Mechanical equatorial sundial, Johann Michael Bergauer, Ante 1745

Johann Michael Bergauer (Simonsfeld, 1676 - Innsbruck, 1745 circa)
Mechanical equatorial sundial
Signed: Michael Bergauer Insprugg ?Innsbruck?
Ante 1745
Gilded and silvered brass; glass.
Measures: closed 1.29 x 3.50 x 4.92 in (33 x 89 x 125 mm); open 5.19 x 3.50 x 3.81 in (132 x 89 x 97 mm).
Weight: the sundial 0.49 lb (224 g); the case 0,20 lb (95 g)
Original wooden case covered in brown leather.
State of conservation: very good. It has some signs of use. The spring that allowed for the two parts of the instrument to remain open is missing (absent even in the comparative specimens kept in museums).

The sundial is composed of two overlapping plates hinged together on the north edge.

The base plate is octagonal and is supported by three turned legs. The upper face is gilded and a compass with a magnetic variation index has been inserted. The rest of the surface is occupied by a rich decoration of engraved scrolls, centered around the inscription “Michael Bergauer Insprugg”. A foldable oval support with a plumb-bob is attached with a hinge on the southern edge. On the reverse of the base plate a table of the latitudes of some European cities (expanded with the vertical writing “Meiland 40” on the edge and “Rome” deleted) and of Jerusalem has been engraved. A cam marked for 0 °-70 ° is applied near the northern edge. This can be adjusted to change the inclination of the upper plate according to the latitude; originally a spring, now lost, made it possible to keep the two plates of the clock open.

The second plate is round, has a toothed edge and measures 3.26 in (83 mm) in diameter: it is slightly smaller than the octagonal base which it rests upon and overlaps when the instrument is closed.
The recto is gilded and there are three concentric graduated circles engraved on it:
- the outermost is the equatorial hour dial, numbered I-XII, I-XII;
- the second-one is that of days 1-30 of the lunar month and has “Aetas lunae” engraved on it;
- the third, silvered, is a subsidiary hour dial, with double numbering 1-12; originally it could have been rotated.
The engravings of the first two circles are enameled in red.
In the center - on the polar axis - there is an alidade, at the end of which is associated the silvered minute dial. This, in turn, is welded, perpendicularly, to a small disc, also silvered, with a triangular gnomon. The plate, alidade and minute dial are connected to each other by toothed mechanisms.

Below is the procedure for measuring the time:
1) Adjust the cam under the base of the clock, based on the latitude of your location;
2) Place the watch on a flat surface using the plumb-bob and with the side closest to the compass facing south;
3) Keeping the instrument still, manually rotate the alidade until the shadow cast by the triangular gnomon on the small silvered disc falls on the line marked below it;
4) The hour and minutes can therefore be read on the hour and minute dials set on the alidade respectively.

Johann Michael Bergauer, who sometimes only signs his works as Michael Bergauer, was born in Simonsfeld, north of Vienna. His apprenticeship as a watchmaker took place in Landshut and he probably worked as a laborer in Augsburg before becoming a watchmaker at the court of Karl Philipp von der Pfalz in Innsbruck in 1708. In the following years, his repeated attempts to obtain Innsbruck citizenship are documented and, in 1721, he is listed as a resident. In 1724 he was admitted to the guild of watchmakers, with which however he had continuous problems. In 1732 he presented a "masterpiece". This is the last reference to his business; he must have died before or in 1745 because in that year his widow appealed to the City Council.

The mechanical sundial is compared with very similar examples preserved in:
- The British Museum, 1888, 1201.326;
- The British Museum, reg. no. OA.384;
- Greenwich Maritime Museum, reg. no. AST0469;
- History of Science Museum, Oxford, new inv. no. 51664;
- Germanisches Nationalmuseum, Nuremberg, WI1216;
- National Technical Museum, Praga.




Bibliography:
- E. Zinner, Deutsche und Niederländische Astronomische Instrumente des 11.18. Jahrhunderts, Munich, 1979, passim;
- Francis Ward, A catalogue of scientific instruments in the Department of Medieval and later Antiquities of the British Museum, London 1981, s. v;
- Wolfgang, Eckhardt, Claude Dunod, Michael Bergauer und Johann Willebrand – Zur Geschichte der Minutensonnenuhr, Jahrbuch des Museums für Kunst und Gewerbe Hamburg 3, 1984, pp. 79-104;
- Gerard, L'E. Turner; Andrew, Turner, Scientific Instruments, 1500-1900: An Introduction, London 1998, p. 22;
- H. Higton, Sundials at Greenwich, Oxford 2002, no. 165, pp. 181-2;
- J. Abeler, Meister der Uhrmacherkunst, 2010, p. 59.