Commentary & Opinion
3:34 pm
Thu September 27, 2012

David Nightingale: Dominique Francois Jean Arago (1786 - 1853)

   And who was the 25th President of France?

   Well, I guess this is information for TV's “Jeopardy”, but it was the physicist Dominique Francois Jean Arago, born 50 years after the birth of another statesman/scientist – Ben Franklin.[Ref.1].

   Some may remember from high school a demonstration called “Arago's disk”, wherein a copper or aluminum disc is spun underneath an ordinary compass, and the compass needle begins to swing round also. This is just one of the many experiments Arago did.

   The Arago family lived near the Pyrenees. At 17 Francois Arago decided to try for entrance into L'Ecole Polytechnique in Paris, by mastering the basic mathematical works of Lagrange, Euler, and Laplace, and he ranked near the top of the 1803 list. Poisson, only five years older, was an assistant professor of mathematics there, and the two soon became friends.

   Near the end of Arago's studies Poisson asked both Arago and Biot [whom all freshmen science students have heard of when studying the Biot-Savart law] to help in locating more exactly the meridian further south from Paris. Meridians, of course, are the north-south lines on a map, and what the Bureau of Longitude wanted was an improved accuracy specifically around Barcelona (which is just over the border in Spain) and the Spanish island of Majorca in the Mediterranean. They set off in 1806 and the measurements of their positions involved signaling with lanterns between mountains.

   In 1808, Napoleon I of France attacked Spain (previously an ally), and that didn't help the young Arago on a mountain top in Majorca. Fortunately, a friendly Spanish boat-captain brought him down in disguise, but at the harbor another mob set on him, and, arrested as a spy, especially with signaling apparatus, he was sentenced to be imprisoned in a nearby castle. With a dagger wound in his leg, and some of the mob still pursuing him, he ran fast towards the dungeon and managed to reach the safety of prison.

   In his autobiography [ref.2]he describes how after two months he escaped, and got onto a fishing vessel bound for Algiers, where he went to the French Consulate, and was put on a ship for Marseilles. From Marseilles of course he would be able to return safely to Paris, with his log book of triangulation measurements.

   However, their ship was intercepted by a Spanish warship. The French ship was taken to Spain and its occupants were taken to a Spanish prison which was incidentally only about 60 miles from his parents' home in southwest France.

   After a while the captured ship was allowed to set off again for Marseilles. But then a violent 'mistral' blew the vessel all the way back to Algeria, somewhere east of Algiers. He and the crew were captured by Moslems but managed to find a Moslem priest to lead them to Algiers, with Arago and the others pretending to be converts to Islam.

   To make the story short he did ultimately get a merchant ship back to Marseilles – having taken 11 months for a 4 day voyage – and he got to Paris and gave his triangulation data to the Academy of Sciences. Still only 23 he was made an assistant professor, based at the Paris Observatory, and elected to the Academy of Sciences.

   He spent the rest of his life working from the Observatory, but was also repeatedly elected Deputy for his native department “Pyrenees-Orientales”, thus sitting as a member of the Chamber of Deputies for many years (1830-1852).

   There isn't time to discuss his extensive work in optics with Fresnel and Fizeau, nor his gravity measurements in Scotland. [Ref.3]

   After the 1848 revolution he was elected President of France's ruling Executive Committee, before losing in a popular vote five months later. Nevertheless he was able to abolish slavery in the French colonies in that short span of time.

   Finally, the dampening of a compass needle's oscillations in the presence of a metal, and 'Arago's disc', were ultimately explained by Faraday as being due to eddy currents, and it is these same eddy currents that today can boil a kettle.

   In his last years he went blind, dying in 1853, at age 67.

   Thus, the 25th President of France.

Refs:

1.   history.mcs.st-andrews.ac.uk

2.  “The history of my youth”, under “Skulls in the Stars”,by Greg Gbur, physics prof at UNC, Charlotte, NC

3.  Complete Dictionary of Scientific Biography, 2008.

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