Floors and roofs are frequently sup ported with arches, in order to render the building more secure from fire.
Arches employed for several of these purposes have been demoninated as fol lows : those over wooden lintels have been called occult discharging arches, or arches of discharge ; those used to prop the sides of a building are called arch boutants, or flying buttresses ; and those over apertures, the intrados of which are horizontal planes, have been absurdly called straight arches ; it is only for the property of its radiating joints this last is called an arch.
Because the courses in every kind of masonry ought to be horizontal, or the nearest position to it that the nature of the arch will admit of, in stone arching, it follows, that when the intrados is a ro tative figure, with a vertical axis, the coursing joints will be conic surfaces, and their intersections upon the intrados ho rizontal circles, and the transverse joints will be planes tending to the axis when the axis is horizontal, the coursing joints will be planes tending to the axis, and the transverse joints will be either ver tical circular rings, or conic surfaces, having the same common axis with the intrados.
Stone Walls. Stone walls are those built of stone, with or without cement in the joints ; the bedding joints have most commonly a horizontal position in the face of the work ; and this ought always to be the case, when the top of a wall ter minates in a horizontal plane or line. In bridge building, and in the masonry of fence walls, upon inclined surfaces, the bedding joints on the face sometimes fol low the direction of the top or terminat ing surface.
The footings of stone walls ought to be constructed of large stones, which, if not naturally near the square from the quar ry, should be reduced by hammer dress ing to that form, and to equal thick ness in the same course ; for, if the beds of the stones of the foundation are suffer ed to taper, the superstructure will be apt to give way, by resting upon angles or points ; or upon inclined sur faces the footings ought to be well bed ded upon each other with mortar, and all the upright joints of an upper footing should break joint ; that is, they should fall upon the solid of the stones below, and not upon the joint.
The following are methods practised in laying the footings of a stone foundation when walls are thin, and stones can be got conveniently, that their length may reach across each footing from one side of the wall to the other, the setting of each course with whole stones in the thickness of the wall should be preferred.
But when the walls are thicker, and bond stones in part can only be conveniently procured, then every other succeeding stone in the course may be a whole stone in the thickness of the wall ; and every other interval may consist of two stones in the breadth of the footing; this is plac ing the header and stretcher alternately, like Flemish bond in nine-inch brickwork. But when bond stones cannot be had con veniently, every alternate stone should be in length two-thirds of the breadth of the footing upon the same side of the wall ; then upon the other side of the wall a stone of one-third of the breadth of the footing should be placed opposite to one of and one of two-thirds op. posite to one of one third ; so that the stones may be placed in the same man ner as those of the other side.
In broad foundations, where stones can not be procured for a length equal to two thirds of the foundation, then build them alternately, with the joints on the upper bed of each footing, so that the joint of every two stones may fall as nearly as pos sible in the middle of the length of the one, or each adjoining stone ; observing to dispose the stones alike on each side of every footing. A wall, the superstruc ture of which is built of unhewn stone laid in mortar, is called a ruble wall. They are of two kinds, coursed and uncoursed. The most common kind of ruble is the uncoursed, of which the greater part of the stones is crude, as they came out of the quarry, and the rest hammer dressed. This kind of walling is very inconvenient for the building of bond timbers ; but if they are to be preferred to plugging, the hacking must be levelled in every height in which the bond timbers are disposed. The best kind of ruble is the coursed; the courses are all of accidental thicknesses, adjusted by a sizing rule, as the slating of a roof; the stones are either hammer dressed or axed. This kind of work is favourable for the disposition of bond tim bers : but as all buildings, constructed either in whole or in part of timber, are liable to be burnt, strong well built walls should never be bound with timber, but should rather be plugged ; for if such ac cident take place, the walls will be less liable to warp.