356 



SCIENCE. 



[N. S. Vol. XXIV. No. 612. 



We appear, therefore, to be driven to 

 one of two assumptions: either {a) that 

 the rate of heat production by radium 

 diminishes as we approach the center of 

 the earth; or (&) that the interior of the 

 earth differs markedly in constitution from 

 the exterior crust. 



It is true that Mr. Makower has shown 

 that there is a slight change of activity in 

 one of the radium products about the tem- 

 perature of 1,200° C, and it is very de- 

 sirable that this inquiry should be pushed 

 to much higher limits. At the same time, 

 it appears evident that but a very slight 

 change in activity takes place at tempera- 

 tures below 1,500° C. 



Now Mr. Strutt has shown, arguing from 

 known data, that the maximum tempera- 

 ture at the bottom of a crust of about 

 forty-five miles in thickness, must be in the 

 neighborhood of 1,530° C, although some 

 amount of uncertainty is necessarily in- 

 duced by our want of knowledge of the 

 conductivity of rock at high temperatures. 

 Anyhow, it is probable that at the depth 

 indicated the temperature does not exceed 

 the melting-point of platinum. Such a 

 crust would contain about one thirtieth of 

 the earth's volume, and if throughout it 

 the radium -heat energy were of the average 

 of that exhibited by many samples exam- 

 ined by Strutt, the temperature of the 

 earth could be maintained until our stores 

 of uranium suffered sensible depletion. 

 Such an assumption would lead to the con- 

 clusion that the whole of the central por- 

 tion of the earth consists of non-radioactive 

 substances at an approximate uniform tem- 

 perature somewhat below the melting-point 

 of platinum. A brief summary of the evi- 

 dence previously at our disposal may not 

 be out of place. 



In the first edition (1867) of Thomas and 

 Tait's 'Natural Philosophy' we find the 

 tidal evidence summarized as follows: 



It seems certain, therefore, that the tidal effect- 

 ive 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 Professor 

 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. Boy. 8oc., 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. Nevertheless, the evi- 

 dence clearly indicated a high degree of 

 effective rigidity. 



Hough {Phil. Trans., A, 1895, 1896) dis- 

 cussed the variation of latitude, and, after 

 correcting a small mistake of Newcomb's 

 (who was the first to suggest the explana- 

 tion), found the prolongation of the Eu- 

 lerian nutation from 305 to 430 days as 

 indicating an effective rigidity of the earth 

 about equal to that of steel. Wiechert 

 {Trans. Boy. Soc. GoUingen, 1897), of 

 Gottingen, found that the mean density, 

 ellipticity and precessional constant were 

 consistent with the hypothesis of homogene- 

 ous core with lighter surface layer. 



Mr. R. D. Oldham (PM. Trans., 1900), 

 in a paper on the ' Propagation of Earth- 

 quake Waves,' came to the conclusion that 

 the evidence pointed to a central metallic 

 core, and to the existence of marked differ- 

 ences 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 in- 

 vestigations of Professor Milne. The dif- 

 ference in the rate of propagation of earth- 

 quake waves through the earth's interior 



