the buttresses, with strong cramps of oak, whereby the buttresses by which the inner vault is supported will be bound and strengthened; these fastenings of oak shall then be covered with plates of iron, on account of the staircases. The buttresses are all to be built of _macigno_, or other hard stone, and the walls of the cupola are, in like manner, to be all of solid stone bound to the buttresses to the height of twenty-four braccia, and thence upward they shall be constructed of brick or of spongite (spugne), as shall be determined on by the masters who build it, they using that which they consider lightest. On the outside, a passage or gallery shall be made above the windows, which below shall form a terrace, with an open parapet or balustrade two braccia high, after the manner of those of the lower tribunes, and forming two galleries, one over the other, placed on a richly decorated cornice, the upper gallery being covered. The rain-water shall be carried off the cupola by means of a marble channel, one third of an ell broad, the water being discharged at an outlet to be constructed of hard stone (pietra forte), beneath the channel. Eight ribs of marble shall be formed on the angles of the external surface of the cupola, of such thickness as may be requisite; these shall rise to the height of one braccia above the cupola, with cornices projecting in the manner of a roof, two braccia broad, that the summit may be complete, and sufficiently furnished with eaves and channels on every side; and these must have the form of the pyramid, from their base, or point of junction, to their extremity. Thus the cupola shall be constructed after the method described above, and without framework, to the height of thirty braccia, and from that height upwards, it may be continued after such manner as shall be determined on by the masters who may have to build it, since practice teaches us by what methods to proceed.'