and prints a carefully executed copperplate engraving of its section.  It is clear from this work that at some time or other a third semi-spherical vault, corresponding to that of the Pantheon, had been contemplated.  This would have been structurally of no value, and would have masked the two upper shells, which at present crown the edifice.  The model shows that the dome itself was from the first intended to be composed of two solid vaults of masonry, in the space between which ran the staircase leading to the lantern.  The lower and flatter shell, which appears also in the model, had no connection with the substantial portions of the edifice.  It was an addition, perhaps an afterthought, designed possibly to serve as a ground for surface-decoration, or to provide an alternative scheme for the completion of the dome.  Had Michelangelo really planned this innermost sheath, we could not credit him with the soaring sweep upwards of the mighty dome, its height and lightness, luminosity and space.  The roof that met the eye internally would have been considerably lower and tamer, superfluous in the construction of the church, and bearing no right relation to the external curves of the vaulting.  There would, moreover, have been a long dark funnel leading to the lantern.  Heath Wilson would then have been justified in certain critical conclusions which may here be stated in his own words.  “According to Michelangelo’s idea, the cupola was formed of three vaults over each other.  Apparently the inner one was intended to repeat the curves of the Pantheon, whilst the outer one was destined to give height and majesty to the building externally.  The central vault, more pyramidal in form, was constructed to bear the weight of the lantern, and approached in form the dome of the Cathedral at Florence by Brunelleschi.  Judging by the model, he meant the outer dome to be of wood, thus anticipating the construction of Sir Christopher Wren.”  Farther on, he adds that the architects who carried out the work “omitted entirely the inner lower vault, evidently to give height internally, and made the external cupola of brick as well as the internal; and, to prevent it expanding, had recourse to encircling chains of iron, which bind it at the weakest parts of the curve.”  These chains, it may be mentioned parenthetically, were strengthened by Poleni, after the lapse of some years, when the second of the two shells showed some signs of cracking.

From Dr. Durm’s minute description of the cupola, there seems to be no doubt about the existence of this third vault in Michelangelo’s wooden model.  He says that the two outer shells are carved out of one piece of wood, while the third or innermost is made of another piece, which has been inserted.  The sunk or hollow compartments, which form the laquear of this depressed vault, differ considerably in shape and arrangement from those which were adopted when it was finally rejected.  The question now remains, whether the semi-spherical shell was abandoned during