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Sleep 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 sleeper, your sleep cycle is as follows . When you first drift off into slumber, your eyes will roll about a bit, your temperature will drop slightly, your muscles will relax, and your breathing will slow and become quite regular. Your brain wavers slow down a bit too, with the alpha rhythm of rather fast waves predominating for the first few minutes. This is called stage 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, the slower your brain waves will be. Then, about 40 to 60 minutes after you lose consciousness, you will have reached the deepest sleep of all. Your brain waves will show the large slow waves that are known as the delta 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 dart around under your closed eyelids 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 that most dreams seem to occur. Provided that you do not wake up during the first REM sleep period,your body will soon relax again, your breathing will grow slow and regular once more, and you 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. 重点单词： When the Group of Seven was formed in 1920, the artists hoped for recognition of their works and ideas. In the catalog to their first exhibition, they actually invited adverse criticismbut only because what they feared most was indifference . At a time when Canada was coming into its own , they felt that a major factor in the development of a strong and healthy nation was a vital and relevant art. They hoped that their works would make a significant contribution to the evolution of a truly Canadian art tradition. Therefore, behind the Group's desire to paint the Canadian landscape lay the genuine conviction that it was the northern landscape that represented and expressed the country's unique character. It was this concept that was to capture the imagination of so many Canadians. Today there is every indication that the Group attained its goals. These artists achieved widespread popular success and acclaim , and their works have been heralded as one of the basic symbols of Canadian culture. They have been honored with exhibitions, degrees, and medals; reproductions of their works can be found on everything from posters to postage stamps . As frequently happens with popular trends , there has been a tendency to romanticize the accomplishments of the Group, which has inevitably caused many misconceptions to develop. The most common of these in the popular belief that the Seven were violently criticized in the first Group shows. However, the truth of the matter is that the reviews for these early shows were nearly all favorable . 重点单词： Marjorie Rice was an unlikely candidate for the role of mathematical innovator. She had no formal education in mathematics save a single course required for graduation from high school in 1939. Nonetheless, in 1975 she took up a problem that professional mathematicians had twice left for dead , and showed how much life as in it still. The problem was tessellation , or tiling of the plane , involves taking a single closed figure a triangle, for example, or a rectangle and fitting it together with copies of itself so that a plane is covered without any gaps or overlap. A region of this plane would look rather like a jigsaw puzzle whose pieces are all identical. Rice worked primarily with polygons, which consist only of straight lines. More specifically, she worked with convex polygons , in which the line joining any two points on the polygon lies entirely within the polygon itself or on one of its edges(A five-pointed star, for example, does not qualify as a convex polygon.) By the time Rice took up tiling, its basic properties had been established. Obviously, any square can tile the plane, as many kitchen floors have demonstrated. Equilateral triangles are also a fairly clear-cut case. There is one other regular polygon(a polygon whose angles, and sides, are equal) that can tile the plane: the hexagon . This fact was established by the ancient Greeks but had long before been exploited by honeybees in building their honeycombs. And what of irregular polygons? As it turns out any triangle or quadrilateral , no matter how devoid of regularity, will tile the plane. On the other hand, no convex polygon with more than six sides can do so, and the three classes of convex hexagons that can were uncovered by the end of the First World War. So the only real question left by the time Marjorie Rice began her work was which convex pentagons tile the plane. 重点单词： The constitutional requirements for holding congressional office in the United States are few and simple. They include age (twenty-five years of age for the House of Representatives , thirty for the Senate ); citizenship(seven years for the House, nine years for the Senate); and residency in the state from which the officeholder is elected. Thus, the constitutional gateways to congressional officeholding are fairly wide. Even these minimal requirements, however, sometimes arouse controversy . During the 1960's and 1970's, when people of the post - Second World War "baby boom" reached maturity and the Twenty-sixth Amendment (permitting eighteen year olds to vote )was ratified , unsuccessful efforts were made to lower the eligible age for senators and representatives. Because of Americans geographic mobility, residency sometimes is an issue . Voters normally prefer candidates with long-standing ties to their states or districts. In his 1978 reelection campaign , for instance, Texas Senator John Tower effectively accused his opponent, Representative Robert Krueger, of having spent most of his life "overseas or in the East" studying or teachinga charge taken seriously in Texas. Well-known candidates sometimes succeed without such ties. New York voters elected to the Senate Robert F. Kennedy (1965-1968) and Daniel Patrick Moynihan(1977-)even though each had spent much of his life elsewhere. While members of the House of Representatives are not bound to live in the district from which they are elected, most do so prior to their election. In the Senate, the "one person. one vote" rule does not apply. Article l of the Constitution assures each state, regardless of population, two Senate seats , and Article V guarantees that this equal representation cannot be taken away without the states' consent . The founders stipulated that senators be designated by their respective state legislatures rather than by the voters themselves. Thus, the Senate was designed to add stability, wisdom, and forbearance to the actions of the popularly elected House. This distinction between the two houses was eroded by the Seventeenth Amendment(1913), which provided for the direct popular election of senators. 重点单词： In 1868 Alfred Ely Beach, the 44-year-old publisher of Scientific American magazine and avid part-time inventor, built New York's first subway . Its site was. 21 feet beneath Broadway, between Warren and Murray streets. The tunnel itself was 9 feet in diameter, 312 feet long, and held one cylindrical car capable of going about 10 miles an hour. A giant 100-horsepower blower propelled the vehicle along a track until it reached the far end, where the fan, reversed by a trip wire , slowed the car to a stop and then pulled it back the other way. The elegance of this first subway will probably never be surpassed . The decor of Beach's tunnel platform included fine paintings, frescoed walls, a gram piano, a fountain, and a goldfish tank bringing the total private expenditure for the tunnel to $350,000. Overshadowed by all this was Beach's substantial contribution to engineering, his method of tunnel construction: a hydraulic shield that looked something like an open-ended tin can and was propelled by pistons that drove it through the earth. Dirt removal and bricklaying went on inside the shield, affording the workers complete protection against cave-ins. The shield would later be used for construction of London's Thames, Glasgow's Clyde, and New York's Hudson tunnels. After the stock exchange collapse of 1873, the subway was sealed up and forgotten. From then on, Beach devoted himself to publishing and did not live to see New York achieve his dream. In 1912, workers digging a new subway tunnel unexpectedly broke into Beach's subway and found the little car sitting on its tracks, the whole tunnel still remarkably intact. Today the Beach tunnel is part of the City Hall subway station, where a plaque commemorates Beach's pioneering and maverick achievement. 重点单词： |
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