Towards the middle of the twelfth century the efforts of the architects were redoubled towards the attainment of these two objects; and the advancement made, both in correcting defects in construction and refining the decorative system, were most strenuously followed up, and all improvements made were founded strictly on reason. The great constructive difficulty met with arose from the powerful outward pressure of the round arch when of great span or carrying any great load, and especially so when used in situations where it was difficult to give it any very massive abutment. The cases of failure from this cause were most frequent; so much so, that besides the numerous instances recorded of buildings wholly or in part falling from the failure of the arches, we find among the buildings still remaining abundant evidences of the insufficiency of the round arches for their load, and of the abutments to resist their pressure. In ordinary architecture we cannot, as in bridges, viaducts, etc., give our arches an unlimited abutment proportioned to the pressure, whatever it may be; we are limited in our means of doing this by innumerable causes: thus, in a central tower, if the arms of the cross have aisles, the natural abutments of the tower arches are reduced to the frail aid of a continuous arcade upon detached pillars; and even if there are no aisles, the abutting walls are perforated with windows. The abutments, again, of a chancel arch are perforated either by arches or windows, while the gable over the arch loads it heavily at its weakest point. The abutment of an arch, again, has often to impinge upon a pier at half its height, as in the case of a nave arcade abutting upon the detached piers of a central tower. In all such situations the undue pressure of the round arch was found to be most prejudicial. Still more strongly was it felt where the nave was spanned by stone vaulting. The Romans had got over this, as in the Baths of Diocletian, by breaking the continuity of the aisles by vast abutting walls across them. But in a church this was impracticable. Its uses demanded continuity of aisle and moderation in the size of the pillars. Failures often occurred from these adverse causes, and the ingenuity of the architects was naturally directed to obviating the defect.