140 LAMBERT: CONSTITUTION OF THE) EARTH 



few months. The modulus of compressibility for these stresses 

 of short period would naturally be larger than the modulus of 

 compressibility deduced from stress continuing unchanged over 

 long periods,^'- and the modulus of rigidity which was zero for 

 the long-continued forces might well be for the short period about 

 3/5 of the corresponding modulus of compressibihty. The con- 

 clusion is that the numbers in the table are too small. The 

 average of the tabular numbers agrees with what was deduced 

 from the variation of latitude, but in the latter calculation the 

 compressibility was neglected, leading to too small a result. 

 This is another reason for believing the values in the table should 

 be somewhat increased. 



You see some of the difficulties in which the subject is involved. 

 How much should those numbers be increased? I don't know; 

 by a variable amount, perhaps not more than twenty per cent, 

 at a guess. As the conclusion of this discussion of the rigidity 

 of the earth, I can merely express my opinion or feeling that the 

 average rigidity of the whole earth is over 12 X 10^ S that the 

 modulus of rigidity must increase with the depth below the sur- 

 face, and that probably the figures of table i, all increased slightly, 

 will give a fair idea of the variations of the modulus of rigidity. 



Any discussion of the elastic constants of the earth would be 

 incomplete without some mention of earthquake phenomena. 

 Any adequate treatment of this, even a mere outline, would be 

 impossible in the time available. The nature of the phenomenon 

 may be described by saying that the earthquake shock is trans- 

 mitted by radiations or waves analogous to those of sound and 

 light. An ordinary elastic solid is a less specialized medium 

 than the ether (if one dare mention the ether in these days of 

 relativity), which transmits only the transverse vibrations of 

 light, less specialized also than the air which transmits sound, 

 for the modulus of rigidity of air is zero, and the air can transmit 

 only the longitudinal waves (waves of condensation and rare- 



'- The modulus is the intensity of force necessary to produce a given deformation, 

 and if the time be limited, it is natural to expect that the force required will be 

 greater. 



