Introduction Section C The material in this section is very diverse. It includes notes and ideas by Appleton, comments and suggestions exchanged with research assistants, collaborators and colleagues, drafts for discussion meetings or for papers and talks, data (charts, graphs, diagrams) and correspondence. The alphabetical sequence in which the material is presented is intended as no more than a guide. Some of the titles, such as ‘Absorption’1 , 'Atmospheric Spectrum', 'Ionized Air', 'Recombination and attachment', ‘B - determinations', etc. correspond to Appleton's own file-headings, but others were adopted for the purpose of this catalogue on the advice of scientists and historians of science. In some cases, the headings cover brief exchanges only, while others describe extensive folders and a wide spread of material and time. Within each topic, the papers are in a chronological sequence, though this is frequently tentative; undated papers come at the end of each sequence. A very substantial proportion of the material was received as loose papers, rarely dated by Appleton and showing little variation in his handwriting over the years which might have permitted closer dating. Although efforts have been made, and advice taken, to assign work to some identified field of study or period of time, it has not always been possible to do so with any certainty. Material of this kind has been grouped under the title 'General Ionospheric Topics' (Coll-37/C.106 - C.165). It includes especially the collaborative work undertaken during Appleton's service as Principal of Edinburgh University 1949-65, when he pursued several lines of enquiry simultaneously with the help of a series of assistants and colleagues at Edinburgh (particularly Mrs. A.G. Pritchard, later Turnbull, A.J. Lyon, C.P. Bell, and the 'Ionospheric Young ladies' from the Department of Mathematics) as well as postal discussion with his lifelong colleagues W.R. Piggott, and W. J. G. (later Sir Granville) Beynon. It should be noted that the data and correspondence in this Section consist only of material either specifically related to the topic in hand, or kept in a named folder by Appleton himself. The correspondence folders in Section E contain similar material; see especially correspondence with W.J.G. Beynon (Coll-37/E.9 - E.16), R. Naismith (Coll-37/E.71 - E.82), W.R. Piggott (Coll-37/E.87- E.98), B. van der Pol (Coll-37/E.117 - E.144). Section F contains extensive additional data, prepared by or under the direction of Appleton in the furtherance of various research projects. Section G includes data and reports sent to him from observatories and ionospheric stations worldwide. These sections should therefore be regarded as supplementary to Section C. In addition, the notebooks in Section B often provide the embryo of Appleton's research ideas.

Additional Material Items Coll-37/C.389-C.425 represent notes, working papers and correspondence, which were received too late for inclusion in the main sequence. Many were in Appleton's original bulky folders or binders, which have been retained together with their titles, but the contents have been divided into more manageable units. The material is presented in the alphabetical order of the index of topics and cross-referenced to the items which it supplements. Appleton's work with W.R. Piggott on ionospheric storms was hardly represented in the collection as first received. It has now been listed as a separate topic in the index and the material appears at Coll-37/C.410-C.421.