THE EARTH’S INTERIOR. > 
“ The Constitution of the Interior of the Earth as revealed by 
Earthquakes ” is the title of an important paper by R. D. Oldham, 
F. G. S., presented to the Geological Society of London, February, 
1906. As the spectroscope extended the scope of astronomical 
knowledge by enabling the astronomer to determine the com- 
position of the heavenly bodies, so the seismograph enables the 
physicist to determine the constitution of the earth at depths re- 
moved from any other possible means of research. 
The distant record of a great earthquake exhibits three distinct 
phases, of which the third represents wave-motion which has 
travelled along the surface of the earth, and gave no information 
regarding the earth’s interior. The other two phases form the 
preliminary tremors, and it is shown that they represent the 
emergence of two distinct forms of wave motion which have been 
propagated through the earth. 
A study of the intervals taken by these waves to reach remote 
points shows, that up to a distance of 120° of arc from the origin 
they are propagated at a rate which increases with the depth of 
the wave-path. The increase being gradual and continuous, may 
be attributed to the effects of increased pressure and temperature, 
and there is no indication of any change in physical constitution 
of the material traversed by waves which merge at 120° or less 
from the origin. * 
Beyond this limit the first-phase waves show a reduction in 
the mean rate of transmission, while the second-phase waves are 
not to be found where they would be expected, but about n' later. 
The interpretation is that the wave-paths emerging at these 
greater distances have entered a central core in which the rate of 
transmission of the first-phase waves is reduced to about nine- 
tenths, and the second-phase waves to about one-half of the rate 
in the outer shell. 
The conclusion reached is that after the outermost crust of 
the earth is passed, there is no indication of any rapid or material 
change of physical condition (nor probably of chemical com- 
position) until a depth of about six-tenths of the radius is reached ; 
but that below this there is a rapid passage to matter which has 
very different physical properties, if not also differing in chemical 
constitution. It will be seen that this central core is but a small 
fraction of the mass of the earth. 562 
