1. Piggott, W. R., Nature, 171, 124 (1953).
2. Watts, J. M., and Brown, J. N., International Scientific Radio Union, Ottawa, October 5–8, 1953 (accepted by J. Geophys. Res. for publication in March 1954).
3. Agy, V., International Scientific Radio Union, Ottawa, October 5–8, 1953 (accepted by the J. Geophys. Res., for publication in June 1954).