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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 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 endof a long piece of string. With the
hand held high in the air hold the strings so thatthe 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 he 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.