Walls faced with squared stones, hewn or rubbed, and backed with ruble stone or brick, are called ashler. The medium size of each ashler measures horizontally in the face of the wall about 28 or 30 inches, in the altitude one foot, and in the thickness 8 or 9 inches. The best figures of stones for an ashler facing are formed like truncated wedges; that is to say, they are thinner at one end than at the other in the thickness of the wall, so that when the stones of one course, or a part of a course, are shaped in this manner, and alike situated to each other,"the back of the course will form an indention like the teeth of a joiner's saw, but more shallow, in proportion to the length of a tooth ; the next course has its indentations formed the same way, andlhe stones so selected, that the upright joints break upon the solid of the stones below.
By these means, the facing and backing are toothed together, and unquestionably stronger than if the back of each ashler had been parallel to the front surface of the wall ; as the stones are mostly raised in quarries of various thicknesses, in an ashler facing, it would greatly contribute suppose a wall 40 feet high, 2 feet thick ; to have a sufficient foundation at 3 feet in breadth, what should be the breadth of a foundation of a wall 60 feet high, 21 feet thick : by proportion it will be 40X2 : 3 :: 60 x 2i : the ans. = qfeet. This calculation will give the breadth of the foundation of the required wall equal to the breadth of the insisting wall itself; when the height of the required wall is equal to the ratio, which is the first term 40 X 2 = 80, divided by the second term 3, that is— 80 3' Thus a wall of 3 feet would have the breadth of its foun dation equal to its thickness above the foundation, and less than 261 feet would have a thinner foundation than even the superstructure. But though the calcula tion in this case gives the foundation less breadth than the thickness of its super structure, it must be considered, that it only calculates the true breadth of sur face that should be opposed to the ground, in order to prevent the wall from penetration by its weight : though the rule gives all the breadth that is necessary, on account of the weignt of the insisting wall, yet the breadth of the substructure should always be greater than that of the superstructure ; as it will stand more firmly on its base when affected by later al pressure, and be less liable to rock by the blowing of heavy winds. The least
breadth that is commonly given to the substructure of stone walls is one foot thicker than the superstructure. In damp foundations, the superstructure should always be separated from the sub structure by lead, tarred paper, or other means.
Stone arch. Stone arch is a number of stones so arranged,that, in consequence of their pressure upon one another and upon their supports, they may be suspended over a hollow space ; every interior stone being such, that, if a plummet be depend ed by a line from any point in that stone, the line will fall within the hollow space.
Stone arches are generally hollow be low, and concave towards that hollow. The interior stones ought to be truncated wedges, and their faces, which fcrm the intrados, of less dimensions than the up per opposite surfaces which form the ex trados : so that when any stone endea vours to descend through the aperture which surrounds it, it will be prevented by the dimensions of the lower part of the aperture being less than those of the top of the stone which has to fall through it.
Wedge-like stones forming an arch are arch stones.
The joints between the arch stones are called sommerings.
The support or supports of an arch are called the reins of that arch.
When the support or supports of an arch are stone walls, the upper course or courses, on which the beds of the ex treme arch stones rest, are called the imposts.
Spring course, or chaptrels of the arch, are called the butments or abutments, or spring beds, or skew backs of the arch.
When an arch is either recessed in any piece of masonry, or forms the head of an aperture through that piece of mason ry, the arch stones, which are common to the intrados and to the face of the mason ry, are called voussoirs, and the middle voussoir is called the key-stone.
Stone arches are used for a variety of purposes, in supporting different parts of a building, over apertures, when the apertures are too wide for lintelling, and over a wooden or stone lintel, to assist in supporting the superincumbent build ing.
Arches are also used to prop the sides of a building, and in soft foundations in verted arches are used, between narrow piers, to prevent the pier from penetrat ing, by opposing a greater surface of re. Instance to the ground.
Floors and roofs are frequently sup ported with arches, in order to render the building more secure from fire.