The d’Arcy family, who are Catholics, suffer under the Penal Laws. In 1739 d’Arcy is sent to Paris for his education. He is tutored in mathematics by Jean-Baptiste Clairaut, and becomes a friend of Jean-Baptiste’s son, Alexis-Claude Clairaut, who is a brilliant young mathematician. d’Arcy makes original contributions to dynamics. He is best known for his part in the discovery of the principle of angular momentum, in a form which is known as “the principle of areas,” which he announces in 1746.
Perhaps seeking more adventure than an academic life can provide, d’Arcy enlists in the French army. He fights in Germany in the regiment of Condé, and serves as an aide-de-camp to Marshal Saxe at the Battle of Fontenoy. He obtains the title of “Count” in the French nobility and is a generous patron of Irish refugees in France. He sails for Scotland in 1745, attempting to fight in “Bonnie” Prince Charlie‘s rising, but his ship is captured and he is taken prisoner.
After his release, d’Arcy returns to France where he continues both his scientific and military careers. In addition to his contributions to dynamics, he performs research on artillery and electricity. One of his experiments on visual perception, reported in 1765, is often referenced. It involves a rotating disk on which a burning coal ember is placed. When the disk is spun at an angular velocity exceeding seven revolutions per second, a full circle of light is perceived. This and other experiments make d’Arcy the first person to demonstrate the illusion of a moving image, although Joseph Plateau greatly elaborates on this initial finding.