Michelangelo’s reputation, not only as an imaginative builder, but also as a practical engineer in architecture, depends in a very large measure upon the cupola of S. Peter’s.  It is, therefore, of great importance to ascertain exactly how far the dome in its present form belongs to his conception.  Fortunately for his reputation, we still possess the wooden model constructed under his inspection by a man called Giovanni Franzese.  It shows that subsequent architects, especially Giacomo della Porta, upon whom the task fell of raising the vaults and lantern from the point where Michelangelo left the building, that is, from the summit of the drum, departed in no essential particular from his design.  Della Porta omitted one feature, however, of Michelangelo’s plan, which would have added greatly to the dignity and elegance of the exterior.  The model shows that the entablature of the drum broke into projections above each of the buttresses.  Upon these projections or consoles Buonarroti intended to place statues of saints.  He also connected their pedestals with the spring of the vault by a series of inverted curves sweeping upwards along the height of the shallow attic.  The omission of these details not only weakened the support given to the arches of the dome, but it also lent a stilted effect to the cupola by abruptly separating the perpendicular lines of the drum and attic from the segment of the vaulting.  This is an error which could even now be repaired, if any enterprising Pope undertook to complete the plan of the model.  It may, indeed, be questioned whether the omission was not due to the difficulty of getting so many colossal statues adequately finished at a period when the fabric still remained imperfect in more essential parts.

Vasari, who lived in close intimacy with Michelangelo, and undoubtedly was familiar with the model, gives a confused but very minute description of the building.  It is clear from this that the dome was designed with two shells, both of which were to be made of carefully selected bricks, the space between them being applied to the purpose of an interior staircase.  The dormer windows in the outer sheath not only broke the surface of the vault, but also served to light this passage to the lantern.  Vasari’s description squares with the model, now preserved in a chamber of the Vatican basilica, and also with the present fabric.

It would not have been necessary to dwell at greater length upon the vaulting here but for difficulties which still surround the criticism of this salient feature of S. Peter’s.  Gotti published two plans of the cupola, which were made for him, he says, from accurate measurements of the model taken by Cavaliere Cesare Castelli, Lieut.-Col. of Engineers.  The section drawing shows three shells instead of two, the innermost or lowest being flattened out like the vault of the Pantheon.  Professor Josef Durm, in his essay upon the Domes of Florence and S. Peter’s, gives a minute description of the model for the latter,