Chap. TII.
ON STONE.
697
ciiap. in.
OX STONE.
Great attention has been deservedly paid to the quality of the materials made use of inbuilding, and the knowledge on the subject has been greatly advanced by the mineralogistand chemist. Vitruvius asserted that all bodies consist and spring from earth, air, fire,and water, but has not given us the quantity of each that enters into their composition :these original elements, he observes, are not only indivisible, but also incapable of changeor of destruction ; this is not exactly the case, for we find that stone is acted upon chemi-cally as well as mechanically, and that it exhibits, when applied to buildings, the samedecomposition to which it is subject when attached to its native rocks. Stone is variouslyacted upon in different situations: in cities, when exposed to the influence of the smokeof coal, some varieties are rapidly decomposed; other kinds are affected by the alternationsof dryness and humidity ; particular streams rapidly destroy some limestones, and materiallyinjure the granites.
The Hmestones used by the Egyptians, Greeks, and Romans, have endured to this day :the sandstones employed in their temples and public edifices in many situations exhibit nodecomposition, and the granite of all varieties remains perfectly entire ; showing that thearchitects of antiquity knew how to select their material, and to apply it in such a mannerthat it was not subjected to any greater action by exposure than it had to encounter in itsnatural bed.
Stone Quarries. — The stone found near the surface, and which has been exposed to theaction of the atmosphere, is not so sound as that which is taken from a depth, where it hasbeen subjected to great pressure, and the greater the depth, generally speaking, the greateris its hardness and density. On opening a quarry, the first consideration is the means ofraising and delivering the stone in the least expensive manner ; an excavation is usually madein the side of a hill, in preference to vncallowing the top, in order that the road leadingfrom it should have as gentle an inclination as possible ; for if the quarry be sunk verylow, the difficulty and expense of drawing out the blocks of stone are seriously increased.
The stone is generally found in beds or masses, divided by joints, at which divisions thenatural adhesion is broken, and there is no difficulty in detaching the blocks, nor risk ofbreaking them : where vertical pressures exist, which is often the case, their removal isfacilitated ; but it is sometimes necessary to break the contiguous blocks, or blast themwith gunpowder. Wedges of steel, driven in by a heavy sledge hammer, are often used toseparate the stone from its bed: in order that the edges may not be broken, or renderedragged by removal, they are elevated to the platform or truck by an instrument called thelewis , which consists of three pieces of strong iron, formed and held together by a shackleand screw bolt; two sides are parallel to each other, or the pieces are of the same thicknessthroughout, which varies from 1^ to 3 inches, according to the weight it is destined tocarry. The two pieces outside spread out, and at the bottom are twice the thickness thatthey are above ; the middle piece is of the same size at top and bottom, consequently whenthe three pieces are put together, the width at the bottom is one third more than at thetop ; by taking out the screw-pin, and withdrawing the central piece, the two others arebrought together, and introduced into a sunk groove in the stone, cut like a dovetail, orspreading at the bottom, of a sufficient dimension to take in the whole lewis, which isplaced in separate pieces, the middle being the hist; the bolt and shackle are then attached,and the lewis now occupying the whole cavity, no force can detach it; by this meansenormous masses are lifted perpendicularly by blocks and falls, worked by a piece ofmachinery called a crab. The upper block, which sustains the weight, is attached to acrane or strong beam of timber, fixed as perpendicularly as possible, with its block over thestone, every precaution being taken to prevent its lower extremity from slipping. Thecrab is so placed as to have the draught of the rope as nearly as possible in the direction oftlie pole, the top of which is either secured or moved by guy ropes, worked also by blocksand falls fastened to the ground ; where the weights are great, two poles are sometimesused for more security.
When the stone in the quarry does not exhibit any natural joints or fissures, but appa-rently forms one compact mass, a line of holes is drilled at short and regular distances, intowhich are put conical steel pins, rather larger than the holes, which, being struck simul-taneously by the hammers of the miners, produce a separation in the direction desired.Wooden pegs are occasionally substituted, where the cleavage is easy, around which a claywall is built, filled with water ; in a short time the pegs swell and separate the stone.