Anglo-French Survey (1784–1790)
This criticism was roundly rejected by Nevil Maskelyne who was convinced of the accuracy of the Greenwich measurements but, at the same time, he realised that Cassini's memoir provided a means of promoting government funding for a survey which would be valuable in its own right.
[1] Approval was granted and Sir Joseph Banks, president of the Royal Society, proposed that Roy should lead the project.
Roy gladly accepted and set matters in motion by submitting to the Crown a grossly underestimated budget for manpower (by far the largest element) and new precision instruments to be constructed by Jesse Ramsden.
[5] The first task of any survey is to establish a baseline and, after a search by Roy and three other members of the Royal Society on 16 April, they fixed upon the heart of what was mainly still common land, the western swathe of Hounslow Heath.
The ground was cleared of bushes and a preliminary measurement of the line was carried out with a 100 ft. steel chain of 100 links prepared by Jesse Ramsden.
The measurement of the baseline to such a high standard of precision was a remarkable achievement and in recognition the Royal Society awarded Roy the Copley Medal in 1785.
Once the baseline had been measured Roy was keen to press on with the triangulation as soon as possible but he was thwarted by Ramsden's failure to produce the new theodolite.
This led to a certain amount of acrimony and Roy went so far as to accuse Ramsden of being remiss and dilatory – in public[8] and in his next report in the Philosophical Transactions.
In this way the mesh of triangles was extended down towards the coast where sights could be made on some of the French stations that had been measured in the course of Cassini's triangulation.
The cross-channel use of these lights entailed careful timing arrangements which could take into account the vagaries of the weather and pocket watches.
The final report of 1790[12] presents figures for the distance between Paris and Greenwich as well as the precise latitude, longitude and height of the British triangulation stations.
At the time of his death he was correcting the final proofs of his report and the work was brought to a conclusion by Isaac Dalby, a senior civilian employee of the Board of Ordnance who had organised the calculations of the triangles.
