说明文中主位推进模式的运用

说明文是一种以说明为主要表达方式的文体，它对客观事物（或事理）的性状、特点、功能和用途等做出科学的说明。说明文主要运用说明的表达方式，侧重对客观事物或事理如实地介绍、简要地解说，力求使人有所知。英语说明文在主位推进模式的应用上有其特点。

Antarctica

By Ian Cameron

Seen from space, the astronauts tell us (T1), the most distinctive feature of our planet is the ice sheet of Antarctica which “radiates light like a great white lantern across the bottom of the world”(R1). This ice sheet (T2) covers 5, 500, 000 square miles (an area greater than the United States and Central America combined); it averages more than 7, 000 feet in thickness; it contains more than 90 percent of the world’s ice and snow, and if suddenly it melted the oceans would rise to such a height that every other person on earth would be drowned (R2). Antarctica (T3) is in fact our planet’s largest and most spectacular natural phenomenon (R3).

Yet 160 years ago (T1) no one had ever set eyes on this vast continent, let alone set foot on it; and even today (T2) man’s tenure of it is unsure and his knowledge comparatively slight. To understand why (T3), we need to appreciate the sort of place Antarctica is.

People (T1) used to regard the Arctic and the Antarctic as much alike. In fact their differences (T) outweigh their similarities. The Arctic is closely hemmed in by the populated landmasses of Europe, America and Asia; the Antarctic in contrast is in splendid isolation, divided from the nearest land by vast reaches of the most tempestuous seas on earth. Another big difference is the climate. We are so inclined to think of both the Arctic and Antarctic as cold, that we tend to forget how much colder the latter is. North of the Arctic Circle, tens of thousands of families live in comfort all the year round; thousands of plants and animals are able to survive; hundreds of children are born every year. South of the Antarctic Circle, in contrast, there is no habitation that a man can describe as home; the only plants are a handful of mosses and lichens; the only landlife is simple one-celled creatures and wingless flies; no human child has ever been born there.

It is not hard to see why. The basic essentials to life are rainfall, warmth and a degree of stillness. The Arctic, at times, provides all three; the Antarctic seldom provides any—witness the descriptions of those who have been there:

As regards precipitation the Southern Continent is a desert with an annual fall no greater than the outback of Australia. The exact accumulation is difficult to measure because of the common occurrence of blown snow, but the central area certainly receives less than five centimeters per year; and there may well be places close to the Pole where snow has never fallen. (US Weather Bureau) Antarctica is by far the coldest place on earth; weather stations have reported temperatures of － 88℃, more than 20℃ below those recorded anywhere else. In this sort of cold if you try to burn a candle the flame becomes obscured by a hood of wax, if you drop a steel bar it is likely to shatter like glass, tin disintegrates into loose granules, mercury freezes into a solid metal, and if you haul up a fish through a hole in the ice within five seconds it is frozen so solid that it has to be cut with a saw (John Bechervaise).

All those who have set foot in Antarctica agree that its main and most cruel characteristic is wind. When we wintered in Adelie Land the wind on 5th July blew nonstop for eight hours at an average speed of 107 mph; gusts were recorded of over 150 mph, and the average wind speed for the month was 63.3. In these conditions it was possible to stand for no more than a few seconds, and then only by leaning forward at an angle of 45° (Douglas Mawson)!

It is worth remembering that wind is as injurious to human health as cold; for by disrupting the cushion of warmth which is trapped by pores and hairs of the skin, each knot of wind has an effect on life equal to a drop of one degree in temperature. So whereas a man can live quite happily at － 20 ℃ in the still air, when the temperature is － 20 ℃ and the wind speed 60 knots he will very quickly die. Small wonder that whereas in the eighteenth and early nineteenth centuries man swiftly explored and occupied the rest of his planet, the southernmost continent remained inviolate.

Yet climate by itself was not the main drawback to the unveiling of Antarctica; an even greater drawback, at least in the early stages, was the nature of the sea—the Southern Ocean—which surrounds it.

(A New English Course Revised Edition, Students’ Book 4, p130-131)

笔者根据前面提及的篇章层级链式主位系统，构建这篇文章的篇章主位、述位结构树形图。

如图5-4-2所示，篇章的主位是标题，篇章的述位是第二段到第六段。篇章的主位是篇章叙述的起点，后续的段落一般应和南极洲有关，由此可以预测后面的论述与南极洲涉及的相关话题有关。

图5-4-2　《南极洲》层级链式主位系统

根据上图，第一段是第二层级的主位，第二段到第六段是第二层级的述位，第三层级的主位是第一段的第一句话，即“Seen from space，the astronauts tell us，the most distinctive feature of our planet is the ice sheet of Antarctica which ‘radiates light like a great white lantern across the bottom of the world’ .”。第一段中的其余句子是第三层级的述位，以此类推，下列各段中的第一句话是主位，其余句子是各段的第三层级述位。根据第一段第一句，作者使用了说明文常用的开篇模式—开门见山提出中心主题，引起读者的兴趣后在担当述位的句子中生动描写了南极洲：“the astronauts tell us，the most distinctive feature of our planet is the ice sheet of Antarctica which ‘radiates light like a great white lantern across the bottom of the world’.”。

第二段到第六段的首句是重要的一级主位单位：“Yet 160 years ago no one had ever set eyes on this vast continent, let alone set foot on it; and even today man’s tenure of it is unsure and his knowledge comparatively slight.”“People used to regard the Arctic and the Antarctic as much alike.”; “It is not hard to see why.”; “It is worth remembering that wind is as injurious to human health as cold; for by disrupting the cushion of warmth which is trapped by pores and hairs of the skin, each knot of wind has an effect on life equal to a drop of one degree in temperature.”; “Yet climate by itself was not the main drawback to the unveiling of Antarctica；an even greater drawback, at least in the early stages, was the nature of the sea—the Southern Ocean—which surrounds it.”。这些句子同时也是每一段的主题句，用于说明南极洲的特点。即使不阅读各段首句后的其余句子，读者也可以知道作者从哪几个方面说明南极洲的特点。