Michael Angelo at the time of his appointment as architect, in the year 1546, was seventy-two years of age. He professed great respect for the original scheme of Bramante, yet he radically changed the form and adjustment of its main feature, the dome. In conformity with Bramante’s project, he made the drum massive at the base and thinner above, but in place of Bramante’s external colonnade surmounted by a solid ring of masonry, forming a continuous abutment at the springing of the vault, he substituted a series of sixteen isolated buttresses, and raised the dome so high above them that they do not meet its thrusts at all. The drum is carried up above the buttresses so as to form an attic over the order with which the buttresses are ornamented, and from the top of this attic the dome is sprung. The stepped circles of abutting masonry at the haunch are omitted, and instead of one solid vault shell, such as Bramante intended, Michael Angelo’s project provided for a variation of Brunelleschi’s double vault, and was to include (as the model, Fig. 26, shows) three separate shells.[44] The inner shell was to be hemispherical (Michael Angelo thus showing that he appreciated the superior character of the dome of the Pantheon and that of Bramante’s scheme to the dome of Brunelleschi as to internal effect), while the other two were to be pointed, with diverging surfaces. Following Brunelleschi, he introduced a system of enormous ribs rising over the buttresses of the drum, and converging on the opening at the crown of the vault. These ribs unite the outermost two shells, extending through the thickness of both, and support the lantern. Of this hazardous scheme only the drum was completed when Michael Angelo died. But the existing dome, which was carried out by his immediate successors, is substantially his design, though the innermost shell of the model was omitted in execution, and the vault was thus made double instead of three-fold (Fig. 30). This dome does not, however, divide into two shells from near the springing, but is carried up in one solid mass almost to the level of the haunch. Michael Angelo may have thought that this would strengthen it, but the solid part has not a form capable of much resistance to thrust, and the isolated buttresses are located so far below the springing that they contribute practically nothing to the strength of the system, as already remarked, and as we shall presently see.