在科技论文写作中，我们经常要用到以时间和过程为主线的段落。比如，当我们要记叙论述对象的发展历史或某一课题的进展过程时，要以时间为主线；当我们叙述生产工艺、流程、机械和装置的工作原理时，要以过程为主线进行描述。

1.时间关系

以时间为主线的段落，可以按照时间的先后顺序进行叙述。但在实际写作中，我们必须根据写作目的和内容的需要，按照段落的内在线索来安排先后顺序。比如，例1和例2就都不是按时间的顺序进行叙述的。

例1：

The emphasis given by both scholars and statesmen to the presumed disappearance of the American frontier helped to obscure the great importance of changes in the conditions and consequences of international trade that occurred during the second half of the nineteenth century.In 1869 the Suez Canal was opened and the first transcontinental railroad in the United States was completed.An extensive network of telegraph and telephone communications was spun:Europe was connected by submarine cable with the United States in 1866 and with South America in 1874.By about 1870 improvements in agricultural technology made possible the full exploitation of areas that were most suitable for extensive farming on a mechanized basis.

例2：

Total U.S.research and development spending is projected to reach a current-dollar level of$66.7 billion in 1981.This is an increase of 10%over the 1980 projected level and nearly double the amount spent on these activities in 1975.Even in constant dollars，U.S.R&D funding grew at an average annual rate of better than 3%.That growth resulted in large part from increased emphasis on searching for means to resolve energy and environmental problems and a resurgence of defense R&D activity.Between 1975 and 1978，the last year for which survey data are available，energy accounted for one-third of the R&D spending increase while，at the same time，amounting to 10%of the national R&D effort.

2.程序和步骤

以程序和步骤为主线的段落常用来叙述生产的工艺流程、机械的工作原理及使用说明等，因此必须按照其过程的先后顺序，一环扣一环地进行叙述。各步骤之间必须要有表明顺序的连接词。

常用的词语和表达有：

begin/start/commence/kicks off/etc.

first，second，then，next，finally，at last

stop/end/finish/wind up/etc.(www.daowen.com)

before，after，further，when，as

The main steps in...are:

One has to go through...steps...

The...process has...major steps...

There are...steps in the process of...

（1）以程序为主线

例1：

Heat pumps circulate a fluid refrigerant that cycles alternatively from its liquid phase to its vapor phase in a closed loop.The refrigerant，starting as a low-temperature，low-pressure vapor，enters a compressor driven by an electric motor.The refrigerant leaves the compressor as a hot，dense vapor and flows through a heat exchanger called the condenser，which transfers heat from the refrigerant to a body of air.Now the refrigerant，as a high-pressure，cooled liquid，confronts a flow restriction which caused the pressure to drop.As the pressure falls，the refrigerant expands and partially vaporizes，becoming chilled.It then passed through a second heat exchanger，the evaporator，which transfers heat from the air to the refrigerant，reducing the temperature of the second body of air.

（2）以步骤为主线

例2：

The purification of water is basically a two-step or three-step process carried out under the strict supervision of public health scientists and engineers.As the first step，natural water from the least contaminated source is allowed to stand in large reservoirs，where most of the mud，clay，and silt settles out；this is called“sedimentation”...As the second step，the water is filtered through beds of sand gravel，which remove other impurities and chemicals in it.During or after filtration，chemicals are ordinarily added to the water to kill any remaining harmful bacteria.Chlorine is one of the most common chemicals used for this purpose.A third step taken by some municipalities is adding to the water other beneficial chemicals such as fluoride to make tooth enamel hard，and soda ash to make the water itself soft.