[al:新概念英语（四）] [ar:MP3 同步字幕版（英音）] [ti:Recording an Earthquake] [by:更多学习内容，请到chazidian.com搜索“新概念”] [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.