192 Mr Larmor, On the Period of the [May 25, 



Were it possible to pick out this tide with fair approximation, 

 we should be able to derive information as to how much the 

 surface of the solid Earth yields to the centrifugal force. 



13. In all the foregoing, the relaxation of strain in the Earth 

 that would follow the removal of the centrifugal force means such 

 change of strain as would occur if the materials retained their 

 existing moduli of elasticity and were of unlimited strength. As 

 a matter of fact, a removal of the centrifugal force of the Earth's 

 rotation would result in strains so great that the solid constitution 

 of the interior would break down, and the materials would flow 

 into new configurations in which the strains would be eased. In 

 so far in fact as the configuration actually existing is due to 

 consolidation from a fluid form, there can be in it no internal 

 stresses other than simple hydrostatic pressure. In all the 

 previous argument, what has been really considered has been the 

 very slight change of configuration arising from a very slight 

 shift of the axis of rotation: and we can treat of that as the 

 difference of the centrifugal strains that would belong to the two 

 positions of that axis, when we assume the elastic constants to be 

 the actual ones that belong to small strains, and any limitation of 

 elastic strength is ignored. The question of imperfect elasticity 

 thus hardly comes up at all in such a case. 



14. The question as to whether the interior of the Earth is 

 in the main solid or fluid, is, when precisely expressed, really a 

 question as to what intensity of deforming stress will make the 

 materials in its interior begin to flow. Every actual solid sub- 

 stance has its own elastic limit: and when subject to deforming 

 forces sufficiently beyond this limit, the substance will flow like oil. 

 The result here arrived at is that for the small stresses involved 

 in the slight changes of the axis of rotation that accompany the 

 free precession, — stresses that are of the order of a moderate 

 number of grammes per square centimetre — the materials of the 

 Earth's interior are solid and elastic, and as will appear below 

 (§ 16) of very high rigidity. Beyond a certain limit they must 

 give way and flow : the stresses due to the astronomical pre- 

 cessional forces are not however in the main beyond this limit. 



As the Earth slowly alters its form owing to change in the 

 distribution of surface materials and loss of heat, strains will 

 arise in its interior and slowly increase until the material gives 

 way somewhere, with earthquake-like effects. At such a weak 

 place there may be a great deal of strain-energy degraded into 

 heat, sufficient it may be to melt the material. The region 

 under a range of volcanic mountains may have a plane of weak- 

 ness of this kind, up which the molten material is forced to the 



