the same part of the transept a square-ended chapel with a vestry
attached was added in place of the original shallow apsidal chapel.
The original chapel on the east side of the north end of the transept
was also removed to make way for another and much larger one. This is
now used as the cathedral library.

The scheme planned after the second fire having been completed by
about the middle of the thirteenth century, little further work was
undertaken in comparison with that then finished; but before 1250 the
wall of the south aisle of the nave was pierced in four bays, and two
more chapels were added. Then, on the north of the nave, the outer
wall of the aisle was cut through in the second bay, going west from
the transept, and a small chapel was built. The other chapels west of
this one were added during the latter half of the century. In each
case the deeply projecting buttresses which had been introduced
against the earlier walls after the second fire were used, where they
were available, to form parts of the masonry of these new chapels, and
were therefore not disturbed unnecessarily. The old walls having been
altered, and the earlier buttresses being changed in their nature, it
became necessary to carry the original thrust from the nave still
farther out from its source in order to find for it some satisfactory
abutment, and in doing this there was that new force, introduced by
the vaulting of these added chapels, to be reckoned with in addition.
Consequently, to the earlier buttressing more was added. The exact
nature and the approximate date of this work are shown by Professor
Willis in the sections and plan given in his monograph on the
cathedral. The addition to each buttress amounted to an elongation of
it as a pierced wing wall which provided lateral support. Upon the end
of it a greater mass of masonry was introduced to serve as a weight
for steadying the structural device; and this necessary structural