Bear with me a while as I complete this Paris trilogy by recounting certain events that occurred in the Paris Pantheon in 1851. The real Pantheon, of course, is found in Rome; it is a temple that, quite literally, belongs to all the gods.
The one in Paris is in similar style, but in latter times it has served merely as a receptacle for the remains of eminent Frenchmen who have passed away; Voltaire, Rousseau and Victor Hugo, for example, are all buried there. And the Paris Pantheon has a great dome which rises some 70 ft above the floor, and it provides the kernel of our story for today.
Pythagoras is credited with being the first to say the Earth was round. Ever since then the concept has been accepted fairly widely, but a sphere is one thing: a rotating sphere is quite another.
It would in all probability never have occurred to William Shakespeare, for example, that his world might be revolving on its own axis. But with the gradual acceptance of the Copernican system during the 17th and 18th centuries the idea became accepted, and all that was necessary was the proof.
Jean Foucault provided it by means of a much-publicised experiment, in which he suspended a vast pendulum from inside the Paris Pantheon dome.
The idea behind the experiment is easier to understand if you imagine a pendulum suspended over the North Pole - a heavy weight swinging to and fro at the end of a long piece of string. As the pendulum swings back and forth, the Earth rotates beneath anti-clockwise. Since the plane of oscillation of the pendulum stays fixed in space, the path traced by the moving weight will appear to rotate in a clockwise direction, completing a full revolution - "around the clock", so to speak - every 24 hours.
The matter is more complex at lower latitudes. Theory predicts that the plane of oscillation of the pendulum should still rotate, but that it will take a much longer time to complete a full revolution. In any case, the changing path of the moving weight is conclusive proof that the Earth must be rotating underneath.
This is precisely what Foucault set out to demonstrate in Paris. His pendulum consisted of a metal sphere weighing 55 lbs, suspended from the dome on a metal thread 63 ft in length.
The floor underneath was suitably marked, so that any change in the path of the swinging weight could be closely observed. Foucault's pendulum behaved exactly as predicted, and we had our first conclusive proof that the world indeed goes round and round.