24 R. D. Oldham — The Interior of the Earth, 



in the depth reached by the two forms of waves; beyond that 

 distance the depth reached increases again, and gradually becomes 

 more equal for the two forms of wave motion till the limit of about 

 120° is reached. In detail these results have been modified by later 

 work and more exact and numerous determinations of the time 

 intervals, which Professor H. H. Turner has, within the last three 

 years, found to require considerable corrections ; it is also 

 a fact that the mathematical methods were not altogether 

 sound, and quite recently the problem has been tackled anew by 

 Dr. C. G. Knott. 1 He informs me that he has succeeded in solving 

 the integrals in an unequivocal manner, no longer needing the use 

 of any assumptions, as had previously been used by himself and by 

 Professor Wiechert. Prom this he has computed and plotted the 

 wave paths for various distances, which show that those emerging at 

 from 4-5° to about 75° are crowded together in their deepest-lying 

 parts, in a zone lying just outside about one-quarter of the radius 

 from the surface, or at a depth of 1,300 to 1,500 km. The 

 flattening of the wave paths in this region shows that the increase 

 in rate of propagation suffers a check, and in the case of waves 

 emerging at 73° the lower part of the path is actually concave 

 towards the centre, indicating a temporary decrease in the rate of 

 travel. 2 



The coincidence of these results make it apparent that a change of 

 some kind takes place in the neighbourhood of 1,400 km., or rather 

 less than a quarter of the radius, from the surface, but it is equally 

 clear that it is not a change of state, for both here and at greater 

 depths the two forms of simple mass waves continue to be propagated 

 at about the same relative rates as is demanded by theory. The 

 change indicated is rather of chemical composition than of physical 

 constitution, and in discussing this it is necessary to hark back. 



There is a considerable body of evidence, principally astronomical, 

 though partly also geological, which goes to show that the central 

 portion of the earth is composed of metal, principally iron, surrounded 

 by a sheath of stony material. In part this deduction is reached 

 from spectroscopic analysis of the sun and stars, in part from the 

 composition of meteorites, which, whether regarded as the fragments 

 of older worlds, or as the material from which worlds are built up, 

 may be regarded as fair samples of the composition of our earth, and 

 in, part from certain geological observations which indicate that deep 

 down in the earth masses of metallic iron are associated with the 



1 Not yet published in full ; for an abstract see Nature, November 21, 1918, 

 p. 239. ' 



2 In this connexion it is noteworthy that just a year ago Dr. G. W. Walker 

 announced his conclusion that many of the earthquakes which give rise to 

 long distance records originate at about this depth (Brit. Assoc. Eep., 1917). 

 The conclusion cannot be regarded as fully established, and there are some 

 difficulties in the way of its acceptance, but it is an important and interesting 

 suggestion, which must receive serious consideration, with the reservation that 

 the origin is not of the earthquake proper but of the bathyseism, of which the 

 surface quake, which is felt and does damage, is a secondary result (see Quart. 

 Journ. Geol. Soc, Ixv, p. 14, 1909). 



