556 
GEOPHYSICS: H. F. REID 
Proc. N. a. S. 
for more concentrated, and still more systematic attacks, from which 
important results may be confidently expected. 
We can summarize the general state of our knowledge of earthquakes 
as follows: 
Nearly all earthquakes are due to the sudden fracture of the rock of 
the earth's crust, which has been strained by slow earth movements 
beyond its strength. Strong vibrations are set up at the fractured surface 
at the time of the fracture, just as vibrations are set up whenever any 
solid is broken. The earth consists of solid material, which is necessarily 
elastic, and therefore these vibrations are transmitted through it as 
elastic waves. There are two kinds of elastic waves: normal waves, 
where the movement is in the direction of propagation, and transverse 
waves, where the movement is at right angles to the direction of propaga- 
tion. These two kinds of vibration advance with different velocities. 
Their velocities near the earth's surface are about 7 and4kms. per second, 
respectively; but the deeper they penetrate below the surface the faster 
they go. Therefore, from the characteristics of elastic waves their paths 
are curved and concave upwards. These important results have been 
obtained by a study of the time necessary for the two types of waves to 
pass from the place of their origin to stations at different distances where 
delicate instruments are installed, which record the time of their arrival. 
It may seem remarkable that the paths followed by the rays and the 
velocity in different parts of the paths could be determined merely from 
the time required to arrive at a number of places on the earth's surface; 
but, by the help of some rather abstruse mathematics, it can be done. 
One interesting conclusion which can be drawn from the passage of the 
transverse waves through the body of the earth is that the earth is a 
solid and not a liquid sphere; for transverse waves can be transmitted 
only by solid substances. 
There are other waves which are transmitted along the surface of the 
earth; they must be started in some way when the body waves, mentioned 
above, arrive at the surface, but we do not know just how near the origin 
their starting place is. 
In many instances submarine earthquakes give rise to great water 
waves, which have been known to travel from one side of the Pacific 
Ocean to the other. It has been the general belief that the first indica- 
tion of these waves along a coast is marked by the withdrawal of the 
water; and the great elevated wave follows. Although this order is 
certainl}^ frequent, it is not general, and in many cases the elevated 
wave is first to appear. 
This is a very summary sketch of what has been learned about earth- 
quakes in the comparatively short time that they have been systematically 
studied; but it will serve as a basis from which to point out the problems 
which are now pressing for solution. 
