Douglas Rayner Hartree (27 March 1897 – 12 February 1958) was an English mathematician and physicist most famous for the development of numerical analysis and its application to the Hartree–Fock equations of atomic physics and the construction of a differential analyser using Meccano. Early life and education Douglas Hartree was born in Cambridge, England. His father, William, was a lecturer in engineering at the University of Cambridge and his mother, Eva Rayner, was president of the National Council of Women of Great Britain and first woman to be mayor of the city of Cambridge. One of his great-grandfathers was Samuel Smiles; another was the marine engineer William Hartree, partner of John Penn. Douglas Hartree was the oldest of three sons that survived infancy. A brother and sister died in infancy when he was still a child, but his two brothers would later also die. Hartree's 7-year-old brother John Edwin died when Hartree was 17, and Hartree's 22-year-old brother Colin William died from meningitis in February 1920 when Hartree was 23. His maternal cousin was the geologist Dorothy Helen Rayner. Hartree attended St John's College, Cambridge but the first World War interrupted his studies. He (and his father and brother) joined a group working on anti-aircraft ballistics under A. V. Hill, where he gained considerable skill and an abiding interest in practical calculation and numerical methods for differential equations, executing most of his own work with pencil and paper. According to Hill, writing in Hartree’s obituary, ‘Quietly one day he improvised a long-base height-finder out of some wires, posts, and a steel tape’. This became known as the Hartree height-finder and was used extensively by British Anti-Aircraft troops until better optical height-finders were introduced. Its advantage was said to be that the height can be calculated from the observed quantities ‘very rapidly by the use of nothing but simple arithmetic’. It was also cheap to manufacture and easy to use. After the end of World War I, Hartree returned to Cambridge graduating in 1922 with a Second Class degree in natural sciences. Atomic structure calculations In 1921, a visit by Niels Bohr to Cambridge inspired Hartree to apply his numerical skills to Bohr's theory of the atom, for which he obtained his PhD in 1926 – his advisor was Ernest Rutherford. With the publication of Schrödinger's equation in the same year, Hartree was able to apply his knowledge of differential equations