NOv. 19, 1923 WILLIAMSON AND ADAMS: DENSITY IN EARTH 417 
Density change due to compression. We shall next use the above 
results to determine to what extent the higher density of the interior 
of the Earth may be due to compression alone. The decrease in 
volume caused by pressure at great depths can not be calculated from 
the measured compressibility of rocks, even if the pressure were 
known, because the compressibility decreases with the pressure, which 
at a depth of only a few hundred kilometers is far beyond the range 
of laboratory measurement. But, fortunately, the velocity of trans- 
mission of earthquake waves yields information as to the variation 
of compressibility (1/K) with depth. The values of K/p at various 
depths were calculated by equation (6), and the results are shown in 
Velocity, Am fer Sec. 
200 400 600 §f00 000 200 /. 7600 /f00 
DeprA 17 Ait lore ters 
00 2000 22V0 2900 2600 2F00 3o0e 3200 
Fig. 1. The velocities of longitudinal and transverse earthquake waves at various 
depths below the surface of the Earth as calculated from seismologic data. 
column 4 of Table 1. Now, it is reasonable to suppose that from 
this information concerning compressibility it would be possible to 
determine the aggregate diminution in volume at a given depth on 
the supposition of a homogeneous earth whose central density is 
made high by compression and not by a change of composition. We 
proceed as follows: 
In general, 
dp _ _ 6.66 X 1078 mp 
PAC isk rf s 
where g is the acceleration of gravity and p is the pressure at distance 
r from the center; and m, the mass of the sphere of radius 7, is obtained 
from the relation 
