Coursed rubble, where proper stones can be obtained, is much preferable to tne other, and is more favourable to the disposition of bond timbers. The courses are of various thicknesses, adjusted by means of a gunge or sizing-rule, and the stones are either hammered, dressed, or axed.
Walls faced with squared stones, and backed with rub ble or brick, are called ashler work, and the stones them selves are called ashlers. The average size of each ashler is from 24 to 36 lilacs in length horizontally, 8 to 12 inches in breadth or depth, and 10 to 16 inches in height. The best figure for the stones of an ashler facing, is that of a truncated wedge, that is, thinner at one end than the other in the thickness of the wall, so that those in one course may form in their back parts in dentations like the teeth of a saw, the next course hav ing its indentations varied from that below it ; the whole is therefore toothed or united with the rubble backing, much more effectually than if the backs of the ashlers were parallel with the face. Bond stones should be in troduced in every course of ashler facing : they should be in quantity equal to one-sixth of the face of the wall, and of a length to reach at least one foot into the back, but the more the better. Every bond stone should, if possible, be placed in the middle between those in the course below. When the jambs of piers are coursed with ashler, or when the jambs are of one entire height, every alternate stone next the aperture in the former case, and next to the jambs in the latter, should bond through the wall ; and every other stone should be placed lengthwise, in each return of an angle, not less than the average length of an ashler. Bond stones should
have no taper in their beds, nor should their ends, or the ends of the return stones, be ever less than 12 inches. (losers should never be admitted, unless they bond at least two-thirds of the thickness of the wall. All upright joints should be square or at a right angle with the fact tor about two inches back, after which they may widen a little towards the back. The upper and lower beds of every stone should be quite level or parallel to each other for their whole breadth. All the joints, for the distance of about one inch from the face, should be cemented with fine mortar, or with a mixture of oil, putty and white lead ; the former is practised at Edinburgh, the latter at Glasgow ; at the latter place the joints of the polished ashler work are uncommonly fine and accurate. Parker's British cement will probably supersede both the former. The remainder of the ashler, and all the rubble, should be laid in good lime mortar ; that for the rubble should be made with coarser sand. All the stones should be laid in their natural beds. Wall plates should always be placed on a number of bond stones, to which they may be either juggled or fixed by iron cramps. For more copious information, see MASONRY.