The mechanical operations at the Bell Rock, during the year 1814, consisted principally in fitting up a thunder-rod or electrical conductor, on the exterior and western side of the building, as may be observed on the elevation of the Light-house in [Plate XVI.] This rod is continuous from the top to the foundation of the building, being connected with the sash-frames of the light-room, and the Railways upon the Rock. The best construction and application of rods for conducting the electric fluid, is a problem upon which scientific men are not altogether agreed; some being of opinion, that the conducting-surface should be large; while the slender bell-wires in dwelling-houses are found capable of conveying it in a body sufficient to destroy the premises. The fixing of a plate of any considerable breadth upon the exterior of the Light-house would have been attended with much difficulty; and the writer having consulted Professors Playfair and Leslie, and Dr Brewster, a rod was prepared by Mr Adie, optician, 2¼ inches in breadth, and, where thickest, 1 inch, the external side being slightly rounded, to suit the circle of the wall, and thus presenting the least possible resistance to the sea. The composition of this rod was one ounce and a half of tin to a pound of pure copper; and with its bats, screws, and connecting pieces, it weighed 556 pounds, or about one-quarter of a ton.
Method of fixing it.
In order to fix it to the building, a groove was cut in the exterior wall, as high as the solid part, about half an inch in depth, which was sufficient to receive the edge or thinnest part of the rod. On the higher parts of the building the wall was not cut, the flat side of the rod being simply applied to it; and in its track, brass-bats, previously perforated and screwed, were sunk in the wall to the depth of three inches, and run up with melted lead, at about two feet apart. The rod being cast in lengths of seven feet, the pieces were successively applied to the building, when the precise positions of the holes in the brass-bats were ascertained, marked on the rod, and bored on the spot for screw-nails of ⅝ths of an inch in diameter; the several lengths of the rod being half checked at each end, and the nail-heads counter-sunk, as shewn in [Plate XIX.] Fig. 10. The cutting of this groove and the bat-holes in the masonry, was done in a very masterly manner by the late Thomas Selkirk, of whom notice is here more particularly taken, as this was the last piece of work which he executed, as well as from his having been acknowledged by his companions to have been one of the best stone-cutters in Scotland.
1814.
A party of the Commissioners and Sir Walter Scott visit the Light-house.
On the 30th of July this year, Mr Hamilton, Mr Erskine, and Mr Duff, Commissioners, along with Mr (now Sir) Walter Scott, and the writer, visited the Light-house; the Commissioners being then on one of their voyages of Inspection, noticed in the Introduction. They breakfasted in the library, when Sir Walter, at the entreaty of the party, upon inscribing his name in the album, added the interesting lines, of which the reader will find a fac-simile on the second title-page.
1815.
Permanent Railways began to be fitted.
Lord President Hope lands at the Rock.