Note 117 — Chapter III

'The dome stands within a polygon of sixteen sides, that rises four metres above the springing line, keeping the dome taut and weighting the haunches. Against this polygonal casing are set buttresses formed by the extension of the piers of the octagon to within m. 11⁄2 from the cornice of the dome. These buttresses are in their turn respectively strengthened, on the rear, by two small buttresses; of which those on the north, south, east, and west sides rest on an arch of the gynecaeum, and carry the thrust to the outer walls of the church, while the others rest on the exhedrae and the vaulting of the gynecaeum. Furthermore, from the summit of the buttresses formed by the piers of the octagon a small buttress is set against the cupola itself up to the cornice.' This marshalling of the buttresses around the dome in three tiers, while securing the stability of the structure, is moreover strikingly artistic. See Fig. 21.