then the said ties of oak must be covered with plates of iron, for the sake of the staircases. The buttresses must be all built of grey-stone and hard-stone, and all the sides of the cupola must be likewise of hard-stone and bound with the buttresses up to the height of twenty-four braccia; and from there to the top the material must be brick, or rather, spongestone, according to the decision of the builder, who must make the work as light as he is able. A passage must be made on the outside above the windows, forming a gallery below, with an open parapet two braccia in height, proportionately to those of the little tribunes below; or rather, two passages, one above the other, resting on a richly adorned cornice, with the upper passage uncovered. The rain water must flow from the cupola into a gutter of marble, a third of a braccio wide, and must run off through outlets made of hard-stone below the gutter. Eight ribs of marble must be made at the angles in the outer surface of the cupola, of such thickness as may be required, rising one braccio above the cupola, with a cornice above by way of roof, two braccia wide, to serve as gable and eaves to the whole; and these ribs must rise pyramidically from their base up to the summit. The two vaults of the cupola must be built in the manner described above, without framework, up to the height of thirty braccia, and from that point upwards in the manner recommended by those masters who will have the building of them, since practice teaches us what course to pursue."