the month of August 1838, operations were commenced by inserting nine of Mitchell's patent mooring-screws, each four feet and a half in diameter, and furnished with shafts of wrought iron about twenty-five feet in length and five inches thick. One of these screws served as a centre to the remainder, which occupied the angles of an octagon forty-two feet in diameter. The screws were turned into the sands to the depth of twenty-one feet and a half, the upper extremities being left standing about five feet above the surface of the sands. For the purpose of fixing the screws, a stage or raft of timber, thirty feet square, was floated over the spot, with a capstan in the centre, which was made to fit on the top of the iron shaft, and firmly keyed to it. A power of about thirty men was employed for driving the screws, and their labours were continued until their united force could scarcely turn the capstan. This stage or raft, which had been formed in two thicknesses crossing each other at right angles, and bolted at their intersection, was, as a precautionary measure, allowed to remain. It covered the whole site within the piles, and also extended some distance beyond them. A curb about eighteen inches high was raised round this stage; on its surface was arranged a quantity of brushwood, and then about two hundred tons of rough stone, which sunk the stage into the sand and prevented it from being displaced. Between the spaces of the stage and the brushwood the sand was allowed to wash its way, and it soon filled the interstices of the stone. The whole mass soon became embedded below the surface of the sand, and gave considerable lateral support to the piles, and formed a solid body for the water to wash upon.