_Domes._--In elaborating his theory of Byzantine dome construction Choisy refers to a passage in Eton’s _Turkish Empire_[343] which describes domes the latter saw built without any kind of centring. The builders put a post in the middle about the height of the walls. To this is fixed a pole reaching to the _inside_ surface of the dome, which is _free to move in all directions_. Below is attached to the post another pole, which reaches to the outside and describes the outside curvature of the cupola. These give the thickness at the top and bottom and at every intermediate point. “Where they build these cupolas of bricks they use gypsum instead of lime, finishing one layer all round before they begin another. Scaffolding is only required for the workmen to close the opening at the top.” Our diagram A, Fig. 42, represents this fascinating scheme of building: with such a rod any point in the whole curvature is defined in a moment; it equally gauges the horizontal courses and the rise of the dome. Choisy suggests a second scheme which will be made clear by B. There is no reason, he points out, why the beds of the bricks in a dome should radiate to the centre of the curve: in the Byzantine domes the beds were flattened so that they radiated more or less accurately to the springing of the opposite side of the dome. The thrusts were thus minimised, and the construction was facilitated. If rods forming a triangle revolve about a vertical post as shown, the horizontal curvature is gauged and the top rod will define the slope for the bed. These rods can then be raised to another position as shown in the figure. We should have supposed that little care would be taken with the slope of the beds, as from the thin bricks used the construction practically became homogeneous. Choisy even thinks that the great dome of S. Sophia may have been built in the air without centring. C, in Fig. 42, gives his representation of the construction of the semidomes, which he thinks were built out some way entirely without support. The outer arch was then built on a centre and the filling completed “in space” (a straight joint between the arch and the dome filling is shown in the figure in Salzenberg’s text). We think it more likely that in all the larger domes auxiliary support was required “to close the opening at the top,” when the space had been so contracted that a light centring resting on the part already completed was all that would be needed.