A rare and unusual John Bennett brass alt-azimuth land-surveying/geodetic instrument, English, mid 18th century,
signed J Bennett London, A fixed conical staff-head carries a six-spoke open base circle engraved along the circumference with a 360° scale divided to 1° and reading to 1' by a plain vernier carried on one of the four feet of the pair of bridges that carry the central compass. This incorporates a pair of bubble levels set at right angles to each other, and an 8-point wind-rose, divided to 1° in four quadrants (0-90-0-90-0) within a raised circumference divided to 360°. Clamping screws are set on each of the four bridge feet.
The body of the instrument is mounted above the centre of the compass on the intersection of the two bridges. It is composed of a vertically set semi-circle with strengthening horizontal and vertical cross struts, and is engraved with a double 90° scale divided to minutes and reading against a double vernier. The two quadrants are graduated from the mid-point of the arc so that either altitudes or depressions may be read. The vernier is carried on an arm pivoted at the centre of the diameter of the semi-circle, which is itself engraved with a linear scale 10-40 divided to 15 minutes of arc. The return arm of the vernier carries the clamping screw and a fixed bubble level while between this return arm and the frame of the instrument is a second bar mounted, like the vernier itself, at the centre of the instrument. This bar is slotted at the pivot end, and counter-changed at its centre to accommodate a vertical scale graduated in each direction from its centre 0-30° reading by dots to 30'. At each 30° level the rule is slotted to pass either side of the upper and lower horizontal struts of the semi-circle, thus allowing the rule to have a small lateral motion the displacement being measured against a scale of 20-0-20° marked on a semi-circle at the centre of the rule. Similarly mounted on this centre beneath the bubble-level through the return arm of the vernier, which runs in a slot in the back of the vertical rule, is a bracket with clamping screw carrying an arc graduated 20-40° and moving in the vertical plane. At the end of the arm carrying the arc, and in the same plane, is a bracket to which is pivoted a short rule, free to move horizontally, divided 0-20°. A slotted arm is fitted with fine lateral adjustment by a key-operated (key-missing) endless screw.
The vernier-arm is attached to the frame of the semi-circle by a wing-nut. Carried on the same axis is a split arm for an adjustable arc, graduated 20-0-40°, with clamping screw, moving in the vertical plane and carrying a sighting telescope on a rule graduated 0-40° and laterally adjustable against a small arc graduated to 90°. The semi-circle and its attachments may be unscrewed from the compass box for transport.
Some tarnishing; slots to the screws on the underside of the compass-box abused.
19in (48cm) high; radius of base plate 9 1/2in (24cm)