From 2 feet 6 inches to 4 feet, is a good length for the face or soffit of the archstones. When they are longer, as the beds can scarcely ever be worked and set exactly true, they are apt to break when the weight comes upon them; and when shorter, there is not sufficient space to overlap or break the joints properly. Each course should be of equal thickness quite through between the headers. The thickness of each course should be from one-third to one-half their depth, and they should be chamfered or rusticated along the bed joints, and also those of the outside heads. The beds should be worked as true as possible for the whole breadth of each stone, the neglect of which destroys every other precaution. Each stone should be laid so as just to swim in the mortar, and be struck with a maul two or three good blows. The joints of the headers should be of equal thickness with those of the other stones in the same course. Inexperienced masons, by laying the headers with thinner joints for show of fine work, frequently create an unequal pressure, which bursts or splinters the headers, before the interior archstones come to an equal bearing.
The French cramp the headers with iron to the inte rior archstones ; but if the masonry is good, this seems totally unnecessary, nor is it practised in Britain.
In setting the archstones, care should be taken to keep each course pointing in the direction of the ra dius ; and in order to enable the workmen to do this cor rectly, the thickness of each course should be marked upon the outer ribs, and its line of direction upon the lower part of the beams of the same ribs. Attention must also be had to carry on the courses equally on each side of the centre, and also to carry up masonry over the solid part of each pier in the spandrels. If this is not attended to, too much weight on one side will de range the form of the centre; and there is sometimes a necessity to lay some temporary weight upon the crown of the centre, until the load approaches the middle. In case of more arches than one, and the arches flat, care must be had not to expose one side of the pier until it has a sufficient weight upon it, or is guarded by resistance on the opposite side. At the bridge of ?Iantz, by a ne glect of this sort, one of the piers was pushed 4-1- inches out of the upright. It was afterwards by loading the op
posite side, made to return 2y inches.
The keystones should be driven to fill their places firmly, but not so as to require much force, otherwise they will derange the rest of the joints, and alter the shape of the arch. As soon as the keys are driven, all the back and end joints of the whole arch should be care fully examined ; and if from the drying of the mortar, or defect of any of the stones, some vacancies appear, they should be run full of mortar, and firmly wedged with slates; the whole should then be left for some time to dry and indurate. The length of this time must depend upon the state of the weather, the qualities of the stone, and also the mortar. While this op, ration is going on, the masonry should be brought up in the spandrels to the level of about of the rise of the arch. This may be of rubble work, but it should be laid closely, in regu lar courses, in good lime mortar, and abutting firmly against the archstones. The outside stones should, in the part over the pier, be carried up to the same height; but immediately adjacent to the archstones, they should be stepped or racked back, and left so until the centre is removed, because if finished close up to the back of the archstones, the least sinking of the arch would cause a fissure. The spandrels having been carried up to the proper height, and the mortar having acquired a proper consistence, the centre may be cased and removed in the way formerly described.
The centres having been removed, the soffit of the arch should be carefully examined, and the joints if ne cessary, pared, cleaned out, and pointed with mortar. The advantage arising from chamfered or rustic joints will now appear: they prevent the edges from chipping, and cover any trifling inequality, so that it is the cross joints only which require paring : the champherecl joints also give an appearance of lightness to the archstones. It is sometimes the practice to lay around the arch a thin course of stone, plain or moulded on the edge, which projects a few inches below the face of the arch stones. This becomes necessary if the headers have any twist, because it admits of the spandrels being set correctly straight, without showing the twist or other defect.