Thus, a circular space may be intersected by four vaults of less width than the sides of a square ([Fig. 410]), leaving portions of the circular walls remaining between them.
The dome, again, may as well be segmental in section as semicircular, in which case the arches supporting it will also be segmental ([Fig. 411]). Again, the figure inscribed need not be equilateral, so that oblong compartments, such as those customary in the nave of a church, may be domically vaulted.
In all the cases which I have enumerated, I have supposed the result to be literally a portion of the original dome. As it happens, however, we have but few ancient examples of so strict an adherence to principle; though in modern works they are more frequent. The purest specimen I know (if I judge rightly from drawings) is the dome of the little church of SS. Nasario and Celso, at Ravenna, the Mausoleum of Galla Placidia, and of her two husbands, the Emperors Honorius and Valentinian II. This is a dome such as I described as standing between four walls, which intersect it in the form of arches.
Fig. 409.
Fig. 410.