In an ashlar Ewing, the stones generally run from twenty eight to thirty- inches in length, twelve inches in height, and eight or Dille inches in thickness. Although both the upper and lower beds of an ashlar, as well as the vertical should be at right angles to the face of the stone, and the fa.•e-bed and vertical joints at right angles to the beds in an ashlar racing, whei.e the stones run nearly of the same thick ness, it is of some advantage, in respect of bond, that the back of the stone be inclined to the thee, and that all the backs thus inclined should run in the same direction, as this gives a small degree of lap in the setting of the next course ; whereas, it the backs were parallel to the front, there could be no lap where the stones run of an equal depth in the thickness of the wall. It is of sonic advantage likewise to stones, so that a thicker one and a thinner one may fidlow each other alternately. The disposition of the stone in the next superior course should follow the same order as in the inferior course, and every vertical joint should 11111 as nearly as possible in the middle of the stone below.
In every course of ashlar facing, with brick or rubble back ing, thorough-stones, (as they are technically termed,) be introduced, and their number should be proportioned to the length of the course, and every such stone of a superior course should fall in the middle of every two similar stones in the course below ; this disposition of bonds should be strictly attended to in all long courses. Some wailers, in order to show or demonstrate that they have introduced suffi cient bonds in their work, choose their bond-stones of greater length than the thickness of the wall, and knock or cut off their ends afterwards. This method is far from being eligi ble, as the wall is not only liable to be shaken by the force applied to break the end of the stone, but the stone itself is apt to be split.
In every pier where the jambs are coursed with the ashlar in front, every alternate jamb .stone ought to go through the wall, with its beds perfectly level. If the jamb-stones are of one entire height, as is frequently the case when architraves arc wrought upon them, and upon the lintel crowning them, in the stones at the ends of the course: of the pier which are to adjoin the architrave jamb, every alternate stone ought to be a thorough-stone; and if' the piers between the apertures be very narrow, no other bond-stones will be necessary in such short courses. But where the piers are wide, the number of bond-stones must be proportioned to the space ; thorough-stones must be particularly attended to in the long courses below and above windows.
Bond-stones should have their sides parallel, and of course be perpendicular to each other, and their horizontal dimen sion in the face of the work should never be less than the vertical one. All the vertical joints, after receding about three quarters of an inch from the face with a close joint, should widen gradually to the back, and thereby form hollow wedge-like figures for the reception of mortar and packing. The adjoining stones should have their beds and vertical joints tilled with oil putty from the thee to about three-quarters of an inch inwards, and the remaining part of the beds with well-prepared mortar. Putty cement will stand longer than most stones, and will even remain prominent when the stone itself is in a state of dilapidation from the influence of the corroding power of the atmosphere. It is true that in all newty-built walls cemented with oil putty, the first appear ance of the ashlar work is rather unsightly, owing to the oil of the putty disseminating itself into the adjoining stones, which makes the joints appear dirty and irregular: but this disagreeable effect is soon removed ; and if care has been taken to make the colour of the putty suitable to that of the stone, the joints will hardly appear, and the whole work will seem as if one piece. This is the practice in Glasgow, but in London and in Edinburgh fine water putty is used instead of it.
All the stones of an ashlar facing should be laid on their beds. From a neglect of this precaution, the stones frequently flush at the joints, and this disposition of the lamina; sooner admits the corrosive power of the atmosphere to take eaet.
In building walls or insulated pillars of very small horizon tal dimensions, every stone should have its bed level, and without any concavity in the middle ; because if the bed: are concave, when the pillars begin to sustain the weight of the fabric, the joints will in all probability flush. It ought like wise to be observed, that every course of masonry of such piers ought to consist of one stone.
An arch, in masonry, is a part of a building suspended over a given plan, supported only at the extremities, and concave towards the plan.
The supports of an arch are called the spring walls.
The whole of the under surface of the arch opposite to the plan is called the intrados of the arch ; and the upper surface is called the extrados.
The boundary line or lines of the intrados or those com mon to the supports and the intrados, are called the springing lines of the arch.