EARTHQUAKES—BULLEN S20 
the density changes due to increasing pressure are related to the in- 
compressibility, so that seismic data can be further brought to bear 
to determine density gradients in these parts of the earth. Second, 
restrictions on the possible density variations are provided from 
knowledge of the mass and moment of inertia of the earth. (The 
moment of inertia, which contains information on the degree of central 
condensation of matter in the earth, is found from the dynamics of 
the earth-moon system in conjunction with measurements of the shape 
of the earth.) Third, by matching P and S velocities in the outer part 
cf the earth against the results of laboratory experiments on rocks, 
an estimate can be made of the density just below the crust. 
Calculations that I carried out some years ago on these lines gave 
density values ranging from 3.3 gm/cm? just below the crust to about 
514 gm/cm® at the bottom of the mantle. The value jumps suddenly 
to 914 gm/cm® at the top of the outer core, and is between 11 and 12 
gm/cm? at the bottom of the outer core. At the earth’s center, the 
density probably lies between 1414 and 18 gm/cm*. The uncertainty 
on the central value cannot be resolved until more is known about the 
character of the transition region F. 
This work also enabled a number of other properties of the deep in- 
terior to be deduced. Atmospheric pressure at sea level is referred to 
as 1 atmosphere (15 pounds weight per square inch). In a steam 
locomotive the pressure may be about 30 atmospheres. At the bottom 
of the Pacific Ocean, the pressure can reach 800 atmospheres. In 
special high-pressure laboratories, values between 100,000 and 300,000 
atmospheres have been reached. Inside the earth, however, the 
pressures become still greater. At the bottom of the mantle, the im- 
mense value of 114 million atmospheres is reached, while at the earth’s 
center the figure lies between 314 and 4 million atmospheres. 
Another section of the results relates to the acceleration g due to 
gravity in the earth. It is remarkable that, down to a depth of 1,500 
miles, g keeps within 2 percent of its surface value of 32 ft./sec’. ‘The 
maximum value, 34 ft./sec’, is reached at the bottom of the mantle. 
Inside the core the value of g steadily diminishes, becoming zero at the 
center. 
The calculations showed further that the rigidity steadily increases 
with depth throughout the entire mantle until at the bottom the value 
is about four times that for steel at atmospheric pressure. The value 
then drops suddenly across the core boundary, and remains close to 
zero throughout the outer core, in keeping with the evidence that the 
outer core is fluid or molten. 
But perhaps the most important fruits in this series of calculations 
were the results for the incompressibility &. Whereas the outer core 
boundary is characterized by sudden large changes in the values of 
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