Septembee 21, 1906.] 



SCIENCE. 



357 



and through the crust has led him to the 

 conclusion that the material below a depth 

 approximating to thirty miles is of a uni- 

 form nature, and that the change in phys- 

 ical constitution is abrupt at some such 

 depth as that indicated. He writes as fol- 

 lows: 



For chords which lie Avithin 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 maxi- 

 mum 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 nu- 

 cleus 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 com- 

 pensation have, therefore, to be based on arbi- 

 trary assumptions of depth. The fact that the 

 plumb-line seems generally to respond readily to 

 the results given by the pendulum perhaps justi- 

 fies 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 

 elfects, especially on plumb-lines, will be less per- 

 ceptible. * * * I have taken several instances 

 of abnormal pendulum results from table, and 

 have found in each case direct response from the 

 plumb-lines at neighboring stations. This con- 

 formity 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 dis- 

 similar sources. On the one hand we have 

 had to deal with effects produced by almost 

 inconceivably small particles traveling with 

 immense velocity ; on the other, with effects 

 dependent upon the behavior of 'the huge 

 terrestrial globe.' That travelers 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 approach- 

 ing that of steel, with an average tempera- 

 ture in the neighborhood of 1,500° 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 radioactivity, and the 

 temptation to dwell on such themes is cor- 

 respondingly 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 



