Signed: H. Cole. 1586
Digges designed the horizontal plate of his 'topographical instrument' as a single geometrical quadrant with an inscribed circle divided by degrees. The instrument in the exhibition, by the man generally regarded as the first native English instrument maker, Humphrey Cole, has a vertical semicircle very similar to that described by Digges, which now sits above a horizontal circle with an inscribed square. Each quarter of the square has sides divided as a geometrical quadrant and this alternative arrangement has the advantage that a single alidade, pivoted at the centre, can be used with the circle or with any of the four geometrical quadrants. By the same token, the alidade fixed to the vertical semicircle can be used for measurements of both coordinates. The arrangement of scales in the Cole instrument is in fact closer to an earlier design of altazimuth instrument, the 'polimetrum' of Martin Waldseemüller.
The confusion of terminology, embracing both 'shadow-square' and 'geometrical quadrant' for the same types of scale, is illustrated on this instrument, where the vertical scales are marked 'Vmbra Versa' and 'Vmbra Recta', and the horizontal 'Quadratum Geometricum'. Certainly the use of such a scale to convert a measurement of a shadow cast on the ground into a height, was an application of the vertical scale.
Despite the refinements to the Digges design, the Cole altazimuth theodolite is conceptually similar to the topographical instrument, which promised that the gunner would 'neuer bestowe vaine shotte'.
Diameter (horizontal circle): 197 mm
St John's College Collection
Inventory no. 55,130