1732-96, American astronomer and instrument maker, b. near Germantown, Pa., self-educated. A clockmaker by trade, he developed great skill in the making of mathematical instruments. He was called upon to determine, with his own instruments, the boundary lines of several states and also part of the boundary known as the Mason-Dixon Line
. In 1769 he was asked by the American Philosophical Society to observe the transit of Venus. His contributions include the use of measured grating intervals and spider threads on the focus of the telescope. Active in public affairs, he was a member of the convention that framed Pennsylvania's constitution and was state treasurer (1777-89) and director of the U.S. mint (1792-95). After the Revolution he was an Anti-Federalist. He succeeded Benjamin Franklin as president (1791-96) of the American Philosophical Society; most of his writings appeared in its Transactions.
See biography by B. Hindle (1964).
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