考研英語出色文章

　　每個人的家庭情況不同，每個人的性格不同，每個人在就業還是考研的問題上選擇也會有所不同。下面是小編給大家整理的，供大家參閱!

　　：The Salinity of Ocean Waters

　　if the salinity of ocean waters is analyzed, it is found to vary only slightly from place to place. nevertheless, some of these small changes are important. there are three basic processes that cause a change in oceanic salinity. one of these is the subtraction of water from the ocean by means of evaporation--- conversion of liquid water to water vapor. in this manner the salinity is increased, since the salts stay behind. if this is carried to the extreme, of course, white crystals of salt would be left behind.

　　the opposite of evaporation is precipitation, such as rain, by which water is added to the ocean. here the ocean is being diluted so that the salinity is decreased. this may occur in areas of high rainfall or in coastal regions where rivers flow into the ocean. thus salinity may be increased by the subtraction of water by evaporation, or decreased by the addition of fresh water by precipitation or runoff.

　　normally, in tropical regions where the sun is very strong, the ocean salinity is somewhat higher than it is in other parts of the world where there is not as much evaporation. similarly, in coastal regions where rivers dilute the sea, salinity is somewhat lower than in other oceanic areas.

　　a third process by which salinity may be altered is associated with the formation and melting of sea ice. when sea water is frozen, the dissolved materials are left behind. in this manner, sea water directly materials are left behind. in this manner, sea water directly beneath freshly formed sea ice has a higher salinity than it did before the ice appeared. of course, when this ice melts, it will tend to decrease the salinity of the surrounding water.

　　in the weddell sea antarctica, the densest water in the oceans is formed as a result of this freezing process, which increases the salinity of cold water. this heavy water sinks and is found in the deeper portions of the oceans of the world.

　　：Cohesion-tension Theory

　　Atmospheric pressure can support a column of water up to 10 meters high. But plants can move water much higher; the sequoia tree can pump water to its very top more than 100 meters above the ground. Until the end of the nineteenth century, the movement of water in trees and other tall plants was a mystery. Some botanists hypothesized that the living cells of plants acted as pumps. But many experiments demonstrated that the stems of plants in which all the cells are killed can still move water to appreciable heights. Other explanations for the movement of water in plants have been based on root pressure, a push on the water from the roots at the bottom of the plant. But root pressure is not nearly great enough to push water to the tops of tall trees. Furthermore, the conifers, which are among the tallest trees, have unusually low root pressures.

　　If water is not pumped to the top of a tall tree, and if it is not pushed to the top of a tall tree, then we may ask: how does it get there? According to the currently accepted cohesion-tension theory, water is pulled there. The pull on a rising column of water in a plant results from the evaporation of water at the top of the plant. As water is lost from the surface of the leaves, a negative pressure, or tension, is created. The evaporated water is replaced by water moving from inside the plant in unbroken columns that extend from the top of a plant to its roots. The same forces that create surface tension in any sample of water are responsible for the maintenance of these unbroken columns of water. When water is confined in tubes of very small bore, the forces of cohesion ***the attraction between water molecules*** are so great that the strength of a column of water compares with the strength of a steel wire of the same diameter. This cohesive strength permits columns of water to be pulled to great heights without being broken.

　　：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.

　　：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.

　　：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.

　　以上是小編整理所得，歡迎大家閱讀和收藏。