197 
1918-19.] The Propagation of Earthquake Waves. 
6. The hypothesis suggested by these facts and deductions is that the 
earth consists of an elastic solid shell down to a depth of about half the 
earth’s radius, that at this depth the rigidity begins to break down, and 
that finally, at a depth of six-tenths of the earth’s radius, the elastic solid 
shell gives place to a non-rigid nucleus of measurable compressibility. 
This hypothesis is broadly similar to the views advanced by R. D. Oldham 
in 1906. 
7. H. Jeffreys’ suggestion that the phenomena may be co-ordinated in 
terms of the theory of firmo-viscosity, which he has found serviceable in 
other lines of research, is worthy of consideration, although here also there 
are some difficulties to surmount. 
Thanks are due to the Carnegie Trust for the Universities of Scotland 
for financial help in preparing and printing the illustrations and tabular 
matter. 
[Appendix 
