Building it


A slogan was commonly heard at meetings preparing the States general: "one king, one law, one weight, one measure".

Early 1790 Talleyrand has yet to become Prince of Bénévent, indeed he was just a representative of Clergy to the National Assembly. Of course looking after anything that could boost his political carrier. At 36 he was only beginning to be known: as a bishop he had triggered the state taking over of the clergy possessions. Thus any chance had to be seized.


Talleyrand could not miss such an opportunity. People wanted uniform weights and measures, a single foot, a single pound in all provinces, they would get them. But that would not stop there: scientists wanted a system relying on unquestionable scientific foundations, merchants needed a system accepted by all nations, political leaders of the time - intellectuals conversant with the political and philosophical principles of the Age of the Enlightenment - dreamed about a universal system. Where could be found the answer to these rarely converging interests ? By Gabriel Mouton and all those who after him had developed the concept of a coherent system of measures relying on a physical, universal standard.

Talleyrand's speech of 9 March 1790 put the process in motion

Work begins enthusiastically - it is still the "dynamic" period of the Revolution. On a proposal from Jean-Charles Borda (a mariner, a physicist, and still a Knight at that time) the Academy of Science sets up a new committee, which included Condorcet, Lagrange, Gaspard Monge, in order to determine the fundamental unit. Already a mistrust of scientists vis-a-vis politicians ? The committee does not follow the text of the decree but considers three options:

- length of pendulum beating the second at the 45° latitude (no longer mention was made of London: in December 1790, after an excited debate, the House of Commons had declared the French proposal "almost impracticable"...),

- a fraction of the quarter of equatorial circle,

- a fraction of the quarter of a meridian.

But not without difficulties, technical (how to measure the meridian) as well as political (the Reign of Terror). Nevertheless work proceeds: the 1st August 1793 decree legalizes the conclusions of the Academy of Science.

Thus, for the first time, in the midst of political turmoil, decimal division is made official, as well as the metre, the name given to the new unit of length. The law reflects the practice of that time to derive new terms from Latin and Greek: metre comes from the Latin word metrum or from its Greek equivalent metron, meaning measure; are from the Latin area; the cade was a measure, kados, used by Athenians; gravis means heavy in Latin; deci- and centi- stem from Latin (later multiples prefixes would be borrowed from Greek). For all that old good measures were not totally forgotten: the pinte was a unit of measure of grain and liquids, equal (in Paris, for liquids) to about 0,93 litre. Of course this created confusion, and by decree of 19 January 1794 the pinte was replaced by the cadil, one thousandth of a cade; finally, two years later, the cadil, for a while named kanne, would become the litre, from the old French word litron, itself stemming from Greek litra, which was a 12 ounce weight. The gram, which will come in 1795, was derived from the Greek gramma, a unit of weight equivalent to the roman scruple.

Robespierre falls on 9th Thermidor, work resumes, a new decree:

On 18 Germinal year III (7 April 1795) is voted the "decree on weights and measures", indeed an organic law which is generally considered as the law establishing the decimal metric system in France.

However those who drafted the law seem to have no illusions about a fast acceptance of the new system: article 1 invites citizens to "show a mark of their affection for the unity and singleness of the Republic by using from now on the new measures in their calculations and commercial transactions". Farther the text orders the making of a new standard: "a rod made of platinum, on which shall be marked the metre determined as the fundamental unit for the whole system of measures"; the standard shall be deposited "close to the Legislative Assembly" in a monument erected to "contain it and to preserve it from time injuries...".


Here it is, new units are decimal, and republican. So republican that, exceptionally, they will deserve a mention in the Constitution of year III, voted on 5th Fructidor (22 August 1795): its article 371 (of 377...) states that "there is in the Republic uniformity of weights and measures".

Now come the Directoire, the Empire. Revolution, and its justice ideal, is somewhat forgotten - including the new measures, so difficult to get acquainted with.

Other projects were developing: in 1811 Laplace proposed to Napoleon a system of "Napoleonian" measures with an Imperial league of 5 kilometer, an Imperial pound of 500 grams, subdivided in 10 ounces of 10 gros. At least, this remained decimal, whilst the decree finally signed by Napoleon on 12 February 1812 allowed, in parallel to the legal system, the use of customary units arranged to more or less follow the people's habits:

- the toise (fathom) with a value of 2 meters, subdivided in 6 pieds of 12 pouces and 144 lignes,

- the aulne (ell) of 120 centimeters, divided in half, third, quarter, sixth, eighth, twelfth, and sixteenth,

- the boisseau (bushel) of 1/8 hectoliter,

- the submultiples of the liter: half, quarter,

- the livre (pound) of 500 grams, dividing in 16 onces (ounces) of 8 gros of 72 grains each.

Was it the end of a nice dream ? No, King Louis XVIII confirms that its implementation will proceed, then King Louis-Philippe signs the Law of 4 July 1837, making compulsory the use of the metric system by 1st January 1840.

How long did it take ? Almost 50 years. In 1794 Fourcroy spoke about "this venture waited for many centuries, undertaken under kings, who never completed something, which had to be achieved by the Republic". Ironically, it is a king, indeed constitutional, who achieved it. After nine different political regimes, from the most revolutionary Committee of Public Safety to the most absolute Empire. After nine Constitutions. After eleven wars, and two invasions of the French territory. After how many fatalities, including some amongst the creators of the decimal metric system. Sure, to survive all these troubles, it had to be a good concept!




Treaty of Metre

United Kingdom

United States