Bennett, J. and Johnston, S., The Geometry of War, 1500-1750 (Oxford, 1996)
34. Clerget's instrument is simply a miniature version of the Le Maire rule and may originally have been supplied as part of a pocket set of silver drawing instruments. Two of the sectoral scales (planes and solids) have been squeezed out due to its smaller size but the other features remain the same, and there is even an extra engraved scale of inches on the outside edge.
As with the larger instrument, the cross-piece is used to fix the instrument as a square for levelling. The cross-piece is attached to one of the two legs and can slide along a groove in the other. A catch in the groove holds the cross-piece when the legs are at right angles to each other. The two legs are bevelled at the ends to give greater stability when it is set up as a square.
Length (closed): 88 mm
Inventory no. 43,341