1. Bullard, E. C., Freedman, G., Gellman, H., and Nixon, J., Phil. Trans. Roy. Soc., A, 243, 67 (1950).
2. Bullard, E. C., in The Earth as a Planet (edit. by Kuiper, G. P.), 57 (Univ. Chicago Press, 1954).
3. Elsasser, W. M., Rev. Mod. Phys., 28, 135 (1956).
4. Rochester, M. G., Phil. Trans. Roy. Soc., A, 252, 531 (1960).
5. Rochester, M. G., and Smylie, D. E., Geophys. J., 10, 289 (1965).