It. D. Oldham — The Interior of the Earth. 23 



will be to outline the position as there presented, and the modifica- 

 tions which have been introduced by subsequent work. In 1905 

 there were twelve earthquakes of which direct and accurate 

 knowledge was available of the place and time of origin, and two of 

 which the place was known, but the time had to be inferred from 

 distant records. Tabulating the records of these earthquakes, it was 

 found that the intervals taken by the first and second phases to 

 reach the place of record increased regularly up to a distance of 

 about 120° from the origin. The rate of transit increased more 

 rapidly at first, less rapidly later, and showed that the deeper the 

 wave path penetrated the greater became the rate of transmission, 

 which meant that the wave paths were curved with a convexity 

 towards the centre of the earth. Up to 120° there was no breach in 

 the regularity of the time curve, and the ratio between the rate of 

 propagation of the condensational and distortional waves remained 

 much the same; beyond 120° distance an irregularity appeared, the 

 first phase, or commencement, was appreciably delayed, and the 

 second phase completely disappeared, only at about 140° did 

 something reappear which was recorded as second phase, but must 

 either be distinct from the second phase at lesser distances, or be 

 delayed by about ten minutes in its arrival. From these facts it 

 was concluded that the earth, down to the depth reached by wave 

 paths emerging at 120°, or to a little more than half the radius 

 measured from the surface, was composed of material capable of 

 transmitting the two primary forms of wave motion, and that down 

 to this depth there was no indication of any change of condition, the 

 increase in elasticity, indicated by the increasing rate of propagation, 

 being no more than might be attributed to the increased pressure 

 and compression of the material. Beyond this depth there is a rapid 

 transition to a material which can only transmit the condensational 

 waves at a somewhat reduced rate, and is either incapable of 

 transmitting the distortional waves, or transmits them with a 

 reduction to about half the velocity attained in the lower parts of 

 the outer shell ; at that time it was impossible to decide between 

 the two alternatives which were both presented, with some leaning 

 towards the former. 



In dealing with subsequent developments it will be convenient to 

 take the two parts of the time curve, and of the resulting parts of 

 the earth's interior separately. Beginning with the outer shell the 

 first work to be noticed is the often quoted paper by Professor 

 AViechert, which appeared in 1907, 1 where the subject is treated in 

 a more elaborately mathematical form, and appended to it is a 

 detailed discussion of the records by K. Zb'ppritz, the most important 

 part of which, from the present point of view, is the discussion of 

 the depth reached by the wave paths. For those emerging up to 

 a distance of about 45° the depth reached by the two wave paths is 

 about the same, and increases rapidly at first, then less rapidly; 

 from 45° to about 70°, where the depth reached is about 1,400 to 

 1,500 km., there is very little increase, but a considerable difference 



1 " Ueber Erdbebenwellen " : Gottingen Nachrichten, 1907. 



