Supplement to the Fourth, Fifth, and Sixth Editions of the Encyclopædia Britannica/Arcy (Patrick D’)

by William Archibald Cadell

ARCY (Patrick D’), Member of the Academy of Sciences of Paris, a general officer in the French army, Chevalier of the order of St Louis, &c. was born in Ireland in 1725. His parents, in consequence of their attachment to the House of Stuart, left Ireland, and settled in France. He studied Mathematics with distinguished success under Clairaut the elder, and had tor his fellow-student the celebrated Clairaut the son. He obtained a commission in the French army; and, in 1746, was embarked in the expedition which was intended to make a descent in Scotland. He was made prisoner, and was treated like other prisoners of war, no notice being taken of his having been born a British subject. He continued attached to the House of Stuart during the whole of his life.

In 1760, he published his Essay on Artillery. To estimate the force of the explosion of gun-powder, he employed a cannon suspended, so that the arc of vibration described by it on its being fired, was a measure of the force of explosion. To measure the initial velocity of a projectile, he used Robins’s machine. Robins showed, that when the velocity of a projectile is great, the resistance opposed by the air is not in proportion to the square of the velocity, as is the case when the velocity is small. D’Arcy made experiments with a view of ascertaining this law, but without success. His work contains an account of experiments made by him to determine the most advantageous length of cannon. He published a paper on hydraulic machines in 1754, where he treats of the maximum of effect of water-wheels. He also published a paper concerning the duration of the impression of light on the retina. He found that the revolution of a luminous point must be rapid, so as to be performed in 8/60 of a second at the least, in order to produce the appearance of a continued luminous circle.

He was of a handsome figure, and passed much of his time in the gay world. A short time before his death he married a young lady his niece, and took the title of Count. Although attached to the House of Stuart, and therefore inimical to the English Government, yet he admired the national character of the English, and was attentive to English travellers. He died in 1779, and was succeeded at the Academy by the Abbé Bossut. (Y.)