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