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Laocoön (which would be rediscovered there in 1506), and mechanical wonders such as pipes concealed in the ceiling of the dining hall that sprayed perfumes on the emperor’s dinner guests. Its most interesting architectural feature, however, is an octagonal room in the east wing that is roofed by a dome whose span is some 34 feet across. The octagonal shape must have interested Filippo, who would have known, of course, that the dome of Santa Maria del Fiore, though much larger, was also intended to be eight-sided.
    Of even more interest to Filippo would have been the Pantheon, the emperor Hadrian’s temple to the gods of all the planets, executed between A.D. 118 and 128. Unlike the octagonal cupola in the Domus Aurea, the dome of the Pantheon is colossal, spanning 142 feet internally and rising to a height of 143 feet. Almost thirteen centuries after its construction it was still the largest dome ever built, and it had escaped plunder because it was now converted into a church, Santa Maria Rotonda. The modern Romans and pilgrims alike were amazed by the immense dome. With no visible signs of support, it seemed to defy the laws of nature. They called it the “house of devils,” attributing its construction not to the skilled engineers of the ancient world but rather to the sinister forces of demons.
    What structural features of this “house of devils” might Filippo have studied? The architects of the Pantheon faced the statical problems encountered by builders of all domes: how to counteract the forces that act on any vault. These forces are separated into “push” and “pull” energies, known respectively as compression and tension. All elements in a building — its columns, arches, walls, roof beams — are subject to one or other of these actions: their stone or timber beams are compressed from above (which causes them to shorten) or pulled from the side (which causes them to stretch). An architect must design a structure that will counteract these pressures by playing them off against each other — a game of action and reaction — and channeling them safely to the ground.

    The Pantheon.
    The first type of pressure does not create insurmountable problems for an architect. Stone, brick, and concrete all possess such enormous compressive strengths that buildings can be raised to colossal heights without the blocks of stone crushing at the base. The tallest spire in England, that of Salisbury Cathedral, stands 404 feet high, and the two towers of the cathedral in Cologne each rise to 511 feet, or the equivalent of a fifty-story building. At this height they are almost a dozen feet taller than the Great Pyramid at Giza, another edifice whose tremendous size is made possible by the strength of the stone blocks from which it was built. Yet not even these soaring structures come close to exhausting the compressive strength of stone: a column of limestone could be built to a height of 12,000 feet, or over two miles high, before starting to crush under its own weight.
    The stones in a dome, however, are not only crushed from above but also thrust outward by the pull energy known as “hoop stress,” in the same way as the rubber of an inflated balloon will bulge outward if one compresses it from above. The problem for architects is that stone and brick do not respond nearly so well to this lateral thrust as they do to compression.
    The Romans seem to have possessed some understanding of the structural problems created by tension and compression, and they attempted to solve them by making extensive use of the new pozzolana concrete. Where the horizontal stress is greatest, at the base of the dome, the concrete wall of the Pantheon was built to a massive thickness of 23 feet. It then tapers to only two feet at the top, at which point a round window or “oculus” is left open. Five thousand tons of concrete were poured in horizontal layers on to wooden formwork, but at the top of the dome lightweight aggregates such