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Henry Foster FRS (1797 – 5 February 1831) was a British naval officer and scientist whose pioneering pendulum experiments advanced the study of Earth's shape and magnetic field. His early career included hydrographic surveys in North and South America, where he collaborated with Basil Hall in measuring Earth's ellipticity using an invariable pendulum. Foster took part in multiple polar expeditions, including Clavering’s 1823 voyage to Greenland and Parry’s 1824–1825 Northwest Passage expedition, where he conducted longitude determinations and geomagnetic observations. His extensive pendulum experiments during these missions earned him the Royal Society’s Copley Medal in 1827. Promoted to commander, Foster led the 1828–1831 British Naval Expedition to the South Atlantic aboard HMS Chanticleer, where he explored the South Shetland Islands, Deception Island, and Cape Horn while continuing geodetic and oceanographic research. His promising career was cut short when he drowned in the Chagres River while conducting astronomical observations in 1831. The results of his pendulum studies were posthumously published in 1834 by Francis Baily, further cementing his legacy in geophysical science.