472 CONSTITUTION OF THE INTERIOR OF THE EARTH. [Aug. I906. 



propagation: that this material undergoes no material change in 

 physical character to a depth of about six-tenths of the radius, such 

 change as takes place being gradual and probably accounted 

 for sufficiently by the increase of pressure ; and that the central 

 four- tenths of the radius are occupied by matter possessing radically- 

 different physical properties, inasmuch as the rate of propagation 

 of the first phase is but slightly reduced, while the second-phase 

 waves are either not transmitted at all, or, more probably, transmitted 

 at about half the rate which prevails in the outer shell. 



If these waves are to be explained as those of condensation and 

 distortion, then the ratio between the modulus of rigidity and the 

 bulk-modulus is only two-thirds of that obtaining in the outer 

 shell; but whether this interpretation be adopted, or that of 

 Mr. Fisher, or some other yet unproposed, we still have a central 

 core the behaviour of which with regard to these waves differs 

 materially from that of the outer shell. I do not propose to enter 

 into speculative grounds, or to offer any opinion as to whether this 

 central core is composed of iron, surrounded by a stony shell, or 

 whether it is the central gaseous nucleus of others. On this 

 occasion, it is enough to have shown that there is a difference 

 which cannot be overlooked, and must be taken into account in any 

 hypothesis that may be formed regarding the constitution of the 

 interior of the earth. 



Table I. 



Group- Average Intervals for Propagation of First- and Second-Phase 

 Waves from their Origin to the Place of Observation. 





First Phase. 



Second Phase. 





Distance : 

 Degrees. 







Earthquake. 

 Ser. No. 











Observ. 



Interval : 



Observ. 



Interval: . 







No. 



Minutes. 



No. 



Minutes. 





23-4 



6 



4-2 







12 



41-7 







"i 



150 



7 



45-9 



17 



8 ! 2 



7 



15-1 



12 



48-5 



5 



8-0 







,, 



53-1 



5 



9-1 







,, 



642 



13 



12-4 



11 



20 ; 7 



5 



78-0 



7 



13-5 



5 



24-4 



11 



83-3 



33 



12-8 



30 



23-5 



9 



85-2 



5 



14-8 







1 



86-3 



5 



14-1 



*8 



24-6 



4 



86-8 



7 



14-5 



10 



25-2 



8 



87-9 







6 



25-7 



3 



88-1 



"9 



151 



7 



24-4 



6 



90-8 



20 



14-8 



14 



251 



11 



92-3 



11 



13-1 



8 



246 



10 



1456 



5 



21-7 







11 



