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, 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.