TIDES AND THE RIGIDITY OF THE EARTH 9 



of one definite rigidity throughout. Being heterogeneous as 

 regards density it may be expected to be so in regard to 

 rigidity as well. It is perhaps not very surprising that it 

 should be possible to fit all the observations by the assump- 

 tion of a core of greater density enclosed in a crust of smaller 

 density, provided the core be stiffer than the crust ; and it is 

 interesting to note that, if the crust be taken to be about 1,000 

 miles thick and to have the average density of surface rocks, 

 whilst the core is taken to have the density of iron, the average 

 rigidity of the core, computed on the hypothesis of incom- 

 pressibility, must be nearly three times that of steel, whilst 

 the average rigidity of the crust, computed on the same 

 hypothesis, may be much less than that of steel and indeed 

 less than that of most hard rocks. 



Rigidity of the Earth 



The inference that the greater part of the body of the 

 Earth must be solid and very rigid has been confirmed in a 

 remarkable way by the results of seismological investigations ; 

 indeed, the perhaps unexpected conclusion that the inner parts 

 must be more rigid than the outer appears to be required as 

 part of the interpretation of seismic records. The systematic 

 recording by suitable instruments of seismic disturbances trans- 

 mitted to great distances has been practised for a relatively 

 short time but the results that have been obtained by means 

 of such records have already proved to be of the highest value 

 for Geophysics. When a great earthquake takes place it affects 

 seismographs all over the world ; the records always conform 

 to one type, a series of minute tremors being followed by a series 

 of much larger oscillations which subside gradually. When 

 the distinction between the preliminary tremors and the large 

 waves was first noticed, it was supposed by some writers that 

 they were to be classed respectively as longitudinal and trans- 

 verse waves, in accordance with the well-known physical 

 principle that waves transmitted through an elastic solid body 

 are of two types— waves of compression or rarefaction, charac- 

 terised by movement parallel to the direction of propagation ; 

 and waves of distortion, unaccompanied by change of volume, 

 characterised by movement transverse to the direction of propa- 

 gation. As the records accumulated and the theory of elasticity 



