Letter from Thomas Gold to Dr Walter H Munk, Institute of Geophysics and Planetary Physics, LaJolla Laboratories, California Telling Dr Munk of an article by Petr Beckmann, 'Shadowing of random rough surfaces' in 'IEEE Transactions on Antennas and Propagation', Volume AP-13, pp 384-388, May 1965, that he thinks may interest him. 30 July 1965