But soon after the completion of the new church the central tower fell, seriously damaging all the adjacent parts, which were rebuilt, including one stage of the tower, by 1248. The foundations, however, on the wet site caused further trouble, and it was not till 1866 that they were properly laid and the tower secured by the late Sir (then Mr.) G. G. Scott. In 1248 an earthquake shook the building in a serious manner, and probably started or aggravated the curious outward inclination of the nave arcade, which can be seen in the illustration, p. 22. The fall of the tower also necessitated alterations in the buildings which had just been erected to the east of the crossing, and no doubt the earthquake also prevented any attempt to complete the stone groining of the nave, for about the year 1500 huge buttresses were added on the north or river side to prevent further settlements. Bishop Martyn (1296-1328) completed De Leiâ’s plan by adding the Lady Chapel; but his immediate successor, Bishop Gower (1328-1347), has, more surely than any prelate--not excepting De Leiâ--left his impress on the present buildings. Under his direction a stage was added to the tower; the south porch was built; the walls of the aisles were raised to their present height and preparations made for vaulting; the eastern chapel to the south transept and alterations to those east of the north transept were carried out; the aisles received Decorated windows and their walls buttresses; and the very remarkable Rood-screen and its adjuncts were then first added. Of his splendid episcopal palace across the Alan more anon. [Illustration: FROM SPEED’S MAP OF PEMBROKE.] In the Perpendicular period the principal alterations were the renewal of the main roofs; the addition of the huge buttresses on the north side of the nave; the vaulting of the chapel just east of the Presbytery, and the addition of one more stage to the tower. It should be noted that this was the only period in which the difficulties of vaulting were overcome, although extensive preparations for a sexpartite system had been made.