**François Jacquier **was a French 18th century mathematician who spent most of his life in the Minim Monastery in Rome. Along with Thomas Le Seur he edited a highly annotated edition of Newton's *Principia*.

- We note that although the spelling of the name Jacquier is constant through generations, the name Desenlis is not, appearing also as de Senlis, Senlis, Sanly, Senly, Sanlys etc.
- The Order believed in poverty, chastity, and obedience but the followers also devoted themselves to study and scholarship which suited Jacquier.
- We note that Le Seur, who was born in 1703, so eight years older than Jacquier, had also studied at the College in Vitry-le-François.
- Jacquier had rapidly gained a high reputation for his mathematical and scientific skills and, in 1734, Cardinal Alessandro Albani consulted him about the hydraulic problems with the harbour of Ravenna and with the river Montone which joins the Ronco before entering the sea just south of Ravenna.
- Jacquier is best known for his work as an editor and commentator on Newton's Principia Ⓣ(Mathematical Principles of Natural Philosophy).
- This annotated edition, published in Geneva in three volumes in 1739, 1740 and 1742, gives Le Seur and Jacquier as the editors (listed in that order since the convention was to list the most senior first).
- It is because of his fame and prominent position in the 'république des lettres' that he expected to be recognised as the editor of the Genevan edition of the 'Principia' Ⓣ(Mathematical Principles of Natural Philosophy): he did not need a mention on the title-page, but rather he was keen on being the object of grateful recognition in the Monita signed by Le Seur and Jacquier.
- There are natural questions regarding how Calandrini cooperated with Le Seur and Jacquier in an era when communication between cities far apart was extremely slow and difficult.
- Be that as it may, a cooperation between scholars living in two cities as distant (geographically and culturally) as Geneva and Rome does not seem the easiest or most obvious choice, but Le Seur and Jacquier were amongst the few in all of Europe who possessed the competence and the stamina to carry out the first, complete, line-by-line, commentary of Newton's 'Principia' Ⓣ(Mathematical Principles of Natural Philosophy).
- In 1739 he met Jacquier who was busy working at Trinità dei Monti on the annotated edition of the Principia Ⓣ(Mathematical Principles of Natural Philosophy).
- Father Jacquier is making a new ladder in order to get to the top of that tower.
- This refers to an interesting aspect of Le Seur and Jacquier's annotated edition of the Principia Ⓣ(Mathematical Principles of Natural Philosophy), namely the problem of whether the Earth orbits the Sun.
- This prohibition was dropped from the Index in 1758 but it was certainly in place in 1739-42 when Le Seur and Jacquier's Principia Ⓣ(Mathematical Principles of Natural Philosophy) was published.
- We note that the Royal Society was very impressed with the annotated Principia Ⓣ(Mathematical Principles of Natural Philosophy) and elected Jacquier a fellow of the Society on 10 December 1741.
- He was a scholar who was well disposed towards learning and impressed with the work of Le Seur and Jacquier.
- Architects had for many years argued over the solution and, in 1742, the Pope asked three mathematicians, Le Seur, Jacquier and Ruggero Boscovich, to advise on stabilising the dome.
- In the same year he also sought advice from Giovanni Poleni who, like Jacquier, Le Seur and Boscovich, proposed fitting large iron rings to strengthen the dome and the rings were fitted in 1748.
- Clairaut was friendly with many of the leading scientists and mathematicians in Paris, in particular with Maupertuis and D'Alembert who Jacquier met.
- Clairaut was also very friendly with Voltaire and Émilie du Châtelet and Jacquier spent two months in the summer of 1744 at the Chateau de Cirey in Cirey-sur-Blaise, Haute-Marne, the home of Voltaire and Émilie du Châtelet.
- Jacquier liked du Châtelet's Fundamentals of Physics and had arranged for an Italian translation to be made which was being used at this time by Laura Bassi.
- The discussions between Jacquier and du Châtelet led to her beginning her French translation of Newton's Principia Ⓣ(Mathematical Principles of Natural Philosophy).
- Cardinal Valenti, Pope Benedict XIV's secretary of state, however, wanted to keep Jacquier in Rome and offered him the chair of experimental physics at La Sapienza.
- In 1755 Jacquier published his Italian translation of Brook Taylor's New principles, with additions, as Elementi di perspettiva secondo li principii di Brook Taylor, con varii aggiunti Ⓣ(Elements of perspective according to Brook Taylor's principles, with various additions).
- Philip died unexpectedly on 18 July 1765 in Alessandria, Italy but Jacquier and Le Seur still went to Parma teaching Ferdinand in the year 1766-67.
- From the Trinità dei Monti, Jacquier conducted a correspondence with many of the leading mathematicians and scientists in Europe, including d'Alembert, Clairaut, Bézout, Lalande, de l'Hôpital, Maupertuis, and Montucla.
- We have seen, however, that Jacquier did not spent his time completely within the Trinità dei Monti.
- For example the French painter Joseph-Marie Vien (1716-1809) was appointed director of the Académie de France in Rome in 1776 and he sent his sons to be taught mathematics and Latin by Jacquier.
- Jacquier, however, was more involved in Roman society than his religious life might suggest.
- Their close friendship is evident, however, when Casanova's lifestyle led to him having to pawn one of his most elegant suits and Jacquier lent him money to repay the loan so that the suit would not be sold by the pawnbroker.
- Teaching at the College was taken over by religious men of other Orders and at this time Jacquier was appointed as Professor of Mathematics.
- Jacquier was a member and gave orations to the Society.
- Jacquier argued, following the Roman Catholic doctrine at that time, that God could shape the static Earth any way he wished, so he could have chose to flatten the poles.
- Although there is no doubt that Jacquier was a devoted Catholic, one is left to wonder if he really believed in a geocentric solar system or whether he argued for it simply because he felt obliged to follow doctrine.
- There is no direct evidence that Jacquier in any way influenced these decisions, but there is just the possibility that he might have done so.

Born 7 June 1711, Vitry-le-François, Champagne, France. Died 3 July 1788, Rome, Papal States (now Italy).

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**O’Connor, John J; Robertson, Edmund F**: MacTutor History of Mathematics Archive