The line of longitude used to determine the length of the metre runs through the centre of the Paris Observatory Credit: Madhvi Ramani. As Dr Alder details in his book, measuring this meridian arc during a time of great political and social upheaval proved to be an epic undertaking.
The two astronomers were frequently met with suspicion and animosity; they fell in and out of favour with the state; and were even injured on the job, which involved climbing to high points such as the tops of churches. The Pantheon, which was originally commissioned by Louis XV to be a church, became the central geodetic station in Paris from whose dome Delambre triangulated all the points around the city. But despite all the technical mastery and labour that had gone into defining the new measurement, nobody wanted to use it.
People were reluctant to give up the old ways of measuring since these were inextricably bound with local rituals, customs and economies. For example, an ell, a measure of cloth, generally equalled the width of local looms, while arable land was often measured in days, referencing the amount of land that a peasant could work during this time.
Eventually, in , Napoleon abandoned the metric system; although it was still taught in school, he largely let people use whichever measures they liked until it was reinstated in This was not just due to perseverance on the part of the state. Of course, it was tricky to do this unless you had clear, standard measures, such as the metre and the kilogram.
Originally established to preserve international standards, the BIPM promotes the uniformity of seven international units of measurement: the metre, the kilogram, the second, the ampere, the kelvin, the mole and the candela. It is the home of the master platinum standard metre bar that was used to carefully calibrate copies, which were then sent out to various other national capitals.
In the s, the BIPM redefined the metre in terms of light, making it more precise than ever. And now, defined by universal laws of physics, it was finally a measure truly based on nature. The small, cylindrical weight cast in platinum-iridium alloy is also, like the metre, due to be redefined in terms of nature — specifically the quantum-mechanical quantity known as the Planck constant — by the BIPM this November. As he explained the principle of the Kibble balance and the way in which a mass is weighed against the force of a coil in a magnetic field, I marvelled at the latest scientific engineering before me, the precision and personal effort of all the people who have been working on the kilogram project since it began in and are now very close to achieving their goal.
I have the honour to be, Sir, with due respect, your most humble and obedient servant, The Bishop of Autun Talleyrand 's proposals for a new measuring method were based on a survey he had done on the measures currently in use in France. Heilbron : The existence of French men and women around was made miserable by, among other things, or differently named measures and untold units of the same name but different sizes.
A 'pinte' in Paris came to 0. The aune, a unit of length, was still more prolific: Paris had three, each for a different sort of cloth; Rouen had two; and France as a whole no fewer than seventeen, all in common use and all different, the smallest amounting to just under lignes, royal measure, the largest to almost France possessed non-uniform measures in law as well as by custom. Their multiplicity went with other relics of the feudal system, which maintained arbitrary rents and duties usually to the disadvantage of the peasant.
A landlord wanted his bushels of grain or hogsheads of beer in the biggest measures in use in the neighborhood, and he preferred to sell according to the smallest. Nor were all seigneurs above enlarging the vessel in which they collected their rents; and since in many cases they possessed the only exemplars of their patrimonial bushel, no one could be certain that it did not grow in time. But one suspected.
A frequent complaint in the cahiers, or notebooks of desiderata brought by representatives of the people to the meeting of the Estates General in , was that 'the nobles' measure waxes larger year by year.
These same representatives castigated the oppressive confusion of customary measures as barbaric, ridiculous, obscurantist, gothic, and revolting, and demanded an end to them, and the establishment of a system of unchanging and verifiable weights and measures throughout the country, or at least throughout their region. Many urged that the King's measure, the royal foot, be made the law of the land.
Sharpers and crooks whose practices were not sanctioned by ancient rights and wrongs and middlemen acting in analogy to money changers opposed the rationalization that menaced their livelihood.
Talleyrand 's concept was for the adoption of a brand new basic standard, ' derived from nature ' pris dans la nature and therefore acceptable to all nations. Talleyrand further suggested that the French National Assembly, the English Parliament, and the Royal Society of London should undertake preliminary work towards this objective jointly. He wrote: Perhaps this scientific collaboration for an important purpose will pave the way for political collaboration between the two nations.
He then made a speech to the British House of Commons proposing extensive measurement reform. His report to the Assembly included a detailed analysis of the extremely muddled state of French weights and measures. Talleyrand proposed to the National Assembly a decimal system of stable, unvarying and simple measurement units. At Talleyrand 's suggestion, the French National Assembly adopted this new measuring system. Louis XVI authorised scientific investigations aimed at reform of all French weights and measures and these investigations led to the development of the 'decimal metric system' as the legal measurement system, firstly in France with the passage of several laws mostly in the s, and then in the rest of the world.
Talleyrand also suggested that the Academy of Sciences in Paris collaborate with the Royal Society of London in defining the new measuring unit. A French politician, La Rochefoucault , had this to say in the National Assembly in support of Talleyrand 's proposal: We cannot make enough haste over promulgating this decree, which should bring about fraternal relations between France and England. As a result, even though it was in the middle of the French revolution, the National Assembly of France requested the French Academy of Sciences to deduce an invariable standard for all the measures and all the weights and to prepare a report on the development of a system of measurement for France and for the world.
The French National Assembly then sent delegates to Britain, Spain and the USA to propose cooperation in developing a universal system of units for measurement.
However, subsequent historians have not been able to find such a letter in the Royal Archives at Windsor Castle. Thomas Jefferson wrote Thomas Jefferson 's report carried considerable influence in the Congress of the USA as he was the first Secretary of State of the USA for President George Washington , but no official action was taken and the Congress passed no legislation relating to weights and measures as a result of Jefferson 's report.
Jefferson 's report used some of the scientific investigations aimed at reform of the French weights and measures but it varied in the detail. Jefferson 's proposals also had a remarkable similarity to the design for a ' universal measure ' outlined by John Wilkins in This conjecture seems more likely when we compare Wilkins ' plan for length with that of Jefferson.
Jefferson defined a standard length using a seconds pendulum then he wrote: Let the foot be divided into 10 inches; the inch into 10 lines; and the line into 10 points. Let 10 feet make a decad; 10 decads one rood; 10 roods a furlong; and 10 furlongs a mile.
And there are many other parallels. In many respects Jefferson's plan might have been taken straight from John Wilkins' essay with only slight changes to the names of the various components of the plan and a few minor differences.
For example Jefferson suggested a pendulum that had a rod instead of a string. Jefferson was a very keen book collector and I, and several very helpful librarians, have searched many catalogs of his extensive collections. The first part of Jefferson 's plan proposed the adoption of a universal length based on the seconds pendulum, measured at 45 degrees North latitude at sea level and the changing of existing old English units to this new universal measure. The second part of Jefferson 's plan proposed the use of a decimal system as a basis for dividing and multiplying the seconds pendulum unit to reduce Talleyrand 's proposal, having been referred to the Committee on Agriculture and Commerce, was recommended to the king, who sanctioned action on August This was the French decree that led to the further development of the metric system.
Their first report, in October, recommended the decimal division of money, weights, and measures. This situation caused difficulties for internal and international commerce, made worse by the need to calculate in twelfths, sixteenths or other fractions when converting from one system to another.
When the metric system was first introduced all units were divided decimally, making calculation easier. As well as explaining decimal arithmetic, Stevin advocated the decimal division of weights, measures and currency 3. Other mathematicians adopted the decimal fractions. An early proposal for a decimal system of measures came in from a Frenchman, Gabriel Mouton — , a parish priest in Lyons with a good knowledge of astronomy and mathematics. He deplored the variety of units of length and proposed a natural unit based on the size of the Earth.
His gold medal depicted at the tap of his label was awarded by the French government for the encouragement of industry and the useful arts, probably as a prize for work displayed at an exhibition of French industry in or Reproducedfrom A. However, Picard did not advocate its decimal division. By now it was suspected that the Earth was not a perfect sphere and that the length of both a degree of longitude and the seconds pendulum which depends on its distance from the centre of the Earth might vary from place to place 5.
This later became an obstacle to international acceptance of the metric units determined in France. During the eighteenth century the lack of an international system of weights and measures affected the development of science as well as commerce. Watt proposed that all chemists should adopt the same pound, preferably that of Paris which was the most widely used in Europe, and that it should be divided decimally 6.
It is noteworthy that Lavoisier expressed some weights as decimal fractions of a pound, as well as the ounces and grains that he had measured in the laboratory. Edinburgh, , p. Professor Robertson has not been identified. English avoirdupois weights were generally used in commerce; troy weights, used for pharmaceuticals and bullion, were preferred by most chemists. In France, public discontent with many aspects of life in an absolute monarchy forced King Louis XVI and his government to call elections to the States-General, the only elected parliamentary body, for the first time in years.
It met in May with the new name of National Assembly and assumed the powers of government. Although the Assembly received many complaints about the lack of uniform weights and measures, it was unable to act immediately. In June the Paris Academy of Sciences independently appointed several members to a Commission of Weights and Measures, with the task of producing a national system.
However, as no progress had been made by May , the Assembly formally asked the Academy to act and provided the necessary funds. One member of the Assembly with a special interest in the project was Charles Maurice Talleyrand — He was not a scientist but was almost certainly advised by members of the Academy. He favoured a system based on the length of the seconds pendulum, with the unit of weight defined as the weight of water filling a cube of side equal to a specified fraction of that length.
He did not, however, recommend the decimal division of the new units. Talleyrand hoped that the system would be adopted by other countries and proposed that the pendulum should be measured at a place that would be internationally acceptable: sea level half-way between the North Pole and the Equator.
This was the 45th parallel, which conveniently crossed the French coast near Bordeaux. Miller lost his seat at the ensuing election and the matter was not raised again in Parliament.
In the United States of America, soon after becoming independent from Britain, adopted a decimal currency, and by Congress was considering a decimal system of measures based on the pendulum. This was proposed by the Secretary of State, Thomas Jefferson, who had an interest in science, but after much discussion it was decided to retain the British weights and measures 8.
Spain was the only country at this time to show an interest in the French proposals. Back in France, in September , the Academy of Sciences instructed several members to determine the length of the seconds pendulum and the measures derived from it, but before work was started there was a dramatic volte-face. No explanation was given for the rejection of the pendulum, which had been the preferred unit for more than a century.
The academicians pointed out that the Paris meridian passed almost exactly through Dunkirk, on the north coast of France, and only a short distance from the Spanish city of Barcelona, both at sea level, which differed in latitude by 9 degrees and 40 minutes, just over a tenth of the quadrant The latitudes could be determined by astronomers with the best available instruments and the linear distance by the well established method of triangulation, starting from a carefully measured base line.
The total length of the quadrant could then be calculated and the fundamental unit derived from it. Much of the meridian had been measured in the s, when a large-scale map of France was being prepared, and since then surveying instruments had been improved. Borda, an engineer with a distinguished naval career as a navigator, had recently perfected his repeating circle, which in skilled hands enabled celestial or terrestrial angles to be determined to within a tenth of a second of circular measure.
There have been suggestions that the desire of the Academy to demonstrate its effectiveness may have been partly responsible for the abandonment of the pendulum as a standard 9. However, as well as measuring the meridian, the Academy decided to determine very accurately the length of the seconds pendulum at Paris, and the task was undertaken by Borda and Jean Dominique Cassini — , director of the Paris Observatory.
They completed it at the Observatory in the summer of , before the meridian survey was started. The archives of the Academy are sparse for this period, so nothing is known about any discussions that went on behind the scenes, but it is possible that the measurement of the meridian was intended to draw attention to the importance of the Academy at a time when, like many institutions of the old regime, it was under attack from extreme revolutionaries The apparatus used by Borda and Cassini was constructed by Etienne Lenoir — , an instrument maker, born in Mers, a village near Blois in the Loire valley.
After being apprenticed to a locksmith he worked in that trade until He then found employment with a mathematical instrument maker in Paris and studied mathematics by attending one of the free courses available to craftsmen. He set up his own business, supplying specialised astronomical instruments of high quality to leading scientists as well as making mathematical instruments for a larger market, and around he collaborated with Borda in perfecting the repeating circle His close association with Borda made him an obvious choice to construct the pendulum apparatus at the Observatory, Figure 2.
This consisted of a platinum sphere about 1. This oscillated with a half-period of about 2 seconds. As air resistance affected the period, platinum was chosen because it was the metal with the highest specific gravity and thus occupied the smallest volume for a given mass. In order to eliminate errors arising from irregularities in the shape of the sphere, which would alter its centre of gravity, it was, with the aid of a little grease, fitted into an inverted hemispherical copper cup at the end of the wire so that readings could be taken with the sphere in several positions and a mean result calculated.
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