A very curious and bold specimen of mediæval engineering may now be mentioned. If the western bays of the great transept next to the central tower be examined, it will be seen that in the clerestory and in the triforium these bays are exceedingly narrow, but that in the ground-story the narrow arch corresponding to the narrow bays of the triforium and clerestory has been moved, and a wider arch substituted. The fact is, when the transept was completed, the Norman nave, which was still standing, had a very narrow aisle. Consequently the builders of the transept built a pair of narrow arches, on either side of the central tower, leading from the transept into the Norman aisle. Later on, as we have seen, the Norman nave and aisles were pulled down, and the present nave was built. Its aisles are exceptionally broad. The result was that the piers of the two narrow arches found themselves in the very middle of the new broad aisles of the nave—a most awkward obstruction to processions passing from aisle to transept, or _vice-versa_. So the triforium and clerestory was underpinned, the Lancet piers next to the tower were taken down and then put up again clear of the aisles, and the two arches also on either side of the tower were taken down and rebuilt with the same stones, each in the place of the other. The result is that, counting from the end-walls of the transepts, in the triforium and clerestory there are three of the bays wide and one narrow, while in the ground-story there are two wide, one narrow, and one wide bay. Similar changes took place on the eastern side of the transept when the choir was built. Later on, when the central tower was built, its great weight sank the piers on which it rests eight inches into the ground, and the adjoining masonry was dislocated. The result was that it was found necessary to rebuild the third piers on both sides of both transepts (counting from the end-walls) as well as the first pier on the western side of the north transept, and at the same time to block up all the four narrow arches with solid masonry (cf. CARLISLE, and EXETER). IV. The west front of the nave was not finished till the very end of the Curvilinear period, to which period belong also the treasury and sacristy, and also Archbishop Zouch’s chapel, unless the latter was rebuilt _c._ 1396. [Illustration: CHAPTER-HOUSE ARCADE.]