:: As an introduction ::

The scene takes place in the Ballroom of the Palate of Tileries. The members of the Committee for weights and measures are waiting silently. Somebody is opening the door, Louis XVI is coming in. He is introduced to the scientists - Monge, Meusnier, Borda, Coulomb, Vandermonde, Brisson, Tillet, Lavoisier, Haüy, Legendre, Méchain. As he is marching past Jacques-Dominique Cassini, director of the Paris Observatory, he stops - the two men have known each other for a long time. "Mister Cassini, I have been told that you are to measure a second time the meridian line of Greenwich your father and your grandfather already did before you, do you really think you will do it better ?"

Cassini, who was surprised, answered nevertheless : "Lord, I would not boast of doing better if I had not a great advantage over them. The instruments my father and my grandfather used only gave the measure to within about fifteen seconds. The Chevalier of Borda here, [he points at Borda, standing amidst his colleagues] has devised one which will give me the measure of angles with a one-minute accuracy, this will be my credit."

The scene takes place on 19 June 1791. The day after this interview, just before midnight, Louis XVI was leaving the Tileries, heading for the kingdom East border. Since a postmaster, Jean-Baptiste Drouet, recognized him, Louis XVI was arrested in Varennes.

But what is this so accurate instrument ? It is a very special theodolite – the repeating circle which was invented a few years before by Jean-Charles Borda, a physicist and sailor.

Based on La revue n° 30 (June 2000), page 56, Denis Guedj

 Return of the royal family in Paris, June 25, 1791, after the escape and the arrest in Varennes.
(Colored engraving , Paris, Musée Carnavalet -  click on the picture to increase it)

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