Bennett, J. and Johnston, S., The Geometry of War, 1500-1750 (Oxford, 1996)
9. To accommodate irregularities, and for the sake of safety, the shot of early modern artillery did not fit tightly in the bore of a gun. The difference between the diameter of the shot and the calibre of the gun was known as the windage. Using the hinged crosspiece and a single series of calibrated notches, these calipers by Nicolas Bion can measure both shot and calibre. Diameters taken between the inner ends of the instrument's points provide the weights of shot from ¼ to 48 pounds. At any given setting for the diameter of shot, the outer tips of the points give a slightly larger dimension, indicating the bore of the corresponding artillery piece; the same scale then indicates different guns from ¼ to 48 pounders, and also allows for windage. This dual measuring function is indicated by the inscriptions on either side of the instrument: 'diamettre des poids des bouleits' and 'Calibres des pieces'.
Bion illustrated this type of gunner's calipers in his Traité de la construction et des principaux usages des instrumens de mathematique of 1709 (catalogue no. 32; figure 46).
Length (closed): 163 mm
Inventory no. 43,257