Jean D'Alembert (1717- 1783). Historical artwork of the French mathematician Jean le Rond D'Alembert. D'Alembert investigated gravitational theory, particularly the precession of the equinoxes. He was a pioneer of the study of differential equations and their application in physics. This led him to study the equilibrium and motion of fluids. He sponsored both Lagrange and Laplace who completed this work. He worked on the great encyclopaedia of Diderot, writing the introduction and many of its mathematics and physics articles.