of the earth considered as a sphere, but the discovery made by Jean Richer turned the attention of mathematicians to its deviation from a spherical form. The determination of the figure of the Earth became a problem of the highest importance in astronomy, inasmuch as the diameter of the earth was the unit to which all celestial distances had to be referred. The French main unit of length was the Toise of Paris whose standard was the Toise of Châtelet which was fixed outside the Grand Châtelet in Paris from 1668 to 1776. In 1735 two geodetic standards were calibrated against the Toise of Châtelet. One of them, the Toise of Peru was used for the French Geodesic Mission to the Equator. In 1766 the Toise of Peru became the official standard of the Toise in France and was renamed as the Toise of the Academy (French: ). In his famous work ('Theory of the Figure of the Earth, drawn from the Principles of Hydrostatics') published in 1743, Alexis Claude Clairaut synthesized the relationships existing between gravity and the shape of the Earth. Clairaut exposed there his theorem which established a relationship between gravity measured at different latitudes and the flattening of the Earth considered as a spheroid composed of concentric layers of variable densities. Towards the end of the 18th century, the geodesists sought to reconcile the values of flattening drawn from the measurements of meridian arcs with that given by Clairaut's spheroid drawn from the measurement of gravity. In 1789, Pierre-Simon de Laplace obtained by a calculation taking into account the measures of meridian arcs known at the time a flattening of 1/279. Gravimetry gave him a flattening of 1/359. Adrien-Marie Legendre meanwhile found at the same time a flattening of 1/305. The Weights and Measures Commission would adopt in 1799 a flattening of 1/334 by combining the arc of Peru and the data of the meridian of Delambre and Méchain. Geodetic surveys found practical applications in French cartography and in the Anglo-French Survey, which aimed to connect Paris and Greenwich Observatories and led to the Principal Triangulation of Great Britain. The unit of length used by the French was the Toise de Paris, which was divided in six feet. The English unit of length was the yard, which became the geodetic unit used in the British Empire. Despite scientific progresses in the field of geodesy, little practical