1. Houtgast, J., Nature, 175, 678 (1955); Solar Eclipses and the Ionosphere (Pergamon Press, London, 1956), Supp. Vol. 6 to J. Atmos. Terr. Phys.
2. I.G.Y. Bull., No. 36 (June 1960); No. 55 (January, 1962).
3. White, M. L., Wyller, A. A., and Fougere, P., Space and Planetary Environments, edit. by Valley, Shea L., Air Force Surveys in Geophysics, No. 139 (Bedford, 1962).