Instead of a massive, gracefully-curving tower, measuring some 40 feet in diameter at the base, these engineers suggested a building of irregular octagonal shape, measuring 14½ and 17 feet across the faces, 80 feet in height, and carried out in ferro-concrete. They advocated its erection upon the Platte Fougère, because there the fog-signal would be brought into the most serviceable position for shipping. A narrow or thin building was advised, to offer the minimum of surface to the waves, which break very heavily on these ridges. The wisdom of this design has been revealed very convincingly since the tower has been in service. The seas fall on either side, divide and rush round the building, so that it does not experience the full brunt of their heavy, smashing blows. As the engineers pointed out, “It is better to avoid heavy sea pressures, where feasible, in preference to courting them.” Still, the Platte Fougère was not an ideal rock from the engineers’ point of view, although it is a solid knot of granite. Its head is visible only at low-water spring-tides, while it is difficult to approach, even in the smoothest weather, owing to the tides and currents. Much of the foundation work had to be carried out under water. The season was unavoidably limited, as the days when both the wind and the sea are calm in this part of the channel are very few and far between. The tower is solid for a height of 46 feet above the rock, and the base is formed of Portland cement placed in iron moulds, with iron bars driven into the solid rock to anchor the concrete firmly. On the side to which the building is exposed to the heaviest seas, massive beams of rolled steel are driven into the rock, so as to impart additional strength to the part of the tower where the greatest strains are likely to be set up. On the entrance level is a compartment containing an electric motor and air-compressor, while on the floor immediately above is a duplicate installation. The siren projects through the top of the tower, the trumpet being so turned as to throw the sounds in a horizontal direction over the water. On the top of the tower is a small automatic acetylene gas plant and light, such as the engineers have employed so successfully in their unattended Scottish light-stations, two air-receivers, and a water-tank. A new type of burner is used, and a clockwork mechanism is incorporated to extinguish the light at dawn and to ignite it at dusk, with a special arrangement to allow for the short summer nights and the long periods of darkness during the winter. [Illustration: THE FORT DOYLE SIREN. This installation on the island is maintained so as to take the place of the automatic lighthouse a mile out to sea, in the remote event of the latter breaking down.]