The dome at first sight seems to rest upon four arches each of 100 feet span; it is, however, only on the east and west that these arches are open. From north to south the main piers are 106 feet apart, and their breadth in this direction is fifteen feet eight inches; but on either side of the nave there are projections, narrowing the opening to 100 feet, and giving the open arches abutments of eighteen feet eight inches. Behind each of these main piers again, at a distance of twenty-nine and a half feet from them, stands one of the buttress or staircase piers, which, including the outside wall, is seventeen feet four inches by twenty-four and a half feet in area. Round arches, which appear below the vaults, transmit the thrust of the great arches from the main piers to these buttressing piers. Above these each of the immense buttress masses which stand right across the aisles, and rise to within eighteen feet of the springing of the dome, bear upon two relieving arches of different radii, so as not to load the vaulting beneath.[250] The cylindrical arches, which, at the ground-floor aisles and the gynaeceum, connect the great piers with the outer buttress piers, are each reinforced by two extra arches, standing on stone additions to the main piers, from which they project five feet.[251] These arches, though thus strengthened, are almost all out of shape; those by the two northern main piers have been pushed out nearly fifteen inches. A drawing given in Salzenberg’s text shows the south arch which supports the dome with the mosaics removed. The piers from east to west are seventy-two feet apart, and accordingly the span of the arch is seventy-two feet, its soffite being fifteen feet eight inches. The arch is five feet deep, formed of two unconnected rings, and on each side the lower part is laid in horizontal courses so that the portion with radiating joints is only three quarters of the whole arch. The window wall which fills the arch opening is four feet thick, and is bonded with the horizontal courses, but a movement of the arches has caused a fissure, which is shown in the diagram. These window walls on the south and north sides have cracked in several places. The upper part of the window wall on the north side is only twenty-nine inches thick. The windows have been reduced and strengthened by inserting stone jambs. On the north and south side are also two large arches, which project on the inside three feet from the window wall and rest on the main piers, having the same height and span as the arches on the east and west. They complete the square form under the cornice of the dome, and give the idea that the dome is carried on four arches of 100 feet span: whereas in reality, as has just been shown, the real supporting arches on the north and south side are concealed in the window wall, and are not suggested in any way in the interior decoration, being only visible on the outside.[252]