The instrument consists of a brass rule with bevelled sides and a scolloped end.There are two scales of inches. The first runs from 0 to 17 and is divided to 1, subdivided to 1/2, and numbered by 1 on the upper face; on the bevel it is further subdivided to 1/4 and 1/8.
Adjacent to this first scale is another scale of inches, each of which is divided into a different number of equal parts, to be used principally in connection with map work. The number of parts is engraved for each inch: 7, divided to 1 on the bevel; 20, divided to 4 on the face and subdivided to 2 and 1 on the bevel; 21, divide to 7 on the face and subdivided to 1 on the bevel; 22, divided to 11 on the face and subdivided to 1 on the bevel; 24, divided to 12 and subdivided to 6 on the face and subdivided to 2 and 1 on the bevel; 26, divided to 2 and subdivided to 1 on the bevel; 27, divided 9 on the face and subdivided 3 and 1 on the bevel; 28, divided to 4 on the face and subdivided to 2 and 1 on the bevel; 30, divided to 10 on the face and subdivided to 2 and 1 on the bevel; 32, divided to 16 on the face and subdivided to 2 and 1 on the bevel; 36, divided to 12 on the face and subdivided to 4, 2 and 1 on the bevel; 40, divided 8 on the face and subdivided to 4, 2 and 1 on the bevel; 42, divided to 14 on the face and subdivided to 2 and 1 on the bevel; 48, divided to 24 on the face and subdivided to 4, 2 and 1 on the bevel; 64, divided to 32 on the face and subdivided to 4 and 2 on the bevel; 56, divided to 8 on the face and subdivided to 4, 2 and 1 on the bevel; and 18, divided to 6 on the face and subdivided to 2 and 1 on the bevel.
The reverse is blank except for the signature: 'H + Cole + 1582 +'.
The two holes mid-way along the rule were probably intended to receive a sight to enable the instrument to function as a plane table alidade. Thomas Digges noted in the second edition of Pantometria (1591) that the upper part of his father's 'topographical instrument' (an altazimuth theodolite) was used in this way. This would have resembled the vertical semicircle on Humphrey Cole's theodolite (Epact 78465).
See G. L'E. Turner, "Mathematical Instrument-Making in London in the 16th Century", in S. Tyacke (ed.), English Map-Making 1500-1650 (London, 1983), pp. 93-106, esp. pp. 102-3.
Stephen Johnston