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 in Galway on 27 Sept. 1725. His parents, being of Jacobite and Roman catholic principles, sent him to be educated in France. As it happened, he was placed in a house where lived M. Clairaut, father of the famous mathematician, whose pupil he became, the two boys being companions. The progress of young D'Arcy in mathematics at the age of seventeen is said to have been extraordinary; it is represented as little short of that of the younger Clairaut, which was unique. He left his studies to enter the army, and after two campaigns went as aide-de-camp to the Count Fitzjames in command of a French force despatched to assist Prince Charles Edward in Scotland. The force was captured at sea by Admiral Knowles, and D'Arcy, although amenable to English laws, had the good fortune to be treated as a French officer. According to Condorcet, D'Arcy was once in London, probably at the time in question, and was treated as a man who did honour to his country. His position prevented his being chosen a member of the Royal Society, although public opinion protected him against the laws. Condorcet states that the position of an Irish catholic in those days was recognised as a sufficient excuse in the opinion of the public for bearing arms against the English government. Condorcet also says that D'Arcy was thoroughly English in his sentiments, and looked upon every success of British arms with pride; but he refused the most tempting offers of a relative in Ireland to induce him to settle under a government which he held to be headed by a usurper, as well as unjust towards his co-religionists. In March 1746–7 a vessel was ordered to convey the Count Fitzjames and his suite back to France on parole. In 1749 D'Arcy became a captain in the regiment of Condé. The same year he became a member of the French Academy of Sciences, to which he contributed two able memoirs on mechanics. In 1750 he wrote a pamphlet on what he called ‘conservation of action’ against the principle of ‘least action’ of Maupertuis. He then devoted himself for a time to the study of electricity, and, in conjunction with M. Roi, invented an electrometer. The same year he began to write on artillery, the collected results being published as a separate work in 1760. He made many experiments, employing the ballistic pendulum, in which the gun, and not the object fired at, is the pendulum, as well as the ordinary one. He was dissatisfied with the common law of resistance, but his experiments did not give him confidence in any other, and not leading to any result, they were lost. Hutton's ‘Dictionary’ states that the experiments were an improvement on those of Robins, but De Morgan believed this to be a quotation from Condorcet rather than a deliberate expression of Hutton's judgment. Condorcet's view has not been endorsed by later artillerists. The outbreak of the seven years' war called D'Arcy back to the colours, and as colonel he fought at the head of his regiment at Rosbach, and was subsequently employed in the preparations for an invasion of England. After the peace he made many experiments on the duration of vision, and wrote a memoir thereon, and others on various other subjects. In 1770 he became a maréchal-de-camp, a rank corresponding with that of assistant adjutant-general holding the rank of major-general in our service. In 1777 he married a niece, who had been educated under his own eye. He died of cholera in Paris on 18 Oct. 1779. His name does not appear in the English ‘Catalogue of Scientific Papers.’

[Some genealogical details will be found in James Hardiman's Hist. of Galway (1820, 4to), pp. 11, 25. The biographical particulars are chiefly taken from a notice by Professor A. De Morgan in Biog. Dict. (Soc. for Diffusion of Useful Knowledge), vol. i., based on Caritat, Marquis de Condorcet's Éloges des Academiciens, 1699–1790 (Paris, 1795). De Morgan observes that in the Biog. Univers. Condorcet is said to have been the object of violent and unjust hatred on the part of D'Arcy, which makes the degree of panegyric with which Condorcet's Éloge is written, accompanied by detailed statement of the grounds thereof, the more remarkable, whether we regard it as reality or affected generosity.] 

D'ARCY, ROBERT, fourth (1718–1778), was the only surviving son of Robert, third earl of Holderness, by his wife, Lady Frederica, the eldest surviving daughter and coheiress of Meinhardt Schomberg, third duke of Schomberg. He was born in June 1718, and while a child succeeded to the title upon the death of his father on 20 Jan. 1722. His mother afterwards married Benjamin Mildmay, earl Fitzwalter, and died 7 Aug. 1751. He was educated at Westminster School under Dr. Freind, and an epigram recited by him on the occasion of the anniversary dinner of 1728, and to which his name is attached, is still preserved (Comitia Westmonasteriensium in Collegio Sancti Petri habita, &c., 1728, p. 50). He afterwards went up to Trinity Hall, Cambridge, but it does not appear that he ever took his degree. In 1740 he was appointed lord-lieutenant of the North Riding of Yorkshire, and was sworn in before the council on 27 Nov. 1740. In April of the following year he