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Laplace thought it unseemly that the less experienced man should be elected first
to the Academy. He sought to undermine Arago's scientific achievements, which
were at the time and later on, considerably less than those of Poisson (even though
Arago boasts of his fine achievements up to then in his autobiography). Arago must
have had mixed feelings about competing with Poisson, since it was he who obtained
Arago his first scientific position at the Paris Observatory. Regardless, Arago's fame,
heroism and potential for later scientific achievement carried the day, and he was
elected by a substantial majority (Poisson was elected three years later).
All this must have been a triumph for Arago, and the adventure and its conse-
quences certainly changed his life. He turned his back on the Polytechnic School,
terminated his leave of absence and his potential career as an artilleryman, and
became a scientist. His subsequent career was less physically adventurous but scienti-
fically and politically more distinguished. He married in 1811, living in an apart-
ment in the Paris Observatory, and had three sons. He became director of the Paris
Observatory in 1834 and took up investigations into the wave theory of light
sparked by the theories of Thomas Young (1773-1829) and applied it to stellar
aberration, the displacement of star positions by the combination of the motion of
light waves from a star and the motion of the Earth round the Sun. He suggested
the crucial test between the wave theory of light and the particle theory by compar-
ing the speed of light in water and air and is well-known to astronomers as the
person who suggested to Urbain Leverrier (1811-1877) that he should investigate
why Uranus was departing from its calculated orbit and might be pulled off track
by a hitherto unknown planet. This led to the discovery of the planet Neptune.
Arago was an extraordinary public lecturer, pursuing his democratic beliefs in
the education of the people, and left a collection of essays on astronomy that
became a best-selling set of 4 volumes called
Astronomie Populaire
(
Popular Astronomy
).
In 1851 he arranged a scientific display that caused a sensation in Paris known as
the Foucault Pendulum and invented by Jean Foucault.
A FOUCAULT PENDULUM is a pendulum that is not fixed to swing in a plane by
any mechanical restriction (as in a pendulum clock, where it is constrained within
the case) and can choose the plane in which it swings. It tries to swing back and
forth along a plane defined relative to the fixed stars (Crease 2003). As the Earth
turns in space, the swing of the pendulum rotates, revealing the Earth's motion.
People
Jean Bernard Léon Foucault (1819-1868)
Foucault was the son of a publisher and at first trained in the field of medicine. Unable to
bear the sight of blood, he turned to physics and with Armand Hippolyte Fizeau he carried
out a series of experiments on light and heat. At the suggestion of François Arago in 1850,
he established that the speed of light in water was less than in air, a prediction of Young's
wave theory of light. His memorial in Montmartre Cemetery records what were thought of
as his greatest discoveries: the photoelectric microscope (it had an intense electric arc light
source), the eponymous pendulum, a gyroscope, a telescope and regulators.
Inspired by seeing how a vibrating rod of steel clamped in a rotating lathe tended to
remain oscillating in the same plane, Foucault became interested in the motion of a
pendulum and whether it would stay in the same plane even though the Earth was rotating
