400 STUDIES FOR STUDENTS 
allowed the times of arrival of the same shocks to be compared. 
The result obtained for the velocity of the surface waves of large 
amplitude (Section 5) was 3.3 kilometers per second, or the same as 
by the usual method. 
It is a general observation that shocks are more violent at the 
earth’s surface than they are in mines. At Przibram, in Bohemia, 
two similar modern Wiechert astatic pendulums have been installed, 
the one at the surface of the ground and the other in a mine 1,150 
meters (or about 3,700 feet) under ground. ‘The falling-off in ampli- 
tude of the shocks at the lower station is confirmed, but otherwise 
the seismograms appear to be nearly identical. 
A most important study, and almost unique within its field, has 
been made by Nagaoka‘ upon the elastic constants of rocks. From 
his results he has obtained the velocity of propagation for waves in 
rock material, and these correspond fairly well with those actually 
measured by seismometers at the time of earthquakes. 
The velocity of propagation of plane longitudinal waves within 
an infinite medium of steel is 6.2 kilometers per second. Within 
the earth’s crust it is hardly to be expected that constant velocities 
will be obtained, since the crust is not homogeneous, and, further, 
is not isotropic, but quasi-crystalline. Several of the rocks investi- 
gated gave values for velocity as high as 6 and 7 kilometers per 
second. Nagaoka shows that a relation exists between the density 
and the elastic constant. In passing from Cenozoic to Archean rocks, 
with an increase of density from 2 to 3, the modulus of elasticity 
increased more than ten times in certain specimens. The mean 
earth density is 5.5+, and Nagaoka argues for a stratum of this 
density quite near to the surface. His studies have since been con- 
tinued by Kusakabe,? using improved apparatus. For the velocity 
of propagation of waves in various types of Archean rocks Kusakabe 
« H. Nagaoka, ‘‘ Elastic Constants of Rocks and the Velocity of Seismic Waves,”’ 
Publications of the Earthquake Investigation Committee in Foreign Languages, No. 4 
(Tokyo, 1900), pp. 47-67. 
2S. Kusakabe, ‘‘ Modulus of Rigidity of Rocks and Hysteresis Function,” Journal 
of the College of Science, Imperial University, Tokyo, Vol. XIX (1904), pp. I-40, 22 
plates and 53 figures. See also by the same author “A Kinetic Measurement of the 
Modulus of Elasticity for 158 Specimens of Rocks and a Note on the Relation 
between the Kinetic and Static Moduli,” Pub. E.I.C. (Foreign Languages’, No. 22 B, 
1906, pp. 27-49. 
