Jean le Rond d'Alembert, French mathematician and philosopher, c 1760s.
© Science Museum / Science & Society Picture Library
Engraving by B L Henriquez after M R Tollain of d'Alembert (1717-1783) who made important contributions to mechanics. D'Alembert was illegitimate; his mother left the newborn baby on the steps of the church of St Jean Le Rond, Paris, whence comes his name. D'Alembert clarified the concept of a limit of the calculus, and also introduced the idea of different orders of infinities. In 'TraitE de dynamique' (1743) he helped resolve the controversy over the conservation of kinetic energy by improving Newton's definition of force. D'Alembert's principle is that Newton's Third Law of Motion holds both for fixed bodies and for those able to move. This led to innovations in the study of fluids, vibration of strings and the general wave equation.