1. Bassi, P., Clark, G., and Rossi, B., Phys. Rev., 92, 441 (1953).
2. See Greisen, K., “Progress in Cosmic Ray Physics”, edit. by J. G. Wilson, 3, chapter 1 (North Holland Pub. Co., Amsterdam, 1956).
3. Abrosimov, A. T., Bedniakov, A. A., Zatsepin, V. I., Nechin, Iv. A., Solov'eva, V. I., Kristiansen, G. B., and Chikin, P. S., J. Exp. Theor. Phys., USSR, 29, 693 (1955).