4.5 inch altazimuth theodolite, mid 18th century.
© Science Museum / Science & Society Picture Library
Theodolites, such as this example made by J Sison, are instruments for measuring angles both in the horizontal and the vertical and have been widely used in surveying since the 16th century. Sison in 1720 introduced the telescope and cros-hairs. This later example includes bubble levels and a better method of setting the vernier.