It usually forms a condition in the Specification of mortar for house-building, that it shall be mixed with pure water, free of earthy or saline particles. In situations where water is scarce, many impurities are apt to be mixed with it, which injure and even destroy the adhesive quality of the mortar. The use of sea or salt water is guarded against in these cases, from its liability to produce an efflorescence on the walls; when the saline particles deliquesce with the changes of the weather, and produce the appearance of dampness.
To have attempted to avoid the use of salt-water in the preparation of the mortar for the Bell Rock Light-house,—which was all prepared on the spot,—independently of the risk of deteriorating the mortar, would have been attended with much additional trouble and expence. Besides the practice at the Edystone, the writer had previously ascertained, that the use of sea-water produced no bad effect upon the tenacity or adhesive quality of the mortar into which it was introduced, and the object of avoiding the appearance of dampness in this building was extremely trifling. The stones were to be very correctly jointed, and the whole of the interior walls to consist of polished masonry, so that the fine lines of the joints exposed to the action of the air were so inconsiderable, as hardly to be taken into account. Salt-water was, therefore, uniformly used in the preparation of the Bell Rock mortar.
Cement.
The recent discovery of a very excellent water-cement, for which Mr Parker of London obtained a patent, under the title of “Roman Cement,” became another matter of importance to the Bell Rock works. This substance is produced from calcined nodules of argillaceous limestone, found upon the southern shores of England. It is of a brownish colour, and from its excellent property of setting in water, when good and fresh, its application as a mortar, for the lower courses of the Light-house, demanded attention. But, for general use as a mortar, it would not only have been expensive, but often highly inconvenient in building, from the speedy manner in which it hardens. It is also of too brittle a nature to be suitable for the general purposes of common mortar; though it forms a paste of great value for lipping or pointing the outward joints of water buildings, not only by preserving the mortar till it gets into a fixed state, but also as forming a durable joint. A considerable supply of this cement was accordingly used throughout the building, for pointing the exterior joints. This cement is sold at the rate of five shillings per Winchester bushel, in the state of powder, packed into casks, lined with paper, to prevent it, in some measure, from imbibing humidity from the atmosphere, by which its adhesive properties are destroyed.
Oaken Trenails and Wedges.
Following out the principle of the Edystone Light-house in most of its details, the oaken trenail and wedge were used for fixing the stones, till the mortar took band, and a superincumbent weight was got upon them to prevent the sea from sweeping them away. These being also introduced into all the lower courses of the Bell Rock Light-house, a sufficient quantity was procured for the probable number of courses that might be built during the ensuing season. The precise lengths of the trenails and wedges could not be fixed, from the uncertainty of the granite quarries, which regulated the thickness of the courses of the building; but, for the present, the trenails were provided of the length of 2 feet, and 1¼th inch in diameter. The wedges were of the length of 18 inches, measuring 3 inches in breadth, 1 inch in thickness at the top, and tapering to ¼th of an inch in thickness at the point, as will be seen in [Plate X.] Figs. 10. and 11. But when we come to speak of the process of building, their respective uses will be described.
1808, March.
The writer visits the Bell Rock.
On the writer’s visit to Arbroath, in the end of March, he was anxious to land upon the Bell Rock, to ascertain the precise state of the Beacon, after the storms of the winter, that he might be enabled to judge of the propriety of converting it into a habitable place for the artificers during the working season. He accordingly sailed from Arbroath on the 30th current, at 1 A. M. in the Light-house Yacht. The wind was from E.NE., but the weather, though cold, was upon the whole favourable for the trip. At 7 the Floating-light was hailed, and all on board found to be well. It was now unfortunately too late in the tide for landing upon the rock this morning; and it became necessary to cruise about till the following day, there being at this season only one tide with daylight. In the mean time, in sailing round the rock, just as it was beginning to be covered with the tide, the base of the Beacon was distinctly seen between the rolling seas, which broke upon it; while at the top, the flag-staff proudly continued to surmount the whole.
Floating-Light.