This Encyclopedia aims, basically, at summanzmg the wealth of well­ established facts and outlining the relevant theories in the different branches of physics. With this as goal, the writers were asked to present their specific field in such a way that access is possible to any scientist without special a priori information in that field; the basic concepts of physics are assumed to be known to the reader. The survey given in each paper was also to be long­ lasting, so that even a few years after publication, each volume would be useful, for example as an introduction for newcomers or as a source of information for workers in a neighbouring field. In the field of geophysics, dealt with in Vols. 47--49 of the Encyclopedia, this task is difficult to achieve because during the last decades there has been a much faster development of basic information and theory than during the decades before. When I came to contribute to this work the famous Julius Bartels, then editor of the geophysical part, told me that Vol. 49 should certainly take into account the results of the "International Geophysical Year" 1957/58 (I. G. Y. ), and that we had better wait until these were accessible than produce a kind of information which might be obsolete in a short time.

Modelling of the Solar Extreme Ultraviolet Irradiance for Aeronomic Applications..- I. Temporal variations of the solar irradiance.- II. Representation and estimation of solar EUV fluxes.- III. Aeronomical implications of the solar EUV radiation.- Transport Coefficients and Collision Frequencies for Aeronomic Plasmas..- A. Transport coefficients derived with a mean-collision-frequency method.- B. Gas kinetic cross sections and collision frequencies.- I. Transfer cross sections and transfer collision frequencies.- II. Transfer cross sections for particular interactions.- 18. Analytic expressions and normalizations for transfer cross sections and transfer collision frequencies.- III. Transport collision frequencies and their calculation.- C. The moment method.- I. System of balance equations for momentum and heat flux.- II. Transport equations.- III. Particular transport coefficients.- D. The Lorentz method.- Append.- General references.- Modelling of Neutral and Ionized Atmospheres..- 1. Fundamental relations.- A. Measurements.- B. Results.- C. Empirical (descriptive) modeling.- D. Empirical modelling of the ionosphere.- E. Aeronomical modeling.- Appendix A: The Role of Photoelectrons in the Heat Balance of the Upper Atmosphere.- General references.- Errata to Volumes XLIX/3-6.