题目内容
Imagine an accident in which a nuclear power plant releases radioactive gas. The cloud starts moving with the wind. Clearly, the authorities will want to evacuate anyone in its path, but what is that path？ Local wind information is meaningless without information about terrain, a mountain range or series of valleys can divert both wind and gas in unpredictable directions.
To make "downwind" a useful term, scientists at Lawrence Livermore National Laboratory have put the United States on a computer, the entire United States-every hill, every valley, every mile of seashore. Within minutes of a disaster, they can give meteorologists a context for weather data, and thus the ability to predict how toxic gases might spread.
The database for this computer map is a series of altitude measurements, made over many years by the Defense Department and the U.S. Geological Survey. They represent the height above sea level of over a billion separate points--a grid of points 200 feet apart, spanning the country. Armed with these data, plus a program that manipulates them, a Cray-1 computer can produce an image of any piece of terrain, seen from any angle, illuminated by an imaginary sun at any time of day placing the "observer" at any altitude from zero to 40,000 feet.
"We use a technique called ray tracing," says Patrick Weidhaas, one of the Livermore computer scientists who wrote the program. The computer is told where the observer is. The program traces an imaginary ray from there outward until it "intersects" with one of the points' of altitude recorded in the machine' s memory. The computer then puts a dot of color at the proper place on the screen, and the program traces another ray.
At its highest resolution of 2,000 horizontal and 1,700 vertical dots per picture, the computer has to trace several million rays. Even on the Cray, the most powerful computer in the world, this takes about a minute. Reducing the resolution to 400～300 (a TV screen has 800 x 700) speeds it up to about eight seconds. "We can't produce a movie simulating flight on the screen in real time," says Weidhaas. There is a way around the problem: Two movies have been made using still pictures generated by the computer as individual frames. "The results were impressive," he says, "but it was cumbersome to do. At twenty-four frames per second, it takes fourteen hundred separate computer images to make a one-minute film." Another limitation. The computer can access only enough memory to cover a 15-mile-square area. An "observer" high up will see blank spaces beyond those limits.
Weidhaas wants to add information about what overlies the terrain-cities, vegetation, roads, and so on. "Making the image as realistic as possible will make our advice more effective," he says, "and might lead to uses we haven't thought of yet."
As used in the first paragraph, terrain most clearly means ______.
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The great advance in rocket theory 40 years ago showed that liquid-fuel rockets were far superior in every respect to the skyrocket with its weak solid fuel, the only kind of rocket then known. However, during the last decade, large solid-fuel rockets with solid fuels about as powerful as liquid fuels have made their appearance, and it is a favorite layman's question to inquire which one is "better". The question is meaningless; one might as well ask whether a gasoline or a diesel engine is "better". It all depends on the purpose. A liquid-fuel rocket is complicated, but has the advantage that it can be controlled beautifully. The burning of the rocket engine can be stopped completely: it can be re-ignited when desired. In addition, the thrust can be made to vary by adjusting the speed of the fuel pumps. A solid-fuel rocket, on the other hand, is rather simple in construction, though hard to build when a really large size is desired. But once you have a solid-fuel rocket, it is ready for action at very short notice. A liquid-fuel rocket has to be fueled first and cannot be held in readiness for very long after it has been fueled. However, once a solid-fuel rocket has been ignited, it will keep burning. It cannot be stopped and re-ignited whenever desired (it could conceivably be stopped and re-ignited after a pre-calculated time of burning has elapsed) and its thrust cannot be varied. Because a solid-fuel rocket can be kept ready for a long time, most military missiles employ solid fuels, but manned space flight needs the fine adjustments that can only be provided by liquid fuels. It may be added that a liquid-fuel rocket is an expensive device; a large solid-fuel rocket is, by comparison, cheap. But the solid fuel, pound per pound; costs about 10 times as much as the liquid fuel. So you have, on the one, hand, an expensive rocket with a cheep fuel and on the other hand a comparatively cheap rocket with an expensive fuel.
The author feels that a comparison of liquid and solid-fuel rockets shows that ______.
The Value of Writing Well
It's that time of year again. No, not "the holiday season". I mean, it is holiday time, but for professors it doesn't start feeling like holiday time until final grades are in and the books are closed on another semester. No, for me, it's paper-grading time, the time of year when I'm reminded over and over of the importance of good writing skills--and of their rarity.
The ability to write well is not a gift. Sure, the special something that sets apart a Tolstoy or Shakespeare or Salman Rushdie or Isabel Allende is a gift, a talent born of disposition, experience, and commitment. But just to be able to communicate clearly with the written word takes no special talent; it's a skill like any other.
Well, not exactly like any other. Because the words we use to write with are the same words we use to think with, learning to write well has ramifications that go beyond the merely technical. As we improve our writing ability, we improve our ability to think--to build an argument, to frame. issues in compelling ways, to weave apparently unrelated facts into a coherent whole.
And despite the recurring hand-wringing and chest-beating about the "end of literacy" and the "death of the printed word", the reality is that we write more than ever these days. While it's a rare person who sits down with pen and paper in hand and writes a letter to a friend or loved one, we pour emails into the ether at an astounding rate. We text message, tweet, instant message, blog, comment, and otherwise shoot words at each other in a near-constant flow of communication. We annotate group portraits, LOL-ify cat pictures, and tag.., well, everything. At work, we write letters, proposals, PowerPoint presentations, Business requirement documents, memos, speeches, mission statements, position papers, operating procedures, manuals, brochures, package copy, press releases, and dozens of more specialized types of documents.
We are, it seems, writing creatures. Homo scribus, if you will.
It's no wonder that Businesses repeatedly cite "communication skills" as the single most desirable trait in new employees. The kicker, though, is that we are as a society incredibly bad at writing. Public schools do a poor job of teaching students how to write well-they barely manage to instill the basic rules of grammar and the miserable 5-paragraph essay, let alone how to write with style. and verve, how to put together an argument that moves steadily from one point to the next to persuade a reader of some crucial point, how to synthesize ideas and data from multiple sources into something that takes those ideas one step further.
It's not just the teachers' fault. Teachers do the best they can with what they're given, and all too often what they're given is inadequate resources with which to teach classrooms full of unmotivated students who could care less about writing. Add to that the requirements of mandatory nation-wide tests that reward conformity, not creativity, and the threat of punishment for any school whose students fail to fall within the fairly rigid boundaries of the test's requirements, and you've got a pretty bad situation all around for instilling in students the power to write well.
That is, alas, a great disservice. Being able to write well vastly improves students'-and others'-potential for success, regardless of the field they find themselves in. As I've already mentioned, people who write well tend to be better able to think through problems and tease out patterns in outwardly dissimilar situations. More importantly, people who write well have the opportunity to make a mark in the world, because their best ideas aren't trapped in their own minds for lack of a means of expression.
This is true whether you're a CEO or a janitor, a marketing expert or an Emergency Medical Technician. The skills
The information used by the computer to make its detailed maps ______. Ⅰ. was gathered by
对于以募集方式设立的股份公司，发起人拟订的章程草案须经出席()的超过认股人所持表决权的()通过。
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