[Sidenote: 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. [Sidenote: 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. [Sidenote: Water.]