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words Stranger than fiction The tale of the epic voyage made to establish the metric system is an intriguing and exciting one. Arago alone to complete the final readings Julyan Cartwright from Majorca. On Majorca, Arago chose ho thinks about science during a S’Eslop, a peak on the northwest coast, as vacation on the Balearic Islands? his viewpoint of Ibiza and Formentera. He WMajorca, Minorca, Ibiza and had a hut built on the summit and settled Formentera are holiday destinations par in with his instruments for the final series excellence, not places where famous scientists of measurements. But events didn’t go of the past lived and worked, or where great according to plan. experiments were carried out. But there was War broke out between France and Spain a moment in history when the Balearic in June 1808, while Arago was on the Islands were crucial for a major scientific summit of S’Eslop. Soon undertaking. Majorcans were com- It was 1806, and the French Bureau des menting that the nightly Longitudes had the task of determining the bonfires were signals Paris meridian — the line of longitude pass- and that Arago must be ing through Paris. At the time there was no a French spy, and a universally agreed prime meridian; it wasn’t detachment of soldiers until 1884 that an international congress was sent up the moun- decided it should be the one that passes tain to capture him. through Greenwich. The reason for deter- Arago got wind of this; Troubled trip: during François mining the meridian with precision wasn’t in his memoirs, he Arago’s year-long journey home map-making but the metric system. A few recounts what hap- from the Balearic Islands, he was years earlier, French revolutionaries had pened next: “We set off accused of spying, held hostage, and EVANS MARY decided to come up with a new, rational for Palma and we imprisoned on several occasions. system of measurement. The metre, the new encountered the troops base unit of length, was to be defined as one that had come to look ten-millionth of the distance from Pole to for me. Nobody recognized me because I received a triumphal reception in Paris. Equator along the Paris meridian. So it was spoke Mallorquin perfectly. I urged the Arago and Biot’s labours confirmed the very important to know exactly how long platoon to continue on the path, and we accuracy of the original measurements; in this meridian was. continued on our route towards the city.” the end, the prototype metre differed from A first series of measurements, from (Arago spoke Mallorquin, a version of Cata- the original meridian definition by just Dunkirk to Barcelona, came up with a provi- lan, as he was born in the French Pyrenees, a 0.02%. sional value for the length and a prototype Catalan-speaking region of France.) His Arago became an eminent physicist, metre bar was produced. Two young physi- escape was only temporary, though — he making many important discoveries in cists, François Arago and Jean-Baptiste Biot, eventually ended up in Bellver Castle, over- optics (Arago’s spot) and electromagnetism were given the task of checking and improv- looking Palma de Majorca. Now a tourist (Arago’s disc). He later entered politics, ing the accuracy of this result by extending attraction, it was then a prison. becoming Minister of War and the Navy, and the observations to the Balearic Islands. In the end, Arago managed to persuade he abolished slavery in French territories. Arago and Biot had to light a bonfire on a the authorities that he wasn’t a spy, and left Biot too became a physicist of note; his best- mountain peak and attempt to view this at the island for Algiers. From there he took a known contribution is the Biot–Savart law in night from another peak to obtain their ship headed for Marseilles, but his bad luck electromagnetism. triangulation readings. continued — the ship was intercepted by Today, Arago is honoured in Paris with South from Paris, the meridian first Spanish pirates and escorted to Catalonia, the Arago monument: a trail of plaques let passes Barcelona on the coast of the Spanish where he was again imprisoned. Once into the pavements of the city to mark the mainland and then runs close to Majorca, more, he got himself released, and again set path of the meridian he plotted. His adven- more than 200 km away across the Mediter- sail for Marseilles. This time it was not tures in the Balearics were fictionalized by his ranean Sea. This was too great a distance to pirates but bad weather that intervened — friend Jules Verne, who mentions these make direct observations, so Biot and Arago it was now December 1808. The ship was episodes in his novels set in the Balearic took triangulations from mountain tops forced by a storm to land at a small port in Islands, Clovis Dardentor and Hector down the Spanish coast to Denia, then across Algeria and was unable to make the winter Servadac (Off on a Comet, in its English the shorter stretch of sea from there to Ibiza crossing to Marseilles, so Arago headed translation). On Formentera, this fiction is and Formentera — still a difficult task with back overland to Algiers. Here he was held remembered more clearly than the reality, visual observations — and then finally to captive again, this time as a hostage by the because on La Mola stands a monument — Majorca. On Ibiza they used the mountain Algerians until the French paid for goods not to Arago and Biot, but to Verne, who Camp Vey, and on Formentera, the highest sent to France. probably never visited the islands. I point on the island, La Mola. At the end of This was resolved in July 1809, and after a Julyan Cartwright is at the Laboratory for 1807, Biot returned to Paris with the obser- year-long odyssey, Arago finally arrived back Crystallographic Studies, CSIC, vations from Ibiza and Formentera, and left in France to file his scientific reports, and E-18071 Granada, Spain. NATURE | VOL 412 | 16 AUGUST 2001 | www.nature.com 683.

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