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Surveying Instrument
Signed by Tobias Volckmer
Dated 1583; Brunswick
Gilt brass; 132 x 132 x 23 mm (box)

The set consists of different components as follows:

A base box, 132 mm square and 23 mm deep. In the upper surface is an aperture 123 mm in diameter. The spandrel spaces are engraved with a fruit and vegetable decoration. The edges bear scales (as follows anticlockwise):

1) A linear scale 0 to 6, numbered by 3 and divided to 1 and 1/2.

2) A linear scale 9 to 12, numbered by 3 and divided to 1 by alternate shading.

3) A linear scale 0 to 24, numbered by 3 and divided to 1 by alternate shading.

4) A linear scale 0 to 24 over half the length of the edge, numbered by 3 and divided to 1 by alternate shading.

5) A linear scale on the same edge 0 to 24, numbered by 3 and divided to 1 by alternate shading.

Three holes in the outside edge of the box suggest that some component was fixed there.

The underside of the box is marked with two semicircular degree scales 90 to 0 to 90, numbered by 10 and divided to 5 and 1. They are intersected by a 'LINEA FIDUCIE' with a scale 0 to 24, numbered by 3 and divided to 1 by alternate shading. The vacant spaces are engraved with a fruit and vegetable decoration.

There are three holes along the fiducial line, two smaller ones towards both ends and a larger one in the centre.

Set into the aperture in the upper face is a rotatable brass drum, 118 mm in diameter and 16 mm in thickness with a top flange 123 mm in diameter. This fits into a recess in the square box. Both sides and the outer edge of the drum bear inscriptions. The upper side has a circular scale along the edge 0 to 90 to 0 to 90 to 0, numbered by 10 and divided to 5 and 1. Further to the inside is a scale 1 to 24, numbered by 1. The sequence 13 to 24 is smaller and duplicated directly underneath the first scale. Further to the inside is a scale with the names of the winds and cherubs' faces as follows: 'BOREAS', 'VVLTVRNVS', 'SVBSOLANVS', 'EVRVS', 'NOTVS', 'AVSTER', 'APHRICVS', 'ZEPHIRVS', 'FAVONIVS', 'CIRCIVS', 'CORVS', 'AQVILO'. The lower half of the centre bears a 'SCALA ALTIMETRA' with the directions labelled 'OR{IENS}' and 'OC{CIDENS} and the sides labelled twice as 'VMBRA VERSA' and 'VMBRA RECTA'.

Inset into the upper half of the centre is a rotatable magnetic compass with a silvered scale 0 to 24, numbered by 1. It is operated by a square arbor for right angled gearing set into the outside edge of the drum. An eared disc in the area of the Scala altimetra covers a second compass box. The needle is missing and there is only a north-south indication. The vacant spaces on the upper surface are decorated with fruit, vegetables, and strapwork.

The outside edge is inscribed with calendrical and zodiacal scales. The zodiacal scale is labelled with the Latin names of the zodiacal signs and their symbols. Each sign has a degree scale 0 to 30, numbered by 10 and divided to 5 and 1. The calendrical scale is labelled with the Latin names of the months and scales with the corresponding number of days, numbered by 10 and divided to 5 and 1. The equinoxes are at 10 March and 14 September.

The underside of the drum is laid out in a similar manner to the plate of an astrolabe for latitude 47? 20' ('CASTINA DISTAT AB ELEVATIO POLI G{RADUS} XXXXVII M{INUTA} XX'). There is an hour scale along the edge clockwise I to XII twice, numbered by 1 and divided to 20 minutes. The tropics and the equator are indicated and the horizon is labelled 'HORIZON OBLIQVVS'. There are hour lines for unequal, Babylonian and Italian hours, numbered twice I to XII and 1 to 24 respectively. The cardinal directions are marked 'MERIDIES, OCCASVS, SEPTENTRIO, ORTVS', with south being at the top.

In the centre is a squre hole into which fits a bent cursor of 175 mm length with a clamping screw. It is marked with a double cross corresponding to the same symbol on the surface to enable it to be fitted properly. The inner part has a scale with the symbols of the zodiacal signs. The extension has a linear scale 0 to 42, numbered by 3 and divided to 1. It is marked 'PROPORTIO' on the reverse.

A vertical rod with a thread can be plugged into the square hole in the centre of the upper surface of the drum. Attached to it is another rod which can rotate around the fixed vertical rod. This second rod is surmounted by a bearded male figure in 16th century clothing carrying a plumb bob support. The plumb bob itself hangs from a thread within a cage consisting of two pierced plates separated by four caryatids. Fixed to the other side of the rod is a pierced disc, 111 mm in diameter, engraved on both sides with two semicircular scales, one 90 to 0 to 90, numbered by 10 and divided to 5 and 1, the other 6 to 0 to 6, numbered by 1. The same 'SCALA ALTIMETRA' as on the upper surface of the drum is repeated here, but halved over front and back. Below this Scala Altimetra on the back are the signature and the date 'THOBIAS VOLCKMER BRAVNSWEIGSENSIS FACIEBAT AN<NO> D<O>M<INI> M D LXXXIII'. On the front above the scale is a small blank shield with the initials 'M G'. A movable compound radial arm with scales scales 3 to 18 (numbered by 3 and divided to 1) on both sides is attached to the centre of the disc. It ends in a pierced shield shaped plate to which a decorated hook is attached. The scales correspond to the Scala Altimetra on both sides of the disc. At the foot of the rod is a fixed index with a pointer and a scale 9 to 18, numbered by 3 and divided by 1. This scale corresponds to the Scala Altimetra on the upper surface of the drum.

Further loose components are:

a) A single ended cursor, marked 'ANGVLUS RECTVS', with scales 0 to 24 on both arms, numbered by 3 and divided to 1 by alternate shading. It is marked with a double cross on the reverse.

b) A double sided counter-changed cursor marked 'LINEA HORIZONTALIS' with a scale 12 to 0 to 12, numbered by 3 and divided to 1. It is marked with a double cross on the reverse.

c) A quadrant marked 'LINEA HORIZONTALIS', with a scale 0 to 90, numbered by 10 and divided to 5 and 1, It is further marked with a 'C' and decorated with the same fruit, vegetables and leaves as the square box.

d) A protractor with two hooks, marked with a scale 90 to 0 to 90, numbered by 10 and divided to 5 and 1.

The instrument was presented by Octavius Morgan in 1867 and is described in F. A. B. Ward, A Catalogue of European Scientific Instruments in the Department of Medieval and Later Antiquities of the British Museum (London, 1981), p. 103, no. 307.

Silke Ackermann

British Museum, London
Registration no. MLA 1867,7-16.1

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Basic URL: http://www.mhs.ox.ac.uk/epact/catalogue.php?ENumber=41531

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