1. H. Spencer-Jones and P. J. Melotte: Mon. Not. Roy. Astr. Soc. Geophys. Suppl., 6, 7 (1953).
2. P. H. Serson and W. L. W. Hannaford: J. Geophys. Res. 62, 1 (1957).
3. K. Whitham: IAGA Bull. 16, 18, Int. Assoc. Geomag. and Aeronomy 1960.
4. See also the contribution by J. R. Balsley in this volume, p. 395.
5. P. H. Serson, S. Z. Mack, and K. Whitham: Publ. Dom. Obs. 19, 15 (1957).