The installation of Foucault’s Pendulum at the Conservatoire des Arts et Métiers becomes, within the fabricated Templar Plan, the crucial conversion of a scientific instrument into a supposed instrument of secret knowledge. The three researchers make its actual placement at Saint-Martin-des-Champs support an invented chain of historical continuity, then imagine the Pendulum marking the world’s Umbilicus—the point from which the Templars could begin to conquer the world. The episode shows how a real object can be made to carry a meaning it does not establish by itself.

Historical Placement and the Problem of Timing

Casaubon first resists the proposed connection. The Pendulum, he points out, had been invented and installed only in the nineteenth century, far too late to have guided a medieval Templar discovery. His answer is to remove it from their chain of evidence: they should skip it. Belbo refuses that boundary. He asks whether Casaubon has seen John Dee’s Monad Hieroglyph, a talisman said to concentrate all the wisdom of the universe, and proposes that its appearance resembles a pendulum. Similarity offers a bridge where chronology does not.

The distinction between the device’s actual history and its use in the Plan is explicit. In the researchers’ reconstructed narrative, Bacon is identified as Viscount St. Albans, and the Conservatoire’s Pendulum is named as a fact that needs a function. The earlier exchange has supplied them with a possible historical setting: the Conservatoire was conceived in homage to Bacon, and a revolutionary convention authorized a house of arts and trades meant to reproduce the House of Solomon described in Bacon’s New Atlantis. That institutional association does not prove any Templar link; the Plan treats it as another suggestive connection to be joined to the rest.

Inventing a Function for the Pendulum

The function emerges through a text submitted to the researchers by one of their Diabolicals, an author writing on the Hermetic secret of cathedrals. The text claims that the builders of Chartres left a plumb line hanging from a vault’s keystone and deduced the Earth’s rotation from it. The researchers use this tale to make the Pendulum’s presence at Saint-Martin seem like part of an old secret. They even fold Galileo’s trial into the construction: one suggestion makes the Church detect a Templar trace in Galileo, while Belbo’s alternative casts the cardinals who condemned him as Templar adepts trying to silence a traitor before the Plan’s fulfillment.

The imagined machinery takes visual form beneath the Pendulum. The researchers interpret a labyrinth drawn on the floor as a stylized diagram of subterranean currents. They compare it to the labyrinth of Chartres, which they can variously read as a solar clock, compass card, vein system, or serpentine course, and finally as a global chart of telluric tides. The shifting descriptions do not resolve into one stable explanation. Their very abundance lets the labyrinth seem capable of supporting whichever connection the Plan requires.

Casaubon then proposes a more operational reading: replace the abstract labyrinth on the floor with a map of the world. At a specified hour, the tip of the swinging Pendulum would mark the Umbilicus. The site, he argues, is beyond dispute: Saint-Martin-des-Champs is the Refuge. But Belbo immediately exposes a difficulty in the mechanism. If the Pendulum’s swing at midnight runs from Copenhagen to Cape Town, the point it indicates could fall in Denmark or South Africa. The proposal has made the supposed secret more concrete, but not yet more determinate.

A Timed Signal and a Paris-Centered Secret

Casaubon answers with another borrowed detail from the cathedral text. At Chartres, the author claims, a fissure in a choir window lets a sunbeam fall at a particular hour on the same stone each time. Casaubon adapts this into a hypothetical feature of the choir at Saint-Martin: a small uncolored spot near the junction of two lead cames, carefully calculated and supposedly preserved for six hundred years. At sunrise on a designated day, the beam would provide the time-and-place reference that the Pendulum alone cannot. The passage leaves the proposed procedure unfinished, but establishes its essential logic: a fixed point of light would discipline the Pendulum’s moving indication.

The device’s location matters because its motion is not uniform everywhere on the globe. Casaubon says that at some places the Pendulum completes its circle in thirty-six hours, at the North Pole in twenty-four, while at the Equator its cycle does not vary with the season. A calculation made at Saint-Martin would therefore be valid in Paris but produce a different curve in Palestine. This geographical dependence forces the researchers to give the Pendulum a local origin even as their imagined signal is meant to identify a worldwide center.

To support Saint-Martin as that origin, Casaubon assembles a succession of clues: the Templars supposedly chose it as their Refuge; control of the site supposedly passed from the prior of Saint Albans to Postel and then to the Convention; and, after Foucault’s first experiments, the Pendulum was installed there. The chain binds medieval legend, later custodianship, and modern science into one story. Each link is presented as a clue, but the episode’s logic depends on the researchers treating accumulation as confirmation rather than testing whether the links establish the same claim.

From Installation to the Plan’s Future Action

The Pendulum’s imagined role extends beyond identifying a location. When Casaubon objects that the final meeting is in Jerusalem rather than Paris, he answers that the Templars would assemble their message there, prepare for a year, and then have all six groups meet in Paris on the following June 23. Only then, under the Plan’s scenario, would they learn the Umbilicus and begin their effort to conquer the world. The installation thus serves as a hinge between a supposed Templar past and a scheduled future: a modern instrument at Saint-Martin is made to authorize the Paris rendezvous on which the conspiracy’s next stage depends.

The episode’s tension is between the Pendulum as a physical instrument, whose behavior changes with location, and the Pendulum as a sign that the researchers can make point toward a hidden center. Casaubon’s objections repeatedly restore limits—its recent installation, the ambiguity of a moving tip, the variation of its cycle—while the Plan absorbs each limit by adding another correspondence: Dee’s talisman, the cathedral labyrinth, a fixed beam of sunlight, and the history of Saint-Martin. The installation is therefore not evidence that the Templars used the Pendulum. It is the real-world anchor around which the researchers construct a story that turns resemblance and accumulated association into an apparent mechanism.