and Mathematics to Seismology. 1 99 



gical action in the neighbourhood of an observatory, though 

 not nearly so important as ocean or estuary tides for obser- 

 vatories situated within a few hundred yards of high-water- 

 mark. 



The considerable fluctuation of the calculated luni-solar 

 effects with the value ascribed to the earth's rigidity may lead 

 eventually to interesting speculations as to the state of the 

 earth's interior. 



Subsidiary Remarks. 



§ 33. Whilst attention has been confined to surface-pressure 

 and luni-so]ar action, it is not intended to imply the non- 

 existence of other agents capable of producing similar 

 phenomena. The sun's direct heating effect is doubtless 

 in some cases a most effective agent in altering the level. 

 A priori one would expect a diurnal variation from this 

 cause, most sensible at stations on rocky ground exposed 

 to the south. 



§ 34. Before quitting the subject, it is desirable to consider 

 what light existing seismological data throw on the credibility 

 of the hypothetical theory adopted. 



It appears pretty generally believed that wave-velocities 

 calculated from observations near and distant from the 

 epicentre of an earthquake are usually different, and the 

 existence of at least two widely different wave-velocities 

 seems on some occasions well established at the distant 

 stations. One of the two wave-velocities has been regarded 

 (on, I think, mistaken grounds) as postulating an elasticity 

 incredibly high for an elastic solid medium. 



These phenomena are easily reconciled with the elastic 

 solid hypothesis. When waves travel between two distant 

 points through the interior of a sphere of large radius they 

 may be expected to behave much as if the medium were 

 infinite. Now in an infinite isotropic medium*", as is well 

 known, there are two wave-velocities ; v x and v 2 , given in our 

 previous notation by 



V\ = V (m + n)lp v 2 = s/nlp. 



Thus, under the conditions supposed, we should expect two 

 eartb quake-waves with velocities similar to i\ and t> 2 . For 

 definiteness, suppose that the velocities are actually i\ and r 2 , 



* See, for instance, Love's ' Treatise on Elasticity, vol. i. pp. 133, 134. 



