294 REID— CONSTITUTION OF THE [April 24, 



year Oldham deduced from the transmission curves a central core 

 of not more than four tenths the earth's radius in which the velocity 

 was distinctly less than in the surrounding shell.'' Neither of these 

 arrangements have been shown to conform to the astronomic re- 

 quirements. Oldham's conclusions are based on what he considers 

 a distinct break in the transmission curve of the transverse waves 

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

 remember that fully 95 per cent, of the energy of an earthquake 

 shock comes to the surface within the hemisphere having the origin 

 as its pole, we see that the data for great distances must be too im- 

 perfect to yield very reliable deductions. 



Many years ago Roche showed that it was quite possible to 

 determine a distribution of density in the earth which would be 

 discontinuous at several levels, but which would still be astronom- 

 ically satisfactory. Wiechert, 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 Asso- 

 ciation in 1907 he announced that they did. At the Manchester 

 meeting of the same association in 191 1 he announced the existence 

 of two shells around the central core. In 1914 Gutenberg (one of 

 Wiechert's assistants) announced 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 in 

 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 dis- 



■^ Constitution of the Interior of the Earth, Quart. Jour. Geol. Soc, 1906, 

 Vol. LXIL, p. 456. 



s " Ueber die Masserivertheilung im Innern der Erde," Nachr. k. Gesells. 

 Wiss. Gottingen, 1897; Math.-phys. KL, p. 221. 



^ " Ueber Erdbebenwellen," VIIA. Nach. k. Gesells. Wiss. Gottingen; 

 Math.-phys. KL, 1914, p. i ; references to the earlier numbers of the series are 

 given in this paper. 



