At the east and west ends of the nave the two cylindrical vaults are each forty-seven feet across. They rest on the four lesser piers, and have an abutment of fifteen and a half feet. The four exedras are each forty-one feet across. All the conchs and semidomes have drums outside, which are pierced by the windows. The conchs which cover the exedras have strong arches, where they intersect the semidomes. The weight of the exedra conchs is chiefly supported by the columns; the upper columns of the south-east exedra, at the time of the last restoration, were much inclined, and had to be brought back to the vertical, by propping the arches, cutting away the old bases, and inserting new pieces--the columns being surrounded and supported by wooden cradling. The thickness of the western barrel vault is four feet; the eastern apse is about three feet thick. The western semidome received an additional thickness at the restoration. _Vaulting of the Aisles._--The three principal divisions of each aisle are covered by domical vaults. The vault arches rest partly on columns; and the spaces between these columns and the outside wall are also vaulted. The middle division of the north and south aisles has two domical vaults, separated by a barrel vault that opens towards the nave arches, and to the window in the outside wall. The arches have iron ties four inches thick, which stretch from the outer wall to the columns of the nave, and grip them tightly. The four columns in the aisles which carry the vault are much lower than those between the aisles and nave, and for this reason the narrow vaulted space, which joins the aisle vault to the nave arcade, is formed by a stilted quadrant. This arrangement only applies to the lower aisles: above is a stilted cylindrical vault, running lengthways between the main gynaeceum vaults, and the arcade towards the nave.[254] Here, besides the iron ties, there are wooden beams. The large arches in the aisles are twenty-nine and a half feet from column to column. The domical vaulting of the aisles is very flat--a combination of cross groining and a dome. For, though it starts with angles at the four corners, it gradually merges into a dome at the apex. The vaulting bricks are arranged in horizontal circles.[255] A diagonal band of mosaic starts from each corner, and merges into a central circle.[256] In the gynaeceum the vaulting is higher and consists of spherical domes, the radii being half the diagonals of the spaces covered. The mosaic decoration here again follows the form.[257]