Material relating to circular letter sent by Edward Appleton to ionospheric stations, February 1954

Material relating to circular letter sent by Edward Appleton to ionospheric stations, dated February 1954. Appleton asks for information about equipment, timing and frequency of readings, in connection with 'Edinburgh studies of the diurnal variations of the critical frequencies of the various layers of the atmosphere'. It includes: draft letter requesting information; list of ionospheric stations throughout the world; Replies to letter, giving or enclosing information, presented in order of stations above, some incorporating correspondence with Appleton; and tabulated summary of information compiled by A.J. Lyon.