"To the end that it may be easier to understand the difficulty of the vault by observing its rise from the level of the ground, let me explain that I have been forced to divide it into three vaults, corresponding to the windows below divided by pilasters; and you see that they go pyramidally into the centre of the summit of the vault, as also do the base and sides of the same. It was necessary to regulate them with an infinite number of centres, and there are in them so many changes in various directions, from point to point, that no fixed rule can be maintained. And the circles and squares that come in the middle of their deepest parts have to diminish and increase in so many directions, and to go to so many points, that it is a difficult thing to find the true method. Nevertheless, having the model, such as I make for everything, they should never have committed so great an error as to seek to regulate with one single centre all those three shells; whence it has come about that we have been obliged with shame and loss to pull down, as we are still doing, a great number of stones. The vault, with its sections and hewn stone-work, is all of travertine, like all the rest below; a thing not customary in Rome." Michelagnolo was excused by Duke Cosimo, hearing of these misfortunes, from coming to Florence; the Duke saying to him that his contentment and the continuation of S. Pietro were more dear to him than anything in the world, and that he should rest in peace. Whereupon Michelagnolo wrote to Vasari, on the same sheet in which he thanked the Duke to the best of his power and knowledge for such kindness, saying, "God give me grace that I may be able to serve him with this my poor person, for my memory and my brain are gone to await him elsewhere." The date of this letter was August in the year 1557. Thus, then, Michelagnolo learned that the Duke esteemed his life and his honour more than he did himself, who so revered him. All these things, and many more that it is not necessary to mention, we have in our possession, written in his hand.