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. [Sidenote: 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. [Sidenote: Oaken Trenails and Wedges.]