The square format of this instrument is unusual and includes several different components, several now incomplete. It is meant to stand upright and would originally have had a stand, now lost.
All otherwise free space on its two faces are occupied by scrollwork and leaf and fruit engraved decoration, except for a small vignette of a firing gun which signals the artillery uses of the instrument.
Two of the instrument's parts are common to both faces: a gunner's sight and a compass. The sliding sight is now lost but would have been placed in the instrument's central slit, which has a scale of inches 0 to 5 on both faces, divided to 1/2, 1/4 and twelfths on one face and to 1/2 and twelfths on the other. The compass is hemispherical and suspended from two pivot points. Its upper face has a circular scale divided as if for hours 1 to 12, 1 to 12. The bowl itself is 18 mm in diameter with a cross of direction and an indication of variation.
On the primary face a plummet would have been hung to the right of the sight. The plummet is lost but a fixing hole, support and engraved vertical line identify the position of its string. There is also a shadow square whose sides run 0 to 12, divided to 3 and 1 and numbered by 3, which surrounds an arc of slightly greater than a quadrant, divided in equal parts 4 to 24 to 4 (divided to 4 and 1 and numbered by 4). Pivoting at the centre of both shadow spare and quadrant is a decorated rule 194 mm long with a hole at one end evidently intended to hold a missing part. Though it would seem that the rule should be used with the shadow square and quadrant, it cannot do so in its current configuration since neither of its edges passes through the centre. Moreover the 'quadrant' scale displaced so that it is not symmetrical and its maximum value of 24 is not at 45?. It seems possible that the rule was originally fixed at the corner of the plate, where there is a hole approximately the same size as the hole at the end of the rule. The hole at the centre of the shadow square could then be more plausibly used to attach a string and plumb bob. However, even with this rearrangement the quadrant is still mysterious.
This face also carries another rule 208 mm with a sight at one end. The rule sits in two holders which can slide independently. The rule has an ungraduated non-linear scale which runs in both directions. This double scale is repeated along the edges of the two slots and also along the bottom edge of the instrument. Its geometrical construction is uncertain, because it begins with a series of equal parts before the intervals become larger, and its exact purpose is also unclear. Two of these scales could also be used with the ungraduated rule if it was fixed in the corner of the instrument.
Finally there is a table for converting between feet ('SHV' - Schuh) and paces ('SHRI' - Schritt), such as is found in range tables like those published in Leonhard Zubler, Nova geometrica pyrobolia. Neuwe geometrische B?chsenmeisterery (1608). The rationale for the choice of values is unclear; the first figures for paces are 12, 13, 16 and 4 inches, 21 and six inches, 27, 33 and six inches. Moreover not all of the entries are correct: 1008 paces (correctly 108) for 324 feet; 392 (192) paces for 576 feet, 440 paces for 1276 feet.
Many of the foot values in this table reappear in another table on the other side of the instrument headed at top and bottom 'SHVC'. This table has not been adequately explained.
Also on the reverse of the instrument is a horizontal sundial marked 'BOIS 48 GRA' with a sun face at the origin of the (lost) folding gnomon. (The sundial could conceivably have been a direct north vertical dial but it would only have worked with early morning and late evening hours, so that most of the hour lines would be redundant.)
Provenance: Lewis Evans Collection G. 582.
See: J. Bennett and S. Johnston, The Geometry of War, 1500-1750 (Oxford, 1996), catalogue no. 15.