The 18th-Century Engineers Who Saved Michelangelo’s Dome At St. Peter’s From Collapsing


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Saving Michelangelo’s Dome: How Three Mathematicians and a Pope Sparked an Architectural Revolution by Wayne Kalayjian; Pegasus Books, 288 pp., $29.95 

By law, no building in Rome’s city center may exceed the height of St. Peter’s Basilica. Its immense, distinctive dome towers above the city’s Seven Hills, dominating the horizon from as far away as the Villa d’Este in Tivoli, nearly 25 miles to the east. Like the Colosseum, St. Peter’s now seems as eternal as the Eternal City itself, yet when Pope Benedict XIV sent a team of three mathematicians to inspect the dome in 1742, they found, as Benedict suspected they might, a cataclysm waiting to happen: 350 feet above ground level, the colossal structure was riddled with cracks, many clearly visible from the floor of the basilica, others noticeable only within its maze of interior staircases, still others hidden beneath the lead plates that sheathe the dome’s exterior. Swiftly, the trio gave the pope a 64-page report with a detailed (and terrifying) map of its manifold fissures, plus recommendations, fortified by a series of calculations, on how to bring the building back into balance.

By its very title, Wayne Kalayjian’s Saving Michelangelo’s Dome anticipates how the story ends, as does the Roman skyline: St. Peter’s still stands. But the story of why it still stands is a marvelous tale, full of vivid characters whom Kalayjian portrays with gusto, beginning with an enlightened pope and the three mathematicians, disciples of Isaac Newton, who believed that their discipline could solve problems of engineering before they arose. Prior to implementing their plans, Pope Benedict sought a series of second opinions from luminaries such as Diego de Revillas, chair of the department of mathematics at the University of Rome; Giovanni Gaetano Bottari, prefect of the Vatican Library; Nicola Salvi, designer of the Trevi Fountain; the architects Ferdinando Fuga and Luigi Vanvitelli; and Francesco Olivieri, director of the Fabbrica di San Pietro, the office that has cared for every aspect of the basilica since its inception. Most of these wise heads found it odd to apply abstract calculation to the practicalities of construction, their suspicions possibly compounded by the foreign nationalities of the three mathematicians, though they all lived and worked in Rome: the Jesuit Roger Joseph Boscovich was born in Dalmatia; the Franciscans François Jacquier and Thomas Le Seur were French. Despite the controversy they stirred up with their report, Pope Benedict’s foreign experts exerted a permanent shift in the way builders approached construction; in effect, as Kalayjian asserts, the science of engineering was born under the reign of the same pontiff who ushered the Enlightenment into Rome.

Benedict swiftly assigned responsibility for St. Peter’s to the brave souls whose sky-high repairs would save the basilica for posterity. An engineer by profession, Kalayjian can pinpoint exactly what ailed the building they set out to stabilize in 1742: Giacomo della Porta’s distinctive pointed cupola, surmounted by a ball of gilded bronze, was too heavy for the dome beneath to support it properly. It had therefore begun to sink, pushing the dome itself outward against the huge iron chains that held its curved surface in place, the way hoops hold a barrel. (To provide an idea of the stresses involved, one of those chains had stretched 18 inches over the course of a century.) At the same time, the whole structure had begun to twist, perhaps after a series of 18th-century earthquakes, perhaps because of the inherent instability of the foundation, which had rested on a combination of alluvial soil and volcanic bluff (the Vatican Hill) ever since Constantine built the first St. Peter’s c. 318. Kalayjian emphasizes that no one person was responsible for the design of the present basilica, which took more than a century to build and includes fundamental contributions by Donato Bramante, Michelangelo, Giacomo della Porta, and Carlo Maderno. Preserving the rickety dome would depend on yet another collective effort, its participants drawn from every level of 18th-century Italian society, each offering an essential, carefully cultivated skill. An epic human balancing act was required to remedy a design originally entrusted to trial and error, intuition, experience, and often, improvisation. Michelangelo, for example, had been compelled to take down the first courses of the dome designed by his predecessor Antonio da Sangallo the Younger because they would never have worked, but Giacomo della Porta faced a similar problem with Michelangelo’s design for the dome and gave it a taller profile. Hence what we see today is really della Porta’s creation rather than Michelangelo’s. In fact, Giacomo della Porta may be the most underrated architect of early modern Rome because it fell to him to finish so many of Michelangelo’s late-life projects.

An epic human balancing act was required to remedy a design originally entrusted to trial and error, intuition, experience, and often, improvisation.

Kalayjian is a deft hand at describing both the thrilling cultural currents animating 18th-century Italy, from Newtonian physics to neoclassical art, and the colorful individuals who provided the convivial fizz for that effervescent culture. The Venetian physicist Giovanni Poleni, invited to take a look at the dome, immediately struck up a close friendship with his designated guide, the Roman architect Luigi Vanvitelli, whose father, the Dutch-born painter Gaspar van Wittel, had Italianized his name as soon as he gained Roman citizenship. (One of van Wittel’s enchanting views of Rome adorns the cover of Kalayjian’s book.) It was this genial pair who took final charge of saving the Roman skyline, together with Nicola Zabaglia, the man who invented the ingenious scaffolding that allowed repairs to take place on high without interrupting the basilica’s day-to-day functions. Zabaglia was nearly 80 when he joined the project, with 65 years of experience at the Vatican under his belt. The huge mobile platforms he invented to bring workmen up to the dome were sturdy enough to handle the six new gigantic iron chains that would ultimately be embedded in its walls to clamp it into a kind of colossal corset. Zabaglia was also a stickler for workers’ safety (his long life its own testament to his success); amazingly, he staged (that is, set up the logistics for) the dangerous, unprecedented project without any serious accidents. By December 1744, 18 months after Benedict had sent out his first trio of experts, the sixth and last chain was fitted into place. Ironically, Poleni and Vanvitelli had arrived at essentially the same solution those three roundly criticized mathematicians had suggested in 1742. Today, the six rings of chains still hold Rome’s tallest building together so that the once-menacing cupola can continue to cast its gentle light over the city, a trusty beacon in the nighttime sky.

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