1. Prentice, Lovell and Banwell, Mon. Not. Roy. Ast. Soc., in the press.
2. Lovell, Banwell and Clegg, Mon. Not. Roy. Ast. Soc., in the press; see also Nature, 159, 119 (1947) and 160, 75 (1947).
3. Appleton and Builder, Proc. Phys. Soc., 45, 208 (1933).
4. Störmer, Terr, Mag. and Atmos. Elect., 501 (Dec. 1946).
5. For example, Appleton, Naismith and Ingram, Phil. Trans., 236, 191 (1937).