Jean Dominique Cassini, generally referred to as Cassini IV

The great-grandson of Cassini I – Jean Dominique Cassini – was also named Jean Dominique and was born at the Observatoire, Paris, on the 30th June 1748, the older of two children. His younger sister was Françoise Elisabeth Cassini. In order to distinguish him from other members of the Cassini scientific family, he has come to be referred to as Cassini IV.

Jean Dominique was the Comte de Cassini and was also known as Cassini de Thury. I have also seen his name given as Jacques Jean Dominique Cassini and as Jacques Cassini in an article of July 1939 in the ‘Légion d'Honneur’.

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Education

As had been the case with other members of his family, Jean Dominique received his early education at the Observatoire before moving to the Collège du Plessis in Paris for his secondary education. From here he went to the Collège Oratorien at Juilly. Although the main aim of the Collège was to educate its students for the priesthood, Jean Dominique concentrated his studies on mathematics, physics and astronomy.

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Work

In 1768, at the age of twenty, he sailed to America and the African coast testing a new marine chronometer invented by Pierre Le Roy. The account was published in 1770.

On the 23rd July 1770 Jean Dominique was elected to the Académie des Sciences.

1771 saw Jean Dominique’s father, César François, made the first Director of the Observatoire with the understanding that the position would be hereditary. With this in mind Jean Dominique began to take an increasing interest in the work of his father, particularly mapping and cartography.

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Marriage

On the 7th April 1773 Jean Dominique married Claude-Marie Louise de la Myre-Mory, daughter of François-Jean, Comte de Mory and Marie-Anne-Thérèse de Chamborand. They had four sons and four daughters:

• Jean Dominique Maurice Cassini – b. 25.10.1773 – d. 22.09.1774
• Dominique Hippolyte Cassini – b. 03.02.1775
• Anne Cécile Cassini – b. 25.02.1777
• Angélique Dorothée Cassini – b. 28.03.1779 – d. 10.02.1806
• Alexandre Henri Gabriel Cassini – b. 09.05.1781 – d. 16.04.1832
• Aglaée-Elisabeth Cassini – b. 20.09.1784 – d. at an old age
• Aline Françoise Cassini – b. 28.03.1787 – d. 02.05.1853
• Alexis Jean Dominique Cassini – b. 19.11.1788 – d. Battle of Braga, Portugal – 20.03.1809.

There is a note in a document, that Anne Cécile Cassini died at the age of 91 on the 5th March ‘dernier dans la Maison des Dames de Sacré-Coeur a Orléans’.

Jean Dominique’s father died in 1784 by which time Jean Dominique was progressing most of the cartographic project to map France. This work continued, but it was not until 1790 that Jean Dominique was able to present the completed work to the National Assembly.

Jean Dominique was offically appointed Director of the Observatory in 1784. He persuaded Louis XVI to accept reorganisation of the Observatory as well as its redevelopment. Although this exercise continued, it was never completed.

In 1787 Jean Dominique was appointed Commissioner together with Legendre and Méchain to determine – in conjunction with English scientists – the precise distance between the observatories of Greenwich and Paris. The English scientists depended on Ramsden’s theodolite whereas the French used Borda’s repeating circle. A memoir on this subject was published in 1791.

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The Revolution

Unfortunately for Jean Dominique – and many others, of course – revolution swept France. On the 14th July 1789 a mob attacked and took over the Bastille in Paris, searching for weapons and munitions. Two days later three hundred men entered the Observatory again looking for food, weapons and munitions but, finding nothing, left but after first stripping lead from the roof to turn into ammunition.

Jean Dominique’s life was further complicated when his wife died on the 24th April 1791, leaving him to look after their five children, Aglaée-Fronçoise having died at an early age.

At that time the Adadémie des Sciences was establishing a project to measure the meridian from Dunkerque to Barcelona in their efforts to establish an accurate measurement for the meter which would be defined as one ten-millionth of the distance from the North Pole to the equator. In the same month as the death of his wife Jean Dominique, together with Legendre and Méchain was appointed to carry out this work. On the 19th June 1791 they had an audience with Louis XVI when Jean Dominique, on being asked by the King if he thought he could improve on the measurements made by his father and grand-father, replied that the Borda repeating circle was fifteen times more accurate than the methods they employed.

The next day the King fled to Varennes, but was recognised, arrested and returned to Paris. Jean Dominique apparently still felt loyalty to the King and was naturally having difficulties with the Revolution. Louis XVI was held by the Revolution but eventually executed on the 21st January 1793.

The Académie des Sciences had required Jean Dominique to lead the surveying as was the tradition in this type of work but, with the pressure of having to look after his children, he asked for permission to direct the work from Paris. This was refused and the Académie appointed Delambre in his stead.

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Professional difficulties

At the Observatoire Jean Dominique was having significant problems. He had three assistants: Nicolas Antoine Nouet, Jean Perny and Alexandre Rouelle. For a variety of reasons, Jean Dominique fell out with all three of them. This operational problem was compounded by the National Assembly creating four posts of Professor, one of them being Jean Dominique appointed at half his previous salary. Perny was appointed Director with the intent that the post would rotate. Jean Dominique was unable to appeal to the Académie des Sciences as it was disbanded in August 1793. Humiliated, Jean Dominique resigned on the 6th September 1793. Coincidentally, this was the year in which Jean Dominique completed his father’s maps and which were published by the Académie des Sciences.

His students now required Jean Dominique to leave his rooms in the Observatoire where he was living with his mother, six children and his cousin, Mlle. Elisabeth Françoise de Forceville, his wife having died in 1791. A few weeks later, he did so. The National Assembly took ownership of his map and, when he complained, he was arrested on the 13th or 14th February 1794 together with his cousin, Mlle. Françoise Elisabeth de Forceville – one of the two children of Elisabeth Geneviève Cassini and Louis François de Forceville – on a charge of their being aristocrats. Sources also say that he was arrested on being denounced by the Revolutionary Committee of Beauvais, near which Thury was situated, and his estates at Thury were confiscated.

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Imprisonment

Jean Dominique was imprisoned in the Convent of the French Benedictines on St. Jacob Street which had been converted into a prison by the Revolution on the 9th October 1793. He shared the prison with, among others, the Dauphin’s governess, Mme. de Tourzel and her seventeen year old daughter, Pauline, the third wife of the playwright, Beaumarchais, Marie-Thérèse-Emilie Willermaulaz, and the wife of the Count of Bourdonnaie. The convent was sold on the 30th August 1799 but returned to the English Benedictines in 1803. A week after Jean Dominique’s arrest, Ruelle, one of the three students was also arrested and imprisoned for manufacturing his astronomical data.

On the 6th June 1794, Mlle. de Forceville, at the age of forty-two, was taken to the guillotine at the Place de la Révolution, an event perhaps provoked by a sarcastic remark she was said to have made. But the next month, on the 27th July, Robespierre fell from power and a less brutal regime took its place. Jean Dominique was released on the 5th August following the intervention of Thermidor and people from the Observatory Quarter who remembered past kindnesses by his family, and went to live at the family home in Thury, said to be a broken man. Although Dalambre and Lalande tried to get him to return and take up his scientific work again, he refused, arguing that his family was more important. He was particularly critical of the new units of measurement brought in by the Revolution.

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Retirement

In 1795 he was offered a position in the Bureau des Longitudes but turned it down along with an offer to join the National Institute which had replaced the Académie des Sciences in January 1796. Two years later he appears to have changed his mind and accepted election to the National Institute though, perversely, rejected for the Bureau des Longitudes, he gave up his work in science.

It is possible that his change of heart may have had something to do with the honour in which some elements of the new regime in Paris held him and his family. In 1797 Napoleon, taking a little time out of his military campaign in Liguria, stayed a night at the Maraldi house in Perinaldo.

A note in a magazine of August, 1798 gives a little more detail on his life towards the end of the century:

Cassini, the fourth of this illustrious name, has retired into the country about 20 leagues from Paris; having declined a seat at the board of longitude, as well as in the national institute, from pecuniary embarrassments. For the credit of the French government, we hope soon to be able to announce that these difficulties have been removed.

Turning to the life of a country gentleman Jean Dominique devoted himself to his personal interests, particularly writing in the defence of the scientific prestige of his family and publishing, in 1810, his memoirs. This was considered to be an invaluable document based on personal records but is now lost, though reputedly having been of particular value in its description of his life during the revolutionary period. The memoirs included eulogies of several academicians together with a history of his great-grandfather, Jean Dominique Cassini.

In this period of his life he became Vice-President of the General Council of Oise, a Justice of the Peace in the canton of Mouy and the Mayor of Thury. He was pensioned both by Napoleon who made him a Senator and Count of the Empire, and then by Louis XVIII.

Jean Dominique died at Thury-sous-Clermont, Oise on the 18th October 1845. He was ninety-seven.