Balance level, 17th century.
© Science Museum / Science & Society Picture Library
Levelling is the method used by surveyors to determine the relative heights of different points on the earth's surface, but originally referred to the simple procedure of determining the horizontal. This crude instrument has a lead tube hanging from a fixed thread inside a protective case, with a slit at one end and a cros-hair at the other. The device would have been so easily disturbed that its use would have been extremely limited.