Pilot balloon slide rule, Met Office MkII model, designed by Francis John Welsh Whipple, and made by James J. Hicks, London, 1927

Pilot balloons were used as tracers for wind in the upper air, and to carry lightweight meteorological instruments. They would be monitored by meteorologists on the ground, either visually using adapted theodolites, or by radar. The slide rule was specially devised to help with the subsequent calculations from the actual readings and many forms were developed. This is the United Kingdom Meteorological Office pattern, Mark Two. It was devised by F J W Whipple, then superintendent of the Kew Observatory, and made by Hicks.