But while taking the scheme of the Baptistery as the basis of his own scheme, Brunelleschi was obliged to make some daring changes in order to give his design the external character which he sought. This great dome (Plate I), like that of the Baptistery, is octagonal in plan and pointed in elevation. It rises from the top of the octagonal drum, and consists of two nearly concentric shells of masonry, with an interval between them. Eight vast ribs of stone rise from the angles of the drum and converge on the curb of an opening at the crown. These ribs extend in depth through the whole thickness of the double vault and unite its two shells. Between each pair of these great ribs two lesser ones are inserted within the interval that divides the two shells, and nine arches of masonry, lying in planes normal to the curve, are sprung between the great ribs and pass through the lesser ones on each side of the polygon (Figs. 7 and 8), while a chain of heavy timbers (_a_, Fig. 8, and Fig. 9), in twenty-four sections clamped together at the ends with plates of iron, binds the whole system between the haunch and the springing. So much of the internal structure can be seen in the monument itself, but further details are described in Brunelleschi’s own account of what he intended to do.[13] From this we learn that the base of the dome, which was to be built solid to the height of 5¼ braccia, was to consist of six courses of long blocks of hard stone (_macigno_) clamped with tinned iron and upon this were to be chains of iron.[14] Mention is also made of a chain of iron over the timber chain (“in su dette quercie una catena di ferro”); but no such chain is visible in the monument, and if it exists, it must be embedded in the masonry of the vault, like the chains at the base. [Illustration: FIG. 8.--Section.] [Illustration: FIG. 9.--Part Plan.] It will thus be seen that while Brunelleschi’s scheme is essentially different from that of the Baptistery, its structural system is little more than an ingenious modification of it. The parts of the one answer to those of the other with singular completeness. The attic wall and pyramidal roof of San Giovanni are transformed into the external shell of the cathedral dome, the angle buttresses of the older monument become the great angle ribs of Brunelleschi’s vault. The intermediate abutments of the Baptistery are changed into the intermediate ribs of the great dome, and the inclined barrel vaults of the Baptistery scheme are represented in the cathedral dome by the arches sprung between the great angle ribs.