358 



NA TURE 



[August 9, 1906 



tidal effective rigidity of the earth must be greater than 

 that of glass." 



In the 1883 edition of the same work a discussion of the 

 question by Prof. George Darwin is given. He states : 

 " On the whole we may fairly conclude, whilst there is 

 some evidence of a tidal yielding of the earth's mass, that 

 yielding is certainly small, and that the effective rigidity 

 is at least as great as steel." 



In a later paper (Proc. Roy. Soc, 1885) Darwin pointed 

 out that this conclusion was based on the assumption that 

 oceanic tides would have their equilibrium value, and that 

 the validity of this assumption was open to doubt. Never- 

 theless, the evidence clearly indicated a high degree of 

 effective rigidity. 



Hough (Phil. Trans., A, 1895, 1896) discussed the vari- 

 ation of latitude, and, after correcting a small mistake of 

 Newcomb's (who was the first to suggest the explanation), 

 found 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 (Trans. Roy. Soc. 

 Gottingen, 197), of Gdttingen, found that the mean density, 

 ellipticity, and precessional constant were consistent with 

 the hypothesis of homogeneous core with lighter surface 

 layer. 



Mr. R. D. Oldham (Phil. Trans., 1900), 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 Prof. 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 : — 



" For chords which lie within a depth of thirty miles 

 the recorded speeds do not exceed those which we should 

 expect for waves of compression 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. -Colonel 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 

 calculations 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-line 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, it 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 

 fiave 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 con- 

 formity could hardly ensue if the variations in density 

 ■extended to greater depths than thirty or forty miles. Our 

 results dp 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 discovereS 

 are apparently superficial." 



It is interesting to notice the agreement between results 

 drawn from such dissimilar sources. On the one hand we 

 have had to deal with effects produced by almost in- 

 conceivably small particles travelling with immense velocity ; 

 on the other, with elTects dependent upon the behaviour 



NO. I919, VOL 74] 



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, con- 

 sidered independently, might be regarded as inadequate, 

 but the cumulative effect is sufficiently strong to justify 

 the belief that some marked physical change in the consti- 

 tution occurs at a depth of some thirty to fifty miles. 



At all events, we have indications that, with the excep- 

 tion 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 in the neighbourhood 

 of 1500° C, and a density at that temperature of about 

 5°-6 C. 



An interesting question awaiting solution is the probable 

 constitution of this core. 



The above is but an example of the many fascinating 

 problems upon which fresh light has been thrown by the 

 revelations of recent discoveries in radio-activity, and the 

 temptation to dwell on such themes is correspondingly great : 

 but I feel that such a task should be committed to hands 

 more capable than mine. 



Fortunately, in the discussions which will take place 

 during our meeting ample opportunity will be afforded 

 those entitled to speak with authority. Nevertheless, there 

 are one or two further aspects of the matter which I will 

 venture to touch upon, although but an onlooker. I would, 

 first of all, urge the importance of a study of what may 

 be termed the natural history of the elements. We require 

 more information as to their comparative proportions in 

 different localities. The fact that, given the amount of 

 uranium in a sample of native rock, we can predict with 

 certainty the amount of radium contained in the same 

 specimen is of startling significance. 



The natural law which governs the proportions of these 

 two substances may have a far wider reaching scope than 

 we at present suspect. Nature appears to present to us a 

 grouping which would not naturally have occurred to the 

 mind of the chemist ; lead and silver, copper and gold, 

 and, again,^ platinum and iridium, seem invariably to be 

 introduced to us by Nature as if bearing to each other 

 some kind of blood relationship. 



The facts we already possess seem dimly to indicate some 

 close relation between elements which we have hitherto con- 

 sidered as outside the bounds of consanguinity, and for a 

 fuller knowledge of this important branch of natural his- 

 tory we require the assistance of the practical engineer, the 

 geologist, the metallurgist, and the chemist. 



Many of the results arrived at by the investigators into 

 the phenomena of radio-activity can apparently only be 

 verified by the lapse of considerable intervals of time. It 

 is probable, for example, that we can estimate with some 

 degree of accuracy the time required for the dissolution of 

 half a given mass of uranium or radium, but the complete 

 verification of our inferences must probably be left to a 

 future generation. If we accept this view, it is our duty to 

 provide our successors with data on which their con- 

 clusions may be based. If, for example, carefully deter- 

 mined masses of the more radio-active substances could be 

 placed in such circumstances as to remain untouched until 

 the meeting of this .Association some hundred years hence, 

 our successors, who would doubtless be equipped with 

 apparatus of research more accurate and more sensitive 

 than any in our possession, would at all events be placed 

 in a position to establish by direct methods the accuracy 

 of inferences based upon the experimental data now at our 

 disposal. This task is one which, it appears to me, might 

 well be undertaken by Section A, and I trust that this 

 suggestion may be held worthy of some consideration. 



It appears probable that one gram of radium dimin- 

 ishes in weight by about half a milligram per annum ; 

 hence, if the funds of this Society admitted of the im- 

 prisonment of some definite mass of radium, our successors 

 a hundred years hence would, even if they possessed only 

 the apparatus now at our disposal, be able to determine 

 its loss with sufficient accuracy to enable them to verify 

 the truth of the conclusions arrived at by the physicist 

 of to-day, while the investigation of the radio-activity of 



