Part of what was known as a Dalton computer, used for solving the navigation triangle of velocities. It comprises a black bakelite or similar hard plastic body, which carries a metal plate with circular scale inscribed 'True AS' (i.e. air speed), 'True alt[itude]', 'miles', 'miles', '10 -95'. Within the circular scale is a further rotatable scale inscribed 'Minutes Ind. AS, Ind Alt'. The scale is divided into statute miles, nautical miles and kilometres. There are two further window scales, each marked 'Air Temp' (in centigrade) and 'Ind Height' (in thousands of feet). A roller chart and rotatable 360 degree calaibrated disc, which comprise the main part of the instrument, are missing from the back.