1. Newton, H. W., Mon. Not. Roy. Astro. Soc. Geophys. Supp., 159 (1949).
2. Chakrabarty, S. K., Nature, 167, 31 (1951).
3. Ferraro, V. C. A., and Parkinson, W. C., Nature, 165, 243 (1950); J. Geophys. Research, 56, 190 (1950).
4. Data taken from J. Geophys. Research (1951).
5. Das, A. K., and Rao, Raja, Obs., 69, 147 (1949).