[00:01.47]Lesson 42
[00:03.45]Recording an earthquake
[00:11.47]What does a pen have to do to record on paper the vibrations generated by an earthquake?
[00:19.70]An earthquake comes like a thief in the night, without warning.
[00:24.83]It was necessary, therefore, to invent instruments that neither slumbered nor slept.
[00:31.38]Some devices were quite simple.
[00:33.83]One, for instance, consisted of rods of various lengths and thicknesses which would stand up on end like ninepins.
[00:41.76]When a shock came, it shook the rigid table upon which these stood.
[00:47.32]If it were gentle, only the more unstable rods fell.
[00:51.64]If it were severe, they all fell.
[00:54.65]Thus the rods, by falling, and by the direction in which they fell,
[00:59.67]recorded for the slumbering scientist the strength of a shock that was too weak to waken him,
[01:05.62]and the direction from which it came.
[01:09.39]But instruments far more delicate than that were needed if any really serious advance was to be made.
[01:17.67]The ideal to be aimed at was to devise an instrument that could record with a pen on paper,
[01:24.33]the movements of the ground or of the table as the quake passed by.
[01:29.99]While I write my pen moves, but the paper keeps still.
[01:35.03]With practice, no doubt, I could in time learn to write by holding the pen still while the paper moved.
[01:42.15]That sounds a silly suggestion,
[01:44.45]but that was precisely the idea adopted in some of the early instruments (seismometers) for recording earthquake waves.
[01:54.25]But when table, penholder and paper are all moving, how is it possible to write legibly?
[02:02.40]The key to a solution of that problem lay in an everyday observation.
[02:07.97]Why does a person standing in a bus or train tend to fall when a sudden start is made?
[02:15.73]It is because his feet move on, but his head stays still.
[02:21.17]A simple experiment will help us a little further.
[02:25.02]Tie a heavy weight at the end of a long piece of string.
[02:29.53]With the hand held high in the air, hold the string so that the weight nearly touches the ground.
[02:36.32]Now move the hand to and fro and around but not up and down.
[02:41.93]It will be found that the weight moves but slightly or not at all.
[02:47.50]Imagine a pen attached to the weight in such a way that its point rests upon a piece of paper on the floor.
[02:56.08]Imagine an earthquake shock shaking the floor, the paper, you and your hand.
[03:02.96]In the midst of all this movement, the weight and the pen would be still.
[03:08.48]But as the paper moved from side to side under the pen point, its movement would be recorded in ink upon its surface.
[03:16.83]It was upon this principle that the first instruments were made, but the paper was wrapped round a drum which rotated slowly.
[03:25.96]As long as all was still, the pen drew a straight line,
[03:30.63]but while the drum was being shaken, the line that the pen was drawing wriggled from side to side.
[03:38.00]The apparatus thus described, however,
[03:40.25]records only the horizontal component of the wave movement, which is, in fact, much more complicated.
[03:48.12]If we could actually see the path described by a particle,
[03:51.63]such as a sand grain in the rock,
[03:54.41]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.
[04:04.45]Instruments have been devised and can be so placed that all three elements can be recorded in different graphs.
[04:13.37]When the instrument is situated at more than 700 miles from the earthquake centre,
[04:18.83]the graphic record shows three waves arriving one after the other at short intervals.
[04:25.57]The first records the arrival of longitudinal vibrations.
[04:30.84]The second marks the arrival of transverse vibrations which travel more slowly and arrive several minutes after the first.
[04:39.70]These two have travelled through the earth.