“Each floor stone forms part of the outward walls, extending inwards to a centre stone, independently of which they are connected by means of copper bats, with a view to preserve their square form at the extremity, instead of dovetailing. These stones are also modelled with joggles, sidewise, upon the principles of the common floor, termed feathering in carpentry, and also with dovetailed joggles across the joints, where they form part of the outward wall.... The floors of the Eddystone Lighthouse, on the contrary, were constructed of an arch form, and the haunches of the arches bound with chains to prevent their pressing outward, to the injury of the walls. In this, Mr. Smeaton followed the construction of the Dome of St Paul’s; and this mode might also be found necessary at the Eddystone, from the want of stones in one length, to form the outward wall and floor, in the then state of the granite quarries of Cornwall. At Mylnefield Quarry, however, there was no difficulty in procuring stones of the requisite dimensions; and the writer foresaw many advantages that would arise from having the stones of the floors to form part of the outward walls, without introducing the system of arching.”
Smeaton in fact adopted an arched form for the floors of his building, which rendered it necessary, in order to counteract the outward thrust, to insert chains, embedded in grooves, cut in the masonry; but Mr. Stevenson, in designing the Bell Rock Lighthouse, improved on Smeaton’s plan, not only by a better general arrangement of the masonry, but by converting the floors into effective bonds, so that, instead of exerting an outward thrust, they actually tie or bind the walls together. This is at once apparent from [Figs. 1] and [2], which show the floor-courses of the Eddystone and Bell Rock in section.
Fig. 1.—Eddystone.
Fig. 2.—Bell Rock.
The engineer of the Bell Rock had all the advantage of Smeaton’s earlier experience, which he ever thankfully acknowledged; but there can be no doubt whatever that the Bell Rock presented peculiar engineering difficulties. The Eddystone Rock is barely covered by the tide at high water, while the Bell Rock is barely uncovered at LOW WATER, rendering the time of working on it, as we shall afterwards find, extremely limited; and the proposal to erect a stone tower on this low-lying isolated reef, at a distance of twelve miles from land, was no less remarkable for its novelty than for its boldness.
PLATE I.
BELL ROCK LIGHT HOUSE.