Spirit level, 1910.
© 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. The spirit level was invented in the 1660s, the horizontal being determined by the position of a bubble in a sealed tube of either alcohol or ether. Attached to a telescope, rapid survey work was be carried out using a graduated pole to measure differences in height. This example was designed in 1910 by Heinrich Wild and manufactured by the company of Carl Zeis.