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1、LECTURE ONEText One:The Age of the EarthThis is a question to which we may never have the exact answer. Man has wondered about the age of the earth since ancient times, and there were all kinds of myths and legends that seemed to have the answer. But he couldnt begin to think about the question scie
2、ntifically until about 400 years ago. When it was proven at that time that the earth revolved around the sun (in other words, that the earth was part of our solar system)，then scientists knew where to begin. To find the age of the earth, it was necessary to explain how the solar system was born. How
3、 did the sun and all the planets come into being?One theory was called the nebular hypothesis. According to this theory, there was once a great mass of white-hot gas whirling about in space and getting smaller and hotter all the time. As the gas cloud grew smaller, it threw off rings of gas. Each of
4、 these rings condensed to form a planet, and the rest of the mass shrank into the center to become the sun.Another explanation is called the planetesimal theory. According to this, millions and millions of years ago, there was a huge mass made up of small, solid bodies called planetesimals, with the
5、 sun at the center. A great star came along and pulled on the sun so that parts of it broke away. These parts picked up the tiny planetesimals the way a rolling snowball picks up snow, and they became planets.Whichever theory is right, astronomers have figured out that it all probably happened about
6、 5,500,000,000 years ago! But other scientists besides astronomers have tackled this question. They tried to find the answer by studying how long it took for the earth to become the way we know it. They studied the length of time it takes to wear down the oldest mountains, or the time needed for the
7、 oceans to collect the salt they now contain.After all their studies, these scientists agree with the astronomers: The earth is about 5,500,000,000 years old!【参考译文】地球的年龄地球已经存在多长时间这一问题，也许很难有个确切的答案。自古以来，人们一直都在猜测地球的年龄，有过各种神话故事和动人传说，也似曾有过一些不同的答案。约从400年前开始，人们才从科学的角度去探索和揭示这个奥秘。当时的科学家证实了“地球围绕太阳运转”的基本事实（换言之
8、，地球当属太阳系的一部分），此后，科学家也就真正懂得应该如何着手去探索和揭示地球的奥秘。假如真想找到有关地球年龄的答案，首先就应了解太阳系的形成过程。那么，太阳和其他行星究竟是如何形成的呢？最初曾有科学家提出一种理论，称之为星云假说。根据这种理论，很久以前曾有一个巨大的白热气团，这个巨大气团总在太空中不停地回旋，气团越变越小，而且愈加炙热。随着气团逐渐缩小，若干气环脱离气团。每个脱离的气环经过冷凝之后演变成为一颗新的行星，逐渐缩小的气团被卷入核心之后最终变成了太阳。另有一种理论称为微星假说。根据这一假说，若干万亿年以前，曾经有个以太阳为中心的大气团，它是由若干小块而坚硬的星体所组成，它们叫微星
9、。有一次，另外一颗大恒星朝着这个大气团的方向移动，那颗大恒星的引力牵动了这个大气团中心的太阳，结果导致了大恒星的局部残裂，若干破裂的小星块滚雪球般地吸引着不同的微星，终而形成一些新的行星。无论上述哪种理论正确，天文学家已经基本测出地球的形成时期，即地球早在55亿年前就已形成！除天文学家外，其他领域的科学家也致力于研究地球的演变过程，测算地球演变至今所需花费或经历的时间，他们几乎殊途同归，同样揭开了千古之谜。这些科学家主要通过研究原始山脉遭受磨损的程度及其历时，抑或通过测算海水所含盐量、海盐累积速率以及海水盐度变化历时等。其他领域的科学家通过不同手段的研究，最终得出相同的研究结论，他们完全赞同天
10、文学家的下述观点：地球如今已有55亿岁的高龄了！Text Two:The Atmosphere of the EarthAs we read about mans plans to explore the moon and other planets, we often come across the question of an atmosphere. Do other planets have an atmosphere, too?As far as scientists know，there is no other planet or star that has an atmospher
11、e like ours. What is atmosphere? We can think of it as a great ocean of air that surrounds the earth and that extends up for hundreds of miles.This ocean of air is just about the same the world over. In general, it consists of certain gases which are always found in the same proportions. About 78 pe
12、rcent of it is nitrogen, about 21 percent oxygen, and the remaining one percent consists of what are called rare gases - argon, neon，helium, krypton, and xenon.The air that blankets the earth has the same chemical composition up to a height of about 18 miles, though it may be that this figure goes a
13、s high as 44 miles. When you reach the top layer of this atmosphere, you are at the end of what is called the troposphere. This is the layer nearest the earth.At 18 to 31 miles above the earths surface, there is a layer of hot air, probably about 108 degrees Fahrenheit. The warmth is caused by the a
14、bsorption of heat from the sun by the ozone which is present. Ozone is a special form of oxygen in which a molecule consists of three atoms of oxygen instead of the usual two. The hot ozone layer serves to protect us from the most active of the ultraviolet rays of the sun. Without it，we could not st
15、and the suns light.Still higher up is a layer, or series of layers, of the atmosphere called the ionosphere. This extends from about 44 miles to 310 miles above the earth. It consists of particles electrified by the sun.The molecules in the air are in constant motion. The atmosphere can be maintaine
16、d only if the molecules keep colliding with others so they cant escape. But as we go higher and higher, the air becomes thinner. The chances become very small that a molecule from below will bounce back from a collision with a molecule above. So the molecules escape to outer space and our atmosphere thins out into nothingness. There is a region called the exosphere where escaped molecules move about freely, which starts at about 400 miles
