Of the ingredients of the Bell Rock mortar, as the pozzolano was not only the most expensive, but, from the distracted state of Europe, during the progress of the works, could hardly be procured at any price, it became an object to be as independent of that article as possible. A train of experiments was therefore undertaken by the writer, when it was ultimately found, that pozzolano and lime, in the state of a dry impalpable powder, and clean sharp sand, in equal proportions by measure, mixed with sea-water, formed a mortar equally good in all respects as when no sand was added. Under favourable circumstances, this mixture did not seem to be more tardy in fixing, than when the sand was excluded, while nothing could exceed the compact and indurated state of the composition. The writer accordingly built some small rubble-walls with it within sea-mark, which were allowed to stand for a few months, and when pulled down, they appeared like so many pieces of conglomerated rock, the mortar being as hard and compact as the sandstone of good quality with which these little walls were built. From the excellency of the Bell Rock mortar, it may not be amiss to go a little farther into detail, by noticing each of the ingredients of which it was composed. [Sidenote: Lime.] In the course of investigating this subject, specimens of lime, from the counties of Edinburgh, Haddington, Fife and Forfar, were subjected to various trials with mixtures of pozzolano and sand. The results were not a little curious, as the experimental balls, made with different proportions of these limes, did not set or harden; but, on the contrary, the particles seemed to repel each other as the mixture became heated, and ultimately crumbled into its constituent ingredients. From these experiments, it was found advisable to bring a cargo of limestone from Aberthaw in Wales, being the same as that used for the Edystone Light-house. This lime is found imbedded in a clayey matrix, in the state of water-worn nodules, varying in size from a cubic inch to that of a cubic foot. This limestone is of a bluish or beautiful French grey colour, of the specific gravity of 2.70. It is easily calcined, and in that state is reducible to the finest powder. It is the mountain or first flœtz limestone of geologists; and, when broken, it pretty generally displays the _Cornua ammonis_, and many other curious animal remains. This limestone is found in great abundance on the sea-shore at Aberthaw, where the softer matters being washed away from the lower stratum of certain high cliffs, containing these rounded masses, the upper parts fall in great quantities, from which the succeeding tides wash away the earthy matters, leaving the limestone upon the beach in the state of debris. When a vessel is to load limestone here, she is grounded on the shore at about half-tide, and loaded when the water recedes. The price paid to the proprietor for a cargo is at the rate of one shilling _per_ ton, as a lordship.