地震来临如同黑夜中的盗贼，毫无预警。
An earthquake comes like a thief in the night, without warning.
因此，有必要发明一种永不瞌睡、永不入眠的仪器。
It was necessary, therefore, to invent instruments that neither slumbered nor slept.
有些装置相当简单。
Some devices were quite simple.
例如，有一种装置由一些长短、粗细各异的杆子组成，这些杆子会竖立起来，就像九柱戏的木柱一样。
One, for instance, consisted of rods of various lengths and thicknesses which would stand up on end like ninepins.
每当震动袭来，这些杆子所站立的那张坚硬桌子就会被震动。
When a shock came, it shook the rigid table upon which these stood.
如果震动轻微，只有较不稳定的杆子会倒下。
If it were gentle, only the more unstable rods fell.
如果震动剧烈，它们就全部倒下。
If it were severe, they all fell.
如此一来，这些杆子通过倒塌的动作以及倒塌的方向，为那位沉睡的科学家记录下了那次震感太轻、不足以唤醒他的地震的强度，以及地震传来的方向。
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.
但是，如果要取得任何真正实质性的进展，就需要比那灵敏得多的仪器。
But instruments far more delicate than that were needed if any really serious advance was to be made.
所追求的理想目标，是设计一种能用钢笔在纸上记录地面或桌面运动的仪器，记录发生在地震经过时的那些运动。
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.
当我书写时，钢笔在动，但纸保持静止。
While I write my pen moves, but the paper keeps still.
假以时日，不消说，通过练习，我完全可以学会反过来——让纸动而钢笔保持不动——来书写。
With practice, no doubt, I could in time learn to write by holding the pen still while the paper moved.
这听起来像个愚蠢的建议，但那恰恰就是某些早期仪器（地震仪）记录地震波所采用的原理。
That sounds a silly suggestion, but that was precisely the idea adopted in some of the early instruments (seismometers) for recording earthquake waves.
但是当桌子、笔架和纸都在运动时，又怎么可能写得清楚呢？
But when table, penholder and paper are all moving, how is it possible to write legibly?
解决这一问题的关键在于一项日常观察。
The key to a solution of that problem lay in an everyday observation.
为什么站在公共汽车或火车里的人在车辆突然启动时会有向后倒的趋势？
Why does a person standing in a bus or train tend to fall when a sudden start is made?
这是因为他的脚随之向前移动，而他的头部则保持静止。
It is because his feet move on, but his head stays still.
一个简单的实验可以帮助我们更进一步理解。
A simple experiment will help us a little further.
将一个重物系在一根长线绳的末端。
Tie a heavy weight at the end of a long piece of string.
将手高高举起，握住绳子，使重物几乎触及地面。
With the hand held high in the air, hold the string so that the weight nearly touches the ground.
现在将手前后左右地移动，但不要上下移动。
Now move the hand to and fro and around but not up and down.
你将会发现，重物几乎纹丝不动，或根本不动。
It will be found that the weight moves but slightly or not at all.
想象将一支钢笔附着在重物上，使笔尖恰好搁置在地板上的一张纸上。
Imagine a pen attached to the weight in such a way that its point rests upon a piece of paper on the floor.
再想象一次地震的震动摇晃着地板、纸张、你和你的手。
Imagine an earthquake shock shaking the floor, the paper, you and your hand.
在这一切运动之中，重物和钢笔却是静止的。
In the midst of all this movement, the weight and the pen would be still.
但是当纸在笔尖下方左右移动时，纸的运动便会以墨水的形式被记录在纸面上。
But as the paper moved from side to side under the pen point, its movement would be recorded in ink upon its surface.
正是基于这一原理，第一批地震仪器被制造出来；但纸张是缠绕在一个缓慢转动的鼓筒上的。
It was upon this principle that the first instruments were made, but the paper was wrapped round a drum which rotated slowly.
只要一切静止，钢笔就会画出一条直线；但当鼓筒被震动时，笔正在画的那条线就会左右扭曲摆动。
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.
然而，如此描述的这一装置，只记录波动的水平分量，而地震波动实际上要复杂得多。
The apparatus thus described, however, records only the horizontal component of the wave movement, which is, in fact, much more complicated.
如果我们能真正看见一个质点（比如岩石中的一粒沙子）所描绘的运动轨迹，它看起来将更像一只蓝苍蝇在房间里嗡嗡乱飞的路径；它会上下、前后、左右地运动。
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.
已经设计出仪器，并且可以将它们如此放置，使得三个方向的分量都能分别记录在不同的图表上。
Instruments have been devised and can be so placed that all three elements can be recorded in different graphs.
当仪器距震中超过700英里时，图形记录会显示三组波依次相继到达，间隔时间很短。
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.
第一组记录的是纵波的到达。
The first records the arrival of longitudinal vibrations.
第二组标示着横波的到达，横波传播更慢，在第一组波到达数分钟之后才到达。
The second marks the arrival of transverse vibrations which travel more slowly and arrive several minutes after the first.
这两种波都是穿过地球内部传播的。
These two have travelled through the earth.
正是通过对这两种波的研究，人类才从中获得了大量关于地球内部的知识。
It was from the study of these that so much was learnt about the interior of the earth.
第三种波，即主波，是速度最慢的，它是沿地球表面岩石传播的。
The third, or main wave is the slowest and has travelled round the earth through the surface rocks.