He was employed in the French War Department in 1790, then, beginning in 1792, he became an engineer-geographer with the French army in the Eastern Pyrenees and in Spain. He returned to Paris in 1796 where he was appointed professor at the École Centrale d'Agen. In 1802 he was promoted to engineer-geographer first class. He was sent to the Island of Elba to carry out triangulations there, then after this was sent to Lombardy to carry out a similar task. This was a time when the French were undertaking many projects measuring the earth. Much of this was related to the introduction of the metre which had become obligatory throughout France in 1801. Delambre and Méchain had been measuring the meridian since 1792 and this project was still not complete when Puissant was carrying out triangulations using similar methods. Puissant was fascinated by this major scientific project, and wrote an book on the problem of the shape of the earth.
He was professor of mathematics at the École Centrale de Lot-et-Garonne, then at the military academy at Fontainebleau. He directed the École de Géographes in Paris from 1809 to 1833. These were difficult years during which Napoleon rescinded the metric system in France, then suffered a military defeat with his Russian campaign in 1812, and was finally defeated in 1815. The Bourbons' constitutional monarchy fell in 1830 and revolutions led to fighting in the streets of Paris. Puissant had risen to the rank of lieutenant-colonel by the time he left the army.
On 3 November 1828 Puissant was elected to the Academy of Sciences to fill the vacancy caused by the death of Laplace in the previous year.
He is best remembered for his invention of a new map projection for a new map of France, and he was involved in the production of the map. The map was produced with considerable detail, the projection used spherical trigonometry, truncated power series and differential geometry.
Puissant wrote on geodesy, the shape of the earth and spherical trigonometry. He wrote a mathematics text Cours de Mathématiques. In 1801 he published Recueil de diverses propositions de Géométrie, résolues ou démontrées par l'analyse algébrique, suivant les principes de Monge et de Lacroix: à l'usage de ceux qui suivent le traité Elémentaire d'Application de l'Algèbre à la Géométrie de ce dernier Ⓣ. In 1807 he published Traité de topographie, d'arpentage et de nivellement Ⓣ. A second enlarged edition was produced in 1820. A supplement to this work contains his theory of map projections.
Article by: J J O'Connor and E F Robertson