of greater thickness. Stone is used only in special cases, as in
the main piers of S. Sophia, but monolithic marble columns are an
important part of the structure. In the later churches stone is used in
courses with the bricks to give a banded effect, and herring-bone,
diamond, and radiating patterns are frequently introduced. The palace of
the Porphyrogenitus, the parecclesion of the Pammakaristos, and Bogdan
Serai, exhibit this style of work. As illustrations of the method
adopted in the construction of walls the following measurements may be
given, the sizes being in centimetres:

+---------------------------------------+----------+--------------+
| | Brick. | Joint. |
+---------------------------------------+----------+--------------+
| Parecclesion of the Pammakaristos | .08 | .04 |
| 4 courses brick, 5 joints | .46 | --- |
| S. John in Trullo | .03 | .07 to .09 |
| Refectory of the Monastery of Manuel | .04 | .04 to .06 |
| 4 course stone, 3 joints | .78 | --- |
| 4 courses brick, 5 joints | .30 | --- |
| | { .0375 | .052 |
| Bogdan Serai | { .035 | .035 |
| | { .04 | .04 |
| 4 courses stone, 8 joints | --- | .55 to .60 |
| 4 courses brick, 5 joints | --- | .43 to .47 |
| Sanjakdar, brick | .045 | --- |
+---------------------------------------+----------+--------------+

_Building Procedure._--The first step in the erection of a building was
to obtain the necessary marble columns with their capitals and bases.
These seem to have been largely supplied ready made, and Constantinople
was a great centre for the manufacture and export of stock architectural
features. Then the main walls were built in brick, the columns were
inserted as required, the vaults were thrown, and the whole building was
left to settle down. Owing to the enormous amount of mortar used this
settling must have been very considerable, and explains why hardly a
plumb wall exists in Constantinople, and why so many vaults show a
pronounced sinking in at the crown or have fallen in and have been
rebuilt. After the walls had set the marble facin