trigonometrical surveys for another twenty years. In 1765 he was appointed surveyor-general under a royal warrant which directed him "to inspect, survey and make reports from time to time of the state of the coasts of this Kingdom and the islands thereunto belonging." This work, which took him to many parts of Britain and abroad (Ireland and Gibraltar), is recorded in the many plans and sketch maps of districts that are now lodged at the British Library. Despite the travel Roy was able to enter fully into the intellectual life of London, and in 1767 he became a fellow of the Royal Society. The only paper he read before the society was in 1783 on Rules for measuring heights with a barometer. Roy was promoted to colonel in 1777, and to major-general in 1781. He was in charge of the departments of the Quartermaster-General and Chief Engineer in 1782, and in 1783 became the director of Royal Engineers. The Anglo-French survey Late in life, when he was 57, Roy was granted the opportunity to establish his lasting reputation in the world of geodesy. The opening came from a completely unexpected direction. In 1783 Cassini de Thury addressed a memoir to the Royal Society in which he expressed grave reservations of the measurements of latitude and longitude which had been undertaken at Greenwich Observatory. He suggested that the correct values might be found by combining the Paris Observatory figures with a precise triangulation (survey) between the two observatories. Sir Joseph Banks, president of the Royal Society, proposed that Roy should lead the project. Roy accepted with enthusiasm for he saw that apart from the specific measurements proposed the survey could be the first step towards the national survey that he had advocated so often. The whole project is described by Roy in three major contributions to the Philosophical Transactions of the Royal Society in 1785, 1787 and 1790. There are shorter accounts of the project in the History of the Royal Engineers, the records of the Royal Society and in every history of the Ordnance Survey. After a preliminary survey by Roy and three other members of the Society on 16 April, they found a suitable location for the starting baseline on Hounslow Heath, between King's Arbour and Hampton Poor-house just over 5 miles to the south-east. A preliminary measurement of the line was carried out with a steel chain prepared