Mahonie Octant

The octant is made from a mahogany frame and limb with a brass index arm and fittings. The octant has a brass stop for the index arm and inlaid ivory plates on the crossbar.
There is a tangent screw and clamping screw , but also a fine adjusting system. Very seldom seen on octants! The tangent screw and 2 clamping screw are fitted to the limb on a separate shoe to the right of the index arm.
The octant has three socket shades in red, orange, and green. Index-glass adjustment is made by a screw and on both horizon glasses by levers, wing nuts and milled clamping screws. The sight vane has two pinholes and a swivelling shutter; the back sight vane has one pinhole. The octant is stored in a stepped oak box, probably not the original.
The instrument has an inlaid ivory scale from -5° to 94° by 20 arcminutes, measuring to 88°. The ivory A-type vernier displays a graduation to 1 arcminute, with zero at the centre (which was used till 1780). The maker’s name is inscribed on the index arm as ‘B. Martin, London’. This is Benjamin Martin (about 1705-82), a lecturer and scientific instrument maker based in London.