for instance, that there is a slight shift in the rocks on
each side of a crack, or fault, at a depth of ten miles. It must be
remembered that the pressure ten miles down would be about thirty-five
tons to the square inch. Even a slight displacement of one extensive
surface over another, the sides being pressed together with a force of
thirty-five tons on the square inch, would be an operation necessarily
accompanied by violence greatly exceeding that which we might expect
from so small a displacement if the forces concerned had been of more
ordinary magnitude. On account of this great multiplication of the
intensity of the phenomenon, merely a small rearrangement of the
rocks in the crust of the earth, in pursuance of the necessary work of
accommodating its volume to the perpetual shrinkage, might produce an
excessively violent shock, extending far and wide. The effect of such a
shock would be propagated in the form of waves through the globe, just
as a violent blow given at one end of a bar of iron by a hammer is
propagated through the bar in the form of waves. When the effect of this
internal adjustment reaches the earth's surface it will sometimes be
great enough to be perceptible in the shaking it gives that surface. The
shaking may be so violent that buildings may not be able to withstand
it. Such is the phenomenon of an earthquake.

"When the earth is shaken by one of those occasional adjustments of the
crust which I have described, the wave that spreads like a pulsation
from the centre of agitation extends all over our globe and is
transmitted right through it. At the surface lying immediately over the
centre of disturbance there will be a violent shock. In the surrounding
country, and often over great distances, the earthquake may also be
powerful enough to produce destructive effects. The convulsion may also
be manifested over a far larger area of country in a way which makes the
shock to be felt, though the damage wrought may not be appreciable.
But beyond a limited distance from the centre of the agitation the
earthquake will produce no destructive effects upon buildings, and
will not even cause vibrations that would be appreciable to ordinary
observation."

THE RADIUS OF DISTURBANCE.

"In each locality in which earthquakes are chronic it would seem as if
there must be a particularly weak spot in the earth some miles below
the surface. A shrinkage of the earth, in the course of the incessant
adjustmen