When the beacon is to be erected upon a sandy bottom, the engineer’s work becomes 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.