The instruments invented for the recording of the motions of the
earth's crust are looked upon by scientists as the most delicate of all
machines. So highly sensitive are they, indeed, that the very slightest
vibratory motion is recorded perfectly. Even the tread of feet cannot
escape this instrument if sufficient to cause a vibration.
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.
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