15000 bài tập tách từ đề thi thử môn Tiếng Anh có đáp án (Phần 65)

Forces other than damaging winds are also at work inside tornadoes. Sometimes, as the writhing, twisting funnel passes over a house, the walls and ceiling burst apart as if a bomb had gone off inside. This explosion is caused by the low air pressure at the center of a tornado.

The pressure at the center of a tornado is usually 13 pounds per square inch. However, inside the house, the air pressure is normal, about 15 pounds per square inch. The difference of 2 pounds per square inch between the inside and outside pressure may not seem like much. But suppose a tornado funnel passes over a small building that measures 20 by 10 by 10 feet. On each square inch of the building, there is 2 pounds of pressure from the inside that is not balanced by air pressure outside the building. On the ceiling, that adds up to an unbalanced pressure of 57, 600 pounds. The pressure on the four walls adds up to 172,800 pounds.

If windows are open in the building, some of the inside air will rush out through them. This will balance the pressure inside and outside the building. But if the windows are shut tightly, the enormous inside pressure may cause the building to burst.

Unfortunately, heavy rain and hail often occur in thunderstorms that later produce tornadoes. So people frequently shut all windows to protect their property. This may cause far worse damage later. For the same reason, tornado cellars must have an air vent. Otherwise, the cellar door might be blown out when a tornado passes over it.

Native Americans from the southeastern part of what is now the United States believed that the universe in which they lived was made up of three separate, but related, worlds, the Upper World, the Lower World, and This World. In the last there lived humans, most animals, and all plants.

This World, a round island resting on the surface of waters, was suspended from the sky by four cords attached to the island at the four cardinal 'points of the compass. Lines drawn to connect the opposite points of the compass, from north to south and from east to west, intersected. This World to divide it into four wedge - shaped segments. Thus a symbolic representation of the human world was a cross within a circle, the cross representing the intersecting lines and the circle the shape of  This World.

Each segment of  This World was identified by  its own color. According to Cherokee doctrine, east was associated with the color red  because it was the direction of the  Sun, the greatest deity of all. Red was also the color of fire, believed to be directly connected with the Sun, with blood, and therefore with life. Finally, red was the color of success. The west was the Moon segment; it provided no warmth and was not life - giving as the Sun was. So its color was black. North was the direction of cold, and so its color was blue sometimes purple, and it represented trouble and defeat. South was the direction of warmth, its color, white, was associated with peace and happiness.

The southeastern Native Americans' universe was one in which opposites were constantly at war with each other, red against black, blue against white. This World hovered somewhere between  the perfect order and  predictability of the Upper World and the total disorder and instability of the Lower World. The goal was to find some kind of halfway path, or balance, between those other worlds.

Arid regions in the southwestern United States have become increasingly inviting playgrounds for the growing number of recreation seekers who own vehicles such as motorcycles or powered trail bikes and indulge in hill-climbing contests or in carving new trails in the desert. But recent scientific studies show that these off-road vehicles can cause damage to desert landscapes that has long-range effects on the area’s water-conserving characteristics and on the entire ecology, both plant and animal. Research by scientists in the western Mojave Desert in California revealed that the compaction of the sandy arid soil resulting from the passage of just one motorcycle markedly reduced the infiltration ability of the soil and created a stream of rain runoff water that eroded the hillside surface. In addition, the researchers discovered that the soil compaction caused by the off-road vehicles often killed native plant species and resulted in the invasion of different plant species within a few years. The native perennial species required many more years before they showed signs of returning. The scientists calculated that roughly a century would be required for the infiltration capacity of the Mojave soil to be restored after being compacted by vehicles.