Lesson52
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 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 every- day 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 strings 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 he 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.
H.H.SWINNERTON The Earth Beneath Us
参考译文
地震之到来有如宵小，悄无前兆。因此需要发明既不打盹水不酣睡的仪器，有些设备十分单间。例如，有一种由长短粗细不等的棒组成，这些棒像九柱戏中那样竖立着。发生地震时棒立于其上的坚硬桌子会摇动。如果是小震，
就只有比较不稳的棒倒下。如果震得厉害，就会全部倒下。于是棒就这样通过倒下和倾倒的方向为沉睡的科学家记录下弱到使他不会醒来的地震的强度以及它所来自的方向。
但是，如果要取得任何真正的重大进展，就必须制造出更为精巧的仪器。其理想目标是设计出一种能在震动经过时把大地或桌子的运动用笔在纸上记录出来的仪器。我写字时笔在动而纸保持静止。而通过练习，我肯定能慢慢
学会笔不动而让纸来动。这似乎是一个愚蠢的建议，但这正是用来记录地震的早期仪器（测震仪）所采用的想法。不过，当桌子、笔夹和纸全都在动时又怎么能写得清楚呢？解决这一问题的关键存大于日常观察之中。公共汽车或
火车起动时，站立的人为什么可能会摔倒？这是因为他的双脚移动了，而头部仍保持静止。一个简单的实验有助于进一步理解。试在一根长线末端绑上一个重物，用手高高提住线的车一端，使重物差一点碰到地面。让手来回动或
兜圈子，但不能上下动。你会发现重物只稍稍移动或干脆不动。设想把笔如是固定在重物上，笔尖正好同地板上的纸接触。假设地震振动了地板、纸、你和你的手。在所有这些运动中，唯独重物和笔会保持静止。当纸在笔尖下方
动来动去时，其运动将被墨水记录在纸面上。最初的仪器正是利用这个原理制成的，只是纸是卷在缓缓转运的滚筒上而已。一切都保持静止时笔将画出一条直线，而当圆筒被振动时，笔画出的线就会蜿蜒起伏。不过，上面所说的
仪器只能记录波动的水平成分，实际上整个波动要复杂得多。如果我们真能看到诸如岩石中的一颗砂粒这样的粒子描绘出的途径，它将很像是在房间里嗡嗡飞的一只绿头大苍蝇；上下、前后、左右飞舞。已设计出这样的仪器，恰当
装置后能在不同图表上记录下所有三种成分。
当仪器位于地震中心700英里外时，图形记录就会出现前后相隔不久的三种波。首先记录到的是纵向振动。第二个记录到的是横向振动，它的速度缓慢，落后于纵波几分钟。这两种波都穿过地球。正是对它们的研究使人们对地球
内部有了许多了解。第三种波（即主波）最慢，它是经过地表岩层绕地球传来的。
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