An extremely rare late 16th-Century lacquered-copper Polyhedral Dial,
circa 1590, signed Pragæ fecit Erasmõs habermel -- 7¼in. (18.5cm.) high; 14in. (35.5cm.) wide; 6 7/8in. (17.5cm.) deep
This strikingly large polyhedral dial is most unusual in being finished in bright copper sheet (thickness between 1mm and 1.5mm), lacquered for protection. The engraving is bold in the typical style of Habermel; the restrained decoration incorporates strapwork and small rosettes in various forms. All engraving is filled with niello. The instrument has seven sides, and they record, variously, the time in mean hours, planetary hours, and Bohemian (or Italian) hours, with conversions between them.
The top is occupied by a square 17.5 x 17.2cm. horizontal dial. The Southern edge is inscribed: Pro Eleuatione Poli 50 grad. This is the latitude of Prague, the modern value being 50°6'. Below is a small right-angled gnomon with its foot at the centre of a sunburst, whose slanted side is for registering mean hours, while the tip registers the Bohemian hours. The gnomon is followed by a curved ribbon marked with mean hours 6am to 6pm from which straight lines (in quarter-hour intervals) run to corresponding hours engraved on the outer edges of the dial. The top edge has space for two additional hours before and after the 6 o'clock lines. There follow seven curves marking the divisions of the year into the Zodiac. At the top is engraved: Tropicus Cancri, and at the bottom: Tropicus Capricorni; the Zodiac sigils are at either side, marking the season. The middle, straight, double line is labelled: Æquator. Across the face of the dial is a set of 13 straight, dotted lines for reading the Bohemian hours; these lines are numbered from 10 to 22, the latter being two hours short of sunset at the 24th hour in this system. The tip of the gnomon gives the Bohemian hour from the place where the Zodiac curve crosses the hour line. At the bottom of the face of this dial is a 4.6cm. diameter magnetic compass, and a hinged brass support for a plumb line (missing). The compass has a replacement base, magnetic needle and cover glass (which is deliquescent). The base is inscribed with two crossing lines, and the four cardinal points are marked in German cursive: N, O, S, W. A further line shows the magnetic declination at 14°West, which is the value for c.1710 and c.1860. It is likely that the replacement compass, along with some other repairs, were effected in the mid-nineteenth century. The original compass would have had a declination of 10°East.
Following on from the South side of the horizontal dial is a vertical dial, the top of which is engraved: Horæ Vulgares Planetariæ. Planetary hours result from the division of daylight into twelve parts, which vary in length according to the season of the year, short in winter and long in summer. A small hinged brass gnomon is attached at the top within a ribbon on which are engraved mean hours, in quarter-hour intervals, from 6am to 6pm, from which lines are cut to the corresponding hours marked on the outer edge of the dial in the usual manner for a vertical dial. These hours are read from a shadow cast by the sloping side of the gnomon. Another set of lines, dotted to distinguish them, show the planetary hours numbered 1 to 12, and these are recorded by the tip of the gnomon. Across the face of the dial is a set of curves identified by the sigils of the Zodiac, and hence the season of the year. Below these curves is the signature.
From the base of the vertical dial is another plane set at 40° (the co-latitude) to the vertical, with a hinged pin gnomon at the centre. At an angle across the face is engraved: Quãntitas diei, indicating the purpose of this dial, which is to give the hours of daylight through the year. These are read from a set of curves labelled both sides in mean hours from 8 to 16. These are the maximum hours of daylight from mid-winter to mid-summer at the latitude of 50°. Additionally, there are vertical lines numbered from 8am to 4pm, in quarter-hours divisions. A vertical strip at the bottom of the instrument is inscribed: AD MERIDIEM.
The panels on either side of the instrument are East and West declining dials, identified on strips at the bottom: AD ORTVM (sunrise), AD OCCASVM (sunset) respectively. The form of both dials is the same, with a hinged pin gnomon (missing on the West side), the central wide diagonal band named: Linea Æquinoctialis. Parallel lines mark the mean hours 4 to 11 on the East; 1 to 8 on the West. All the hours are divided in quarters. Curves, marked by Zodiac sigils, measure Bohemian hours from 9 to 18 East, 18 to 24 West, their positions shown by dotted lines.
The northern side of the instrument has two dials, one polar, the other North vertical. The latter is a panel named: Tabúla Planetarum diei, with seven rows and fourteen columns. The rows are marked firstly Diei, and secondly with the days of the week: Solis, Lunæ, Martis, Mercurÿ, Iouis, Veneris, Sabbati. The columns are labelled by planetary sigils, beginning with .., the Sun, and ending with .. Mars. Using this table it is possible to find the planet that governs any hour in the week. At the centre is a hinged brass gnomon to show the time in mean hours from 4 to 6, and 6 to 8, thus complementing the South facing vertical dial on the South side of the instrument. In a band at the bottom is engraved: AD SEPTENTRIONEM.
Unusually for a polyhedral instrument, the polar dial has a pin gnomon fitted parallel to the axis of the poles with a plate at right angles (i.e., parallel to the Equator) marked with mean hours in Roman numerals from VIII through XII to IIII, the quarter-hour subdivisions alternately shaded. In addition, rotating about the centre is a volvelle to convert mean hours to Bohemian hours and vice versa. The rim of the volvelle has a portion cut out, leaving a space for a pointer. This pointer registers over an arc with four scales, which, from top, are:
a) Ortus Solis: scale 8 to 4 in quarters.
b) Occas9 (Solis shared with previous scale): 4 to 8.
c) Quantitas deie: 8 to 16.
d) (Quantitas shared with previous scale) noctis: 16 to 8.
The first pair of scales gives the hour of sunrise and sunset respectively, thus spanning the year for the latitude of 50°. The second pair gives the length of the day or the night through the year. Thus at mid-Winter sunrise is at 8am and sunset at 4pm, with the length of day 8 hours and the length of night 16 hours.
This dial is labelled in an arc round the centre of the volvelle: Maius et minus Horologium inuicem oppositum et Adæquatum (The larger and lesser dials are set against each other and matching). Most of the edge is divided into Bohemian hours from 8 to 24. Thus with the pointer set to the time of sunrise (or sunset), which is in mean hours, the scale of Bohemian hours is set against the chapter ring of mean hours. The time may be read in either of the hour systems. Older sundials commonly give Bohemian hours, whereas clocks give mean hours, and since clocks are corrected by sundials, a ready means of conversion is necessary.
The hollow centre of the instrument, bounded by copper plates, shows the dark tarnish of the copper, small metal straps screwed to the plates to hold them together, and some later repairs using solder. The instrument is attached by four grub screws (seemingly hand-made) to a frame of oak fixed to a fine-grained wooden base that is ebonised. There are four, small bun feet.
See Colour Illustrations and Details
A metal analysis of this instrument has been conducted. Details of this analysis are available on request.