第42课

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An earthquake comes like a thief in the night, without warning.

地震如同夜间的小偷，不打招呼就来了。

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It was necessary, therefore, to invent instruments that neither slumbered nor slept.

因此，有必要发明出永不休眠的仪器。

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Some devices were quite simple.

有些装置非常简单。

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One, for instance, consisted of rods of various lengths and thicknesses which would stand up on end like ninepins.

例如，其中一种是由长短粗细不同的杆子组成，它们像九柱戏的木柱一样直立着。

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When a shock came, it shook the rigid table upon which these stood.

当震动来临时，它就会摇晃这些杆子所立的坚硬桌子。

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If it were gentle, only the more unstable rods fell.

如果震动轻微，只有那些更不稳定的杆子会倒下。

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If it were severe, they all fell.

如果震动剧烈，它们会全部倒下。

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Thus the rods, by falling, and by the direction in which they fell,

这样，杆子通过倒下，以及它们倒下的方向，

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recorded for the slumbering scientist the strength of a shock that was too weak to waken him,

为熟睡的科学家记录了不足以将他惊醒的震动的强度，

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and the direction from which it came.

以及它传来的方向。

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But instruments far more delicate than that were needed if any really serious advance was to be made.

但如果想要取得任何真正重大的进展，就需要比这精巧得多的仪器。

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The ideal to be aimed at was to devise an instrument that could record with a pen on paper,

理想目标是设计出一种能够用笔在纸上记录的仪器，

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the movements of the ground or of the table as the quake passed by.

当地震发生时，记录地面或桌子的移动。

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While I write my pen moves, but the paper keeps still.

我写字时笔动纸不动。

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With practice, no doubt, I could in time learn to write by holding the pen still while the paper moved.

毫无疑问，经过练习，我最终能学会笔不动而纸动来写字。

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That sounds a silly suggestion,

这听起来是个愚蠢的建议，

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but that was precisely the idea adopted in some of the early instruments (seismometers) for recording earthquake waves.

但这正是早期一些用于记录地震波的仪器（地震仪）所采用的原理。

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But when table, penholder and paper are all moving, how is it possible to write legibly?

但当桌子、笔架和纸都在移动时，如何才能清晰地书写呢？

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The key to a solution of that problem lay in an everyday observation.

解决这个问题的关键在于一个日常观察。

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Why does a person standing in a bus or train tend to fall when a sudden start is made?

为什么站在公共汽车或火车上的人，在突然启动时会倾向于摔倒？

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It is because his feet move on, but his head stays still.

那是因为他的脚随之移动，但头部却保持静止。

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A simple experiment will help us a little further.

一个简单的实验会帮助我们进一步理解。

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Tie a heavy weight at the end of a long piece of string.

在一根长绳的末端系上一个重物。

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With the hand held high in the air, hold the string so that the weight nearly touches the ground.

将手高举在空中，握住绳子，使重物几乎接触地面。

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Now move the hand to and fro and around but not up and down.

现在，前后左右移动手，但不要上下移动。

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It will be found that the weight moves but slightly or not at all.

你会发现重物只移动很小，甚至完全不动。

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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.

想象一支笔连接在重物上，笔尖落在地板上的一张纸上。

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Imagine an earthquake shock shaking the floor, the paper, you and your hand.

想象一场地震震动着地板、纸、你和你的手。

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In the midst of all this movement, the weight and the pen would be still.

在所有这些运动中，重物和笔会保持静止。

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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.

但当纸在笔尖下左右移动时，它的运动就会以墨迹的形式记录在纸面上。

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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.

第一批仪器就是根据这个原理制造的，但纸是缠绕在一个缓慢旋转的鼓上的。

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As long as all was still, the pen drew a straight line,

只要一切静止，笔就画出一条直线，

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but while the drum was being shaken, the line that the pen was drawing wriggled from side to side.

但当鼓被摇晃时，笔所画的线就会左右扭动。

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The apparatus thus described, however,

然而，这样描述的装置，

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records only the horizontal component of the wave movement, which is, in fact, much more complicated.

只记录波浪运动的水平分量，而实际上，波浪运动要复杂得多。

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If we could actually see the path described by a particle,

如果我们能真正看到一个粒子所描述的路径，

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such as a sand grain in the rock,

例如岩石中的一个沙粒，

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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.

它会更像一只苍蝇在房间里嗡嗡飞舞；它会上下、前后和左右移动。

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Instruments have been devised and can be so placed that all three elements can be recorded in different graphs.

已经设计出仪器，并且可以这样放置，使得所有三个要素都能在不同的图表中记录下来。

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When the instrument is situated at more than 700 miles from the earthquake centre,

当仪器位于距震中700多英里之外时，

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the graphic record shows three waves arriving one after the other at short intervals.

图形记录显示三波在短时间内一个接一个地到达。

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The first records the arrival of longitudinal vibrations.

第一波记录了纵向振动的到达。

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The second marks the arrival of transverse vibrations which travel more slowly and arrive several minutes after the first.

第二波标志着横向振动的到达，它们传播速度较慢，比第一波晚几分钟到达。

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These two have travelled through the earth.

这两波已经穿过地球。

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It was from the study of these that so much was learnt about the interior of the earth.

正是通过对它们的研究，人们才对地球内部有了如此多的了解。

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The third, or main wave is the slowest and has travelled round the earth through the surface rocks.

第三波，即主波，速度最慢，它通过地表岩石环绕地球传播。