Inventory no. 45944 - Epact entry
Epact number: 17446
Theodolite
Signed by Erasmus Habermel
circa 1600; Prague
Gilt brass; 290 mm in height (minimum)
Main text
This fine and early example of an altazimuth theodolite has some detailing that is interesting in itself and typical of its maker, Erasmus Habermel. The clever provision of additional functions was clearly prized in fine instruments, perhaps in those that would be more enjoyed for their ingenuity than used to practical ends. Here the instrument can be removed from its mounting and the supporting structure for the vertical semicircle can be replaced by an extended arm with a linear scale. In this configuration the surveyor has a protractor, for setting out on paper the angles previously measured in the field.
A second feature of note is the plumb-bob. Usually separate plumb-bobs are missing from surviving instruments. Here the maker has guarded against loss by screwing it into a cross-piece to keep it on the instrument, or perhaps by providing a spare. Either way, the idea has worked and the bob has survived.
Source museum: Museum of the History of Science, Oxford
Museum number: Inventory no. 45,944
Detailed text
Turned screw mount for supporting the theodolite, fitting inside a sleeve with a clamping screw working on a knurled edge. This rises to an altazimuth mount with clamping screw for a diametric bar which supports the horizontal circle. The rotation of the bar can be clamped by a screw and the circle is secured to it by two screws with pierced heads.
The horizontal circle, in the form of a ring carried by an open cross, has an outer degree scale 0 to 360, divided to 10, subdivided to 5 and to 1, numbered by 10, and an inner scale of hours 1 to 24, divided to 1, subdivided to |1/2| and to |1/4| with alternate hatching.
A central glazed circular compass box, not rotating with the vertical semicircle, 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.
A diametric bar rotates around the circle with two short uprights at either end with holes accommodate fixing studs at the lower ends of the 'A' frame rising to the pivot for the vertical semicircle; the studs are secured by hinged clamps . A third arm, at right angles to the diametric bar, is for supporting the lower end of a lateral reinforcing strut rising to a hinge on the semicircle; this strut is missing. The 'A' frame has a scrollwork cross piece, with a plumb-bob stowed in a screw-hole in the centre.
The vertical semicircle is decoration with mirror gilding on one side and on the other a degree scale 0 to 90 to 0, divided to 10, subdivided to 5 with alternate mirror gilding and to 1, numbered by 10. Circle and circle-centre sights at either end of the diameter of the semicircle. The motion of the semicircle is constrained by two clamping screws.
The vertical framework can be removed and replaced by a shaped rule extending beyond the horizontal circle, so that without the mounting beneath the circle, this can be used as a protractor. The scale on the arm is 0 to 24, divided to |1/2| with alternate hatching and to |1/4|, numbered by 1.
Jim Bennett