100 link steel surveying chain, 1784.
© Science Museum / Science & Society Picture Library
Made by Jese Ramsden, this chain was used in the Primary Triangulation of Great Britain. This was a network of measured triangles acros 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.