让-费利克斯·皮卡德Jean Picard

让-费利克斯·皮卡德出生于卢瓦尔河畔的拉弗莱什镇。在8中奥姆斯特德的研究发表之前，人们对他的家庭和早年生活一无所知。我们给出的关于皮卡德的家庭和早年生活的细节均取自这部著作。皮卡德的父亲也叫皮卡德，是拉弗莱什的一名书商，而他的母亲玛丽·雷泽原籍勒芒。本传记的主人公皮卡德来自一个相当大的家庭，其中我们了解两个年幼弟妹的细节：勒内生于1623年，夏洛特生于1625年。

皮卡德曾在拉弗莱什的耶稣会学院学习。拉弗莱什的城堡由亨利四世的祖父母建造，他于1603年将其赠予耶稣会士以创办一所学院。该学院将被称为亨利大帝皇家学院，并获得了“选拔和培养当时最优秀的人才”的指示。学院成立后最初几年最著名的学生之一是勒内·笛卡儿。到皮卡德在那里学习时，该学院已享有法国最好的教育机构之一的声誉[4]：-

它有超过一千名学生就读——有些是走读生（据推测皮卡德就是这种身份），另一些是寄宿生——由于规模庞大，它提供了耶稣会所能提供的最全面的教育。至于皮卡德的学业在多大程度上被17世纪30年代和40年代法国那一地区如此普遍的民间动乱所打断，这只能靠推测；而耶稣会学院本身也是一个极不稳定的机构，因为社会秩序的崩溃也影响了学生的行为。在17世纪30年代末和40年代初（即赤脚党起义时期）出现了几起违纪问题。

皮卡德于1644年左右离开拉弗莱什的耶稣会学院，前往巴黎居住。皮埃尔·伽桑狄于1645年被任命为皇家学院的数学教授，很可能皮卡德在那里听过他的讲座。无论如何，皮卡德大约在这时成为皮埃尔·伽桑狄的弟子，并帮助他观测了1645年8月21日的日食。从那时起直到1655年皮埃尔·伽桑狄去世，两人一直保持联系；例如，有记录显示皮卡德在1646年和1647年协助皮埃尔·伽桑狄观测月食。他在这些观测中很大程度上是助手，因为我们知道他的职责是架设设备并在皮埃尔·伽桑狄的指导下使用设备。尽管奥姆斯特德做了研究，关于皮卡德的生平仍有许多谜团，此处应提及一个。他从拉弗莱什的耶稣会学院——在那里他本会学到一些数学，但不会很多——来到巴黎，不久之后他的知识就足以成为顶尖科学家皮埃尔·伽桑狄的助手。没有证据表明他听过任何高等数学或天文学讲座，因此合理的猜测是，他在这些科目上主要是自学成才。他父亲是书商这一事实可能与此相关，因为这使皮卡德能够接触到高级文本，他可以自学这些文本。

同样不清楚的是，皮卡德在巴黎这些年是如何维持生计的。除了他与皮埃尔·伽桑狄合作的少量记录外，没有其他证据表明他做过天文学研究（或任何其他研究）。当然，与皮埃尔·伽桑狄的任何合作都在1648年结束，因为那一年皮埃尔·伽桑狄因健康原因离开巴黎前往普罗旺斯。1655年，皮卡德在同年10月皮埃尔·伽桑狄去世后成为巴黎法兰西公学院的天文学教授，但并非凭借任何已发表的著作，因为当时没有任何著作问世。到这时他已获得杰出声誉，1655年首次访问巴黎的克里斯蒂安·惠更斯会特意找他讨论研究课题。皮卡德于1666年成为Académie Royale des Sciences的成员，就在该院成立后不久，从此致力于为科学院工作。同样值得注意的是，他在当选科学院院士之前没有发表过任何东西。

此时我们还需要考察另一个谜团。在他后来的著作中，他自称“里耶修道院长”。确实，在安茹有一个叫里耶的镇子，离皮卡德出生的拉弗莱什不远，人们推测这就是他担任神父和修道院长的地方。如果是这样，那么他何时接受神职培训，又是在人生的哪个阶段实际履行里耶修道院长的职责？斯特迪和其他一些历史学家一样，对里耶是否就是安茹的那个地方表示怀疑，并建议[4]：-

……他可能只是一个受俸修道院长，即一个领取圣俸收入的俗人。

这也会给皮卡德带来收入，并解释关于皮卡德生平的其他谜团。现在让我们看看他为科学院所做的工作。

皮卡德设计了一种测微计来测量太阳、月亮和行星等天体的直径。1667年，他在象限仪上添加了望远镜瞄准器，使其在观测中更加有用。亚伯拉罕·阿德里安·艾伯特 van Helden写道[11]：-

测微器和望远镜瞄准器不是皮卡德的发明；这些我们必须归功于其他人——William Gascoigne、克里斯蒂安·惠更斯、阿德里安·奥祖和Pierre Petit。皮卡德是一位腼腆而谦逊的修道院长，他采纳了他们的创新，并将其系统地应用于天文学、大地测量学和水准测量。皮卡德的热爱是精确性，在这一领域他无与伦比；就连约翰·佛兰斯蒂德也无法匹敌他的精确度。皮卡德是一位不知疲倦的工作者，当其他人在巴黎出风头时，他常常远赴外省或国外从事某个重要项目。

皮卡德使用威理博·斯涅尔的三角测量方法，极大地提高了地球测量的精确度。他测量了子午线的弧长；这些测量结果出现在Mesure de la TerreⓉ（测量地球）（1671年）中。他首先精确测量了从Villejuif到Juvisy-sur-Orge的基线，然后使用十三个三角形，通过三角测量沿巴黎子午线从巴黎南郊的Malvoisine到亚眠附近Sourdon的钟楼测量了一度纬度的长度。最后他测量了恒星的高度。他用于完成这项工作的所有仪器都装有望远镜瞄准器，这使他得到的值精确到10角秒（第谷·布拉赫仅达到4角分的精确度），他得出的地球半径值仅比正确结果低0.44%。这些技术的使用意味着皮卡德是最早将科学方法应用于地图制作的人之一。他制作了巴黎地区的地图，然后加入了一个绘制法国地图的项目。他关于地球的数据被艾萨克·牛顿用于他的引力理论。皮卡德因这些贡献而受到表彰，1740年在他基线一端的Juvisy-sur-Orge竖立了一座金字塔。

1671年，皮卡德前往位于瑞典文岛上的第谷·布拉赫天文台，以便准确测定其位置，并使第谷的观测能与其他人的观测直接比较。在那里，他与Ole Rømer一起工作了几个月，观测了约140次木星卫星Io的食，同时巴黎的乔凡尼·多美尼科·卡西尼进行了相同的观测。1673年，皮卡德搬到巴黎天文台，在那里他与让·里歇尔、乔凡尼·多美尼科·卡西尼以及稍后的菲利普·德拉伊尔合作。Rømer在访问文岛后也去了巴黎天文台，并与皮卡德一起继续观测木星的卫星。正是从皮卡德和Rømer收集的数据中，Rømer通过注意到由于光传播到地球所需时间而导致的食时间差异，能够计算出光速。我们还应该提到皮卡德的另一项重要发现，即他在1675年发现的气压计发光。这是在摇动气压计管时，在水银上方的真空中看到的发光现象。这一发现促使艾萨克·牛顿进行了导致光谱学的研究。

皮卡德 也参与了秒摆长度的测量。发现该长度并非恒定，引出了第一个证明：地球并非完美球体，而是在两极处扁平。同样在巴黎天文台，皮卡德 参与了火星视差的测量。这很重要，因为一个准确的数值将给出太阳系的大小。皮卡德 与 乔凡尼·多美尼科·卡西尼 和 让·里歇尔 合作，推断出的地球到太阳的距离比真值低估了8.5%。同样在巴黎天文台，皮卡德 试图测量附近恒星的视差，从而验证地球绕太阳运行这一事实。1680年，他发现了第一个可证实的变动，但当时光行差尚未被理解，因此不能将首次恒星视差的发现归功于他（更多细节见[9]）。

皮卡德的其他技能是水力学，他不仅就此写作，还将他的技能付诸实践。他解决了为凡尔赛宫的喷泉供水的问题。艾伯特 van Helden写道[11]：-

凡尔赛宫的许多水池和喷泉需要大量的水，而宫殿在这方面位置不佳。为解决这个问题投入了巨大的工程努力，皮卡德带有望远镜瞄准器的新水准测量仪器帮助确定了路线并避免了代价高昂的错误。

皮卡德与他那个时代的许多顶尖科学家通信，包括拉斯穆·巴多林、约翰·赫维留、Hudde和克里斯蒂安·惠更斯。他出版了1679年的第一本Connaissance des tempsⓉ（时间知识），并在接下来的四年里继续出版这部年度著作。自那时起它一直连续出版。然而，他的天文观测在他生前没有发表。事实上，在他去世60年后，才由Pierre-Charles Le Monnier发表。艾伯特 van Helden以对皮卡德的这番评价结束了他的评论[11]：-

……现在显而易见，现代精密测量在多大程度上归功于这位安静、谦逊、远离聚光灯的人。

Jean Picard was born in La Flèche, a town on the banks of the Loire. Nothing was known about his family and early life until the research of Olmsted presented in [8]. The details we give about Picard's family and early life are taken from this work. Jean Picard's father, also called Jean Picard, was a bookseller in La Flèche while his mother, Marie Rezé, came originally from Le Mans. Jean, the subject of this biography, was one of a fairly large family of which we know details of two younger children; René born in 1623 and Charlotte born in 1625.

Jean studied at the Jesuit College at La Flèche. The castle in La Flèche, which had been built by the grandparents of Henry IV, was given by him to the Jesuits in 1603 to found a College. It was to be called the Collège Royal Henry-Le-Grand and was given instructions "to select and train the best minds of the time". One of the most famous students at the College in the years immediately after its foundation was René Descartes. By the time Picard studied there, the College had the reputation of being one of the best educational establishments in France [4]:-

It had more than a thousand students in attendance - some as day boys (presumably Picard's status) and others as borders - and because of its size, provided as comprehensive an education as could be offered by the Jesuit Order. It must remain a matter of conjecture as to how far Picard's studies were interrupted by the civil unrest which was so prevalent in that part of France in the 1630s and 1640s; and the Jesuit College itself was a highly volatile institution as the breakdown of social order affected the conduct of the students too. There were several problems of indiscipline in the late 1630s and early 1640s (the time of the revolt of the Nu-Pieds).

Picard left the Jesuit College at La Flèche around 1644 and went to live in Paris. Pierre Gassendi was appointed as Professor of Mathematics at the Collège Royale in 1645 and it is likely that Picard attended his lectures there. At any rate Picard became a disciple of Gassendi around this time and helped him with observations of a solar eclipse on 21 August 1645. The two would remain in contact from that time on until Gassendi's death in 1655; for example there are records of Picard assisting Gassendi in observations of lunar eclipses in 1646 and 1647. He was very much the assistant for these observations, for we know that his responsibility was setting up the equipment and using it under Gassendi's directions. There are a number of puzzles which, despite Olmsted's research, still remain about Picard's life and one should be mentioned at this point. He had gone from the Jesuit College at La Flèche, where he would have learnt some mathematics but not a great deal, to Paris where very soon after he was knowledgeable enough to be an assistant to the leading scientist Gassendi. There is no evidence that he attended any advanced mathematics or astronomy lectures, so it is a reasonable guess to assume that he was largely self-educated in these topics. The fact that his father was a bookseller could be relevant here, for it would have given Picard access to advanced texts which he could have studied on his own.

What is also unclear is how Picard was supporting himself during these years in Paris. Other than the few records of his work with Gassendi, there is no other evidence of his astronomical researches (or any other for that matter). Certainly any work with Gassendi ended in 1648 for in that year Gassendi left Paris and went to Provence for health reasons. In 1655 Picard became professor of astronomy at the Collège de France in Paris, following the death of Gassendi in October of that year, but not on the strength of any published work for none had appeared. By this stage he had gained an outstanding reputation and Huygens, who made his first visit to Paris in 1655, would seek him out to discuss research topics. Picard became a member of the Académie Royale des Sciences in 1666, just after its foundation, and from this time on devoted himself to working for the Académie. Again it is worth noting that he had published nothing before his election to the Academy.

There is another puzzle that we need to examine at this point. In his later writings he referred to himself as the 'Abbé of Rillé.' Certainly there is a town of Rillé is in Anjou, not that far from La Flèche, where Picard was born, and it has been assumed that this was where he had worked as a priest and prior. If this is so, then when did he train for the priesthood, and at what point in his life did he actively fill the role of Abbé of Rillé? Sturdy, like a number of other historians, casts doubt on whether Rillé is the place in Anjou, and also suggests [4]:-

... he may simply have been an abbé commendataire i.e. a layman who drew that revenues of a benefice.

This would also have given Picard an income and explain other puzzles about Picard's life. Let us now look at his work for the Academy of Sciences.

Picard devised a micrometer to measure the diameters of celestial objects such as the Sun, Moon and planets. In 1667 he added a telescopic sights to the quadrant making it much more useful in observations. Albert van Helden writes [11]:-

The micrometer and telescopic sights were not Picard's ideas; for these we must go to others - William Gascoigne, Christiaan Huygens, Adrien Auzout, and Pierre Petit. Jean Picard was a shy and modest abbot who took their innovations and applied them systematically to astronomy, geodesy, and levelling. Picard's passion was precision, and in this area he was unsurpassed; not even John Flamsteed could match his accuracy. Picard was a tireless worker, often away in the provinces or abroad on some important project while others were in the limelight in Paris.

Picard greatly increased the accuracy of measurements of the Earth, using Snell's method of triangulation. He measured the length of the arc of the meridian; the measurements appear in Mesure de la Terre Ⓣ (1671). He began by accurately measuring his baseline from Villejuif to Juvisy-sur-Orge then, using thirteen triangles, measured by triangulation one degree of latitude along the Paris Meridian from Malvoisine, in the southern suburbs of Paris, to the clock-tower of Sourdon near Amiens. Finally he measured the altitudes of stars. All the instruments he used to carry out this work were fitted with telescopic sights which gave him values correct to 10 seconds of arc (Tycho Brahe had only attained an accuracy of 4 minutes of arc) and he produced a value for the radius of the Earth which was only 0.44% below the correct result. The use of these techniques meant that Picard was one of the first to apply scientific methods to the making of maps. He produced a map of the Paris region, and then went on to join a project to map France. His data on the Earth was used by Newton in his gravitational theory. Picard was honoured for these contributions with a pyramid erected at Juvisy-sur-Orge, one end of his baseline, in 1740.

In 1671 Picard went to Tycho Brahe's observatory at Hven Island in Sweden so that its location could be determined accurately and so Tycho's observations could be directly compared with others. There over a period of several months, working with Ole Rømer, he observed about 140 eclipses of Jupiter's moon Io, while Giovanni Domenico Cassini in Paris carried out the same observations. In 1673 Picard moved to the Paris Observatory where he collaborated with Jean Richer, Giovanni Domenico Cassini and, slightly later, with La Hire. Rømer also went to the Paris Observatory following the visit to Hven Island and, with Picard, continued to observe the moons of Jupiter. It was from the data that Picard and Rømer collected that Rømer was able to calculate the speed of light by noting the discrepancies in the timings of the eclipses due to the time light takes to travel to the Earth. We should also mention another significant discovery by Picard, namely his discovery of barometric light in 1675. This is the luminous glow which is seen in the vacuum above the mercury in a barometer when the tube is shaken. This discovery prompted Newton to make studies which led to spectrometry.

Picard was also involved with the measurement of the length of the second pendulum. Finding that the length was not constant led to the first proof that the Earth was not a perfect sphere but was flattened at the poles. Also at the Paris Observatory, Picard was involved with measuring the parallax of Mars. This was important since an accurate value would give the scale of the solar system. The value Picard, working with Giovanni Domenico Cassini, and Jean Richer, deduced for the distance from the Earth to the sun underestimates the true value by 8.5%. Also at the Paris Observatory, Picard tried to measure the parallax of nearby stars and so verify the fact that the Earth orbits the sun. In 1680 he found the first certifiable variation but at this time aberration was not understood, so he cannot be credited with the discovery of the first stellar parallax (see [9] for further details).

Among Picard's other skills were hydraulics, a topic on which he wrote but also one where he put his skills into practice. He solved the problem of supplying the fountains at Versailles with water. Albert van Helden writes [11]:-

The many ponds and fountains of Versailles needed a copious supply of water, and the palace was not well situated in this respect. Prodigious engineering efforts went into the solutions to this problem, and Picard's new levelling instruments with telescopic sights helped determine routes and avoided costly errors.

Picard corresponded with many of the leading scientists of his time including Bartholin, Hevelius, Hudde and Huygens. He published the first Connaissance des temps Ⓣ for the year 1679 and continued to produce the annual work for the following four years. It has been published continuously since then. However, his astronomical observations were not published in his lifetime. In fact it was 60 years after his death before they were published by Pierre-Charles Le Monnier. Albert van Helden ends his review with this assessment of Picard [11]:-

... it is now evident just how much modern precision measuring owes to this quiet, unassuming man who stayed out of the limelight.