[00:00.49]An earthquake comes like a thief in the night, without warning.
[00:05.68]It was necessary, therefore, to invent instruments that neither slumbered nor slept.
[00:12.10]Some devices were quite simple.
[00:14.49]One, for instance, consisted of rods of various lengths and thicknesses with would stand up end like ninepins.
[00:22.16]When a shock came, it shook the rigid table upon which these stood.
[00:27.50]If it were gentle, only the more unstable rods fell.
[00:31.54]If it were severe, they all fell.
[00:34.62]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.
[00:48.45]But instruments far more deliecate than that were needed if any really serious advance was to be made.
[00:55.76]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.
[01:07.37]While I write my pen moves, but the paper keeps still.
[01:12.20]With practice, no doubt, I could in time learn to write by holding the pen still while the paper moved.
[01:19.08]That sounds a silly suggestion,
[01:21.73]but that was precisely the idea adopted in some of the early instruments (seismometers) for recording earthquake waves.
[01:31.01]But when table, penholder and paper are all moving, how is it possible to write legibly?
[01:38.50]The key to a solution of that problem lay in an everyday observation.
[01:43.69]Why does a person standing in a bus or train tend to fall when a sudden start is made?
[01:51.07]It is because his feet move on, but his head stays still.
[01:56.12]A simple experiment will help us a little further.
[01:59.84]Tie a heavy weight at the end of a long piece of string.
[02:04.10]With the hand held high in the air, hold the string so that the weight nearly touches the ground.
[02:10.69]Now move the hand to and fro and around but not up and down.
[02:16.17]It will be found that the weight moves but slightly or not at all.
[02:21.36]Imagine an earthquake shock shaking the floor, the paper, you and your hand.
[02:36.45]In the midst of all this movement, the weight and the pen would be still.
[02:41.86]But as the paper moved from side to side under the pen point, its movement would be recorded in ink upon its surface.
[02:49.81]It was upon this principle that the first instruments were made,
[02:54.29]but while the drum was being shaken, the line that the pen was drawing wriggled from side to side.
[03:01.24]The apparatus thus described,however, records only the horizontal component of the wave movement, which is, in fact, much more complicated.
[03:11.13]it would be more like that of a bluebottle path described by a particle,such as a sand grain in the rock,
[03:17.54]it would be more like that of a bluebottle buzzing round the room;
[03:22.23]it would be up and down, to and fro and from side to side.
[03:27.46]Instruments have been devised and can be so placed that all three elements can be recorded in different graphs.
[03:35.52]When the instrument is situated at more than 700 miles from the earthquake centre,
[03:40.93]the graphic record shows three waves arriving one after at short intervals.
[03:47.55]The first records the arrival of longitudinal vibrations.
[03:52.46]The second marks the arrival of transverse vibrations which travel more slowly and arrive several minutes after the first.
[04:01.27]These two have travelled through the earth.
[04:04.56]It was from the study of these that so much was learnt about the interior of the earth.
[04:10.62]The third, or main wave, is the slowest and has travelled round the earth through the surface rocks.
