Irish-French mathematician and army officer.

Aide-de-camp to Count Fitzjames, commander of a French force dispatched to assist the Jacobite forces in Scotland (1745). Captured by the English; paroled (1747). Captain in the Condé regiment (1749); served in Seven Years’ War and rose to the rank of maréchal de camp (1770). Member of the Académie des sciences (1749). Published works on mechanics and artillery. Helped invent an electrometer.

Son of John and Jane (Lynch) Darcy. Irish Catholic; educated in France, where he subsequently settled. Married his cousin Jane Darcy, who was a lady-in-waiting to Marie Antoinette.