2 Zinc discs 7” dia. painted white one side, with broad red line across centre
Surveying projects were important to James Watt as he made the transition from being a scientific instrument maker to being an engineer capable of working on large projects such as steam engines. He was engaged to work on schemes for a number of canals, and he took the opportunity to continue to innovate with the tools and instruments at his disposal. This pair of painted discs was intended to be used to measure distance, along with a telescope which also survives in Watt’s workshop (inv. no. 1924-792/1897. His assistant placed one disc at the bottom of a pole, and moved the other until it lined up with the upper of two cross hairs in the telescope eyepiece. Watt used them whilst surveying Scotland’s Great Glen in 1773 and could measure half a mile to an accuracy of within a yard using this method.