Lesson 42 Recording an earthquake 記錄地震
An earthquake comes like a thief in the night, without warning. It was necessary, therefore, to invent instruments that neither slumbered nor slept. Some devices were quite simple. One, for instance, consisted of rods of various lengths and thicknesses with would stand up end like ninepins. When a shock came, it shook the rigid table upon which these stood. If it were gentle, only the more unstable rods fell. If it were severe, they all fell. Thus the rods, by falling, and by the direction in which they fell, recorded for the severe, they all fell. 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.
But instruments far more deliecate than that were needed if any really serious advance was to be made. 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. While I write my pen moves, but the paper keeps still. With practice, no doubt, I could in time learn to write by holding the pen still while the paper moved. That sounds a silly suggestion, but that was precisely the idea adopted in some of the early instruments (seismometers) for recording earthquake waves. But when table, penholder and paper are all moving, how is it possible to write legibly? The key to a solution of that problem lay in an everyday observation. Why does a person standing in a bus or train tend to fall when a sudden start is made? It is because his feet move on , but his head stays still. A simple experiment will help us a little further. Tie a heavy weight at the end of a long piece of string. With the hand to and fro and around but not up and string so that the weight nearly touches the ground. Now move the hand to and fro and around but not up and down. It will be found that the weight a piece of string. With the hand held high in the air, hold the string so that the weight nearly touches the ground. Now move the hand to and fro and around but not up and down. It will be found that ten weight moves but slightly or not at all. Imagine an earthquake shock shaking the floor, the paper, you and your hand. In the midst of all this movement, the weight and the pen would be still. But as the paper moved from side to side under the pen point, its movement would be recorded in ink upon its surface. It was upon this principle that the first instruments were made, but while the drum was being shaken, the line that the pen was drawing wriggled from side to side. The apparatus thus described, however, records only the horizontal component of the wave movement, which is, in fact, much more complicated. If we could actually see the path described by a particle, such as a sand grain in the rock, it would be more like that of a bluebottle path described by a particle, such as a sand grain in the rock, 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. Instruments have been devised and can be so placed that all three elements can be recorded in different graphs.
When the instrument is situated at more than 700 miles from the earthquake centre, the graphic record shows three waves arriving one after at short intervals. The first records the arrival of longitudinal vibrations. The second marks the arrival of transverse vibrations which travel more slowly and arrive several minutes after the first. These two have travelled through the earth. It was from the study of these that so much was learnt about the interior of the earth. The third, or main. The third, or main wave, is the slowest and has travelled round the earth through the surface rocks. 參考譯文
地震就像夜間的小偷，不打招呼就來了。因此，有必要發明一種儀器，既不打盹兒，也不睡覺。有些裝置非常簡單。例如，有一種裝置是由一些長短、粗細不同的木棒組成，就像九柱戲的木棒一樣堅立著，一旦有地震，就會震動豎立在堅硬的桌上的木棒。如果地震輕微，只有不穩定的木棒倒下；如果地震劇烈，所有的木棒都會例下。由於地震太弱而未驚醒科學家時，木棒倒下的多少和倒下的方向就為科學家記錄下了地震的強度和地震方向。
但是，如果要取得真正重大的進展，需要有比這種裝置精細得多的儀器。理想的目標是設計出這樣一種儀器：當地震發生時，它能用筆在紙上記錄下大地和桌子運動情況。我寫字時，筆是移動的，紙是靜止的。毫無疑問，經過練習，我最終能夠學會筆不動而紙動來寫字。這聽起來似乎是一種愚蠢的想法，但是早期記錄地震波的儀器（地震儀）正是采用了這中思路。可是，當桌子、夾筆裝置、紙都在移動時，怎麽能書寫得清楚呢？可以從我們的日常生活觀察中找到這個問題的答案。一個站在公共汽車或火車上，當車突然開動時，他為什麽會傾倒呢？這是因為他的腳動了，而他的頭保持著靜止。再做一個簡單的實驗可以幫助我們進一步理解這個問題。把一個生物拴在一根長繩子的一端，把手高高舉在空中握住繩子，讓重物幾乎接觸地面。然後把手前後左右以及旋轉擺動，但不要上下擺動。結果會發現，重物是動了，但動得很小，甚至沒動。假定把一支筆拴在重物上，筆尖落在地板上的一張紙上，假定地震發生了，地板、紙、你和你的手都會動，重物和筆卻不動。由於紙在筆下來回運動，紙的表面就會用墨水記錄下地板運動的情況。根據這一原理，制造出了最初的地震儀器，但是紙是卷在慢慢放置的圓筒上的。只要一切都是靜止的，筆就會劃出一條直線；但是，圓筒受到震動，筆所畫出的線就會就會左右擺動。然而，這裏所說的儀器記錄下來的只是地震波運動中的水平部份，地震波的運動實際比這要復雜得多。假如我們真能看到諸如巖石中一個沙粒子的運動軌跡，那就像一只嗡嗡叫的綠頭蒼蠅在屋內飛行的軌跡，呈現出上上下下、來來回回、左左右右3種性質的運動。已經設計出了一些儀器，它按照一定的安放方式就可測繪出這三種運動的曲線圖。
如果把這種儀器安裝在距震源700多英裏遠的地方，曲線記錄就能顯示出前後相同的這3種地震波。首先記錄下的是縱向波的到達；然後記錄下的是橫向波的到達，橫向波比縱向波傳播得慢，在縱向波到過幾分鐘後能到達。這珍兩種波都是穿過地球而來的。正是從這兩種波中的研究中，我們可以了解到地球內部的許多情況。第三種波，即主波，是最慢的，是圍繞地球通過表面巖石傳來的。

新概念4-42