radical English pamphleteer, member of the House of Commons (1774–80, 1782–84), and inventor, son of the philosopher David Hartley (Hartley, David). As British plenipotentiary he signed the Treaty of Paris (Paris, Peace of) (September 3, 1783), ending the American Revolution, which he had opposed (see Paris, Peace of).

About 1776 Hartley became well known for his method of protecting buildings against fire, and he attracted crowds to experiments performed at his house on Putney Common, London. In London he became a close friend of Benjamin Franklin (Franklin, Benjamin), with whom he corresponded for several years following Franklin's return to America. Believing that a peaceful reconciliation between Great Britain and the North American colonies was possible, he attacked the conflict both in parliamentary speeches and in his Letters on the American War (1778–79). He was also sympathetic toward the French Revolution and critical of the African slave trade.