Patrick d'Arcy

Patrice d'Arcy
M. d'Arcy, by Hubert Drouais (1742).
Born	Patrick Darcy 27 September 1725 Ireland
Died	18 October 1779 (54 years old) Paris (France)

Patrick d'Arcy (27 September 1725 – 18 October 1779) was born in the west of Ireland. His family, who were Catholics, suffered under the penal laws. In 1739 d'Arcy was sent abroad by his parents to an uncle in Paris. He was tutored in mathematics by Jean-Baptiste Clairaut, and became a friend of Jean-Baptiste's son, Alexis-Claude Clairaut, (Alexis Clairaut), who was a brilliant young mathematician. d'Arcy made original contributions to dynamics. He is best known for his part in the discovery of the principle of angular momentum, in a form which was known as "the principle of areas," which he announced in 1746. See the article on areal velocity. d'Arcy also had an illustrious military career in the French army. He obtained the title of "Count" in the French nobility. He was a generous patron of Irish refugees in France. In addition to his contributions to dynamics, he performed research on artillery and on electricity. An experiment of his, reported in 1765, on visual perception is often referred to: it involved a rotating disk on which a burning coal was placed; when the disk was spun at an angular velocity exceeding seven revolutions per second, a full circle of light was perceived. This and other experiments makes d'Arcy first person to demonstrate the illusion of a moving image; although Joseph Plateau greatly elaborated on this initial finding. d'Arcy was elected to the Academie Royale des Sciences in 1749. He died from cholera in Paris in October 1779.

There is a copy of a portrait of d'Arcy in Wade (1997), [1], which was found in Charbonnier (1928).

References

• C.S. Gillmor, "D'Arcy, Patrick," Dictionary of Scientific Biography, Vol. III, pp. 561–2, Scribner's, New York (1970).
• http://www.iol.ie/~mfinn/kiltullaghho.html
• M. le Chevalier D'Arcy, "Principe Géneral de Dynamique,"Mémoires de l'Académie des Sciences de Paris, 1747, pp. 348–356.
• J. Casey, "Areal Velocity and Angular Momentum for Non-Planar Problems in Particle Mechanics," American Journal of Physics, Vol. 75, pp. 677–685, 2007.

• M. le Chevalier D'Arcy, "Sur la Durée de la Sensation de La vue," Mémoires de l'Académie des Sciences de Paris, pp. 439–451, 1765.
• N.J. Wade, Guest Editorial, Perception, Vol. 26, 1997, [2].
• P.J. Charbonnier, Essais sur l'Histoire de la Balistique, Société d'Éditions Géographiques, Paris (1928)