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 and the north coast of France, in order to determine the relative positions of the observatories of Paris and Greenwich. The scheme was suggested by the French government. Roy selected Hounslow Heath for a base line, which was measured in the summer of 1784 three times over by means of cased glass tubing, seasoned deal rods, and a coffered steel chain made by Ramsden, the length being 27,404 feet, and the discrepancy between the several measurements under three inches. This work took nearly three months, and excited considerable scientific interest, the king, the master-general of the ordnance, and many distinguished savants visiting Hounslow during its progress. The result of a remeasurement of the base on Hounslow Heath in 1791 by Captain Williams, Mudge, and Dalby was only 23/4 inches different from Roy's measurement, and the mean of the two was accepted as the true measurement.

In 1785 Roy contributed a paper to the ‘Transactions’ of the Royal Society on the measurement of this base, which was separately published the same year in a quarto volume. On 30 Nov. he was presented with the Copley medal of the Royal Society for the skill with which he had conducted the measurement of the base line on Hounslow Heath, accompanied by a highly complimentary speech from the president. He also wrote a paper for the Royal Society, entitled ‘An Account of the Mode professed to be followed in determining the Relative Situations of the Royal Observatories of Greenwich and Paris.’ This was read in 1787, and published separately in the same year in a quarto volume.

In the summer of 1787 Roy carried his triangulation from the Hounslow base to the Kentish coast, and on 23 Sept. met the French commissioners at Dover, and, after a conference with them, the observations connecting the English with the French triangulations were made from both sides of the Channel. A base of verification, 28,535 feet long, was measured on Romney Marsh under Roy's direction, and found to differ only twenty-eight inches from its calculated length as determined by the triangulations of the Hounslow base. Roy continued in 1788 and the following year the observation of a great number of secondary triangles, which became the foundation of the topographical survey of Middlesex, Surrey, Sussex, and Kent. He wrote for the Royal Society ‘An Account of the Trigonometrical Operations by which the Distance between the Meridians of the Royal Observatories of Greenwich and Paris has been determined;’ but Roy's health had failed, and he was able to give it only the leisure which illness and his military avocations permitted. In November 1789 he was obliged to go to Lisbon for the winter, returning to England in April 1790. He died suddenly at his house in Argyll Street, London, while correcting the proof-sheets of the above-mentioned paper, on 1 July 1790.

Roy left ready for the printer his ‘Military Antiquities of the Romans in Britain, and particularly their Ancient System of Castrametation illustrated from Vestiges of the Camps of Agricola existing there.’ His executors presented the manuscript to the Society of Antiquaries, who published it at the expense of the society, in a handsome folio volume, in 1793.

In addition to the works enumerated above, there are in the British Museum the following maps and plans drawn by Roy between 1752 and 1766: Roman Post at Ardoch; Culloden House; Roman Camp, Dalginross, Glenearn; Esk River; Kent, New Romney to North Foreland; Louisbourg; Milford Haven; Roman Temple at Netherby, Cumberland; Strathgeth Roman Post, near Innerpeffrey, Strathearn; Coast of Sussex; South-east part of England; Country between Guildford and Canterbury; Hindhead to Cocking; Lewes Road from Croydon to Chailey; Country from Dorchester to Salisbury; Country from Gloucester to Pembroke; Marden Castle, near Dorchester.

In Sir Walter Scott's ‘Antiquary’ Jonathan Oldbuck of Monkbarns relates his discovery of the site of the final conflict between Agricola and the Caledonians, and reflects on Roy for having permitted the spot to escape his industry.

[War Office Records; Royal Engineers' Records; Parish Records of Carluke; Transactions of the Royal Society, vols. lxvii. lxxv. lxxvii. lxxx. and lxxxv.; Dod's Ann. Reg. 1790; Gent. Mag. 1785 and 1790, vols. lv. and lx.; Weld's Hist. of the Royal Society; Anderson's Scottish Nation; Chambers's Eminent Scotsmen; Watt's Bibliotheca Britannica; Notes and Queries, 2nd ser. vol. vii.; Cornwallis Correspondence, vol. i.; King's Warrants; European Mag. 1789, vol. xv.; Wright's Life of Wolfe; Porter's Hist. of the Corps of Royal Engineers; Portlock's Life of Major-general Colby; White's Ordnance Survey of the United Kingdom; Society of Antiquaries, 1793.] 

ROYDON, MARMADUKE (1583–1646), merchant-adventurer, son of Ralph Roydon or Rawdon of Rawden Brandesby in Yorkshire, by Jane, daughter of John Brice of Stillington, was baptised at Brandesby on 20 March 1583. At sixteen years of age he went to London, where he was apprenticed to Daniel Hall, a Bordeaux