produce the sensation of sound are waves of condensation, whose
motion is in the direction of their propagation and they come
later. In the case of the jars of earth, the reverse is true.
The first set of waves to arrive are the waves which are due to
compression--vibrations in the direction in which the waves are
produced--and correspond to sound waves. Later come waves which
are transverse sidewise disturbances of the solid mass of the
earth. As we can easily see, in an earthquake jar traveling
from the opposite end of the earth, there should be no
insurmountable difficulty in recognizing the jar, which is a
direct upthrow from one which would tilt it to the right or
left. Now there is a law of Laplace by which the velocity of
spread of sound waves through gas may be calculated. That this
law should hold at temperatures and pressures so high as those
that must exist in the middle of the earth is, of course, a
question, but it will be interesting to see how nearly the
actual velocity of about 10 kilometers a second compares with
the velocity which such waves should have in gas of a density
and under a pressure such as a gas near the center of the earth
must have. Using Oldham's figures (and they seem to be
confirmed by the recent investigations of E. Rudolph and S.
Szirtes[18]), we find that the time of transmission of these
first and fastest preliminary compression tremors is about
twice the velocity of such a jar according to Laplace's law in
as dense a mass of gas, provided the ratio of the specific heat
of a gas at constant pressure to that of a gas at constant
volume remains 1.4, which is for many substances. But as it is
1.6 for mercury the discrepancy is not more than I had
expected.