Unfinished portrait of Michelangelo di Lodovico Buonarroti (circa 1545) at age seventy and attributed to Daniele da Volterra. This was painted around the time when the great artist had been appointed by Pope Paul III as capomaestro of Saint Peter’s Basilica. Note the way that Da Volterra modeled Michelangelo’s left hand, and its striking similarity to Adam’s iconic pose on the Sistine Chapel’s ceiling. This subtlety may have been an intentional tribute paid by one artist to another. Courtesy of The Metropolitan Museum of Art, Art Resource, New York.

The south elevation of Saint Peter’s Basilica. It was rendered by Étienne du Pérac in 1569 and shows Michelangelo’s design around the time of the great artist’s death. Note the hemispheric shape of his dome, which was subsequently altered by Della Porta in 1588. The decision to extend the eastern nave around 1610 ruined much of the balance within Michelangelo’s architectural intent. Courtesy of The Metropolitan Museum of Art, Art Resource, New York.

An architectural elevation of the dome soon after construction. It was rendered by Alessandro Specchi and shows the notable change in silhouette to an oval shape. This departure from Michelangelo’s design required the express approval of Pope Sixtus V in 1588. © Biblioteca Ambrosiana/ De Agostini Editore/ agefotostock.

Portrait of Luigi Vanvitelli at age sixty-nine by Giacinto Diano. By this time, in 1769, Vanvitelli was late in life and deep into his design for the Royal Palace at Caserta (Reggia di Caserta), near Naples. He was an architect to the Vatican for some twenty-five years and oversaw the dome’s repairs between 1742–48. Note the compass in his left hand, which was—and is—an instrument commonly used when drafting a set of architectural drawings. Used with permission from the Italian Minister of Culture and the Reggia di Caserta.

Michelangelo’s dome was badly fractured by the spring of 1743, and most notably within its drum and attic. Some cracks were frightening in length and depth, and “wide enough to step through.”

They were documented and numbered on paper by Luigi Vanvitelli and then presented to Benedict XIV on nineteen drawings that were entitled the Stato de’ Difetti (“the State of the Defects”). These same drawings were subsequently packaged within Giovanni Poleni’s famous book, Memorie Istoriche, four years later in 1747. Courtesy of Getty Research Institute

Many ominous separation cracks had formed beneath the dome’s lantern; they were observed along the “unstable” spiral stairway that held the inner and outer shells together. Courtesy of Getty Research Institute, Los Angeles (85-B17849).

Portrait of Thomas Le Seur, who was an avid Newtonian physicist and the lead author of Parere. © Biblioteca Ambrosiana/ De Agostini Editore/ agefotostock. below: Portrait of François Jacquier at fifty-three as rendered in the style of Laurent Pécheux. By this time, in 1764, the dome had been safely repaired. In 1739, Jacquier partnered with Thomas Le Seur to publish a commentaryto Newton’s Principia that became famously known as their “Jesuit Edition.” The book placed them among the leading Newtonians of their day. © Lyon Musee des Beaux Arts. Photo by Alain Basset.

Portrait of Roger Joseph Boscovich, one of Europe’s most notable polymaths and scientific minds of the eighteenth century. © Biblioteca Ambrosiana/De Agostini Editore/ agefotostock. below: The unassuming title page to Parere as prepared by Le Seur, Jacquier, and Boscovich in the fall of 1742. The ideas contained within their thirty-six page pamphlet had thunderous implications and led to a revolution in architectural thought. Parere invented the practice of modern engineering, with its imaginative use of applied science and mathematics to solve complex problems in the everyday world. Courtesy of the Getty Research Institute, Los Angeles (2928-606).

Portrait of the Marquis Giovanni Poleni, physicist and Italy’s leading scientist of the eighteenth century. He taught at the University of Padua for some fifty years, and created there his renowned “physics cabinet.” Courtesy of Science Source / Science Photo Library.

Title page to Memorie Istoriche which Poleni compiled and wrote between 1743–1747. His book became the model for modern forensic analytics. Courtesy of the Getty Research Institute, Los Angeles (85-B17849).

Poleni’s earliest effort to model the dome’s structural behavior was contained in his pamphlet entitled Riflessioni. It was published in March 1743, in direct response to the innovative scientific ideas outlined a few months earlier within Parere. Before these two pamphlets, thorny problems in construction were solved by subjective intuition. Here, Poleni shows the dome’s ‘hinge points’ as shown in “Tavola. H.” inside Memorie Istoriche. Courtesy of the Getty Research Institute, Los Angeles (85-B17849).

Between 1743 and 1747, Poleni continued to refine his ideas about the dome, its cracks, and its structural behavior. To do this, he borrowed heavily from the inverted catenary curve invented in the 1670s by Robert Hooke (“FIG. XI.”). Poleni presented this idea in Memorie Istoriche, though he never gave credit to Hooke. Courtesy of the Getty Research Institute, Los Angeles (85-B17849).

According to Poleni, the dome was never in peril because its thrusting force always remained within the confines of its catenary curve. He was wrong about this. Poleni prepared a visual representation of this concept in Memorie Istoriche, and it defined a new way to think about structural stability which we now know as “Limit Analysis.” Courtesy of the Getty Research Institute, Los Angeles (85-B17849).

The only known portrait of Niccola Zabaglia. Here, he is shown industriously at work inside his personal workshop at the Vatican’s munizione. (Opening plate from Castelli e Ponti, with etching by Pietro Leone Ghezzi, a noted caricaturist of the day). Courtesy of Marquand Library of Art and Archaeology, Princeton University.

Plate XXIV of Castelli e Ponti. Zabaglia designed and installed wondrous scaffolds in and around Rome and most famously at Saint Peter’s Basilica. Without these scaffolds, none of the repairs at Michelangelo’s dome could have been performed. (Etching by Francesco Restagni). Courtesy of Marquand Library of Art and Archaeology, Princeton University

Plate XXIV of Castelli e Ponti. Zabaglia designed and installed wondrous scaffolds in and around Rome and most famously at Saint Peter’s Basilica. Without these scaffolds, none of the repairs at Michelangelo’s dome could have been performed. (Etching by Francesco Restagni). Courtesy of Marquand Library of Art and Archaeology, Princeton University

Plate LVI of Castelli e Ponti. Zabaglia’s scaffolds were revolutionary in design and influenced the practice of construction for 200 years, until the advent of tubular steel framing in the 1940s. (Etching by Francesco Restagni). Courtesy of the Getty Research Institute, Los Angeles (85-B16478).

The title page to Zabaglia’s masterwork—Castelli e Ponti—which showcased his “precious talent as a ‘mechanic.’” Zabaglia spent all of his professional life at the Vatican and the book elevated his stature across Europe. It made him an influential best-selling author, though he could neither read nor write. Courtesy of Marquand Library of Art and Archaeology, Princeton University.

above and below: Poleni and Vanvitelli specified that Michelangelo’s dome be reinforced and fixed by wrapping five new iron chains around its outside shell. This mirrored the recommendations within Parere. These chains were fabricated from the highest quality iron that could be cast and were installed in 1743–1744 (“Tavola. F.—FIG XVIII” and “FIG XX,” and “Tavola. H.—FIG. XXIII”). Courtesy of the Getty Research Institute, Los Angeles (85-B17849).

The floor plate of Saint Veronica’s Pier, which was built by Bramante between 1506–1515. It lies at the southwest corner of the basilica’s crossing which encircles Saint Peter’s tomb. Note the circular stairway “D” through which many of the new iron chains may have been hoisted up and into the dome. Courtesy of the Getty Research Institute, Los Angeles (85-B17849).

The location of Poleni’s five new iron chains were shown in Memorie Istoriche. They were installed in 1743–1744 and shown as A, B, C, Z, and D. The sixth iron chain was embedded just beneath the lantern in 1748. The iron chains installed by della Porta from 1588–1590 were shown as dashed lines at locations n, u, and E. Courtesy of the Getty Research Institute, Los Angeles (85-B17849).

Portrait of Benedict XIV (Prospero Lorenzo Lambertini) at seventy-one by Pierre Hubert Subleyras. It was painted in 1746 and around the time when the dome was under repair. This pope’s forward-thinking ideas, intelligence, moral fiber, humor, wit, and warmth made him “without question, the most remarkable occupant of the throne of Saint Peter during the eighteenth century.” Courtesy of The Metropolitan Museum of Art. Image source: Art Resource, NY.

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I'm a civil and structural engineer who has designed and built all kinds of buildings, bridges, and tunnels around the world. Of them all, my most memorable was bringing an historic lighthouse out of harm's way and into safety while guarding an eroding bluff on Cape Cod National Seashore. Readers will find that my book, "Saving Michelangelo's Dome," features a similar sense of drama, daring, and uncertainty -- as well as a colorful cast of characters -- which make the world of construction so compelling.