The difficulties involved in constructing a lighthouse on solid rock have been shown, and it was at one time thought absolutely impossible to erect—with any prospect of permanent duration—one upon storm-exposed sands. _Nous avons changé tout cela._ It is no longer necessary to place floating lights in places of great danger, although for other reasons they are constantly used. One of the greatest modern triumphs of engineering is Mitchell’s screw-mooring apparatus. To describe it fully would necessitate several pages of technical matter. Suffice it to say that enormous cast-iron screws, having hollow cylindrical centres, through which wrought-iron spindles pass, are literally screwed down into the sand, or its substratum of other soil. One of the earliest experiments was made on the verge of the Maplin Sand, at the mouth of the Thames. Nine of the mooring-screws were inserted into the sand 21½ feet, one in the centre, the rest forming an octagon 42 feet in circumference, having standards or posts which stood 5 feet above the surface of the sand. A raft of timber was floated over the spot, and a capstan in its centre drove the screws to the required depth. This raft was afterwards sunk, by covering it with 200 tons of rough stone. Two years were allowed to elapse, at the termination of which time the whole mass was found firmly embedded, and then a lighthouse, raised on a strong open framework, was erected over this sub-structure. During these long preparations a very similar structure was commenced and finished at Port Fleetwood, on the River Wyre, near Lancaster. The preparatory steps were similar to those already described. The foundation of the lighthouse was formed of seven screw-piles, six of them occupying the angles of a hexagon 46 feet in diameter, the seventh being in the centre. From each screw proceeds a pile 15 feet in length, having at the upper end another screw for securing a wooden column. These columns are of Baltic timber, the one in the centre being 56 feet, the others 46 feet in length, firmly secured with iron hoops and coated with pitch. The platform, upon which the house stands, is 27 feet in diameter, the house itself being 20 feet in diameter and 9 feet high. From the summit of the house rises a twelve-sided lantern, 10 feet in diameter and 8 feet high. Altogether the light is elevated about 46 feet above low-water level, and ranges over an horizon of eight miles. The light is of the dioptric kind—bright, steady, and uniform, and when the weather is too foggy to allow it to be seen, a bell is tolled by machinery, to give the needful warning.