100 link chain made by Ramsden, used by General Roy in 1784 and 1787

Made by Jesse Ramsden, this chain was used in the Primary Triangulation of Great Britain. This was a network of measured triangles across the country, used to underpin maps. The network was checked in a number of places by setting up a base line, several miles long, and comparing measured and calculated dimensions. The first line was on Hounslow Heath and General William Roy of the Royal Engineers used this chain for its measurement.

It was proposed by Cassini de Thury in 1783 that the relative positions of the Royal Observatories at Greenwich and Paris should be determined with great accuracy by the process of triangulation. This process is a standard technique in surveying, allowing tracts of undulating country of considerable extent to be mapped using a relatively short and level base line, the length of which is measured very precisely, the remainder of the territory being linked by a continuous network of triangles, only the angles of which need to be determined.

Cassini's proposal gained influential support from King George III and the Royal Society with the result that the exercise was swiftly put in hand under the direction of Major-General Roy, whose work laid the foundations of a general survey of the British Isles. The principle base line on this side of the Channel was laid out on Hounslow Heath, with a further base for verification on Romney Marsh.

The 100-foot steel chain shown here was commissioned by Roy and made by Ramsden, the noted instrument maker; it was used to make the first experimental measurement of the Hounslow Heath base in 1784. The definitive measurement was made using a number of glass tubes having ends fitted with steel-tipped devices intended to allow adjoining tubes to be brought together in a consistent manner with considerable accuracy. However, Roy was so impressed with his trial run using the chain that he used the latter exclusively for measurement of the Romney Marsh base in 1787.