Recording an earthquake
原文
Lesson 42
Recording an earthquake
What does a pen have to do to record on paper the vibrations generated by an earthquake?
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 which would stand up on 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 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 delicate 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 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 the weight moves but slightly or not at all.
Imagine a pen attached to the weight in such a way that its point rests upon a piece of paper on the floor.
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 the paper was wrapped round a drum which rotated slowly.
As long as all was still, the pen drew a straight line,
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 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 the other 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 wave is the slowest and has travelled round the earth through the surface rocks.
译文
第42课
记录地震
一支笔要如何才能在纸上记录下地震产生的震动呢？
地震就像夜间的小偷一样，毫无预兆地突然降临。
因此，有必要发明那些既不会打盹也不会睡觉的仪器。
有些设备的设计非常简单。
例如，其中有一个装置由长度和厚度各不相同的杆子组成；这些杆子可以像九柱戏（ninepins）中的柱子一样竖立起来。
当震动来临时，它震动了这些杆子所放置的坚硬桌子。
如果震动轻微，只有较不稳定的杆子会倒下。
如果震动剧烈，它们就全部倒下。
因此，这些杆子通过倒下以及倒下的方向，
为那位正在沉睡的科学家记录下了那次冲击的强度——不过那次冲击实在太微弱了，根本无法将他唤醒。
以及它来自的方向。
但是，如果要取得真正重大的进展，就需要使用比那些仪器更加精密的设备。
理想的目标是设计出一种能用笔在纸上记录的仪器，
记录地震经过时地面或桌子的运动。
当我写字时，笔在动，但纸却静止不动。
毫无疑问，通过练习，我最终能学会保持笔不动而让纸动来写字。
那听起来真是个愚蠢的建议。
但这正是某些早期地震仪在记录地震波时所采用的设计理念。
但是当桌子、笔架和纸张都在移动时，怎么可能还能写出清晰可读的文字呢？
解决那个问题的关键在于一个日常的观察结果。
为什么当公交车或火车突然启动时，站在上面的人容易摔倒呢？
这是因为他的脚向前移动，而头部却保持静止。
一个简单的实验能帮助我们进一步理解这个原理。
在一段较长的绳子末端系上一个重物。
将绳子高高举过头顶，使重物几乎触碰到地面。
现在让手前后左右移动，但不要上下移动。
你会发现重物几乎不动，或者只轻微移动。
想象一下，有一支笔被固定在重物上，其笔尖接触着地面上的一张纸。
再想象一场地震，地震使地面、纸张以及你的手都在震动。
在所有这些运动中，重物和笔会保持静止。
但当纸张在笔尖下左右移动时，它的运动就会以墨迹记录在纸面上。
最早的测量仪器就是基于这个原理制造的——只不过当时的纸张是缠绕在一个缓慢旋转的鼓上的。
当一切保持静止时，笔会画出一条直线；
但当鼓被震动时，笔所画的线就会左右摆动。
然而，上面描述的装置
只能记录波动的水平分量，而实际上波动要复杂得多。
如果我们能真的看到粒子所描绘的路径，
就像岩石中的一粒沙子一样……
它会更像一只绿头苍蝇在房间里嗡嗡飞舞：上下、前后、左右地移动。
已经设计出可以适当放置的仪器，从而能将这三个要素记录在不同的图表上。
当该仪器距离地震中心超过700英里时，
图纸上会显示出三列波以较短的时间间隔相继到达。
第一列波记录了纵向振动的到达。
第二列波标志着横向振动的到达；这些振动传播较慢，在第一列波到达几分钟后才传来。
这两列波已经穿过地球。
正是通过研究这两列波，人类才对地球内部有了如此多的了解。
第三列波即主波传播最慢，它是通过地表岩石环绕地球传播的。