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73 years

   
In 1795 Jean Baptiste Joseph Delambre was admitted to the Bureau des Longitudes, becoming President in 1800. In 1801 he was appointed secretary to the Académie des Sciences making him the most powerful figure in science in France.

In 1790 The Académie had set up a Commission of Weights and Measures consisting of Borda, Condorcet, Laplace, Legendre and Lavoisier to advise on a metric system of weights and measures. It eventually recommended, in a report of 19 March 1791, that the system be based on a metre defined as one ten millionth part of one quarter of the Earth's meridian. The report was approved by the National Assembly one week later and it remained to calculate a more accurate value of the length of the meridian. The Académie des Sciences appointed Méchain, Legendre and Dominique Cassini to carry out this task.

It was decided to measure, using the method of triangulation with sightings made with the Borda repeating circle which was an extremely accurate new instrument, that part of the meridian between Dunkerque and Barcelona. This was divided into two unequal parts, Dunkerque to Rodez and Rodez to Barcelona. The northern part was much the longer since it had been accurately measured by Cassini de Thury in 1740. Although Dominique Cassini was keen to take charge of the project, he refused to personally measure one sector and, on 5 May 1792 Delambre was given charge of the Dunkerque to Rodez sector and Méchain the Rodez to Barcelona sector. The task proved much harder than anticipated because of the French Revolution and wars with France's neighbours.

By June of 1799 a definitive platinum bar of length one metre was made to become the basis of the metric system. Details of the whole project were published by Delambre in Base du système métrique.
 
 
In 1795 Jean Baptiste Joseph Delambre was admitted to the Bureau des Longitudes, becoming President in 1800. In 1801 he was appointed secretary to the Académie des Sciences making him the most powerful figure in science in France.

In 1790 The Académie had set up a Commission of Weights and Measures consisting of Borda, Condorcet, Laplace, Legendre and Lavoisier to advise on a metric system of weights and measures. It eventually recommended, in a report of 19 March 1791, that the system be based on a metre defined as one ten millionth part of one quarter of the Earth's meridian. The report was approved by the National Assembly one week later and it remained to calculate a more accurate value of the length of the meridian. The Académie des Sciences appointed Méchain, Legendre and Dominique Cassini to carry out this task.

It was decided to measure, using the method of triangulation with sightings made with the Borda repeating circle which was an extremely accurate new instrument, that part of the meridian between Dunkerque and Barcelona. This was divided into two unequal parts, Dunkerque to Rodez and Rodez to Barcelona. The northern part was much the longer since it had been accurately measured by Cassini de Thury in 1740. Although Dominique Cassini was keen to take charge of the project, he refused to personally measure one sector and, on 5 May 1792 Delambre was given charge of the Dunkerque to Rodez sector and Méchain the Rodez to Barcelona sector. The task proved much harder than anticipated because of the French Revolution and wars with France's neighbours.

By June of 1799 a definitive platinum bar of length one metre was made to become the basis of the metric system. Details of the whole project were published by Delambre in Base du système métrique. More...

 
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