THE INTEEIOR OF THE EARTH. 



237 



distinct break in the transmission curve of the transverse waves at 

 distances between 120° and 150° from the origin; but when we re- 

 member that fully 95 per cent of the energy of an earthquake shock 

 com.es to the surface within the hemisphere having the origin as its 

 pole, we see that the data for great distances must be too imperfect 

 to yield very reliable deductions. 



Many years ago Roche showed that it was quite possible to deter- 

 mine a distribution of density in the earth which would be discon- 

 tinuous at several levels, but which would still be astronomically 

 satisfactory. Weichert, in 1897,^ showed that such a system might 

 consist of a central core of radius about 4,900 km. or three-fourths 

 of the earth's radius, consisting of iron with a density of about 8.3, 

 surrounded by a stony shell about 1,500 km. thick and with density 

 varying from 3 to 3.4. It was natural that he should examine the 

 transmission curves to see if they supported his ideas; and at The 

 Hague meeting of the International Seismological Association in 

 1907 he announced that they did. At the Manchester meeting of 

 the same association in 1911 he announced the existence of two shells 

 around the central core. In 1914 Gutenberg (one of Wiechert's 

 assistants) amiounced the existence of three shells.^ In addition to 

 ordinary times of transmission, Gutenberg also used the times of 

 waves reflected at the earth's surface and the variations m the 

 amplitude; it is evident that a wave which crosses the boundary of 

 the core will experience reflection and refraction ; and whichever 

 part is later observed at the surface of the earth will have a distinctly 

 smaller amplitude than the wave which just missed penetrating 

 ' into the core. The following table shows the positions of the boun- 

 daries of the shells and of the core, and the velocities of the longi- 

 tudinal 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. 



1 Ueber die Massenvertheilung im Innern der Erde," Nadir, k. Oesells. Wiss. OUttingen, 

 1897 ; Math.-phys. Kl., p. 221. 



2 Ueber Erdbebenwelleu," VIIA. Nach. k. Oesells. Wiss. Odttingen; Math.-phys. Kl., 

 1914, p. 1 ; references to the earlier numbers of the series are given in this paper. 



