Haghia Sophia - Strolling Through Istanbul: The Classic Guide to the City - page 41

We learn from the Book of Ceremonies that in the days of Byzantium
the imperial chamberlains called praepositii stood there, and we are
reminded once again of the great antiquity of this place.
The first and abiding impression created by the interior of Haghia
Sophia is that of a vast contained space, pierced by shafts of moted
sunlight. Walking forward, we can now see the whole of the immense
interior at once and appreciate its beauty and its grandeur: the fabled
dome, which the ancients pictured as being suspended from heaven
by a golden chain; the enormous expanse of the nave, its central area
flanked by the graceful two-tiered colonnade which Procopius likened
to a line of dancers in a chorus; all elements of the vast structure
interrelated in perfect harmony.
Justinian and his architects, Anthemius and Isidorus, chose to
disregard the plan or the earlier churches on this site and, indeed,
all earlier plans of which any trace or record has come down to us.
The essential structure of their astonishingly original building can be
briefly described. Four enormous and irregularly shaped piers, built of
ashlar stone bound together with lead, stand in a square approximately
31 metres on a side. From these piers rise four great arches between
which four pendentives make the transition from square to circle.
Upon the cornice of the circle so formed rests the slightly elliptical
dome, of which the east-west diameter is about 31 metres, the north-
south diameter about 33 metres, the crown being 56 metres above
the floor - that is, about the height of a 15-storey building. The dome
has 40 ribs which radiate out from the crown, separated at the base by
40 windows, of which four towards the west were blocked up during
repairs in the tenth century. To east and west, pairs of subsidiary piers
support the two great semidomes, each with five windows, which give
the nave its vast length, a full 80 metres. The central arches to north
and south are filled with tympanum walls pierced by 12 windows,
seven in the lower row, five in the upper, of which the three central
ones originally formed a kind of triple arcade. All these windows
have in Turkish times been considerably reduced in size, probably by
the architect Sinan in the sixteenth century. Between the great piers
on the north and south, four monolithic columns of verd antique
support the galleries, while above six columns of the same type carry