Signed 'Pragae fecit Erasmus Habermel', the instrument is one of several hybrid artillery instruments by Habermel to survive.
The instrument combines three distinct devices: a gunner's quadrant, a sight and a gauging rod. The quadrant, when seen stripped of other components such as the stand and sight, is of disarming simplicity and is one of the few surviving early instruments to resemble the form illustrated by Niccol? Tartaglia in his Nova scientia (1537). Its plumb bob and line were to be read against an arc graduated in points rather than degrees, from 0 to 12, divided to 1, subdivided to 1/2 and 1/8, and numbered by 1.
In use, the long leg of the quadrant was meant to inserted into a gun's muzzle. However, two small sights attached to the side of this long leg enable the quadrant to be used for more general observations of altitudes.
Also present on the leg are gauge scales for determining the weight of spherical shot. They cover the three standard materials found on German gunner's gauges: iron ('FERRVM'), lead ('PLVMBV<M>') and stone ('LAPIS'). All the scales run from fractions of the unit weight to 100, divided to 10, subdivided to 5 and 1, and numbered by 10 (though the first ten units are also individually numbered). The first divisions on the iron scale are obscured by one of the quadrant's additional sights.
On the fourth side of the leg is a one foot scale of equal parts, divided as inches but graduated in tenths. The scale runs from 0 to 120, divided to 10, subdivided to 5 and 1, and numbered by 10. When the instrument is assembled this scale is partly obscured by the stand.
To transform the instrument from a quadrant into a gunner's sight that can be placed on the breech of a gun, the quadrant is turned upside down and made to serve as no more than a frame, to which both the stand and the sight attachment are screwed. The slot for the sliding sight is graduated using both numbers and letters. The numerical scale lies on the left hand side of the slot and runs from 0 to 12, divided to 1, subdivided to 1/2, 1/4 and 1/8, and numbered by 1. The alphabetical scale is on the right side with the Roman letters from A to M marked against divisions using the same unit size.
The gunner's quadrant and sight share no common structural features but their combination creates an impressive and elaborate device. Habermel's instrument is an ingenious tour-de-force whose primary purpose was presumably to grace the collection or cabinet of a noble or wealthy patron.
The instrument is illustrated and described in J. Bennett and S. Johnston, The Geometry of War, 1500-1750 (Oxford, 1996).