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Fligh speeds of bird have been clocked many times, but usually at ground speed, and it is asserted that migrating birds travel faster when migrating than they travel at other times. The speed of most small migrating birds seldom exceeds 30 miles per hour. Most shorebirds average between 40 and 50 miles per hour, whereas many ducks travel 50 to 60 miles per hour. At such speeds migrating birds may cover several hundreds of miles in a day or a night. It is routine for North American migrants to travel more than 500 miles nonstop across the Gulf of Mexico, where there are no midway islands. A turnstone, one of a cosmopolitan species of shorebird banded in Europe, covered 510 miles in 25 hours. Blue geese make the 1,700 miles from James Bay in Canada to coastal Louisiana in 60 hours. Most routine migration probably takes place within 3,000 feet of the Earth, although David Lack, computing the altitude of British nigh migrants with radar, found that some small land birds often travel at 5,000 feet and may sometimes go as high as 14,000 feet. Radar has recorded birds at altitudes of 20,000 feet, perhaps not too surprising since many species cross both the Andes and the Himalayas during migration. 重点单词： The Alaska pipeline starts at the frozen edge of the Arctic ocean. It stretches southward across the largest and northernmost state in the United States, ending at a remote ice-free seaport village nearly 800 miles from where it begins. It is massive in size and extremely complicated to operate. The steel pipe crosses windswept plains and endless miles of delicate tundra that tops the frozen ground. It weaves through crook canyons, climbs sheer mountains, plunges over rocky crags, makes its way through thick forests, and passes over or under hundreds of rivers and streams. The pipe is 4 feet in diameter, and up to 2 million barrels (or 84 million gallons) of crude oil can be pumped through it daily. Resting on H-shaped steel racks called " bents," long sections the pipeline follow a zigzag course high above the frozen earth. Other long sections drop out of sight beneath spongy or rocky ground and return to the surface later on. The pattern of the pipelines's up-and-down route is determined by the often harsh demands of the arctic and subarctic climate, the tortuous lay of the land, and the varied compositions of soil, rock, or permafrost (permanently frozen ground). A little more than half of the pipeline is elevated above the ground. The remainder is buried anywhere from 3 to 12 feet, depending largely upon the type of terrain and the properties of soil. One of the largest in the world, the pipeline cost approximately $8 billion and is by far the biggest and most expensive construction project ever undertaken by private industry. in fact, no single business could raise that much money, so 8 major oil companies formed a consortium in order to share the cost. Each company controlled oil rights to particular shares of land in the oil fields and paid into the pipeline-construction fund according to the size of its holdings. Today, despite enormous problems of climate, supply shortages, equipment breakdowns, labor disagreements, treacherous terrain, a certain amount of mismanagement, and even theft, the Alaska pipeline has been completed and is operating. 重点单词： The ability of agriculture to smooth out the seasonal scarcities of wild foodstuffs had major consequences for the sizes of Native American populations in New England, The nonagricultural Native Americans of Maine sustained population densities, on average, of perhaps 41 persons per 100 square miles. the crop-raising native Americans, on the other hand, probably maintained 287 persons on an identical amount of land, a sevenfold difference. When these two broad groups were combined, the total Native American population of New England probably numbered somewhere between 70,000 and 100,000 people in 1600, ( Lest this seem unimpressive, one should remember that the English population of New England was smaller than this even at the beginning of the eighteenth century, having reached only 93,000 people by 1700.) the crucial role of agriculture in maintaining so large a Native American population in pre colonial New England is clear; although agricultural and nonagricultural peoples inhabited roughly equal areas of southern and northern New England, respectively, those who raised crops contributed over 80 percent of the total population. 重点单词： The North American frontier change some of the characteristics of the pioneers of the 1750's and intensified others. They were , as a group, semiliterate, proud, and stubborn, as dogged in their insistence on their own way of life as pine roots cracking granite to grow. Perhaps their greatest resource was their capacity to endure. They outlasted recurrent plagues of smallpox and malaria and a steady progression of natural accidents. They were incredibly prolific. Squire Boone's family of eight children was small by frontier standards. James Robertson, an eventual neighbor of Boone's and the founder of Nashville, had eleven children. Twice-married John Sevier, the first governor of Tennessee, fathered eighteen; his longtime enemy, John Tipton, also twice married, produced seventeen. The entire assets of one of these huge families often amounted, in the beginning, to little more than an axe, a hunting knife, an auger, a rifle, a horse or two, some cattle and a few pigs, a sack of seed corn and another of salt, perhaps a crosscut saw, and a loom. those who moved first into a new region lived for months at a time on wild meat, Indian maize, and native fruits in season. Yet if they were poor at the beginning. they confidently expected that soon they would be rich. In a way almost impossible to define to urban dwellers, a slice of ground suitable for farming represented not just dollars and cents. but dignity. The obsession brought shiploads of yearners every week to Boston, New York, Philadelphia, Baltimore, Charles Towne, and Savannah. It sent them streaming westward into the wilderness after their predecessors to raise still more children who wanted still more land. 重点单词： Over the years, and especially since the Second World War, the realm of landscape architecture has been diversified and its activities classified in response to the needs of a changing world. There now appear to be three clearly definable related stages. First there is landscape planning and assessment. This has a strong ecological and natural science base and is concerned with the systematic evaluation of large areas of land in terms of the land's suitability or capability for any likely future use. The process usually involves a team of specialists. It may result in a land use plan or policy, affecting, for example, the distribution and type of development or land use, the alignment of highways, the location of industrial plant, the conservation of water, soil, and amenity values, and the use of countryside for recreation. The study area usually coincides with a natural physiographic region such as the watershed of a major river or some other logical unit of land; unfortunately these seldom coincide with the legal jurisdiction of county and state boundaries. The planning function may at times be less comprehensive and concentrate on the impact of major proposals on the environment or the identification of land suitable for one major use such as recreation. The second stage is site planning. This represents the more conventional kind of landscape architecture and within this realm lies landscape design. Site planning is the process in which the assessment of the site and the requirements of the program for the use of the site are brought together in creative synthesis. Elements and facilities are located on the land in functional relationships and in a manner fully responsive to the characteristics of the site and its region. finally, there is detailed landscape design. This the selection of components, materials, and plants and their combination as solutions to limited and well-defined problems; paving, steps, fountains, and so forth. This is the process through which specific quality is given to the diagrammatic spaces and areas of the site plan. 重点单词： |
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