An experimental gyrocompass, believed to have been designed by William Thomson, Lord Kelvin (1824-1907). The compass card is of the sort Thomson originally patented in 1876 for magnetic dry compasses. It is marked in points and degreees, with two degree scales, one printed with the figures reversed. The degree scales are graduated from 0 to 90 in each quadrant. The card has eight needles. The card is printed with the manufacturing details: SIR W. THOMSON'S PATENT No. 23664 REGISTERED' and 'KELVIN & JAMES WHITE LTD. GLASGOW'. There are two handwritten inscriptions on the underside of the card: 'P.31' and '13.35 / {...}'. The compass is held in a brass housing with a small door in the side, and has a single sighting vane attached to the body. A fixing for a second vane is on the opposite side of the body. Within the body is a gyroscopic mechanism powered by compressed air to keep it running. The aim was to use the properties of a gyroscope to create a compass that gave a true north reading without being susceptible to magnetic variation and deviation.