Pierre-Simon Girard (4 November 1765 – 30 November 1836) was a French mathematician and engineer, who worked on fluid mechanics.
Girard was born in Caen.
A prodigy who invented a water turbine at the age of ten, he worked as an engineer at the École Nationale des Ponts et Chaussées.
He was in charge of planning and construction of the Amiens canal and the Ourcq canal.
He collaborated with Gaspard de Prony on the Dictionnaire des Ponts et Chaussées (Dictionary of Bridges and Highways, 1787).
He wrote on fluids, and in 1798 he published a monograph, Traité analytique de la résistance des solides on beam theory, including possibly its first history, within the topic of strength of materials.
The complicated beam equations were not of practical much use, since he applied Euler's non-linear theory.
In 1799 he led a cultural and scientific expedition to Upper Egypt.