There existed a scientific rivalry, often not friendly, between d’Alembert and Clairaut. At the age of 24, d’Alembert was admitted to the French Academy. In 1743 he published his Traite de dynamique, based upon the great principle of kinetics that now bears his name. In 1744 he applied his principle in a treatise on the equilibrium and motion of fluids, and in 1746 in a treatise on the causes of winds. In each of these works, and also in one of 1747 devoted to vibrating strings he was led to partial differential equations, and he became a pioneer in the study of such equations. With the aid of his principle he was able to the equinoxes. D’Alembert showed interest in the foundations of analysis, and in 1754 he made the important suggestion that a sound theory of limits was needed to put analysis on a firm foundation, but his contemporaries paid little heed to his suggestion. It was in 1754 that d’Alembert became permanent secretary of the French Academy. During his later years he worked on the great French Encyclopedie. Which had been begun by Denis Diderot and himself.