A large-format surveying compass? / circumferentor constructed from patinated brass. The instrument features a central dial plate and a rotating alidade arm fitted with two vernier scales to allow for precise readings against the outer degree ring. The outer ring is graduated from 0 to 360 degrees in single-unit increments, with cardinal points marked; a large N is visible at the north position and a W at the west. The dial face is engraved with the literal transcription ALEX. MABON & SONS above GLASGOW. Set into the brass dial plate are two circular spirit levels for horizontal alignment. The instrument is equipped with two folding brass sighting vanes and a substantial mounting assembly to the underside, featuring a threaded collar and adjustable levelling screws. The magnetic needle and protective glass cover are absent.
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