An earthquake comes like a thief in the night, without warning. It was
necessary, therefore, to invent instruments that neither slumbered nor slept.
Some devices were quite simple. One, for instance, consisted of rods of
various lengths and thicknesses which would stand up on end like ninepins.
When a shock came, it shook the rigid table upon which these stood. If it were
gentle, only the more unstable rods fell. If it were severe, they all fell.
Thus the rods, by falling, and by the direction in which they fell, recorded
for the slumbering scientist the strength of a shock that was too weak to
waken him, and the direction from which it came.#
But instruments far more delicate than that were needed if any really serious
advance was to be made. The ideal to be aimed at was to devise an instrument
that could record with a pen on paper, the movements of the ground or of the
table as the quake passed by. While I write my pen moves, but the paper keeps
still. With practice, no doubt, I could in time learn to write by holding the
pen still while the paper moved. That sounds a silly suggestion, but that was
precisely the idea adopted in some of the early instruments (seismometers) for
recording earthquake waves. But when table, penholder and paper are all
moving, how is it possible to write legibly? The key to a solution of that
problem lay in an everyday observation. Why does a person standing in a bus or
train tend to fall when a sudden start is made? It is because his feet move
on, but his head stays still. A simple experiment will help us a little
further. Tie a heavy weight at the end of a long piece of string. With the
hand held high in the air, hold the string so that the weight nearly touches
the ground. Now move the hand to and fro and around but not up and down. It
will be found that the weight moves but slightly or not at all. Imagine a pen
attached to the weight in such a way that its point rests upon a piece of
paper on the floor. Imagine an earthquake shock shaking the floor, the paper,
you and your hand. In the midst of all this movement, the weight and the pen
would be still. But as the paper moved from side to side under the pen point,
its movement would be recorded in ink upon its surface. It was upon this
principle that the first instruments were made, but the paper was wrapped
round a drum which rotated slowly. As long as all was still, the pen drew a
straight line, but while the drum was being shaken, the line that the pen was
drawing wriggled from side to side. The apparatus thus described, however,
records only the horizontal component of the wave movement, which is, in fact,
much more complicated. If we could actually see the path described by a
particle, such as a sand grain in the rock, it would be more like that of a
bluebottle buzzing round the room; it would be up and down, to and fro and
from side to side. Instruments have been devised and can be so placed that all
three elements can be recorded in different graphs.#
When the instrument is situated at more than 700 miles from the earthquake
centre, the graphic record shows three waves arriving one after the other at
short intervals. The first records the arrival of longitudinal vibrations. The
second marks the arrival of transverse vibrations which travel more slowly and
arrive several minutes after the first. These two have travelled through the
earth. It was from the study of these that so much was learnt about the
interior of the earth. The third, or main wave, is the slowest and has
travelled round the earth through the surface rocks.&