PRESIDENTIAL ADDRESS, 347 
The existence of two groups of earthquake-waves—those passing through, 
and those passing near the surface around the Earth—has long been reeognised ; 
but R, D. Oldham * has shown that the waves passing through the Earth are 
of two kinds, travelling at two different speeds. 
The record on the distant seismograph thus shows three well-marked phases : 
the first phase, due to waves of compression passing through the Earth’s interior ; 
the second phase, due to waves of distortion,*® also passing through the Earth’s 
interior; and the third phase, recorded by the waves which pass around the 
are along the superficial crust. 
The third phase is always recorded at a time after the occurrence of the 
shock proportional to the arcual distance of the recording seismograph from the 
earthquake centre, the records of several large earthquakes showing an average 
speed for the waves of about three kilométres per second. The rates of propa- 
gation of the waves giving the first and second phases are both much greater 
than of those forming the third phase; and up to an arcual distance of about 
120° from the earthquake’s centre the rate of their propagation increases with 
the distance. It is thus assumed that the waves giving rise to the first and 
second phases in each distant seismographic record, by following approximately 
along the chord of the are between the place of origin and the instrument, 
pass through deeper layers of the Earth when the seismograph is farther away, 
the material at greater depths being presumably more elastic as well as denser. 
But Oldham” has shown that when the seismograph is as much as 150° from 
the earthquake centre there is a remarkable decrease in the mean apparent 
rate of propagation of the waves giving the second phase in the record, from 
over six to about four and a half kilométres per second. There is also a drop, 
although not nearly so marked, in the apparent speed of the waves of the first 
phase when transmitted to a seismograph 150° or more distant from the earth- 
quake origin. Oldham concludes that this decrease of apparent rate for waves 
travelling through the Earth to places much more than 120° distant is due to 
their passing into a central core, four-tenths of the radius in thickness, com- 
posed of matter which transmits the waves at a markedly slow speed. Thus 
the earthquake waves which emerge at a distance not greater than 120° from 
their origin do not enter this central core, while those which pass into the Earth 
to a greater depth than six-tenths of the radius are supposed to be refracted on 
entering, and again on leaving the postulated core, in which the rate of trans- 
mission of an elastic wave of distortion is very much slower than in the main 
mass of the Earth around. In consequence of the refraction of these waves on 
passing through the central core, places situated at about 140° from an earthquake 
origin should be in partial shadow, due to the great dispersion of the distortional 
waves, and the few records made so far by seismographs thus situated with 
regard to great earthquakes show that there is either no, or at most a doubtful, 
record for the second phase, which is known to be due to the so-called dis- 
tortional waves. 
Oldham’s deductions are based confessedly on a small number of earthquake 
records—he considered fourteen examples only—but the conclusions based on a 
small number of trustworthy records, from which variations due to the different 
methods of marking the phases are eliminated, are more reliable than those for 
which there are imperfect distant records as well as doubts regarding the exact 
times of the disturbances. If these observations, however, be confirmed by 
further records, we are justified in assuming that below the heterogeneous 
crust there is a thick shell of elastic material, fairly homogeneous to about six- 
tenths of the radius, surrounding a central core, four-tenths in thickness, which 
possesses physical properties utterly unlike those of the outer layers; for in 
this core the ‘ distortional’ waves are either damped completely or are trans- 
mitted at very much lower speeds than in the shell. 
* Phil. Trans., Ser. A., vol. exciv. (1900), pp. 135-74. 
° There is more complete agreement regarding the fact that two distinct 
sets of waves give rise to the so-called preliminary tremors indicated by a 
seismographic record than about the nature of the waves. Confer. R. D. Oldham, 
Phil. Trans., loc, cit.,and O. Fisher, Proc. Cambr. Phil. Soc., vol. xii. pp. 354-361, 
" Quart. Journ. Geol, Soc., vol. 62, pp. 456-475 (1906). 
