新概念英語(yǔ)第四冊(cè)(含lrc文本)Lesson 42 Recording an earthquake

[ti:δ?] [ar:] [al:] [by:https://www.lrc.com.cn] [00:02.25]Lesson 42 [00:04.24]Recording an earthquake [00:10.93]What does a pen have to do [00:12.53]to record on paper the vibrations generated by an earthquake? [00:19.77]An earthquake comes like a thief in the night,without warning. [00:24.80]It was necessary,therefore, [00:26.61]to invent instruments that neither slumbered nor slept. [00:31.37]Some devices were quite simple. [00:33.97]One,for instance,consisted of rods of various lengths and thicknesses [00:37.85]which would stand up on end like ninepins. [00:41.60]When a shock came,it shook the rigid table upon which these stood. [00:46.96]If it were gentle,only the more unstable rods fell. [00:51.18]If it were severe,they all fell. [00:54.25]Thus the rods,by falling,and by the direction in which they fell, [00:59.28]recorded for the slumbering scientist [01:01.83]the strength of a shock that was too weak to waken him, [01:05.74]and the direction from which it came. [01:08.89]But instruments far more delicate than that were needed [01:12.81]if any really serious advance was to be made [01:17.03]The ideal to be aimed at [01:18.88]was to devise an instrument that could record with a pen on paper, [01:23.61]the movements of the ground or of the table as the quake passed by [01:28.95]While I write my pen moves,but the paper keeps still. [01:33.94]With practice,no doubt [01:35.42]I could in time learn to write by holding the pen still while the paper moved. [01:40.84]That sounds a silly suggestion, [01:43.20]but that was precisely the idea adopted in some of the early instruments [01:48.43](seismometers) [01:49.99]for recording earthquake waves. [01:53.00]But when table, penholder and paper are all moving, [01:57.71]how is it possible to write legibly? [02:00.91]The key to a solution of that problem lay in an everyday observation [02:06.56]Why does a person standing in a bus or train tend to fall when a sudden start is made? [02:14.22]It is because his feet move on,but his head stays still. [02:19.59]A simple experiment will help us a little further. [02:23.27]Tie a heavy weight at the end of a long piece of string. [02:27.73]With the hand held high in the air, [02:30.12]hold the string so that the weight nearly touches the ground. [02:34.55]Now move the hand to and fro and around but not up and down. [02:39.97]It will be found that the weight moves but slightly or not at all [02:45.63]Imagine a pen attached to the weight in such a way [02:49.60]that its point rests upon a piece of paper on the floor. [02:54.00]Imagine an earthquake shock shaking the floor,the paper,you and your hand. [03:00.88]In the midst of all this movement,the weight and the pen would be still. [03:06.34]But as the paper moved from side to side under the pen point, [03:10.18]its movement would be recorded in ink upon its surface. [03:14.57]It was upon this principle that the first instruments were made, [03:18.78]but the paper was wrapped round a drum which rotated slowly. [03:23.64]As long as all was still,the pen drew a straight line, [03:28.19]but while the drum was being shaken, [03:30.33]the line that the pen was drawing wriggled from side to side. [03:35.42]The apparatus thus described,however, [03:37.79]records only the horizontal component of the wave movement, [03:41.80]which is,in fact,much more complicated. [03:45.49]If we could actually see the path described by a particle, [03:48.79]such as a sand grain in the rock, [03:52.03]it would be more like that of a bluebottle buzzing round the room [03:56.15]it would be up and down,to and fro and from side to side. [04:01.60]Instruments have been devised and can be so placed [04:05.34]that all three elements can be recorded in different graphs. [04:10.34]When the instrument is situated at more than 700 miles from the earthquake centre, [04:15.92]the graphic record shows [04:17.67]three waves arriving one after the other at short intervals. [04:22.58]The first records the arrival of longitudinal vibrations. [04:27.70]The second marks the arrival of transverse vibrations [04:31.71]which travel more slowly and arrive several minutes after the first. [04:36.69]These two have travelled through the earth. [04:39.68]It was from the study of these that so much was learnt about the interior of the earth. [04:45.91]The third,or main wave is the slowest [04:49.73]and has travelled round the earth through the surface rocks.

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