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Inventory no. 35939 - Epact entry

Epact number: 90959

Surveying Instrument

Signed 'D O'
circa 1600; German ?
Wood and brass; 14 x 277 x 145 mm

Main text

This is an unusual combination instrument which incorporates several distinct elements. The square brass plate has an alidade with folding sights which can be read against circular scales of degrees and hours. An inset compass allows this part of the instrument to be used for surveying. The compass can also be used in conjunction with a string gnomon horizontal dial arranged symmetrically with the compass.

Also mounted on the decoratively carved wooden support is a brass ruler with sights which could, in principle, have been used on a plane table.

Source museum: Museum of the History of Science, Oxford
Museum number: Inventory no. 35,939

Detailed text

The instrument is made of a block of wood onto which are screwed a brass rule and plate. The exposed section of wood features scrollwork scrolls for decorative effect.

The rule has a linear scale from 0 to 225, divided to 10, 5 and 1, and numbered by 10 (200 divisions equal to 242.5 mm). There is a bead sight at the end and a pin-hole sight almost half-way. The reverse has decorative line engraving.

The square brass plate has an alidade with folding sights moving over two circular scales: a scale of degrees 0 to 90 to 0 to 90 to 0, divided to 10, 5 and 1, and numbered by 10; and a scale of hours 0 to 12, 0 to 12, divided to 1, |1/2|, |1/4| and numbered by 1. Inside the hour scale the cardinal points are indicated 'SEPTENTRIO', 'ORIENS', 'MERIDIES', 'OCCIDENS'.

Within the circular scales is a compass with an ivory bowl (32 mm in diameter), with a circular scale of directions 0 to 48, divided to 4 and 1 and numbered by 4. A cross indicates the cardinal directions which are identified as 'SE', 'OR', 'ME', 'OC'. Magnetic variation is marked at about 8° east; the line indicating variation has the same form as the needle.

Arranged symmetrically with the compass and set slightly into the plate is a horizontal string gnomon dial. The hours run 4 to 12 to 8, with half hours marked. The signature 'D O' is placed on the meridian line. The folding gnomon support is sprung to the vertical and the dial is for about 48°.

Provenance: Billmeir Collection (57-84/252).

Stephen Johnston

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