until the day following was any noticeable disturbance felt in
Honolulu, but on April 19th shocks were plainly felt for six minutes and
the water in the harbor rose rapidly. Panic seemed imminent just before
the shocks subsided. While earthquakes are by no means infrequent in
these islands, this was more severe than any recorded in recent years,
causing buildings to sway to and fro and partly demolishing some of
frail construction.

If, as the majority of men qualified to discuss earthquakes seem to
think, the San Francisco earthquake had no connection with volcanic
action, but was caused by what is technically known as a "fault" in the
formation of the crust of the earth, it seems easy enough to account
for these wave motions travelling round the earth. How widely this may
really have made itself felt it is not possible to say. Several of the
great earthquakes in Japan have been recorded in the seismographs of
the observatories on every continent and in Australia, showing that in
severe disturbances of this kind the whole surface strata quiver, alike
under the oceans and over the continents and islands. At the time of a
shock, of course, half of the world is in darkness and asleep. This is
taken to account for the fact that so far only a few observatories have
reported catching the San Francisco vibrations.

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