Card's Traverse Calculator comprises a grid with a movable plastic rule, which is attached by a rivet to the lower left corner. Instructions for its use are printed on the bottom. The instrument is used to calculate the distance and compass bearing between two points. This is calculated from their difference in latitude (north-south position) around their average latitude, and the difference in their longitudes (east-west position). The latitudes and longitudes of the two points would be read from a map. The instrument therefore simplifies the process of reading off the difference in latitude using a pair of dividers and the latitude scale at the map’s edge. Once the calculator is set, the required distance is read off in ground (statute) miles or sea (nautical) miles, up to 1000 miles. Although intended as an air navigation instrument, it does not take account of wind speed or direction, which might affect the true heading required.

This is one of a number of items formerly belonging to Henry Theodore Augustus Bosanquet (1870-1959), who was a leading figure in the development of air navigation in Britain during the First World War.