Jacques Cassini, generally referred to as Cassini II

Jacques Cassini, born in the family house at the Observatory, Paris, on the 8th February 1677, was the younger son of Jean Dominique Cassini and Geneviève Delaistre, daughter of Lieutenant-Général au baillage de Clermont en Beauvoisis. In order to distinguish between the different Cassinis, particularly those associated with astronomy and cartography, Jacques has come to be known as Cassini II.

Incidentally, the above illustration of him was found on the Internet. I’m told that researchers in the Observatoire in Paris have been unable to discover a portrait of him, so I can’t be absolutely sure that this is actually his portrait.

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Jean Baptiste Cassini

Jacques’ older brother, Jean Baptiste Cassini, born in 1674 or 1675, was killed at the age of eighteen when, as a Lieutenant of the King’s Ships, he fought at the Battle of La Hogue or La Houque off the Cotentin, Normandy. The battle is named after the place where the last in a series of engagements was enacted where a combined and superior force of British and Dutch navies fought the French under the Compte de Tourville. It is believed that Jean Baptiste’s untimely death cut short a promising career which would have seen him carry out practical research on the application of astronomy to navigation.

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Education

Both children received an excellent education at home in their early years. The family circumstances were good with Jean Dominique being relatively wealthy, in touch with some of the best scientists of the day, and having a loving relationship with his wife, Geneviève. He was, by all accounts, a happy man, secure in his religion, and this is likely to have passed itself on to his children.

Jacques was an eager student and moved on to study at the Collège Mazarin in Paris where Varignon had been appointed professor of mathematics in 1688. Varignon supervised Cassini’s thesis on optics which, in August 1691, at the age of fourteen, Cassini defended. An early biographer states that, when he was fifteen years old, Jacques dedicated a thesis on mathematics to the Duc de Bourgogne. It is unclear whether these two references to theses relate to the same work or to two separate ones.

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Work with his father

At the age of seventeen, Jacques began to assist his father in his scientific work, and was admitted to the prestigious Académie des Sciences as a Student. On the 23rd September 1694, he set out with his father on a long journey through France to Italy where they carried out a number of geodesic observations and repaired the gnomon at the Church of San Petronio which Cassini’s father had re-designed nearly thirty years previously. The trip through France and Italy lasted a year and a half and was followed by trips to Flanders and England where, not only did he meet Newton, Flamsteed and Halley, but where he was also elected to the Royal Society of London in 1698.

Jacques’ main scientific failing can be traced to work he carried out with his father in 1700. Their project to measure the meridian from Perpignan to Paris resulted in Jean Dominique’s stating that the Earth was elongated at the poles. Later, in 1713, and using a different method of measurement based on the moon’s eclipses of stars and planets but with the data of the 1700 experiments, Jacques confirmed his father’s theory, a position he maintained throughout his life.

It is not known why he continued to hold this position in the face of more likely theories, but it has been ascribed both to nationalism – opposing the English Newton’s theories – or to loyalty to his father, Jean Dominique.

Jacques was married in about 1702 to Suzanne Françoise Charpentier de Charmois, daughter of a Captain of the Guards. They had three sons and two daughters:

• Dominique Jean Cassini
• César François Cassini – Cassini III
• Dominique Joseph Cassini
• Suzanne Françoise Cassini, and
• Elisabeth Geneviève Cassini.

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Death of his father

During the early years of the new century Jacques continued with his work, essentially assisting the scientific work of his father. Increasingly, however, he took over this work as Jean Dominique’s poor sight led to his becoming infirm around 1709, blind by 1711, and dying on the 11th September 1712.

Not only did Jacques operate as a scientist, but by virtue of some legal training he was appointed to the Chambre des Comptes in 1706. This was a financial court with administrative and legal duties relating to the King's accounts, in particular to land owned by the Crown. He had a reputation for honesty but was sometimes criticised as being indecisive. In 1716 he became an advocate in the Court of Justice and, in 1722, a Councillor of State.

The work of the Paris Observatory was, officially, run by the Académie des Sciences. In reality it was run by Jean Dominique Cassini and, upon his death in 1712 the day to day running of the Observatory was taken over by Jacques Cassini.

Jacques continued the work of his father, in particular the work associated with determining the elongation of the Earth at its poles. In 1718 he measured the meridian from Paris to Dunkerque, publishing the results in 1722. Maupertuis of the Académie des Sciences continued to oppose him, supporting the Newtonian theory that the Earth was flattened at its poles. From 1733 to 1734 Jacques measured the meridian from Saint Malo to Strasbourg, taking with him his son, César François, then aged nineteen, in a like manner to which he had accompanied his father to Bologna when he was of a similar age.

The data they collected appeared to support his position on the elongation of the Earth, but this only galvanised Maupertuis to develop his own, contradictory theory. Experiments in Peru by Bouguer and La Condamine in 1735, and in Lapland by Maupertuis in 1736 measuring the length of a meridian degree, supported the Newtonian theory of the flattening of the Earth at its poles and Jacques, while still maintaining his own position, began to scale back his scientific work around 1740, the year in which he published ‘Eléments d'astronomie’.

In this same year Jacques trained and organised a team of young geometers to assist in drawing up maps of France, much of which was inaccessible due to the lack of roads. As a part of this work, a group of them followed the course of the Loire towards its source. Much of France was not only unmapped but unoccupied and untravelled by the majority of its inhabitants who were extremely wary of incomers, to the extent that when the team of geometers were seen in Les Estables, a commune in the Haute-Loire region of France, and carrying a collection of strange instruments, one of their number was set upon and hacked to death by the villagers.

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Work with his son

Jacques’ son, César François was, by then, making his own way in the scientific world, particularly in the field of cartography. It was in this area that Jacques continued his work, assisting César François.

Despite his position on the shape of the Earth, Jacques Cassini was responsible for furthering scientific work and traditions, not just in astronomical observations where his most important contribution in 1738 was the proper motion of the stars, but also in mathematics, electricity, barometers, the recoil of firearms, and mirrors.

Jacques died of injuries following an accident on the 15th April 1756 when his carriage overturned near his home in Fillerval à Thury-sous-Clermont, Oise. He died the following day, on the 16th April 1756 at the age of seventy-nine.