THE EVIDENCE

The Greek dioptra served several functions. When self-suspended it took heights and levels, and when mounted horizontally it took angles in a more or less horizontal plane. For these purposes the Romans, in contrast, had not one but two instruments. The groma worked purely in the horizontal plane and was applied particularly to land surveying, while the libra worked purely in the vertical plane, most notably in levelling for aqueducts. Libra is therefore not a simple synonym for dioptra, as Vitruvius makes plain in distinguishing them (Source 3). Indeed it is quite clear from what the sources say – and from what they do not say – that Greek and Roman instruments differed markedly. No source written in Greek mentions the libra, or anything that might be interpreted as the libra, although it remains perfectly possible, and indeed likely, that the libra was used for surveying aqueducts in the Greek world once Roman technical influence had made itself felt there. Similarly there is a dearth of references in Latin sources from the western half of the Mediterranean to the dioptra as a surveying instrument. This dearth is not total, since Vitruvius is aware of it; but his words in no way prove that it was ever used by the Romans.