1915-] INTERIOR OF THE EARTH. 295 



tinctly smaller amplitude than the wave which just missed penetrat- 

 ing into the core. The following table shows the positions of the 

 boundaries of the shells and of the core, and the velocities of the 

 longitudinal waves P and of the transverse waves S ; it will be 

 noticed that it is only at the boundary of the central core that any 

 marked sudden change in velocity occurs. 



The remark regarding Oldham's results applies also here, namely 

 that it is questionable whether the observations at distances greater 

 than 100° or 120° are sufficiently accurate to justify such definite 

 conclusions. Gutenberg had the advantage, however, of more ac- 

 curate observations than Oldham, and also of measures of ampli- 

 tudes. There is no a priori reason why the earth might not be made 

 up of a number of shells, but there should be satisfactory evidence 

 for any proposed system ; and it must be shown to satisfy the 

 astronomic requirements ; or, at least, not to contradict them. 

 Gutenberg's system does not correspond with Wiechert's system of 

 1897. In the latter a marked change in physical properties occurs 

 at a depth of 1,500 km. ; in the former, at a depth of 2,900 km. ; and 

 in crossing into the core, the ratio of the elasticity to the density, 

 according to Gutenberg, rapidly loses six tenths of its value. This 

 change might be the result of a great increase in density or a great 

 decrease in elasticity ; it may be questioned whether the former is 

 compatible with the astronomic requirements, and whether the latter 

 is compatible with the high rigidity which we know the earth, as a 

 whole, has. So far no answer has been given to these questions. 



In 1879 George and Horace Darwin attempted to determine the 

 rigidity of the earth by measuring the deviation of the vertical under 



PROC. AMER. PHIL. SOC, LIV. 2I9 T, PRINTED SEPT. 4, I915. 



