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 to the strength of the work to select the stones in each course, so that every alter nate ashler may have broader beds than those of every ashler placed in each alter nate interval. In every course of ashler facing bond stones should be introduced, and their number should be proportioned to the length of the course. This should be strictly attended to in long ranges of stones, both in walls without apertures, and in the courses that form wide piers ; when they are wide, every bond stone of one course should fall in the middle of every two bond stones in the course be low. In every pier where the jambs are coursed with the ashler, and also in every pier where the jambs are one entire height, every alternate stone next to the aperture in the former case, and every al ternate stone next to the jambs in the lat ter case, should bond through the wall ; and also every other stone should be plac ed lengthwise, in each return of an angle, not less than the average length of an ash ler. Bond stones should have no taper in their beds; the end of every bond stone, as well as the end of every return stone, should never be less than a foot. There should be no such thing as a closer per mitted, unless it bond through the wall. All the uprights, or joints, should be square, or at right angles to the front of the wall, and may recede about iths of an inch from the face, from thence gradu ally widen to the back, and thereby make hollow, wedge-formed figures, which will give sufficient cavities for the reception of packing and mortar.
Both the upper and lower beds of every stone should be quite level, and not form acute angles, as is often the case ; the joints from the face to about iths of an inch within the wall should be either cemented with fine mortar, or with a mix ture of oil, putty, and white lead : the for mer is the practice both in London and Edinburgh, and the latter in Glasgow. The putty cement will stand longer than most stones, and will be prominent when the face of the stones has been corroded with age. The whole of the ashler, ex cept that mentioned of the joints toward the face of the wall, the ruble work, and the core, should be set and laid in the best mortar, and every stone laid on its natural bed.
All wall-plates should be placed upon a number of bond stones, and particularly those of the roof; by which means they May either be joggled upon the bonds, or fastened to them by iron and lead. In building walls or insulated pillars of very short horizontal dimensions, not exceed ing a length of stones that can be easily procured, every stone should be quite level on the bed, without any degree of concavity, and should be one entire piece between every two horizontal joints: This should be particularly attended to on piers, where the insisting weight is great, otherwise the stones will be in danger of splintering and crushing to pieces, and perhaps occasion a total demolition of the fabric. Vitruvius has left us an account of the manner of the construction of the walls of the ancients, which were as fol lows : the reticulated is that wherein the joints run in parallel lines, making angles of 43° each with the horizon in contrary ways, and consequently the faces of the stones form squares, of which one diago nal is horizontal, and the other vertical. This kind of wall was much used by the Romans in his time. The incertain wall was formed of stones, of which the one direction of the joints was horizontal, and the other vertical ; but the vertical joints of the alternate courses were not always arranged in the same straight line : all that they regarded, was to make them break joint. This manner of walling was used by the Romans in times antecedent to the time of Vitruvius. Vitruvius di rects, that in both the reticulated and in certain walls, instead of filling up the spaces between the sides with ruble pro miscuously, they should be strengthened with abutments of hewn stone or bricks, or common flints, built in walls two feet high, and hound to the front with cramps of iron. The emplection consisted of two sides or shells of squared stone, with alternate joints, and a ruble core in the middle.