The graphometer is similar in principle to a simple theodolite, but with a semicircle divided into degrees instead of a full circle. There are four sights: the diameter of the semicircle has fixed sights at either end, while at the centre of this diameter a pivot carries a sighting rule or alidade that moves across the divided semicircle.

In early examples the compass is small, is incorporated into the pivot and is used for orientation; later, larger compasses were set in the central space of the semicircle and had a divided scale beneath the needle so that the instrument could also be used as a circumferentor or surveying compass, by sighting with the fixed sights. In the earlier instruments these fixed sights were used only for aligning the instrument with another station, though with the instrument mounted vertically they could have a levelling function.

The use of different orientations - vertical as well as horizontal - is explained in the treatise, Declaration de l'usage du graphometre (Paris, 1597), by Philippe Danfrie, who designed the instrument.

Jim Bennett
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