1. H. Jeffreys, Reports on Progress in Physics 10, 52 (1945).
2. J. B. Macelwane: Internal constitution of the Earth, Chapter X. Dover, New York: 1951.
3. G. Herglotz: Phys. Z. 8, 145 (1907).
4. H. Bateman: Phys. Z. 11, 96 (1910).
5. K. E. Bullen: Introduction to the Theory of Seismology. Cambridge 1953.