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Signed by Tobias Volckmer
Dated 1608; Brunswick
Gilt brass; 360 x 360 mm

The gilded metal instrument, which is square in shape, is provided with small removable feet, which allow it to maintain a vertical position. It is decorated with elaborate engravings.

Face a: four linear scales are present, one on each side, divided into twelve parts. In the lower part on the left is a compass which is 65 mm in diameter, that rotates on its base and is removable. Inscribed underneath the instrument is the wind rose (diameter 120 mm), divided into 360? and 24 hours, with subdivisions related to the cardinal points up to thirty-second parts. The base of the compass bears an astrolabe plate on its lower face, which bears the inscription 'Elevatio poli 48 G', referring to a latitude of 48?. Inside the instrument is a nonius, composed of five concentric circles intersected by oblique lines. The pointer is joined to the compass and bears divisions into five parts, allowing a precision of an order of 6' in readings.

The compass bears the wind rose, subdivided into eighths. The ends of the needle are marked in such a way as to allow North to be easily distinguished from South. On the base of the compass is a marking allowing geographical North to be distinguished from magnetic North, which is placed 5? to the East of geographic North in this instance. The compass is provided with a cover which bears a sundial complete with a folding gnomon, hinged to the base, suited to a latitude of 48?. A hole allows the underlying compass to be read. The back of the cover bears a lunar nocturnal dial ('Horologium noctis secundum Lunae'). The remaining part of the quadrant is made up of an angular scale of 9? and a grid for trigonometric readings of sines and cosines. Two independent alidades run along the quadrant. These are around 33 mm long, and are pivoted to the upper right hand corner. One of these bears two removable sights. The angular scale is subdivided into degrees and bears a nonius with divisions of 10'. The alidades, in correspondence with the nonius, are divided into minutes. The grid is traced in the quarter of circumference inside the nonius. The bounding radii are divided into 90 equal parts, numbered in groups of 5. On this base equidistant straight lines are traced in both a horizontal and vertical direction. The same subdivisions are present on the alidade.

To measure the trigonometric functions of an angle, one of the alidades is brought to the chosen angle and the line (horizontal or vertical) in correspondence to which the alidade intersects the circumference that delimits the grid is observed. The corresponding number is read from one of the delimiting radii. Both the angle and its complement are given on the scale of angles: the readings of sine and cosine can thus be made both in the horizontal direction and in the vertical direction.

Face b: Here we have an altimetric scale with indications of Umbra versa, Umbra recta and Umbra media, a two-limbed horary quadrant of the Stoffler type, for unequal or planetary hours and a second horary quadrant, of the same kind, for equal or common hours. Both quadrants are delimited by indications of the signs of the zodiac, both for horary use and for the typological definition of the signs and to indicate the influence of the planets on each sign. The two quadrants are delimited by the equinoctial lines and by the line of the tropics. The 'scala iaciendi' on one side and the 'calendarium et locus solis' follow, with the distribution of signs and months providing the subdivisions of the seasons. Below, the nonius is engraved, allowing the usual reading of the angles. At the extreme right is an astrological calendar, which bears a sophisticated inscription on the right hand side. A plumb-line, fixed to the graduated alidade (which bears two sliding pointers) completes this face of the instrument. Each pointer can be extended to a length equal to the breadth of one of the horary quadrants, which it can be used to read. The sides are marked with four scales: Pes monacensis cum suis unciis, Plumbum, Ferrum, Lapis.

On the sides of the pivot which holds the alidade is the signature of the maker and the date: 'Auctorae Tobia Volckmero Brunsvicensi Serenis. Ducis Bavarorum etc. Mathe. et Aurifabro faciebat 1608'.

Mara Miniati

Istituto e Museo di Storia della Scienza, Firenze
Inventory nos 2465 and 1495

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