Semi-circumferentor in brass with slit & wire sights bubble level on the alidade and small inset compass marked N, S, E & W and 0-360 degrees on the ring dial. Socket mount with ball and socket joint and clamped against the motion. The 165 mm diameter scale with verniers reads both ways to 1 minute of arc. The outer part is divided into 180 degrees in both directions. Continental origin (possibly French).
Semi-circumferentors were used rather more on the continent than in Britain and particularly in France where this item was most likely manufactured. To economise on weight and brass the alidade allows the instrument to be used through 360 degrees.
Used for underground surveying or for surveying in thickly wooded country where the accuracy of a theodolite is not required.