Joseph Louis Lagrange. Coloured portrait of the Italian-born French mathematician Joseph Louis, Comte de Lagrange (1736-1813). Lagrange became a professor at the Turin Royal Artillery School in Italy when he was 19. He later moved to Berlin, Germany, and then to Paris, France. Although his major work, Analytical Mechanics, was started when he was 19, it was only finished when he was 52. In it he treated general mechanical problems using a powerful combination of two forms of calculus. He contributed to number & gravitational theory, and found solutions and proofs for various unsolved problems. He worked on weights & measures and the development of the metric system.