474 constitution of the interior of the earth. [aug. i906, 



Discussion. 



The President referred to recent physical speculations as to the 

 condition of the earth's interior, and particularly to those of 

 Arrhenius. There could be little doubt that we were on the eve 

 of important additions to the data hitherto available for discussing 

 this subject. The establishment of accurate self-registering instru- 

 ments all over the world for recording earthquake-movements had 

 provided a new method of investigation, as the Author had shown 

 by his previous papers and by the interesting communication that he 

 had now made to the Society. In his diagrams, the Author had 

 represented the path of the wave as abruptly refracted against the 

 inner nucleus of the globe ; but he would no doubt admit that there 

 must be a gradual transition from the material of the nucleus into 

 that of the external portions. The President was glad to hear the 

 Author's generous recognition of the value of the work accomplished 

 by Prof. John Milne, who must be regarded as the great pioneer of 

 modern seismology. Prof. Milne had been recently invited by the 

 Royal Society to give the Bakerian Lecture next month, on recent 

 advances in seismology, a lecture to which physicists and geologists 

 were looking forward with much interest. 



Dr. J. W. Evans enquired as to the nature of the difference 

 between the waves of the first and second phases recorded by 

 seismographs. 



Mr. A. P. Young wished to ask whether he was right in supposing 

 that the Author's hypothesis required a division of the earth's sphere 

 into two portions — a highly-refringent core, separated by a distinct 

 boundary from a rind of greater elasticity. If that were the case, the 

 speaker thought that he saw the necessity for some postulate as to 

 the conditions of elasticity in the outer shell, on the basis of which 

 the curve of the rays, and ultimately the diameter of the core, might 

 be estimated. 



Mr. G. W. Young understood that, if one type of seismograph 

 were used, the waves of the third phase were much larger than those 

 of the second phase, while with another type of instrument the reverse 

 was the case; so, apparently, the relative sizes of the waves depended 

 upon the instrument used. Did not that make it difficult to determine 

 which of the two phases was really the more powerful ? 



The Author, in reply, said that he believed that the passage from 

 the outer shell to the central core was probably gradual, but certainly 

 rapid. The supposition of an abrupt change was convenient, as the 

 passage seemed to be so rapid that it would only be represented 

 by a line on the scale used in the diagram ; whether gradual or 

 abrupt, there would be no difference in the ultimate amount of 

 refraction. The difference in character of the seismograms depended 

 on the fact that pendula, of short period were usually fitted with 

 arrangements for a large multiplication of the actual displacements. 

 The record of the first and second phases being mainly due to 

 iuertia, and the actual displacement being approximately the same 

 in each case, the resulting record was naturally larger in the case of 



