Born: 8 December 1594 in Montluçon, France
Died: 20 August 1677 in Lagny-sur-Marne, France

Pierre Petit was the son of a minor provincial official in Montluçon. The town, in the Auvergne region of central France, is situated northwest of Clermont-Ferrand. The old town in which Pierre was brought up was on a hill dominated by a château. After being educated in Montluçon he followed his father into local government and spent the first years of his adult life as 'contrôleur de l'election' in the town of his birth. As Petit was much involved in the government of France, we should look briefly at political events of the times in which he lived.

Louis XIII had become King of France in 1610, around the time Petit was thinking about a career in government. The fact that Louis XIII's father Henry IV had been assassinated by a fanatical Roman Catholic, and Louis was only nine years old when he succeeded to the throne, already gives some idea of the political situation. It was a time of revolt by the French peasants, and officials loyal to the government were in some danger of assassination.

In 1633 Petit moved to Paris to play a role in national rather than local government. Being in Paris had another advantage for Petit since it allowed him to become more involved with men of science than he could be in a town like Montluçon. Richelieu, a major figure in the French government, had begun with a career in the Church and, as Bishop of Luçon, became involved with the government in the difficult period when Louis XIII was a minor. The King managed to get Richelieu made a Cardinal, then appointed him as secretary of state in 1624, giving him the title of first minister four years later. When Petit arrived in Paris in 1633 he was appointed 'Commissaire Provincial de l'Artillerie' by Richelieu.

In Paris Petit joined with the group of scientists meeting with Mersenne at the Minims de l'Annociade near Place Royale. In the year Petit arrived in Paris, Mersenne published Traité des mouvements, and in the following year he published Les Mécanique de Galilée which was a version of Galileo's lectures on mechanics. Etienne Pascal had arrived in Paris two years before Petit so that his son could have the best education. He and his son Blaise Pascal were also members of Mersenne's group as were others such as Gassendi, Roberval, and Beaugrand.

Petit was an influential figure with important government positions which enabled him to try to influence national science policy. A firm believer, as were the other members of Mersenne's group, of the experimental method rather than the philosophical approach advocated by Descartes, Petit argued strongly for better astronomical facilities in France. He wanted the King to establish a Royal Observatory to allow France to again take a leading role in astronomy. Petit argued that France had fallen behind some other European countries and was relying on observations made in other countries. Petit himself had a fine collection of astronomical instruments and several of these were of his own invention. In particular, late in his life, Petit devised a filar micrometer to measure the diameters of celestial objects such as the Sun, Moon and planets. By this time Petit was 'Intendant Général des Fortifications', a position he was appointed to in 1649, and he wrote a letter which describes the instrument. This was published in the Journal des sçavans in May 1667 as Extrait d'une lettre de M Petit Intendant des Fortifications ... touchant une nouvelle machine pour mesurer exactment les diamètres des astres. Petit's instrument was later used by Giovanni Cassini in the Royal Observatory in Paris.

Among many collaborations, Petit worked with Etienne Pascal and his son Blaise Pascal in October 1646 in repeating Torricelli's experiment on the barometric vacuum. Petit wrote an account of these experiments which were published as Observation touchant le vuide faite pour la première fois en France in 1647. Petit kept up a regular correspondence with the secretary of the Royal Society in London after its foundation in 1660 and was one of the first foreign fellows of that Society when elected in April 1667. Petit had written Dissertation sur la nature des comètes in 1665 which had been highly praised by the Royal Society and must have played a large role in him being elected a fellow.

1. M Fichman, Biography in Dictionary of Scientific Biography (New York 1970-1990). 
   http://www.encyclopedia.com/doc/1G2-2830903375.html

Articles:

1. J Michaud (ed.), Biographie universelle XXXII (Graz, 1968), 588-589.
2. J P Nicéron, Mémoires pour servir à l'histoire des hommes illustres dans la république des lettres 42 (Paris, 1741), 191-195.