American black bears appear in a variety of colors despite their name. In the eastern part of their range, most of these bears have shiny black fur, but in the west they grow 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, roots, 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 eat during the winter months,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 litter 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。

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. Since the First World War, coal fired power plants have accounted for about half of the electricity produced in the United States each year. In 1986 such plants had a combined generating capacity of 289, 000 megawatts and consumed 83percent 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, coalfired 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 one 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 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 environmental 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 evolutionary and 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。