de of the apse.[540]

_The original plan of the church_ (Fig. 102). The greater part of the
alterations made in the church date from Byzantine times, and the marble
coverings then placed upon the walls have effectually covered up any
traces which might have given a clue to the original form of the
building. In consequence any attempt at restoration must be of a very
tentative character.

It is evident that there has been a serious movement in the structure
due to the weight of the dome and the thrust of the dome arches, for the
walls below the dome are bent outwards in a very pronounced manner. It
was in order to check this movement that the flying buttress was applied
to the apse, and in all probability the enormous thickness of the walls
surrounding the central cross is due to the same cause. Had the walls
originally been as thick as at present it is hard to imagine that
movement could have taken place.

The axial line from east to west, passing through the doors of both
narthexes, divides the present building into two dissimilar parts. We
know that the parecclesion is a later addition, and if it be removed and
the plan of the north side repeated to the south the resulting plan
bears a striking resemblance to S. Sophia at Salonica (Fig. 101). The
position of the prothesis and diaconicon in particular is identical in
the two churches.

Some proof that this was the original form of the building is given by
the small chamber in the wall thickness between the church and the
parecclesion. For it corresponds to the angle of the south 'aisle,' and
on its west wall is a vertical break in the masonry which may be the
jamb of the old door to the narthex.

This plan gives a narthex in five bays--the three centre ones low, the
two outer covered by domes and leading to the 'aisles.' When the
parecclesion was added, the south gallery and two bays of the inner
narthex were swept away. The third door leading into the church was
built up, and the present large domed bay added to the shortened
narthex.

[Illustration: FIG. 101.--S. SOPHIA, SALONICA.]

Traces of the older structure remain in the wall between the church and
the parecclesion. The space already described, which originally opened
from the passage at the higher level to the south cross-arm, corresponds
in width both to the window above and to the space occupied by the doors
below. At S. Sophia, Salonica, the side-arms are filled in with arcades
in two stories form