itectural
requirements. The square plan and the enormous size of the dome in S.
Sophia demanded the great buttresses on the sides; while in SS. Sergius
and Bacchus the eight buttresses show only on the outside of the dome
and are not carried over the aisles as they are in S. Sophia. Below the
roof the arches and piers of the galleries and aisles are arranged so as
to carry the thrust to the external walls, and following the tradition
of Roman vaulting all buttressing is internal. In S. Irene, where the
true drum dome first appears, the buttresses between the windows of the
dome still remain, though much reduced in size. A dome raised on a drum
can evidently no longer exercise a thrust against the dome-arches; its
thrust must be taken by the drum, and only its weight can rest on the
arches.

The weight of the drum and dome rests on the pendentives and
dome-arches. Their thrust is neutralized by the use of ties and by the
barrel vaults of the cross arms, and these in their turn depend on the
thickness of the walls. The lower buildings attached to the church in
the form of side-chapels and the narthex also helped to stiffen and
buttress the cross walls. The system is by no means perfect in these
late churches. It was apparently found impossible to construct drum
domes of any size, except at the extreme risk of their falling in, and
probably it is for this reason that many of the larger domes in late
churches, like SS. Peter and Mark, S. Theodosia, the Chora, have fallen.
No system of chainage appears to have been used for domes in
Constantinople.

Flying buttresses probably of the ninth century are used at the west end
of S. Sophia. The double-flying buttress to the apse of the Chora does
not bond with the building and is certainly not original. It may be set
down as part of the Byzantine restoration of the church in the
fourteenth century. In any case, such external flying abutments are
alien to the spirit of Byzantine architecture, and may be regarded as an
importation from the West. Flying buttresses, it may here be noted, are
not uncommon in the great mosques of the city. They are found in Sultan
Bayazid, Rustem Pasha, Sultan Selim, the Suleimanieh, and the Shahzade.
But they are generally trifling in size, and are rather ornaments than
serious attempts to buttress the dome.

_Walls._--The walls of the earlier churches are built of large thin
bricks laid with mortar joints at least as thick as the bricks, and
often