精品写作素材(五)

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 hibernate². Although the bear does not during the winter moths³, sustaining itself from body fat, its temperature remains⁴ 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 dense⁵ thickets⁶. A little of one to four cubs⁷ is born in January or February after a gestation⁸ period of six to nine months, and they remain with their mother until they are fully⁹ 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.


42.Coal-fired power plants 

The invention of the incandescent¹⁰ 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 uncertainty¹¹ 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 emission¹² 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 purely¹³ economic perspective. 

Changes in the technological¹⁴ 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. 


43.Statistics 

There were two widely divergent influences on the early development of statistical¹⁵ methods. Statistics had a mother who was dedicated¹⁶ to keeping orderly records of government units (states and statistics come from the same Latin root status) and a gentlemanly gambling¹⁷ father who relied on mathematics to increase his skill at playing the odds¹⁸ in games of chance. The influence of the mother on the offspring, statistics, is represented by counting, measuring, describing, tabulating¹⁹, 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, depicting²⁰ and describing collections of data. These data may be quantitative²¹ such as measures of height, intelligence or grade level------variables that are characterized by an underlying²² continuum---or the data may represent qualitative²³ 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 superintendent²⁴ wishes to determine the proportion of children in a large school system who come to school without breakfast, have been vaccinated²⁵ for flu, or whatever. Having a little knowledge of statistics, the superintendent would know that it is unnecessary and inefficient²⁶ 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.


44.Obtaining Fresh water from icebergs²⁷ 

The concept of obtaining fresh water from icebergs that are towed to populated areas and arid²⁸ 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 outgrow²⁹ its fresh water supply faster than it runs out of food. 

Glaciers³⁰ 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 continental³¹ 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 entirely³² 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 steer³³ 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 funneling³⁴ 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.


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 nuclei³⁵ 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 wavelength³⁶. This gamma radiation is absorbed by atoms inside the Sun to be reemitted at slightly longer wavelengths³⁷. 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 spectrum³⁸ 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. 


46.Vision 

Human vision like that of other primates⁴⁰ has evolved in an arboreal⁴¹ environment. In the dense complex world of a tropical forest, it is more important to see well that to develop an acute sense of smell. In the course of evolution members of the primate³⁹ line have acquired large eyes while the snout has shrunk to give the eye an unimpeded view. Of mammals only humans and some primates enjoy color vision. The red flag is black to the bull. Horses live in a monochrome world .light visible to human eyes however occupies only a very narrow band in the whole electromagnetic spectrum. Ultraviolet rays are invisible to humans though ants and honeybees are sensitive to them. Humans though ants and honeybees are sensitive to them. Humans have no direct perception of infrared⁴² rays unlike the rattlesnake which has receptors tuned⁴³ into wavelengths longer than 0.7 micron. The world would look eerily⁴⁴ different if human eyes were sensitive to infrared radiation. Then instead of the darkness of night, we would be able to move easily in a strange shadowless world where objects glowed with varying degrees of intensity⁴⁵. But human eyes excel in other ways. They are in fact remarkably⁴⁶ discerning in color gradation. The color sensitivity of normal human vision is rarely surpassed even by sophisticated technical devices.


47 Folk Cultures 

A folk culture is a small isolated⁴⁷, cohesive⁴⁸, conservative, nearly self-sufficient group that is homogeneous in custom and race with a strong family or clan⁴⁹ structure and highly developed rituals. Order is maintained through sanctions based in the religion or family and interpersonal. Relationships are strong. Tradition is paramount⁵⁰, and change comes infrequently and slowly. There is relatively⁵¹ little division of labor⁵² into specialized⁵³ duties. Rather, each person is expected to perform a great variety of tasks, though duties may differ between the sexes. Most goods are handmade and subsistence economy prevails. Individualism is weakly developed in folk cultures as are social classes. Unaltered folk cultures no longer exist in industrialized countries such as the United States and Canada. Perhaps the nearest modern equivalent in Anglo America is the Amish, a German American farming sect¹ that largely renounces⁵⁴ the products and labor saving devices of the industrial age. In Amish areas, horse drawn⁵⁵ buggies still serve as a local transportation device and the faithful are not permitted to own automobiles⁵⁶. The Amish's central religious concept of Demut "humility⁵⁷", clearly reflects the weakness of individualism and social class so typical of folk cultures and there is a corresponding strength of Amish group identity. Rarely do the Amish marry outside their sect. The religion, a variety of the Mennonite faith, provides the principal mechanism⁵⁸ for maintaining order. 

By contrast a popular culture is a large heterogeneous⁵⁹ group often highly individualistic and a pronounced many specialized professions. Secular⁶⁰ institutions of control such as the police and army take the place of religion and family in maintaining order, and a money-based economy prevails. Because of these contrasts, "popular" may be viewed as clearly different from "folk". The popular is replacing the folk in industrialized countries and in many developing nations. Folk-made objects give way to their popular equivalent, usually because the popular item is more quickly or cheaply produced, is easier or time saving to use or leads more prestige to the owner. 

48 Bacteria 

Bacteria are extremely small living things. While we measure our own sizes in inches or centimeters, bacterial⁶¹ size is measured in microns. One micron is a thousandth of a millimeter: a pinhead is about a millimeter across. Rod-shaped bacteria are usually from two to four microns long, while rounded ones are generally one micron in diameter. Thus if you enlarged a rounded bacterium⁶² a thousand times, it would be just about the size of a pinhead. An adult human magnified by the same amount would be over a mile(1.6 kilometer) tall. 

Even with an ordinary microscope, you must look closely to see bacteria. Using a magnification of 100 times, one finds that bacteria are barely visible as tiny rods or dots. One cannot make out anything of their structure. Using special stains, one can see that some bacteria have attached to them wavy-looking "hairs" called flagella⁶³. Others have only one flagellum. The flagella rotate, pushing the bacteria through the water. Many bacteria lack flagella and cannot move about by their own power, while others can glide⁶⁴ along over surfaces by some little-understood mechanism. 

From the bacteria point of view, the world is a very different place from what it is to humans. To a bacterium water is as thick as molasses is to us. Bacteria are so small that they are influenced by the movements of the chemical molecules⁶⁵ around them. Bacteria under the microscope, even those with no flagella, often bounce about in the water. This is because they collide with the watery⁶⁶ molecules and are pushed this way and that. Molecules move so rapidly that within a tenth of a second the molecules around a bacteria have all been replaced by new ones; even bacteria without flagella are thus constantly exposed to a changing environment.


49 Sleep 

Sleet is part of a person's daily activity cycle. There are several different stages of sleep, and they too occur in cycles. If you are an average sleeper⁶⁷, your sleep cycle is as follows. When you fist drift off into slumber⁶⁸, your eyes will roll about a bit, you temperature will drop slightly, your muscles will relax, and your breathing well slow and become quite regular. Your brain waves slow and become quite regular. Your brain waves slow down a bit too, with the alpha rhythm of rather fast waves 1 sleep. For the next half hour or so, as you relax more and more, you will drift down through stage 2 and stage 3 sleep. The lower your stage of sleep. slower your brain waves will be. Then about 40to 69 minutes after you lose consciousness you will have reached the deepest sleep of all. Your brain will show the large slow waves that are known as the delta⁶⁹ rhythm. This is stage 4 sleep. 

You do not remain at this deep fourth stage all night long, but instead about 80 minutes after you fall into slumber, your brain activity level will increase again slightly. The delta rhythm will disappear, to be replaced by the activity pattern of brain waves. Your eyes will begin to dart⁷⁰ around under your closed eyelids⁷¹ as if you were looking at something occurring in front of you. This period of rapid eye movement lasts for some 8 to 15 minutes and is called REM sleep. It is during REM sleep period, your body will soon relax again, your breathing will slip gently back from stage 1 to stage 4 sleep----only to rise once again to the surface of near consciousness some 80 minutes later.


50. Cells and Temperature 　

Cells cannot remain alive outside certain limits of temperature and much narrower limits mark the boundaries of effective functioning. Enzyme⁷² systems of mammals and birds are most efficient only within a narrow range around 37C;a departure of a few degrees from this value seriously impairs⁷³ their functioning. Even though cells can survive wider fluctuations⁷⁴ the integrated actions of bodily systems are impaired⁷⁵. Other animals have a wider tolerance⁷⁶ for changes of bodily temperature. 

For centuries it has been recognized that mammals and birds differ from other animals in the way they regulate body temperature. Ways of characterizing the difference have become more accurate and meaningful over time, but popular terminology⁷⁷ still reflects the old division into "warm-blooded" and "cold-blooded" species; warm-blooded included mammals and birds whereas all other creatures were considered cold-blooded. As more species were studied, it became evident that this classification was inadequate⁷⁸. A fence lizard⁷⁹ or a desert iguana-each cold-blooded----usually has a body temperature only a degree or two below that of humans and so is not cold. Therefore the next distinction was made between animals that maintain a constant body temperature, called home0therms, and those whose body temperature varies with their environments, called poikilotherms. But this classification also proved inadequate, because among mammals there are many that vary their body temperatures during hibernation⁸⁰. Furthermore, many invertebrates⁸¹ that live in the depths of the ocean never experience change in the depths of the ocean never experience change in the chill of the deep water, and their body temperatures remain constant.