William John Macquorn Rankine FRS (5 July 1820 – 24 December 1872) was a Scottish engineer and physicist. He was a founding contributor, with Rudolf Clausius and William Thomson (1st Baron Kelvin), to the science of thermodynamics. Rankine developed a complete theory of the steam engine and indeed of all heat engines. His manuals of engineering science and practice were used for many decades after their publication in the 1850s and 1860s. He published several hundred papers and notes on science and engineering topics, from 1840 onwards, and his interests were extremely varied, including, in his youth, botany, music theory and number theory, and, in his mature years, most major branches of science, mathematics and engineering.
Born in Edinburgh to British Army lieutenant David Rankine and Barbara Grahame, of a prominent legal and banking family. Rankine was initially educated at home owing to his poor health but he later attended Ayr Academy (1828-9) and, very briefly, the High School of Glasgow (1830). Around 1830 the family moved to Edinburgh; in 1834 he studied at a Military and Naval Academy with the mathematician George Lees; by that year he was already highly proficient in mathematics and received, as a gift from his uncle, Newton's Principia (1687) in the original Latin.