Petty, Sie William (1623-87), was the son of a clothier at Romsey, in Hampshire. He studied at several foreign universities, and in 1651 was appointed professor of anatomy at Oxford and of music at Gresham College. In 1652 he went to Ireland as physician to the army, and in 1654-55 completed a new survey of the lands forfeited in 1641. He was made surveyor-general of the island by Charles II. His chief invention was a doublebottomed ship, which sailed successfully from Dublin to Holyhead. He published a Treatise on Taxes and Contributions, Political Arithmetic, etc.