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. Étienne 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 Étienne 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.
Petit had argued strongly in favour of setting up an official scientific organisation in France so it is rather surprising that when the Académie des Sciences was founded in Paris in 1666 by Jean-Baptiste Colbert, at that time controller general of finance in France, Petit was not made a member. Colbert's programme of economic reconstruction was largely responsible for making France the leading power in Europe. There surely were personal reasons why Colbert, whose aims were so close to those of Petit, chose to ignore him when he drew up the initial list of members.
Article by: J J O'Connor and E F Robertson