The Life of Michelangelo Buonarroti · Chapter 69IX
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Down to the last days of his life, Michelangelo was thus worried with the jealousies excited by his superintendence of the building at S. Peter's; and when he passed to the majority, he had not secured his heart's desire, to wit, that the fabric should be forced to retain the form he had designed for it. This was his own fault. Popes might issue briefs to the effect that his plans should be followed; but when it was discovered that, during his lifetime, he kept the builders in ignorance of his intentions, and that he left no working models fit for use, except in the case of the cupola, a free course was opened for every kind of innovation. So it came to pass that subsequent architects changed the essential features of his design by adding what might be called a nave, or, in other words, by substituting the Latin for the Greek cross in the ground-plan. He intended to front the mass of the edifice with a majestic colonnade, giving externally to one limb of the Greek cross a rectangular salience corresponding to its three semicircular apses. From this decastyle colonnade projected a tetrastyle portico, which introduced the people ascending from a flight of steps to a gigantic portal. The portal opened on the church, and all the glory of the dome was visible when they approached the sanctuary. Externally, according to his conception, the cupola dominated and crowned the edifice when viewed from a moderate or a greater distance. The cupola was the integral and vital feature of the structure. By producing one limb of the cross into a nave, destroying the colonnade and portico, and erecting a huge façade of barocco design, his followers threw the interior effect of the cupola into a subordinate position, and externally crushed it out of view, except at a great distance. In like manner they dealt with every particular of his plan. As an old writer has remarked: "The cross which Michelangelo made Greek is now Latin; and if it be thus with the essential form, judge ye of the details!" It was not exactly their fault, but rather that of the master, who chose to work by drawings and small clay models, from which no accurate conception of his thought could be derived by lesser craftsmen.
We cannot, therefore, regard S. Peter's in its present state as the creation of Buonarroti's genius. As a building, it is open to criticism at every point. In spite of its richness and overwhelming size, no architect of merit gives it approbation. It is vast without being really great, magnificent without touching the heart, proudly but not harmoniously ordered. The one redeeming feature in the structure is the cupola; and that is the one thing which Michelangelo bequeathed to the intelligence of his successors. The curve which it describes finds no phrase of language to express its grace. It is neither ellipse nor parabola nor section of the circle, but an inspiration of creative fancy. It outsoars in vital force, in elegance of form, the dome of the Pantheon and the dome of Brunelleschi, upon which it was actually modelled. As a French architect, adverse to Michelangelo, has remarked: "This portion is simple, noble, grand. It is an unparalleled idea, and the author of this marvellous cupola had the right to be proud of the thought which controlled his pencil when he traced it." An English critic, no less adverse to the Italian style, is forced to admit that architecture "has seldom produced a more magnificent object" than the cupola, "if its bad connection with the building is overlooked." He also adds that, internally, "the sublime concave" of this immense dome is the one redeeming feature of S. Peter's.
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, 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 Michelangelo's lifetime and with his approval. There is good reason to believe that this may have been the case: first, because the tambour, which he executed, differs from the model in the arching of its windows; secondly, because Fontana and other early writers on the cupola insist strongly on the fact that Michelangelo's own plans were strictly followed, although they never allude to the third or innermost vault. It is almost incredible that if Della Porta departed in so vital a point from Michelangelo's design, no notice should have been taken of the fact. On the other hand, the tradition that Della Porta improved the curve of the cupola by making the spring upward from the attic more abrupt, is due probably to the discrepancy between the internal aspects of the model and the dome itself. The actual truth is that the cupola in its curve and its dimensions corresponds accurately to the proportions of the double outer vaulting of the model.
Taking, then, Vasari's statement in conjunction with the silence of Fontana, Poleni, and other early writers, and duly observing the care with which the proportions of the dome have been preserved, I think we may safely conclude that Michelangelo himself abandoned the third or semi-spherical vault, and that the cupola, as it exists, ought to be ascribed entirely to his conception. It is, in fact, the only portion of the basilica which remains as he designed it.