Sison's Rule, 1784.
© Science Museum / Science & Society Picture Library
Made by Jonathan Sison (fl 1736-1788) and divided by a Mr Bird. When in 1784, Major-General Roy (1726-1790) was set the task of setting a base line on Hounslow Heath, he used this rule for his measurements.This enabled him to fix the relative positions of Greenwich and Paris observatories with greater accuracy than before. By the end of August 1784 the baseline was finally calculated to be 27404.7219 feet. After Roy's death, it became a model for the extension of the project to map the rest of Britain.