174 Dr. C. Chree on Applications of Physics 



familiar with at the surface. The enormous pressure, and 

 the presumably high temperature, very likely combine to 

 produce a state to which the terms solid, viscous, liquid, as 

 we understand them, are alike inapplicable. But be the state 

 what it may, the material must respond to the action of 

 periodic forces ; such forces must produce varying strains and 

 stresses ; and these strains and stresses can hardly fail to 

 produce effects at the surface. No numerical estimate of 

 these effects can claim to be in any sense final, as the mathe- 

 matical work by which it is evolved must depend on physical 

 data which are at best unproved, It appears desirable, how- 

 ever, that such numerical estimates should be made, on the 

 least objectionable physical basis available, if only for the 

 reason that their existence supplies a guide and incentive to 

 direct observation. 



In my opinion, for reasons previously discussed*, the 

 treatment of the earth as an incompressible elastic solid is 

 exposed to perhaps a minimum of objections. Most probably 

 the material increases in density and temperature as we 

 approach the centre, and a treatment which assumes the 

 material to vary with the radial distance would possess higher 

 a priori claims to regard than one which treats the earth as 

 homogeneous throughout. The problem of a gravitating 

 mass of varying elastic properties has, however, still to be 

 published, and as the assumed law of variation would probably 

 be mainly guess-work, the advantages for practical ends 

 might be less than would appear at first sight. 



Variation of the material with the radial distance, I may add, 

 could hardly affect the general character of the phenomena. 

 Surface heterogeneity, in which the material varies rapidly 

 w T ith latitude or longitude, is not unlikely to modify largely 

 the magnitude of some of the results at individual stations, 

 but is most unlikely to produce a large effect on the order of 

 magnitude of the mean lunar or solar tidal effects at a 

 moderate number of stations scattered over the earth's 

 surface. 



The treatment of local surface pressures in the first part 

 of the paper is in some respects on a less uncertain basis. 

 We can assure ourselves, if need be, of the solidity of the 

 ground surrounding a station ; and though the mathematical 

 work treats the solid as going down to infinity, this only 

 means in practice that the depth must be large compared to 

 the shortest distance of the loaded area. There are reasons, 

 however, even here for regarding the numerical results as in 

 general but rough approximations. 



* Phil. Mag. Sept. 1891, p. 232. 



