毕业评语新概念英语第四册听力内容：Recording an
earthquake
中元节的来历和传说Recording an earthquake
记录地震
古钱币图片及价格
First listen and then answer the following question.
听录音，然后回答以下问题。
What does a pen have to do to record on paper the vibrations generated by an earthquake?如何共享文件夹
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 thickness which would stand up on end like ninepins. When a shock came, it shook the rigid table upon which the stood. If it were gentle, only the more unstable rods fell. If it were vere, 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 rious advance was to be made. The ideal to be aimed at was to devi an instrument that could record with a pen on paper, the movements of the ground or of the table as the quake pasd 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 precily the
idea adopted in some of the early instruments (ismometers) 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 obrvation. Why does a person standing in a bus or train
tend to fall when a sudden start is made? It is becau 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 e 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 devid 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 cond marks the arrival of transver vibrations which
travel more slowly and arrive veral minutes after the first. The two have travelled through the earth. It was from the study of the 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
东湖御景slumber
v. 睡眠
ninepin
n. 九柱戏中的木柱
看不见的角落
rigid
adj. 坚硬的
重阳节诗词delicate
对教师的评价用语adj. 灵感的
