Vol. 62.] OP THE INTERIOR OF THE EARTH. 471 



be a zone, at about 140° from the origin, where the second-phase 

 waves would be so dispersed, and consequently feeble, that it would 

 practically amount to a shadow, and the second phase should be 

 absent in records from this distance, or much more feebly marked 

 than in those from greater or lesser distances. 



The effect will be modified by the fact that the transition from 

 central core to outer shell is not abrupt but gradual, though com- 

 paratively rapid ; yet it is worth noting that, so far as the limited 

 amount of available material may be trusted, the second phase is 

 certainly much less marked at about 140° from the origin than at 

 distances of less than 130° or more than 150°. For instance, the 

 Guatemala earthquake was recorded at Bombay, 144° from the 

 origin, by three instruments. On one record no indication of a 

 second phase can be found ; on another it is so indistinctly marked 

 that it can hardly be recognized as such ; and even on the Milne 

 pendulum, which shows what I take to be the second phase most 

 distinctly, it is not at all characteristic. The Batavian record of 

 this same earthquake, at a distance of 160° from the origin, shows 

 it much more distinctly ; and on the Cape-Town records of the 

 Alaskan earthquakes at 150° from the origin it is easily recognizable. 



These considerations lead to the conclusion that the time-curve 

 of the second-phase waves is not a continuous line. Up to about 

 130° it is continuous, and represents the emergence of waves which 

 have travelled directly from the origin ; beyond that distance it 

 represents waves which have been refracted, after passing on the 

 opposite side of the centre of the earth, and it would be misleading 

 to join the two into one continuous curve. For these reasons, the 

 second-phase time- curve has been drawn as it is shown in fig. 1 

 (p. 462). 



We may now turn to the first-phase waves, and see how they are 

 affected by the central core. At 120° the increase in mean apparent 

 rate of transmission is maintained, but at 150° the rate has dropped 

 to 9*76 km. sec, and the value is a good one. There can be no 

 doubt that the drop is real, but it is much less than in the case of 

 the second-phase waves, and merely represents a diminution of the 

 rate of propagation by about one-tenth. 



From this it will be seen that the central core behaves differently 

 from the outer shell with regard to the first, as with regard to the 

 second phase, but the change is much less in amount, and would 

 probably have remained undetected were it not for the very con- 

 spicuous alteration in the case of the second-phase waves. 



IV. Conclusions. 



From the considerations detailed in the foregoing page3, 1 conclude 

 that the interior of the earth, after the outermost crust of hetero- 

 geneous rock is passed, consists of a uniform material, capable of 

 transmitting wave-motion of two different types at different rates of 



