474 TRANSACTIONS OF SECTION A. 



the prolongation of the Eulerian Nutation from 305 to 430 days as indicating 

 an effective rigidity of the earth about equal to that of steel. Wiechert,^ of 

 Gottingen, found that the meau density, ellipticity, and processional constant 

 were consistent with the hypothesis of homogeneous core with lighter surface 

 layer. 



Mr. E.. D. Oldham," in a paper on the * Propagation of Earthquake Waves,' 

 came to the conclusion that the evidence pointed to a central metallic core, and to 

 the existence of marked differences in the physical constants of the core and the 

 surrounding crust. He, however, assigned a comparatively small radius to this 

 core, viz., about 0'55 that of the earth. 



I will now call your attention to the light thrown on this subject by the recent 

 investigations of Professor Milne. The difference in the rate of propagation of 

 earthquake waves through the earth's interior and through the crust has led him 

 to the conclusion that the material below a depth approximating to thirty miles is 

 of a uniform nature, and that the change in physical constitution is abrupt at some 

 such depth as that indicated. He writes as follows : — 



Tor chords which lie within a depth of thirty miles the recorded speeds do not 

 exceed those which we should expect for waves of compresf ion in rocky material. 

 This, therefore, is a maximum depth at which we should look for materials having 

 similar physical properties to those we see on the earth's surface ; beneath this limit 

 the materials of the outer part of this planet appear rapidly to merge into a fairly 

 homogeneous nucleus with a high rigidity.' 



In the ' Transactions of the Royal Society for 1905 ' will be found a paper by 

 Lieut.-Ool. S. G. Burrard on ' The Intensity of the Force of Gravity in India.' 

 Colonel Burrard writes as follows:— 'Geodetical observations have shown that 

 the density of the earth's crust is variable, but they have not given any positive 

 indications of the depths to which these observed variations extend. All calcula- 

 tions of the depths of subterranean variations in density and of the mountain 

 compensation have, therefore, to be based on arbitrary assumptions of depth. The 

 fact that the plumb-liue seems generally to respond readily to the results given by 

 the pendulum perhaps justifies the inference that the observed variations in the 

 density of the earth's crust are not deep-seated. If an abnormal amount of matter 

 exists in the crust near the surface, ic will exercise direct effects upon plumb-lines 

 and pendulums in the vicinity, but if it lies at a great depth its effects, especially 

 on plumb-lines, will be less perceptible. ... I have taken several instances of 

 abnormal pendulum results from table, and have found in each case direct response 

 from the plumb-lines at neighbouring stations. This conformity could hardly 

 ensue if the variations in density extended to greater depths than thirty or forty 

 miles. Our results do not justify us in asserting that no deep-seated variations 

 in density exist, but they do justify the belief that the variations in density which 

 have been discovered are apparently superficial.' 



It is interesting to notice the agreement between results drawn from such 

 dissimilar sources. On the one band we have had to deal with effects produced by 

 almost inconceivably small particles travelling with immense velocity ; on the 

 other, with effects dependent upon the behaviour of 'the huge terrestrial globe.' 

 That travellers starting from such opposite extremes should arrive at a common 

 destination is in itself a striking example of the scope and accuracy of the work 

 undertaken by investigators in physical science. 



_ It is possible that the evidence from each source, considered independently, 

 might be regarded as inadequate, but the cumulative effect is sufficiently strong to 

 justify the belief that some marked physical change in the constitution occurs at a 

 depth of some thirty to fifty miles. 



_ At all events, we have indications that, with the exception of a comparatively 

 thin crust, the earth consists of a non-radio-active substance with a rigidity 

 approaching that of steel, with an average temperature m the neighbourhood of 

 1500° 0., and a density at that temperature of about 5°'6 C. 



' Trans. Roy, Soe. G'ottingen, 107, « Phil, Trans., 1900, 



