来源：网络 发布时间：2015-02-11 作者：上外培训网
The city water pipes in Rome were usually of baked clay or lead; copper was sometimes used and also hollowed stone. For the large supply conduits leading to the city the Romans used covered channels with free water surfaces, rather than pipes. Perhaps this choice was a matter of economics, for apparently they could make lead pipes up to 15 inches in diameter. While pipes can follow the profile of undulating ground, with the pressure increasing in the lower areas, channels cannot. They must slope continuously downwards, because water in channels does not normally flow uphill; and the grade must be flat, from 1 in 60 in small channels to perhaps 1 in 3,000 in large ones, to keep the water speed down to a few feet per second. Thus the main supply channels or aqueducts had long lengths of flat grade and where they crossed depressions or valleys they were carried on elevated stone bridges in the form of tiered arches. At the beginning of the Christian era there were over 30 miles of these raised aqueducts in the 250 miles of channels and tunnels bringing water to Rome. The channels were up to 6 feet wide and 5 to 8 feet high. Sometimes channels were later added on the tops of existing ones. The remains of some of these aqueducts still grace the skyline on the outskirts of Rome and elsewhere in Europe similar ruins are found.
Brick and stone drains were constructed in various parts of Rome. The oldest existing one is the Cloaca Maxima which follows the course of an old stream. It dates back at least to the third century B.C. Later the drains were used for sewage, flushed by water from the public baths and fountains, as well as street storm run-off.
The truly surprising aspect of the achievements of all the ancient hydraulic artisans is the lack of theoretical knowledge behind their designs. Apart from the hydrostatics of Archimedes, there was no sound understanding of the most elementary principles of fluid behaviour. Sextus Frontinus, Rome’s water commissioner around A.D. 100, did not fully realize that in order to calculate the volume rate of flow in a channel it is necessary to allow for the speed of the flow as well as the area of cross-section. The Romans’ flow standard was the rate at which water would flow through a bronze pipe roughly 4/3 inch in diameter and 9 inches long. When this pipe was connected to the side of a water-supply pipe or channel as a delivery outlet, it was assumed that the outflow was at the standard rate. In fact, the amount of water delivered depended not only on the cross-sectional area of the outlet pipe but also on the speed of water flowing through it and this speed depended on the pressure in the supply pipe.
21. The Romans used all of the following to make water pipes EXCEPT _________.
(A) earth (B) wood (C) copper (D) stone
22. Covered channels were used instead of pipes to supply large quantities of water probably because _________.
(A) the Romans could build them more cheaply
(B) these channels could follow uneven ground more easily
(C) the Romans could not build large pipes
(D) these channels avoided rapid changes of pressure
23. The use of ‘grace’ in line 15 suggests that the aqueducts today are _________.
(A) hideous (B) divine (C) useful (D) attractive
24. In order to calculate the volume of water flowing through a pipe, it is important to know its speed and ________.
(A) the area across the end of the pipe (B) the length of the pipe
(C) the water pressure in the pipe (D) the level from which the water falls
25. The main subject of the passage is concerned essentially with __________.
(A) the classical scientific achievements
(B) the theoretical Greek hydrostatics
(C) the ancient Roman hydraulic system
(D) the early European architectural designing
Every day of our lives we are in danger of instant death from small high-speed missiles from space—the lumps of rocky or metallic debris which continuously bombard the Earth. The chances of anyone actually being hit, however, are very low, although there are recorded instances of ‘stones from the sky’ hurting people, and numerous accounts of damage to buildings and other objects. At night this extraterrestrial material can be seen as ‘fireballs’ or ‘shooting stars’, burning their way through our atmosphere. Most, on reaching our atmosphere, become completely vaporised.
The height above ground at which these objects become sufficiently heated to be visible is estimated to be about 60-100 miles. Meteorites that have fallen on buildings have sometimes ended their long lonely space voyage incongruously under beds, inside flower pots or even, in the case of one that landed on a hotel in North Wales, within a chamber pot. Before the era of space exploration it was confidently predicted that neither men nor space vehicles would survive for long outside the protective blanket of the Earth’s atmosphere. It was thought that once in space they would be seriously damaged as a result of the incessant downpour of meteorites falling towards our planet at the rate of many millions every day. Even the first satellites showed that the danger from meteorites had been greatly overestimated by the pessimists, but although it has not happened yet, it is certain that one day a spacecraft will be badly damaged by a meteorite.
The greatest single potential danger to life on Earth undoubtedly comes from outside our planet. Collision with another astronomical body of any size or with a ‘black hole’ could completely destroy the Earth almost instantly. Near misses of bodies larger than or comparable in size to our own planet could be equally disastrous to mankind as they might still result in total or partial disruption. If the velocity of impact were high, collision with even quite small extraterrestrial bodies might cause catastrophic damage to the Earth’s atmosphere, oceans and outer crust and thus produce results inimical to life as we know it. The probability of collision with a large astronomical body from outside our Solar System is extremely low, possibly less than once in the lifetime of an average star. We know, however, that our galaxy contains great interstellar dust clouds and some astronomers have suggested that there might also be immense streams of meteorite matter in space that the Solar system may occasionally encounter. Even if we disregard this possibility, our own Solar system itself contains a great number of small astronomical bodies, such as the minor planets or asteroids and the comets, some with eccentric orbits that occasionally bring them close to the Earth’s path.
26. According to the writer, the Earth is being continuously bombarded by _________.
(A) big bright stars from space
(B) man-made space vehicles
(C) great interstellar dust clouds
(D) small high-speed pieces of rock from space
27. The word “vaporised” (para.1) means _________.
(A) turned from stones into missiles
(B) turned from a fireball into black
(C) turned from a solid into a gas
(D) turned from meteors into shooting stars
28. Why was it once thought that no spacecraft would survive for very long in space?
(A) People believed that spacecraft would be destroyed in a black hole.
(B) People believed that spacecraft would be misguided by missiles.
(C) People believed that spacecraft would be collided with a star.
(D) People believed that spacecraft would be damaged by meteorites.
29. What is the greatest danger to life on Earth?
(A) Collision with small high-speed missiles.
(B) Collision with an astronomical body.
(C) Collision with stones from the sky.
(D) Collision with spacecrafts.
30. According to the passage, which of the following statements is true?
(A) Our galaxy contains great interstellar dust clouds.
(B) Near misses of bodies smaller than our own planet could be disastrous.
(C) The probability of collision with a large astronomical body is very high.
(D) The chances of anyone actually being hit by missiles are very high.
21-25 B A D A C 26-30 D C D B A