92 Mr. Matter on the Dynamics of Earthquakes. 
the depth or thickness of the crust or bent plate. This amount of extension 
will decrease as we descend, until it reaches the neutral plane, where it will be 
evanescent, and at which point compression commences, and which will probably 
be equal to the extension above, at equal distances below the neutral plane. 
Now, at the moment that fracture takes place, and the particles are released from 
this their state of constraint, as action and reaction will be equal, a wave of elastic 
compression will spread itself in every direction from the surfaces of fracture, 
and will pass through the substance and surface of the adjacent country, until 
finally lost and obliterated, owing to the imperfect elasticity of the masses com- 
posing it. 
The amplitude of this wave at the surface will be, guam proxime, half the 
amount of elastic compression or extension due to the substance, and to its dimen- 
sions; thus, in the case assumed, the amount of lateral oscillation or amplitude of 
the great earth wave or shock will be about nine feet at the surface of the sur- 
rounding country. 
As homogeneous elastic bodies are equally elastic in all directions, and as the 
earth’s crust is free to expand upwards, but not downwards, at least not nearly to 
the same extent, so the transit of the elastic wave will be attended with an actual 
undulation of the surface of the country, at the point of transit of the elastic earth 
wave; for every compression or extension of particles in the horizontal ordinate 
must, in its reaction, produce corresponding extension and compression in the 
vertical ; hence, the vertical height of the wave of horizontal transit will depend 
upon the elasticity and the depth of the rock through which the wave passes. 
We have thus shewn that the dimensions of the wave producible by the elas- 
ticity of the earth’s crust are fully sufficient to answer the facts observed in 
earthquake shocks: the example I have given is a mean case ; for harder and 
more elastic rocks the amplitude of the wave will be less, its transit velocity greater ; 
and for formations of less hardness and elasticity, the amplitude greater and the 
transit speed of the wave less. 
But we have further to consider the actual rate of progress of such a wave 
through the solid crust of the globe. This, which may be called the specific 
period of wave transit, will vary for each formation, and depend upon the specific 
elasticity of the mass considered as uniform, and probably, also, in some degree, 
upon its depth. The velocity of transit will be the same as that of sound in 
