Alexander Dalrymple FRS (24 July 1737 – 19 June 1808) was a Scottish geographer and the first Hydrographer of the British Admiralty. He was the main proponent of the theory that there existed a vast undiscovered continent in the South Pacific, Terra Australis Incognita. He produced thousands of nautical charts, mapping a remarkable number of seas and oceans for the first time, and contributing significantly to the safety of shipping. His theories prompted a number of expeditions in search of this mythical land, until James Cook's second journey (1772–1775) led to the conclusion that, if it did exist, it was further south than the 65° line of latitude South.
He went to London in 1752 and was appointed a writer in the British East India Company, being first posted to Madras. He arrived in Madras in 1753. While with the East India Company he became interested in the possibilities of trade with the East Indies and China, and subsequently negotiated a treaty with the sultan of Sulu and visited Canton (Guangzhou) at age 22. He was elected Provisional Deputy Governor by the Manila Council with the departure of Dawsonne Drake on 29 March 1764. He then departed Manila 10 April for Sulu with Sultan Alimuddin I.:2, 5-6, 104-106
In 1765 he returned to London where was elected a fellow of the Royal Society. There he became acquainted with the civil engineer John Smeaton, who during the course of his studies on windmills had devised a descriptive scale for grading wind speed. This scale was included in the paper for which he was awarded the Copley Medal. In Defining the Wind: The Beaufort Scale and How a 19th-Century Admiral Turned Science into Poetry, author Scott Huler relates that Dalrymple's voyages had convinced him that a standard scale for measuring the speed of wind at sea would be of great value to sailors, and that he had included Smeaton's scale in his work Practical Navigation, which was written around 1790 but never published. It is believed that Dalrymple conveyed this information to Francis Beaufort, who later refined the wind scale that bears his name and that is still in use today.
Whilst translating some Spanish documents captured in the British occupation of Manila in 1762, Dalrymple had found Luis Váez de Torres' testimony proving a passage south of New Guinea now known as Torres Strait. This discovery led Dalrymple to publish the Historical Collection of the Several Voyages and Discoveries in the South Pacific Ocean in 1770-1771, which aroused widespread interest in his claim of the existence of an unknown continent. Meanwhile, James Cook had been appointed in his place to lead an expedition to the South Pacific which in 1770 discovered the east coast of Australia.
In 1772 Dalrymple published his first set of charts, four of which were of the seas around Sulu. In 1779, he headed the hydrographic office of the East India Company, and that of the Admiralty in 1795.:124-125 He remained Hydrographer of the Royal Navy until his death in 1808.
In 1782 he was elected a foreign member of the Royal Swedish Academy of Sciences.