vertical plane of the walls. The other series was the immediate
counter-poise to any direct thrust exerted by the arching of the vault
against the upper section of the same walls. There was, in fact, a
large buttress added to support these nave walls at that point from
which each set of vault-carrying ribs began to rise. This buttress,
though apparently sub-divided, was one thing, but of composite
structure. It was pierced first by the aisle, next by the triforium,
and then again above the roof of the triforium. It will be seen that
most of these alterations were the direct result of the introduction
of a stone vault. But the almost entire renewal of the eastern part
of the cathedral was made possible by the destruction and total
removal of the apsidal terminations of the earlier work. It has been
suggested that the fire may have so badly damaged this portion as to
allow no alternative but rebuilding. What may have been the actual
cause of its removal it is impossible for us now to know; but the
substitute is quite a perfect piece of work of its kind. This
ambulatory, or presbytery, as it is commonly misnamed, was nearly all
newly built from the foundations during the first half of the
thirteenth century. The continuation of the arcade, the triforium, the
clerestory, and the vault, the vaulting of the aisles and the chapels
forming their terminations eastwards,--all this, with the new arch at
the entrance to the earlier lady-chapel, was work of the same date.

[Illustration: PIER-CAPITALS IN THE RETRO-CHOIR. _From a photograph by
S.B. Bolas & Co_.]

Some new buttressing had been added to the south-west tower when the
upper part of the tower itself was rebuilt; but the larger works were
the addition of a vaulted sacristy in the corner between the west side
of the south end of the transept and the nave. On the opposite side of