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underneath it. He carried out some preliminary experiments to make a free-swinging
pendulum in the basement of his house in the Rue de Vaugirard just to the north of the
Paris Observatory at the intersection of the Rue d'Assas (the house is now demolished,
but its site is marked with commemorative stonework). The results were encouraging;
however, he found that to make the results clearer it was necessary to use a massive
pendulum bob less likely to be disturbed by air currents. To see the rotation of the plane
of the swing of such a large pendulum bob, it was necessary to get a large swing and
therefore a long pendulum. With the cooperation of Arago, he installed one that was
suspended from the high ceiling of the Salle Méridienne of the Paris Observatory in
February 1851. He confirmed that the slow swing of the long pendulum clearly turned
over the floor as a result of the Earth's rotation. Through the Academy, Arago arranged
for the experiment to be repeated in public, sending out dramatic invitations inviting all
“to come to watch the Earth turn in the central hall of the Observatory.”
As a result of all the interest, Foucault set up the experiment under the even higher
dome of the Panthéon where it caused a public sensation (Foucault's original pendulum
bobs are preserved in the Musée des Arts et Metiers in the Marais on the Right Bank).
The Panthéon still has a Foucault exhibition and a replica of the pendulum experiment
which still holds the fascination Foucault described a few weeks after his discovery:
The phenomenon unfolds calmly: it is inevitable, irresistible… Watching it being born and
grow, we realize that it is not in the experimenter's power to speed it up or slow it down…
Everyone in its presence grows thoughtful and silent for a few seconds, and generally takes
away a more pressing and intense feeling of our ceaseless mobility in space.
Few scientific experiments have met with such instant and long-lasting fame and even
fewer have been the subject of a novel ( Foucault's Pendulum by Umberto Eco).
Arago combined a scientific and political career in what continued to be turbu-
lent times. While still a pupil at the Polytechnic School he had shown signs of
political principle by refusing to swear allegiance to Napoleon when he seized the
crown. He was saved from retaliation by his position as a brilliant mathematician,
and Napoleon appears to have been magnanimous in forgiving this youthful ideal-
ism; he authorized Arago's appointment as professor at the Polytechnic School
soon after his election to the Academy. Arago then visited Britain in 1818 or 1819
with Biot, his mentor at the Paris Observatory, who accompanied him to Spain (see
above), to connect the geodetic systems of Britain and France. The pair also visited
Unst, the most northern of the Shetland Islands, to measure the period of a pendu-
lum in order to further test the shape of the Earth at high-northerly latitudes. He
made a second visit to Britain in 1834 to meet British scientists.
Napoleon abdicated in 1814 and was forced into exile in Elba, where he escaped
only to be exiled again in St. Helena. The Bourbon monarchy was restored as the
government of France, with the accession of Louis XVIII from 1814-1824 and fol-
lowed by Charles X from 1824-1830. From 1830 on, Arago was drawn into politics
by his younger brothers and was active as a prominent republican. He was a mem-
ber of the provisional government which took power after the 1848 Revolution and
became known as the Second Republic. Sixty years after the Revolution, the politi-
cal changes had not altered the condition of the people, the socialists declared, and
the government was forced to set about a program of reform.
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