recent
years point to the conclusion that no part of the earth's surface is
quite exempt from movements which, though not readily perceived, can
be made visible by the use of appropriate instruments. With an
apparatus known as the horizontal pendulum it is possible to observe
vibrations which do not exceed in amplitude the hundredth part of an
inch. This mechanism consists essentially of a slender bar supported
near one end by two wires, one from above, the other from below. It
may readily be conceived that any measurable movement will cause the
longer end of the rod to sway through a considerable arc. Wherever
such a pendulum has been carefully observed in any district, it has
been found that it indicates the occurrence of slight tremors. Even
certain changes of the barometer, which alter the weight of the
atmosphere that rests upon the earth to the amount indicated by an
inch in the height of the mercury column, appears in all cases to
create such tremors. Many of these slight shocks may be due to the
effect of more violent quakings, which have run perhaps for thousands
of miles from their point of origin, and have thus been reduced in the
amplitude of their movement. Others are probably due to the slight
motion brought about through the chemical changes of the rocks, which
are continuously going on. The ease with which even small motions are
carried to a great distance may be judged by the fact that when the
ground is frozen the horizontal pendulum will indicate the jarring due
to a railway train at the distance of a mile or more from the track.

In connection with the earth jarring, it would be well to note the
occurrence of another, though physically different, kind of movement,
which we may term earth swayings, or massive movements, which slowly
dislocate the vertical, and doubtless also the horizontal, position of
points upon its surface. It has more than once been remarked that in
mountain countries, where accurate sights have been taken, the heights
of points between the extremities of a long line appear somewhat to
vary in the course of a term of years. Thus at a place in the
Apennines, where two buildings separated by some miles of distance are
commonly intervisible over the crest of a neighbouring peak, it has
happened that a change of level of some one of the points has made it
impossible to see the one edifice from the other. Knowing as we do
that the line of the seacoast is ever-changing, uprising taking