Inventory no. 52538 - Epact entry
Epact number: 48838
Theodolite
Signed by Erasmus Habermel
circa 1600; German
Gilt brass; 235 mm in height (minimum)
Main text
A theodolite for measuring both horizontal and vertical angles at once was an innovation described in surveying textbooks of the later 16th century, but apparently little used in practice. It was probably too complicated for general use and the functions it offered were not really needed. Thus, despite the enthusiasm and the confident claims of designers and authors, such instruments are very rare. Neither does this somewhat delicate altazimuth theodolite in gilt brass look like an instrument that has seen much service in the field.
Source museum: Museum of the History of Science, Oxford
Museum number: Inventory no. 52,538
Detailed text
Base of four legs, each foot with two mounting or securing holes, rises to a cross with a central square hole, through which a screw with a broad, round head secures a flat disc with two short arms rising to the underside of the horizontal circle, to which they are attached by two wing nuts. The underside of the circle is decorated with scrollwork in the manner of Habermel.
The horizontal circle comprises a ring of gilt brass, which accommodates a central silvered brass circular glazed compass box, around which rotates a silvered brass ring with three mounting plates for the supports to the vertical semicircle. The compass plate has lines for the four cardinal directions, marked 'SEPT|Ebar|TRIO', 'ORIENS' (to the right of north), 'MERIDIES', 'OCCIDENS', and a target line offset for variation; steel needle with brass cap to mount. Each mounting plate has two holes, one for a locating pin, the other for a securing screw. The circle has an outer degree scale 0 to 90 four times, divided to 10, subdivided to 5 and to 1, numbered by 10. Inside this is a scale of hours numbered 0 to 24 and 0 to 12 trice, divided to 1 hour, subdivided to |1/2| and to |1/4| with alternate hatching, numbered by 1 hour.
A vertical semicircle is carried by an 'A' frame rising to the pivot for the semicircle, with a decorated scrollwork joining piece incorporating the pointed target for a plumbline (missing), and a strut hinged to the semicircle giving lateral support. The lower part of the 'A' frame has two studs with holes for a line. One side of the semicircle has decoration with mirror gilding, which is also used on other parts of the instrument, the other has an outer degree scale 0 to 90 to 0, divided to 10, subdivided to 5 and to 1, numbered by 10, and an inner scale 0 to 12, divided to 1, subdivided to |1/2| and to |1/4| with alternate hatching, numbered by 1. Pinhole sights are attached by screws to either end of the diametric arm.
Jim Bennett