From the similarity of the situations and the buildings upon the Edystone and Bell Rocks, and from Mr Smeaton’s celebrity as an engineer, his Narrative of the former work became a text-book to the writer in the erection of the latter. In considering the importance of this subject, with a view to the erection of the Bell Rock Light-house, the judicious remarks, and numerous experiments, of Dr Higgins on water cement, were carefully examined, and they will always be consulted with interest by the professional reader. The composition of the Edystone mortar consisted of equal portions, by measure, of Aberthaw lime and Pozzolano earth, both in the state of powder, mixed up with sea-water. When the writer first visited Plymouth in the year 1801, the Edystone Light-house had been erected about 42 years; and he was informed by Mr Tolsher, agent for the establishment, that the original pointing of the joints of that building had never required repair.
Bell Rock Mortar.
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.
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.
Pozzolano.
Pozzolano, the second mentioned ingredient of the Bell Rock mortar, is a kind of earthy lava, of a brownish red or greyish colour. It contains in the hundred parts, silica 55, alumina 20, lime 5, and oxide of iron 20. It was not so easily procured as the limestone from Aberthaw. It is very abundant on the coast of Italy and shores of Sicily, where it is found in considerable masses, and is usually imported in a crude earthy state, requiring to be pounded, or beat in a mill, to fit it for the finer purposes of mortar. It is generally brought to this country as ballast; and, in time of peace, when the ports are open, is sold for about L. 5 per ton. During the progress of the Bell Rock works, however, from the long continued and almost universal restrictions upon British trade with foreign ports, as much as L. 15 per ton has been paid for pozzolano for the use of these works. The writer having had great difficulty in procuring a supply of it for commencing the works, got a quantity of Tarras from Holland, the Dutch ports being, at this time, open to British vessels. Tarras or Trass is found near Andernach, and is brought down the Rhine to Holland. It is very similar in its nature and properties to pozzolano, and, like it, is of a reddish or greyish colour. It contains 37 parts silica, 28 alumina, 6.5 lime and 8.5 iron. Its property of setting in water is very remarkable, and, when good, it admits even a greater proportion of lime in the composition of mortar than pozzolano. The Dutch are very attentive in ascertaining the quality of the trass before using it in building their dikes. The following simple experiment is always employed. A small vessel, made of a mixture of lime and trass, is filled with water, and if at the end of three days the water does not filter through the vessel, the trass is considered good; if, on the contrary, water passes through, the trass is rejected as bad.
Sand.
According to Dr Higgins, the sand used for mortar should be free of earthy particles, and have as many sharp angular points as possible. The writer having accordingly examined the shores in the neighbourhood of Arbroath, for sand answerable to this description, he found it of excellent quality about a mile and a half westward from that place. Upon examining this sand with a magnifying-glass, its appearance was like so many small shining crystals shooting into numerous points.—It is often a difficult question with mineralogists, to account for the production of sand in many situations upon the coast; but here the question involves no perplexity, as St Andrew’s Bay, which is generally understood to extend from Fifeness to the Redhead, a distance of about 25 miles, is not only bounded by sandstone, but forms the embouchure of the river Tay, and of several other considerable streams, which fall into the sea upon its shores.
Water.