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Lesson 42
Recording an earthquake
记录地震
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.
H.H,SWINNERTON The Earth beneath Us
New words and expressions 生词与短语
earthquake
n. 地震
slumber
v. 睡眠
ninepin
n. 九柱戏中的木柱
rigid
adj. 坚硬的
delicate
adj. 灵感的
seismometer
n. 地震仪
penholder
n. 笔杆
legibly
adv. 字迹清楚地
drum
n. 鼓状物
wriggle
v. 扭动
bluebottle
n. 绿头苍蝇
graph
n. 图表
graphic
adj. 图示的
longitudinal
adj. 纵向的
transverse
adj. 横向的
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发表于 2016-7-11 22:47:59