28 SCIENCE PROGRESS 



therefore consider the constitution of the earth itself, and, from 

 what little knowledge we have, attempt to deduce the probable 

 distribution of radium. It is no more absurd for the geologist 

 to speculate on the interior of the earth than it is for the 

 physicist to postulate the architecture of an atom. Both earth 

 and atom have an outer shell with which we are to some extent 

 familiar. Both have a central core or nucleus which is not 

 amenable to direct observation ; but the one is not more in- 

 accessible than the other. The atom is penetrated by the 

 Becquerel rays ; the earth is inwardly explored by earthquake 

 waves. 



In 1900, Dr. R. D. Oldham 1 showed that the disturbances 

 due to an earthquake may be analysed into three distinct 

 forms of wave motion, which, after passing around or through 

 the earth, give rise to three different phases in their distant 

 record. The third phase is attributable to surface waves, and 

 with these we need not here concern ourselves. The first and 

 second phases are due respectively to waves of compression 

 and waves of distortion, the former travelling much more rapidly 

 than the latter. Traversing the heterogeneous rocks of the 

 earth's crust, the two types of waves cannot be readily dis- 

 tinguished. However, as soon as they sink (at a depth of say 

 20 miles) into the homogeneous material that lies below, they 

 are sorted out in virtue of their different velocities, and at a 

 distance of 700 miles from their source two distinct records 

 may be detected, each referable to the same original shock. 



If the velocity of waves transmitted through a medium re- 

 mains constant, then the wave motion is propagated in all 

 directions in straight lines. If, however, the velocity varies 

 owing to internal constitutional changes, the wave path is re- 

 fracted. Now, since refraction of earthquake waves actually 

 occurs within the earth, we are able to learn something about 

 the variation of the physical conditions of the interior. When 

 massive waves are propagated through the body of the earth, 

 they travel faster as they penetrate to greater depths, provided 

 that the maximum depth does not exceed 2,000 miles. There 

 appears to be, according to Oldham, 2 a well-marked surface 

 of physical discontinuity at a depth of between 2,000 and 2,400 

 miles, for, when the waves penetrate still more deeply, there 



1 Phil. Trans. Roy. Soc, A., vol. cxciv. p. 135, 1900. 

 1 Nature, August 21, 1913, p. 635. 



