让-多米尼克·德·卡西尼Jean-Dominique, comte de Cassini

让-多米尼克·德·卡西尼是塞萨尔-弗朗索瓦·卡西尼·图里的儿子，雅克·卡西尼的孙子，乔凡尼·多美尼科·卡西尼的曾孙。德·卡西尼也被称为德·卡西尼四世。他的母亲夏洛特·德鲁安·德·旺德伊于1747年与塞萨尔-弗朗索瓦德·卡西尼结婚，他们住在巴黎天文台，雅克·卡西尼实际上是那里的台长，尽管这个正式职位是后来才设立的。多米尼克出生在天文台，是他父母两个孩子中的长子。他有一个妹妹弗朗索瓦丝-伊丽莎白。他八岁时，他的祖父雅克·卡西尼去世了。

多米尼克在巴黎天文台接受了早期教育，然后他在巴黎的普莱西斯学院和由奥拉托利会众在瑞伊管理的奥拉托利学院上学。耶稣和圣母无玷奥拉托利会众由皮埃尔·德·贝吕尔于1611年创立，其主要目标之一是提供教育以培养神职候选人。然而，多米尼克德·卡西尼并不希望加入神职，而是希望继承家族传统，因此他继续学习物理、数学和天文学。

1768年，他乘船进行了一次科学航行，任务是测试皮埃尔·勒罗伊发明的一种新型航海天文钟，如果成功，将允许船只在海上确定经度。这次航行带他到了美洲，然后到非洲海岸，最后回到布雷斯特。1770年，他在Voyage fait par ordre du roi en 1768 pour éprouver les montres marines inventées par M Le RoyⓉ（1770年奉国王之命进行的一次航行，以测试勒罗伊先生发明的航海钟）中发表了他的航行记述。在1770年同年，实际上是7月23日，德·卡西尼被选入Académie des Sciences。

1771年，德·卡西尼的父亲塞萨尔-弗朗索瓦·卡西尼·图里被国王任命为巴黎天文台台长，条件是继承权将保留给德·卡西尼家族。在这个阶段，德·卡西尼知道他将在父亲去世后成为天文台台长，因此他怀着这个想法生活。在他成长过程中，他一直知道父亲的主要项目是法国地图，在1770年代，他开始越来越多地参与协助父亲完成这个项目。

1773年，德·卡西尼与克洛德-玛丽-路易丝·德·拉·米尔-莫里结婚。他们有五个孩子：塞西尔、安热莉克、阿琳、亚历克西斯和成为植物学家的亚历山大·亨利·加布里埃尔。1784年父亲去世后，德·卡西尼担任了巴黎天文台台长的角色。他说服国王路易十六修复天文台，并开始重组其运作。然而，他的首要任务是完成他父亲多年来辛勤工作的项目，即法国地图。只有布列塔尼的两张图需要完成，但该项目直到1790年德·卡西尼向国民议会提交地图时才完成。

在完成他父亲的地图时，德·卡西尼从事了另一个测量项目。1787年，他参与了一个与英国科学家的联合项目，以确定格林尼治天文台和巴黎天文台之间的精确距离。这将通过结合两个天文台的数据获得极其有用的科学结果。政府任命德·卡西尼为专员，与阿德里安-马里·勒让德和皮埃尔·梅尚一起，对法国一侧进行三角测量。英国一侧将使用杰西·拉姆斯登的经纬仪进行测量，而法国方面提议使用让-夏尔·德博尔达的重复圆。[关于重复圆的描述见让-夏尔·德博尔达的传记。]让-夏尔·德博尔达重复圆的图片在THIS LINK。德·卡西尼有皮埃尔·梅尚作为他的助手，但他掌控了局面并用重复圆进行测量，而皮埃尔·梅尚则被赋予用旧设备检查结果的任务。德·卡西尼对让-夏尔·德博尔达重复圆的精度印象深刻，写道（见[2]）：-

通常在艺术和科学中，越接近完美，困难的数量就越多地倍增和积累；以至于人们有时会倾向于认为存在一个界限，即使是人的天才和双手也无法跨越，要不是那出乎意料的成功[让-夏尔·德博尔达圆]没有来重新激发我们的信任，并向我们证明对于探究和坚持不懈的人来说没有什么是不可能的。

1789年7月14日早晨，一群武装人员向巴士底狱推进，想要夺取那里储存的武器和弹药。监狱长拒绝让他们进入，这群人便攻占了这座国家监狱，这一行动标志着旧制度的终结和法国大革命的开始。两天后，约300名武装人员强行闯入巴黎天文台，寻找食物、武器和弹药。他们迫使德·卡西尼带他们进入建筑下方的地窖，但他们在那里没有找到任何有用的东西。他们从屋顶取下铅，用来制造弹药。

1791年，悲剧降临到德·卡西尼身上，他的妻子去世，留下五个年幼的孩子需要他抚养。此时，Académie des Sciences正在设立其项目，以精确测量从敦刻尔克到巴塞罗那的子午线，从而获得米的精确值，米将被定义为从北极到赤道距离的一千万分之一。尽管塞萨尔-弗朗索瓦·卡西尼·图里在1740年几乎精确测量了这一距离，但正是让-夏尔·德博尔达重复圆的发明使科学院确信可以实现新的、更精确的测量。1791年4月，Academy任命德·卡西尼、阿德里安-马里·勒让德和皮埃尔·梅尚执行这项任务。6月19日，德·卡西尼、阿德里安-马里·勒让德、皮埃尔·梅尚和让-夏尔·德博尔达觐见了国王路易十六。德·卡西尼后来报告了他与国王的谈话，国王问道：

你愿意再次测量你父亲和祖父在你之前测量过的子午线吗？你认为你能比他们做得更好吗？

德·卡西尼回复道：-

陛下，如果我没有明显的优势，我不敢自认为能超越他们。我父亲和祖父的仪器只能测量到十五秒以内；这里让-夏尔·德博尔达先生的仪器可以测量到一秒以内。

他们会面的第二天，国王逃往瓦雷讷，但被认出并逮捕。德·卡西尼对国王怀有深深的忠诚，但对现在控制法国的革命力量则毫无忠诚可言。他本应出发测量子午线的北段，但既然他必须亲自抚养五个年幼的孩子，他现在不愿这样做。他仍然觉得传统要求他领导这次探险，因此他提议自己留在巴黎指挥，由助手们进行测量。Academy不希望这样安排，认为如果不了解日常工作，他就无法指挥这个项目，因此在1792年5月，他被让·巴蒂斯特·约瑟夫·德朗布尔取代。

对于完全不同情大革命的德·卡西尼来说，事情变得越来越困难。随着期望的变化，管理天文台变得越来越困难。德·卡西尼有三名助手，即[2]：

……最年长的是一位五十岁、性情温和的修士，名叫Nicolas-路易·安托万 Nouet，他也担任天文台牧师，他告诉德·卡西尼他想娶自己的私人女仆。德·卡西尼大为震惊，两人原本关系融洽，从此再未交谈。第二个学生是一位名叫Jean Perny的年轻天文学人才，一天深夜在他所属的革命俱乐部聚会后醉醺醺地回到天文台，用剑柄猛敲他恩人的门，喊道：“必须杀死那个贵族德·卡西尼！他不得不被制服并送上床。……第三个学生Alexandre Ruelle，一个从龙骑兵团逃出的年轻逃兵，德·卡西尼曾收留并培养他，直到他获得赦免，后来却成了他恩人最激烈的敌人。

天文台的学生们指责 德·卡西尼 以他自己的名义发表他们的工作而不给予他们荣誉。事实上 德·卡西尼 一直承认他们的工作，但在这个时期，真相在法国没有立足之地。国民议会改变了天文台的状况，设立了四个教授职位，其中一个给了 德·卡西尼，但薪水只有他之前的一半，而他的学生被任命为其他三个教授职位。Perny 被任命为主任，并达成共识该职位将轮换。德·卡西尼 感到羞辱，于 1793 年 9 月 6 日辞职。

Académie des Sciences已于1793年8月被解散，尽管德·卡西尼尽了最大努力阻止此事发生，因此他无法向那里求助。他的学生告诉他，他必须离开天文台的房间，几周后他照做了。国民议会随后将德·卡西尼地图据为己有，当他抱怨时，他于1794年2月14日被捕入狱。然而，一周后，吕埃勒，这位被提升为天文台教授的学生之一，因伪造观测数据而被监禁。德·卡西尼后来获释，前往塞萨尔-弗朗索瓦·卡西尼·图里的家族城堡居住。让·巴蒂斯特·约瑟夫·德朗布尔和拉朗德恳求他重返科学工作，但他拒绝了。

德·卡西尼 在 1795 年于 Mon apologie 中写道（例如见 [2]）：-

“但你的天文学呢？”你问。我承认，现在对我来说它无关紧要了 .... “但是，”你问，“你的荣耀、你的声誉、你作为学者的职责，不都反对这种退隐吗？”我的朋友，父亲的职责胜过院士的职责。... 至于我的声誉、我的荣耀，我已经牺牲了它们，这对我来说代价很小。... 被迫逃离天文台，我看到 科学院 被交给了无套裤汉政府。而最让我痛心的是，我看到学者们自己武装起来，彼此分裂，参与革命暴徒的狂热和愤怒，采纳他们的道德、他们的举止，甚至他们的语言。... 当他们改变了我们旧的计算方式、旧的度量衡，而我们一天不是十小时，而是二十四小时，也没有四百度的圆周... 我如何能在这些变化中认出自己？一切都变了，而我太老了，无法放弃旧的习惯和想法。年份、月份、历书、天文表，全都变了。如果 伽利略、艾萨克·牛顿 或 约翰内斯·开普勒 从天而降，出现在 科学院，他们不会理解公民 拉朗德 在陈述中的一句话，当他告诉他们，在雾月 20 日，月亮与太阳成 200 度对冲，在五时经过子午线...

1795 年，当被提供经度局的一个职位时，他拒绝了，就像 1796 年 1 月当被提供新国家学院的一个位置时一样，该学院现在取代了 科学院。大约在 1798 年，他确实改变了主意，当时他接受了国家学院的选举，但他进入经度局的尝试被拒绝，他放弃了重返科学的尝试。

Dominique Cassini was the son of César-François Cassini de Thury, the grandson of Jacques Cassini and the great-grandson of Giovanni Domenico Cassini. Dominique Cassini is also known as Cassini IV. His mother, Charlotte Drouin de Vandeuil, married César-François Cassini in 1747 and they lived in the Paris Observatory where Jacques Cassini was effectively the director, although this official position was only created somewhat later. Dominique was born in the Observatory and was the eldest of his parents two children. He had a younger sister Françoise-Elisabeth. When he was eight years old his grandfather Jacques Cassini died.

Dominique received his early education in the Paris Observatory, then he attended the Collège du Plessis in Paris and the Collège Oratorien run by the Congregation of the Oratory at Juilly. The Congregation of the Oratory of Jesus and Mary Immaculate had been founded by Pierre de Bérulle in 1611 with one of the main aims being to give an education to train candidates for the priesthood. Dominique Cassini, however, did not wish to join the priesthood but wished to follow in the family tradition, so he continued his education by studying physics, mathematics and astronomy.

He sailed on a scientific voyage in 1768, given the task of testing a new marine chronometer invented by Pierre Le Roy which, if successful, would allow ships to determine their longitude while at sea. The voyage took him to America, then to the coast of Africa, and finally back to Brest. In 1770 he published an account of his voyage in Voyage fait par ordre du roi en 1768 pour éprouver les montres marines inventées par M Le Roy Ⓣ. In the same year of 1770, actually on 23 July, Cassini was elected to the Académie des Sciences.

In 1771 Cassini's father, Cassini de Thury, was made Director of the Paris Observatory by the King with the conditions that succession would be preserved for the Cassini family. At this stage Cassini knew that he would become Director of the Observatory on the death of his father and so he lived his life with this in mind. As he was growing up he always knew that his father's major project was the map of France and in the 1770s he began to become more and more involved in assisting his father with the project.

In 1773 Cassini married Claude-Marie-Louise de la Myre-Mory. They had five children Cécile, Angélique, Aline, Alexis, and Alexandre Henri Gabriel who became a botanist. After the death of his father in 1784, Cassini assumed the role of Director of the Paris Observatory. He persuaded King Louis XVI to restore the Observatory and he began a reorganisation of its operation. His first task, however, was to complete the project his father had laboured on for so many years, namely the map of France. Only two sheets of Brittany required completion but the project was not finished until 1790 when Cassini presented the map to the National Assembly.

While he was completing his father's map, Cassini worked on another surveying project. In 1787 he was involved in a joint project with English scientists to determine the precise distance between the observatories at Greenwich and at Paris. This would allow extremely useful scientific results to be obtained by combining data from the two observatories. The Government appointed Cassini as a commissioner, along with Legendre and Méchain, to triangulate the French side. The English side would be surveyed using Ramsden's theodolite while the French proposed using Borda's repeating circle. [See Borda's biography for a description of the repeating circle.] A picture of the Borda repeating circle is at THIS LINK. Cassini had Méchain as his assistant but he took control and made the measurements with the repeating circle while Méchain was given the task of checking the results with older equipment. Cassini was very impressed with the accuracy of the Borda repeating circle writing (see [2]):-

Usually in the arts and sciences, the closer one approaches perfection, the more the number of difficulties multiply and accumulate; so that one is sometimes tempted to think that there is a limit beyond which even the genius and hand of man cannot cross, were not that unhoped for success [the Borda circle] did not come to reanimate our trust, and prove to us that nothing is impossible for men of inquiry and perseverance.

On the morning of 14 July 1789 a mob of armed men advanced on the Bastille wanting the arms and munitions stored there. The prison governor refused them entry and the mob stormed and captured the State Prison, an action which came to signify the end of the ancien régime and the beginning of the French Revolution. Two days later, around 300 armed men forced their way into the Paris Observatory looking for food, arms and munitions. They forced Cassini to take them into the cellars beneath the building but they found nothing of use to them. They removed lead from the roof to use to make ammunition.

In 1791 tragedy struck Cassini when his wife died and he was left with five young children to bring up. At this time the Académie des Sciences was setting up its project to accurately measure the meridian from Dunkerque to Barcelona in order to obtain an accurate value for the metre which was to be defined as one ten millionth of the distance from the North Pole to the equator. Although Cassini de Thury had surveyed almost exactly this in 1740, it was the invention of the Borda repeating circle which made the Academy confident that a new much more accurate measurement could be achieved. In April 1791 the Academy appointed Cassini, Legendre and Méchain to carry out the task. On 19 June Cassini, Legendre, Méchain, and Borda had an audience with King Louis XVI. Cassini later reported his conversation with the King who asked:-

Will you again measure the meridian your father and grandfather measured before you? Do you think you can do better than they?

Cassini replied:-

Sire, I would not flatter myself to think I could surpass them had I not a distinct advantage. My father and grandfather's instruments could but measure to within fifteen seconds; the instrument of M Borda here can measure to within one second.

The day after their meeting the King fled to Varennes but was recognised and arrested. Cassini felt deep loyalty to the King, but none to the revolutionary forces now in control of France. He was supposed to set out to measure the northern part of the meridian, but he was reluctant to do so now that he had to bring up his five young children himself. He still felt that tradition required him to lead the expedition and so he proposed that he remain in command in Paris while assistants undertook the surveying. The Academy did not wish this arrangement, believing that he could not command the project without knowledge of the day to day work, so in May 1792 he was replaced by Delambre.

Things were becoming more and more difficult for Cassini who was completely out of sympathy with the Revolution. Running the Observatory became increasingly difficult as expectations changed. Cassini had three assistants, the [2]:-

... eldest, a mild fifty-year old monk named Nicolas-Antoine Nouet, who also served as the Observatory chaplain, informed Cassini that he wished to marry his personal serving-woman. Cassini was horrified and the two men, once cordial, never spoke again. The second student, a young man of astronomical talent named Jean Perny, returned drunk to the Observatory late one night after a meeting of his Revolutionary club and banged on his patron's door with the butt of his sword, shouting, "Cassini the aristocrat must be killed! He had to be subdued and taken to bed. ... The third student, Alexandre Ruelle, a youthful deserter from a dragoon regiment, whom Cassini had harboured and trained until his amnesty came through, became his benefactor's most bitter enemy.

The students in the Observatory accused Cassini of publishing their work under his own name without giving them credit. In fact Cassini had always acknowledged their work but truth had no place in France during this period. The National Assembly changed things at the Observatory by making four posts of Professor, one of which went to Cassini but on half his previous salary, while his students were appointed to the other three professorships. Perny was appointed Director, with the understanding that the post would rotate. Cassini was humiliated and resigned on 6 September 1793.

The Académie des Sciences had already been disbanded in August 1793, despite Cassini's best efforts to prevent this happening, so he could not turn there for help. His students told him that he had to leave his rooms in the Observatory which he did a few weeks later. The National Assembly then took the Cassini map as their own property and when he complained he was arrested and imprisoned on 14 February 1794. However, a week later Ruelle, one of the students who had been elevated to professor at the Observatory, was imprisoned for making up his observational data. Cassini was later released and went to live in the family château at Thury. Delambre and Lalande begged him to return to his scientific work but he refused.

Cassini wrote in Mon apologie in 1795 (see for example [2]):-

"But what of your astronomy?" you ask. I confess, it is nothing to me now .... "But," you ask, "does not your glory, your reputation, your duty as a savant all speak against this retreat?" My friend, the duty of a father surpasses that of an academician. ... And as for my reputation, my glory, I have sacrificed them, and it has cost me little. ... obliged to flee the Observatory, I saw the Academy of Sciences delivered to the government of the sans-culottes. And what grieved me most, I saw the savants themselves up in arms, divided against one another, partaking of the delirium and the rage of the Revolutionary horde, adopting their morals, their manners, and even their language. ... How can I recognise myself in the changes they have wrought in our old ways of calculating, our old measures, when we had not ten hours in a day, but twenty-four, and no circles of four hundred degrees ...? Everything has changed, and I am too old to abandon my old habits and ideas. The year, the months, the almanac, the astronomical tables, all are changed. If Galileo, Newton or Kepler were to descend from heaven and appear at the Academy, they would not comprehend a word in the presentation of Citizen Lalande when he told them that on 20 brumaire, the moon, in a 200 degree opposition to the sun, passed the meridian at five hours ...

When offered a position at the Bureau des Longitudes in 1795, he refused as he did when offered a place at the new National Institute, which now replaced the Academy, in January 1796. He did have a change of heart around 1798 when he did accept on election to the National Institute but his attempts to enter the Bureau des Longitudes were turned down and he gave up on his attempted return to science.