Geoscience Reference
In-Depth Information
Observatory building is now a school). He was also welcomed by the naturalist
Alexander von Humboldt (1769-1859), who was fresh back from his own adventures
exploring South America. He had taken up work in Paris with Gay-Lussac, and
he invited Arago to share an apartment upon his return to Paris (from 1809-1811).
The fact that Arago was feted in this way by such illustrious scientists indicates the
respect that he had earned from the scientific community.
When his quarantine expired Arago visited his mother and father in Perpignan.
They had feared him dead because his father had seen a captured Spanish soldier
in Perpignan with his son's watch. Arago had sold it in Rosas the previous sum-
mer for food such as herrings, grapes and lard. Assuming the worst, his mother
had had church masses said for his soul and she was emotional at seeing him
return, as she thought, from the dead. After this joyful reunion, Arago returned to
Paris.
In all his adventures, Arago had safeguarded his precious observations, carrying
papers stuffed in the front of his shirt and protecting them as he protected himself.
He was thus able to deposit them at the Paris Observatory and when the observa-
tions were reduced, he was able to confirm the shape of the Earth by tabulating the
length of one degree of latitude across Europe, from London to Formentera. Even
over the ten-degree latitude difference from the north to the south it was clear that
the degree got longer towards the pole.
Length of one degree on the meridian
Name of arc
Mean latitude
Length of a degree in toises
Greenwich to Dunkerque
51° 15′
57,097.6
Dunkerque to the Pantheon
49° 56′
57,087.7
The Pantheon to Evaux
47° 30′
57,069.3
Evaux to Carcassonne
44° 42′
56,988.4
Carcassonne to Montjuïc
42° 17′
56,960.5
Montjuïc to Formentera
40° 01′
56,955.4
With such convincing observations and a heroic story to tell, Arago's return was
a sensation and on 18 September, 1809, at the age of 23, he was nominated to the
Academy of Sciences to replace Joseph Jérôme Lalande. His election was not,
however, straightforward; Pierre-Simon Laplace had been lobbying to elect another
scientist, Siméon-Denis Poisson, at the time of the first available vacancy in the
Academy. Siméon-Denis Poisson (1781-1840) was five years older than Arago and
when Arago was still a student, Poisson had been his professor at the Polytechnic
School. Poisson was not only a gifted mathematician but also had cultivated social
connections in the salons of Paris, the world of the theatre and cultural life. He had
been taught by Lagrange and Laplace, both of whom had a high opinion of his
mathematical ability. Their high opinion was justified by Poisson's eventual mathe-
matical achievements in the theories of gravity, probability and the flow of radiation;
modern mathematicians still speak of “Poisson's Equation” and a “Poissonian
distribution of random variables.”
