段落的写作（3）

    (4)段中主题句:当我们对两种事物或某事物的两个方面进行比较或对照时,主题句常常在段落的中间出现。在这种段落中,通常先提出一个观点并稍加发挥,接着主题句出现,笔锋一转,提出不同或相反论点,然后展开论证。论述的中心是二者的相异点。其特点是不仅使问题多样化,而且使内容协调一致。

    例1: Californians and New Englanders are both American.They speak the same language and abide by the same federal laws.But they arevery different in their ways of life.Mobility--- both physical and psy-chological --- has made a GREat impression on the culture of Californians;lack of mobility is the mark of the customs and morality of New Eng-landers.

    例2: Many people have heard about the famous Leaning Tower of Pisa,but few know that Pisa also has a university of ancient origin.In about the year 1580 its most illustrious student was a mathematical genius named Galileo,who was soon to invent the hydrostatic balance,a thermometer,and a proportional compass.But Galileo's invention was his telescope,which enabled him to make some significant contributions to astronomy.It was this instrument that made it possible for Galileo to discover satellites around Jupiter,the unevenness of the moon's surface,and the fact that the Milky Way is a vast collection of stars.By studying sunspots,he was also able to infer the rotation of the sun.