a22 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
The P waves travel through solid parts of the earth about 1% 
times as fast as S waves. In fluid regions, the S waves are not trans- 
mitted. Thus the detection of S in addition to P waves in a part of 
the earth is positive evidence of solidity (in a sense to be defined a 
little later). Likewise, failure to detect S waves is negative evidence 
of fluidity. Near the outer surface of the earth, P waves travel at 
a speed of about 3 miles per second. The greatest speed, 814 miles 
per second, is reached at a depth of 1,800 miles below the surface. 
One of the great labors of seismologists over the first 40 years of 
this century was to evolve, by successive approximation from crude 
beginnings, seismic travel-time tables which give the travel times 
along seismic rays in terms of the angular distances subtended by the 
rays at the earth’s center. The accuracy of the Jeffreys-Bullen 
tables, which are the ones used in compiling the International Seis- 
mological Summary, is of the order of 1 or 2 seconds in travel times 
of the order of 20 minutes, in the best instances. (Twenty minutes is 
the time taken for a seismic P pulse to go straight through the earth 
from one side to the other. ) 
REGIONS OF THE EARTH’S INTERIOR 
From the travel-time tables, it is possible by a mathematical proc- 
ess to estimate the speeds of P and S waves at points throughout a 
large part of the earth’s interior. In the case of P waves, the speeds 
are moderately well known throughout nearly the whole interior, 
while for S waves this knowledge is available down to a depth of 1,800 
miles. On the basis of the calculations, it has become possible to 
divide the interior into a number of concentric regions. The bound- 
aries between these regions are indicated by the levels at which the P 
and § velocities, or their gradients with respect to depth, change 
abruptly with increase of depth. 
In continental regions, there is a fairly marked jump in the P and 
S velocities some 25 miles below the surface. The first evidence for 
this was obtained by the seismologist Mohorovitié in investigating 
local features of a Balkan earthquake in 1909, though he thought 
the change occurred rather deeper than 25 miles. Later work by 
others showed the change of property to be worldwide, and the bound- 
ary where the change occurs has come to be called the Mohorovicié 
discontinuity. In oceanic regions, work to date indicates that the 
depth of the discontinuity is of the order of only 5 miles below the 
ocean floor. The region which lies above the discontinuity is some- 
times called the earth’s crust, though it is well to realize that the 
term has only a conventional meaning nowadays. For more than a 
thousand miles below the crust, the earth is solid, with increasing ri- 
gidity. Above the crust, the velocities of the P and S waves vary 
rather erratically and can vary in the horizontal as well as the ver- 
