Stones

in military architecture, may be distinguished into two sorts; that is, into hard and soft: hard stone is that which is exposed to the open air, such as rocks, and which lie loose upon the surface of the earth: the soft stone is that which is found in quarries, and underground. It is undoubtedly true that the hardest stones make the most durable works; but as there is seldom a sufficient quantity to build the whole fortification, the best serve in the facings of the work, in the foundations, and where the works are exposed to the violence of the waves. The stones of some quarries are very soft, and easily worked, when first cut out; but, when exposed for some time to the open air, become very hard and durable. As there is undoubtedly a kind of sap in stones as well as in timber, by which the same sort of stone, taken out of the same quarry, at one season, will moulder away in a few winters, but, when dug out in another season, will resist the weather for many ages: stones should always be dug in the spring, that they may have time to dry before the cold weather comes in; for the heat of the sun will extract the greatest part of the moisture, which otherwise expands in frosty weather, and causes the stone to splinter, although it be otherwise hard and good. As stones lie in the quarries in horizontal beds or strata, (that is, they cleave in that direction) and have likewise a breaking vein, which is perpendicular to the former; both these directions must be observed in cleaving, as well as in raising them out of their beds. Stones that will not easily cleave must be blown up by gunpowder. Marble , is of various sorts and colors; the most beautiful of which is exported from Italy. The marble found in England is mostly blackish, and so very hard and difficult to polish, that very little use is made of it, except to burn and make lime. The American marbles are various, and every day produces new discoveries of marbles of the most beautiful colors. Fire - Stone , or Soap Stone , serves chiefly for chimneys, hearths, ovens, furnaces, and stoves; being a dry, porous, gritty stone, which bears the heat without breaking: on account of this quality, it is called fire-stone . Purbeck - Stone , is a hard, greyish stone, and serves chiefly for paving, coping of walls, and for all such other uses where strength is required, it being the most hard and durable stone. Rag - Stone , is of a bluish color, and commonly used in paving: but there is a stone called Kentish rag , that is very useful in building: it splits very easily, and yet is very hard. Free - Stone , more generally called Portland stone : it is a fine whitish stone, without any veins. This stone is very soft when it comes out of the quarry, is easy to be worked, and becomes very hard in time. Hence it is very fit for military works. Gypsum , is a clear whitish stone, not unlike coarse marble. It is plentiful in some parts of Italy; in France; and very abundant in Nova Scotia, whence it has been lately imported to a vast amount to be pulverised for manure; it is to be had in great abundance in Scotland, and makes the very best lime. Whin , or Aberdeen whin , is of a greyish color, intermixed with veins, not unlike coarse marble. This stone is the fittest of any for military works: because it withstands the weather, and the violence of the waves, better than any stone found in England.