1. Workman, E. J., and Reynolds, S. E., Phys. Rev., 78, 254 (1950).
2. Vonnegut, B., and Moore, C. B., Recent Advances in Atmospheric Electricity, edit. by Smith, L. G., 399 (Pergamon Press, New York, 1958).
3. Matthias, B. T., and Buchsbaum, S. J., Nature, 194, 327 (1962).
4. Humphreys, W. J., Proc. Amer. Phil. Soc., 76, 613 (1936).
5. Schonland, B. F. J., The Flight of Thunderbolts (Oxford Univ. Press, 1950).