There are three classes of instruments for the automatic
recording of earth tremors, each with its own particular
function. First is the seismoscope, which will
merely detect and record the fact that there has been
such a tremor. Some of these are so equipped as
to indicate the time of the disturbance.
Second, is the seismometer, the function of which
is to measure the maximum force of the shock, either
with or without an indication of its direction.
The third instrument is the seismograph, which is so
arranged that it will accurately record the number,
succession, direction, amplitude and period of successive
oscillations. This last instrument is by far
the most delicate of the three.
In the construction of this earthquake recording machine
the maker must so suspend a heavy body that when its
normal position is disturbed in the most infinitesimal
degree no reactionary force will be developed tending
to restore it to its original position. The inventor
has never been found who could accomplish this suspension
of a body to perfection. The seismograph of to-day,
however, has reached a stage of perfection where close
approximations are obtained in the records made.
Vesuvius Devastates the Region of Naples.
We have in other chapters described the terrible work
of Mount Vesuvius in the past, from the far-off era
of the destruction of Pompeii down to the end of the
last century. There comes before us now another
frightful eruption, one of the greatest in its history,
that of 1906. For thirty years before this outbreak
the mighty volcano had been comparatively quiet, rarely
ceasing, indeed, to smoke and fume, but giving little
indication of the vast forces buried in its heart.
It showed some sympathy with Mont Pelee in 1902, and
continued restless after that time, but it was not
until about the middle of February, 1906, that it
became threatening, lava beginning to overflow from
the crater and make its lurid way down the mountain’s
side.
It was in the middle of the first week of April that
these indications rose to the danger point, the flow
of lava suddenly swelling from a rivulet to a river,
pouring in a gleaming flood over the crater’s
rim, and meeting the other streams that came streaming
down the volcano’s rugged flank. While
this went on the mountain remained comparatively quiet,
there being no explosions, though a huge cloud of volcanic
ash and cinders rose high in the air until it hung
over the crater in the shape of an enormous pine tree,
while from it a shower of dust and sand, soon to become
terrible, began to descend upon the surrounding fields
and towns.
Dangerous as is Vesuvius at any time, the people of
the vicinity dare its perils for the allurement of
its fertile soil. A ring of populous villages
encircles it, flourishing vineyards and olive groves
extend on all sides, and the hand of industry does
not hesitate to attack its threatening flanks.
The intervals between its death-dealing throes are
so long that the peasants are always ready to dare
destruction for the hope of winning the means of life
from its soil.
