This insrument was ised to find the magnetic variation at a particular place by aligning the instrument along a meridian and reading the needle's variation from true north. The instrument has a brass frame and is signed on the scale 'Ramsden London'. The brass scale is marked from -5°to 47° from left to right, by 15', with a brass vernier measuring to ½'. There are three adjusting feet (one now missing). The arm is moved by rack and pinion and carries a rolled brass, glass-covered variation compass, with pivoted needle (missing), measuring from +9° to -9°. The two stands on eiter side of the frame are for carrying a level (now missing).

The instrument is held in a wooden keystone box from which some parts are missing. A hole has been made in the lid to make room for one of the sights, suggesting that the box is probably not original to this instrument.