41.American black bears
American black bears appear in a variety of colors despite their name. In the eastern part of their range, most of these brown, red, or even yellow coats. To the north, the black bear is actually gray or white in color. Even in the same litter, both brown and black furred bears may be born.
Black bears are the smallest of all American bears, ranging in length from five to six feet, weighing from three hundred to five hundred pounds Their eyes and ears are small and their eyesight and hearing are not as good as their sense of smell.
Like all bears, the black bear is timid, clumsy, and rarely dangerous , but if attacked, most can climb trees and cover ground at great speeds. When angry or frightened, it is a formidable enemy.
Black bears feed on leaves, herbs. Fruit, berries, insects, fish, and even larger animals. One of the most interesting characteristics of bears, including the black bear, is their winter sleep. Unlike squirrels, woodchucks, and many other woodland animals, bears do not actually hibernate. Although the bear does not during the winter moths, sustaining itself from body fat, its temperature remains almost normal, and it breathes regularly four or five times per minute.
Most black bears live alone, except during mating season. They prefer to live in caves, hollow logs, or dense thickets. A little of one to four cubs is born in January or February after a gestation period of six to nine months, and they remain with their mother until they are fully grown or about one and a half years old. Black bears can live as long as thirty years in the wild , and even longer in game preserves set aside for them.
美国黑熊
美国黑熊虽然被叫做黑熊但却有各种各样的颜色。在它们生活区域的东部，大部分黑熊长有富有光泽的黑毛，但在西部，他们则长着棕色、红色甚至是黄色的毛。
在北部，黑熊其实长着灰色或白色的毛。就是在一胎所生的小熊中，都可能混杂棕毛和黑毛。黑熊是所有美洲熊中最小的，5～6英尺长，300～500
磅重。它们的眼睛和耳朵都很小，他们的视力和听觉不如嗅觉那样好。像所有的熊一样，黑熊胆小，笨拙，很少具有危险性。但如果
受到攻击，大部分黑熊会以很快的速度爬上树和奔跑。当发怒或受惊吓时，黑熊会成为可怕的对手。
黑熊以树叶、草、树根、水果、浆果、昆虫、鱼，甚至更大的动物为食。熊类，包括黑熊的最有趣的一个特点是他们的冬眠。
与松鼠、旱獭和其它别的林地动物不同，熊并不真正地冬眠。虽然熊在冬天的几个月中不吃东西，靠体内脂肪维持生命，但它们的体
温保持正常，并有规律地一分钟呼吸4或5次。除交配季节外，大多数黑熊独自生活。他们喜欢住在洞里、空心的大木头里或茂密的树丛里。经过6
到9个月的怀孕期后一胎1～4个小熊在1月或2月出生。它们同母熊住在一起，直到它们完全长大，即1岁半左右。黑
熊在野外可以活到长达30年，在专门的保护区中甚至能活得更长。
42.Coal-fired power plants
The invention of the incandescent light bulb by Thomas A. Edison in 1879 created a demand for a cheap, readily available fuel with which to generate large amounts of electric power. Coal seemed to fit the bill, and it fueled the earliest power stations. (which were set up at the end of the nineteenth century by Edison himself). As more power plants were constructed throughout the country, the reliance on coal increased throughout the country, the reliance on coal increased. Since the First World War, coal-fired power plants had a combined in the United States each year. In 1986 such plants had a combined generating capacity of 289,000 megawatts and consumed 83 percent of the nearly 900 million tons of coal mined in the country that year. Given the uncertainty in the future growth of the nearly 900 million tons of coal mined in the country that year. Given the uncertainty in the future growth of nuclear power and in the supply of oil and natural gas, coal-fired power plants could well provide up to 70 percent of the electric power in the United States by the end of the century.
Yet, in spite of the fact that coal has long been a source of electricity and may remain on for many years(coal represents about 80 percent of United States fossil-fuel reserves), it has actually never been the most desirable fossil fuel for power plants. Coal contains less energy per unit of weight than weight than natural gas or oil; it is difficult to transport, and it is associated with a host of environmental issues, among them acid rain. Since the late 1960’s problems of emission control and waste disposal have sharply reduced the appeal of coal-fired power plants. The cost of ameliorating these environment problems along with the rising cost of building a facility as large and complex as a coal-fired power plant, have also made such plants less attractive from a purely economic perspective.
Changes in the technological base of coal-fired power plants could restore their attractiveness, however. Whereas some of these changes are intended mainly to increase the productivity of existing plants, completely new technologies for burning coal cleanly are also being developed.
火力发电厂托马斯?爱迪生1879年发明的白炽灯导致对便宜、易得、可生产大量电能的燃料的需求。
煤似乎符合这个要求，并成为第一批电厂的燃料(正是爱迪生本人在19世纪末建造了第一批电厂)。全国到处兴建电厂时，对煤的依赖加深了。
自第一次世界大战以来，美国每年约有一半的电力是以煤为燃料的电厂提供的。1986年这些电厂的总发电能力达到28，900
千瓦并且消耗了当年全国开采的九亿吨煤的83%。考虑到核能发展以及石油、天然气供应中的不确定因素，到本世纪末，火力发电厂仍可能为美国提供多达
70%的电力。然而，尽管煤长期以来一直是电力的原料之一并且可能会继续如此(煤占美国化石燃料储量的80%)，它却不是电厂的理想燃料。
煤的单位能量含量低于石油和天然气，而且会导致包括酸雨在内的一系列环境问题。从1960年以来，排放控制和垃圾处理的问题极大地
削弱了燃煤电厂的魅力。由于减轻这些环境问题需要大量资金，而且建造庞大复杂的燃煤电厂的费用不断上涨，也使得这些电厂从经济角度上不具备吸引力。
改变火力发电厂的基础技术却可能恢复它们的吸引力。虽然某些技术改进是渐进的，其目的只是提高现有电厂的生产率，但人们正在开发全新的清洁燃煤的技术。
43.Statistics
There were two widely divergent influences on the early development of statistical methods. Statistics had a mother who was dedicated to keeping orderly records of government units (states and statistics come from the same Latin root status) and a gentlemanly gambling father who relied on mathematics to increase his skill at playing the odds in games of chance. The influence of the mother on the offspring, statistics, is represented by counting, measuring, describing, tabulating, ordering, and the taking of censuses—all of which led to modern descriptive statistics. From the influence of the father came modern inferential statistics, which is based squarely on theories of probability.
Describing collections involves tabulating, depicting and describing collections of data. These data may be quantitative such as measures of height, intelligence or grade level------variables that are characterized by an underlying continuum---or the data may represent qualitative variables, such as sex, college major or personality type. Large masses of data must generally undergo a process of summarization or reduction before they are comprehensible. Descriptive statistics is a tool for describing or summarizing or reducing to comprehensible form the properties of an otherwise unwieldy mass of data.
Inferential statistics is a formalized body of methods for solving another class of problems that present great of problems characteristically involves attempts to make predictions using a sample of observations. For example, a school superintendent wishes to determine the proportion of children in a large school system who come to school without breakfast, have been vaccinated for flu, or whatever. Having a little knowledge of statistics, the superintendent would know that it is unnecessary and inefficient to question each child: the proportion for the sample of as few as 100 children. Thus , the purpose of inferential statistics is to predict or estimate characteristics of a population from a knowledge of the characteristics of only a sample of the population.
统计学
统计方法的早期发展受到两种截然不同的影响。统计学有一个"母亲"，她致力于井井
有条地记录政府机构的文件(国家和统计学这两个词源于同一个拉丁语词根，status)，还有一
个有绅士般的赌博"父亲"，他依靠数学来提高赌技，以便在几率的游戏中取胜。"母亲"对
其子女统计学的影响表现在计数、测量、描述、制表、归类和人口普查。所有这些导致了现代描述统计学的诞生。
由于"父亲"的影响则产生了完全基于概率论原理的现代推理统计学。描述统计学涉及对所收集数据的制表、制图和描述。这些数据可以是数量性的数据，
如高度、智商、或者是层级性的数据--具有连续性的变量--或数据也可以代表性质变量，如性别、大学专业或性格类型等等。
数量庞大的数据通常必须经过概括或删减的程序才能为人所理解。描述统计学就是这样一个工具，它对极其庞杂的数据进行描述、概括或删减，
使其变成能为人理解的东西。推理统计学是一套已定形了的方法体系，它解决的是光凭人脑极难解决的另一类问题。
这类问题的显著特点是试图通过取样调查来作出预测。例如，有一位教育督察想知道在一个庞大的学校系统中，不吃早饭就上学的学生、已经做过防感冒
免疫的学生，或其它任何类型的学生占多大比例。若具备一些统计学的知识，这位督察应明白，询问每个孩子是没有必要而且没有效率的，只要用100
个孩子为样本，他就可以相当精确地得出这些孩子占整个学区的比例了。因此，推理统计学的目的就是通过了解一个群
体中一些样本的特性，从而对整个群体的特性进行推测和估算。
44.Obtaining Fresh water from icebergs
The concept of obtaining fresh water from icebergs that are towed to populated areas and arid regions of the world was once treated as a joke more appropriate to cartoons than real life. But now it is being considered quite seriously by many nations, especially since scientists have warned that the human race will outgrow its fresh water supply faster than it runs out of food.
Glaciers are a possible source of fresh water that has been overlooked until recently. Three-quarters of the Earth’s fresh water supply is still tied up in glacial ice, a reservoir of untapped fresh water so immense that it could sustain all the rivers of the world for 1,000 years. Floating on the oceans every year are 7,659 trillion metric tons of ice encased in 10000 icebergs that break away from the polar ice caps, more than ninety percent of them from Antarctica.
Huge glaciers that stretch over the shallow continental shelf give birth to icebergs throughout the year. Icebergs are not like sea ice, which is formed when the sea itself freezes, rather, they are formed entirely on land, breaking off when glaciers spread over the sea. As they drift away from the polar region, icebergs sometimes move mysteriously in a direction opposite to the wind, pulled by subsurface currents. Because they melt more slowly than smaller pieces of ice, icebergs have been known to drift as far north as 35 degrees south of the equator in the Atlantic Ocean. To corral them and steer them to parts of the world where they are needed would not be too difficult.
The difficulty arises in other technical matters, such as the prevention of rapid melting in warmer climates and the funneling of fresh water to shore in great volume. But even if the icebergs lost half of their volume in towing, the water they could provide would be far cheaper than that produced by desalinization, or removing salt from water.
从冰山中获取淡水把冰山拖到世界上人口稠密的地区和干旱地带，再从中获取淡水，这个想法曾一度被认
为是一个笑话，更适合于卡通画，而非现实生活。然而现在，许多国家正相当认真地考虑这件事情，特别是在科学家们发出警告之后。
科学家们认为人类将在耗尽粮食之前首先耗尽淡水资源。冰川是一个直到最近以前一直被忽视的可能的淡水源。全球四分之三的淡水还锁在冰川的冰块中。
冰川就是一个蓄水池，其中未开发的淡水量是如此巨大，足够支持全世界的江河1000年。每年有7，659万亿公吨冰漂流在海洋中。
它们包含在10，000座从极地冰帽中断裂出来的冰山中。这些冰山的90%以上来自南极。一年四季里，覆盖
在浅层大陆架上的巨大冰川生成了众多冰山。冰山和海水的冰不同，后者是海水自身结冰形成的，而冰山则完全是在陆地上形成的。
当冰川伸展到海水中时，冰山就断裂下来。当漂离极地地区时，冰山有时会在底层洋流的推动下颇为神秘地逆风移动。由于冰山比小块
的冰融化要慢，因此有的冰山在大西洋中向北飘到了赤道以南35°的地方。把冰山蓄拦起来并拖到世界上需要它们的地方将不会太困难。
有困难的是其它的技术事宜。比如，如何防止冰山在较暖的气候中迅速融化以及如何把大量的淡水收集到岸上去。但是，即便在拖
的过程中冰山失去了一半体积，这样做也远比从海水中脱盐取得淡水便宜。
45.The source of Energy
A summary of the physical and chemical nature of life must begin, not on the Earth, but in the Sun; in fact, at the Sun’s very center. It is here that is to be found the source of the energy that the Sun constantly pours out into space as light and heat. This energy is librated at the center of the Sun as billions upon billions of nuclei of hydrogen atoms collide with each other and fuse together to form nuclei of helium, and in doing so, release some of the energy that is stored in the nuclei of atoms. The output of light and heat of the Sun requires that some 600 million tons of hydrogen be converted into helium in the Sun every second. This the Sun has been doing for several thousands of millions of year.
The nuclear energy is released at the Sun’s center as high-energy gamma radiation, a form of electromagnetic radiation like light and radio waves, only of very much shorter wavelength. This gamma radiation is absorbed by atoms inside the Sun to be reemitted at slightly longer wavelengths. This radiation , in its turn is absorbed and reemitted. As the energy filters through the layers of the solar interior, it passes through the X-ray part of the spectrum eventually becoming light. At this stage, it has reached what we call the solar surface, and can escape into space without being absorbed further by solar atoms. A very small fraction of the Sun’s light and heat is emitted in such directions that after passing unhindered through interplanetary space, it hits the Earth.
能量的来源
概说生命的物理和化学特性必须始于太阳--确切地说，是太阳的核心，而非地球。能量来自太阳的核心。
在这里，太阳不停地以光和热的形式向空间倾泻出能量。数十亿计的氢原子核在太阳的核心碰撞并且聚变生成氦。在此过程中一部分原本储存于原子核中的能
量被释放出来。太阳所产生的光和热需要每秒将六亿吨氢转化为氦。这样的转化在太阳中
已经持续几十亿年了。核能在太阳的核心被释放为高能的伽马射线。这是一种电磁射线，就象光波和无线电波一样，只是波长要短得多。
这种伽玛射线被太阳内的原子所吸收，然后重新释放为波长稍长一些的光波。这新的射线再次被吸收，而后释放。在能量由太阳内
部一层层渗透出来的过程中，它经过了光谱中X射线部分，最后变成了光。在此阶段，能
量到达我们所称的太阳表层，并且离散到空间而不再被太阳原子所吸收。只有很小一部分太阳的光和热由此方向释放出来，并且未被阻挡，穿越星空，来到地球。
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